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INSECTS AND HUMAN
WELFARE

AN ACCOUNT OF THE MORE IMPORTANT RELATIONS

OF INSECTS TO THE HEALTH OF MAN, TO

AGRICULTURE, AND TO FORESTRY

BY

CHARLES THOMAS BRUES

ASSISTANT PROFESSOR OF ECONOMIC ENTOMOLOGY, BUSSEY INSTITUTION

HARVARD UNIVERSITY

CAMBRIDGE
HARVARD UNIVERSITY PRESS

LONDON: HUMPHREY MILFORD

OXFORD UNIVERSITY PRESS

1920

COPYRIGHT, 1920
HARVARD UNIVERSITY PRESS

TO
MY WIFE

PREFACE

THE present volume is an attempt to present some of the
principles and practices of economic entomology in a form
that will illustrate the biological relationships of insects to
their environment. Like nearly all applied sciences, economic
entomology was early developed as an art, with little refer-
ence to biology, aiming toward the empirical application of
certain methods for the destruction or abatement of noxious
insects.

The past few decades have witnessed great changes
whereby the field of the entomologist has been greatly ex-
tended, and he has been compelled, not unwillingly, to im-
prove his methods of investigation and to take advantage
of the rapid progress made not only in zoology and botany,
but in medicine and chemistry as well. He has naturally
greatly improved his efficiency, and has been enabled to in-
crease his usefulness to humanity many fold.

The general public rarely appreciates fully the many
economic problems in relation to insects which continually
present themselves. Even the zoologist, be he morphologist,
ernbryologist, geneticist or student of animal behavior, often
regards the entomologist as a collector or cabinet naturalist,
who spends the greater part of his time impaling specimens
upon slender pins, and continually rearranging them in cork-
lined boxes. Unfortunately, this means toward an end seems
to be a bugbear from which the entomologist may never es-
cape, since he has to deal with a wonderfully varied and ex-
tensive series of animals. This very fact makes it difficult to
deal with insects in the brief and generalized manner applica-
ble to other groups of animals. As a consequence, entomo-
logical books and treatises naturally tend to assume ency-
clopaedic form.

vi PREFACE

In the following pages I have considered few of the details
which may be found in many other carefully prepared vol-
umes, but have rather attempted to avoid, as consistently as
possible, matters not directly necessary for a brief considera-
tion of insects as they affect human welfare.

Most of the material contained in the chapter on Insects
and Public Health appeared in the Scientific Monthly, and
in its recast and revised form is reprinted with their kind
permission. The illustrations have come from several sources,
the photographs almost entirely from negatives made by the
author; of the maps, charts, etc., some have been made es-
pecially for the present book and others are redrawn or
modified from various authors.

Owing to changes in nomenclature during the past few
years, many of our common insects now have different Latin
names. All of these changes have not been made in the
present book, as the author feels that they should become
well established before they are presented to the non-expert
reader.

CHARLES THOMAS BRUES

BUSSEY INSTITUTION

HARVAKD UNIVERSITY
February, 1919

CONTENTS

PAGE

INTRODUCTION . ix

CHAPTER I

INSECTS AND THE PUBLIC HEALTH 3

CHAPTER II

INSECTS AND THE FOOD SUPPLY 39

CHAPTER III

FOREST INSECTS 63

CHAPTER IV

HOUSEHOLD INSECTS 87

CHAPTER V

THE OUTLOOK FOR THE FUTURE . . 100

INTRODUCTION

IN his ceaseless strides towards a domination of the material
world, man has encountered many obstacles. Some of these
are palpably of his own making. For many of those pre-
sented by his living environment, man is responsible only in
so far as he forms a part of the intimately interdependent
myriad of organisms which make up that environment.

In his battle with the elements, he has continually im-
proved his condition and he has seemingly mastered many
problems which did not even disturb the minds of previous
generations. It is in relation to other animals and to plants,
more like himself in their plastic constitution and powers of
reproduction, that he has so far experienced the greatest dif-
ficulty in turning the balance to suit his fancy.

The great variety and usefulness of domesticated animals
and cultivated plants that have been selected and developed,
shows that there is much in man's living environment that
may be diverted to useful ends. These have been sought for
and remain, at least in their present state of improvement,
only as unwilling guests, ready to depart, or to revert to
their former savage state at the first opportunity.

Another, and far more extensive series, of unbidden guests,
is made up of numerous other animals and plants, some
originally associated with the human species, and others
attracted to it as the result of changes wrought by civiliza-
tion. The organisms of this class consist of various disease-
causing microbes and parasites, and of living things that
either find a more congenial environment where the face of
nature has been altered, or have actually been transferred to
parts of the world where they did not formerly exist in the
orderly arrangement of nature.

This great train of undesirable animals and plants is con-
tinually augmented by new arrivals, mainly through exten-

x INTRODUCTION

sions in the distribution of those already affected by a
changed environment. A quite considerable part of this
motley assemblage consists of insects, which enter into our
life and activities in many ways. This group of animals
really vies with the mammals, and with man in particular, in
its attempt to dominate the animal world. With this end in
view, insects have several points in their favor which must
not be forgotten at the outset, although they are dealt with
at greater length in the succeeding chapters. There are many
more different kinds of insects than there are of other animals
and it follows from this, if for no other reason than as a mat-
ter of chance, that they will cross our path at many places.
They are small, extremely tenacious of life and endowed with
such great powers for reproduction and multiplication that
the abundance of any particular species responds very
rapidly to changes in food supply, or other variable factors
in their surroundings. Although in great part terrestrial or
aquatic during their preparatory stages, they are typically
winged and adapted to an aerial life in their mature condition.
In addition, some are parasites of various plants, animals,
and even in other insects, till it would seem that about every
method of earning a living, or of securing it otherwise, may
find its counterpart in some insect. Endowed as a natural
consequence also with a great fixity of purpose dependent
upon their purely instinctive behavior, insects do not respond
readily as a class to our efforts to thrust them aside and out
of our way.

What can be more persistent than the well-directed flight
of the hungry mosquito in its search for human blood. Much
has been learned concerning the hidden dangers that lie in
the wake of the mosquito bite, but little has been accom-
plished toward banishing the mosquito tribe. Quite properly
no biologist has entertained the idea that the instincts of the
mosquito or for that matter of any other insect could be
altered in the least. The only remedies which can be pro-
posed at the present time for dangerous, annoying, or de-
structive insects depend upon actually destroying them.

INTRODUCTION xi

preventing their reproduction or in actually imprisoning the
objects that might be damaged, as we do for ourselves with
barricades of fly screens and mosquito nets. Crude as such
methods may appear to be in general principle, their applica-
tion is by no means as simple as might appear at first sight,
and in practice they are becoming much more efficient from
year to year.

Unfortunately the vast group of insects has furnished al-
most no species that are directly useful to man. Only two
stand out in sharp contrast; the honeybee and the silkworm.
Like some of our higher domesticated animals and cultivated
plants, they have been associated with man for many cen-
turies. Indeed the silkworm has its origin hidden in antiq-
uity and like our familiar plant, Indian corn, has never been
found in the wild state. Through selection, several improved
races of the honeybee have been evolved and apiculture as
now followed in many countries has added greatly to the
material wealth of the world through its extensive produc-
tion of honey and beeswax.

The silkworm is likewise an extremely valuable insect,
although its cultivation is limited to a smaller part of the
world. All the real silk of commerce is produced by insects,
although certain kinds are spun by other caterpillars than
those of the true silkworm (Bombyx mori). Like the honey-
bee, the silkworm is known in several races with special
characteristics of cocoon-color and rate of development,
brought about by selection and inbreeding. For many cen-
turies the silkworm has produced not only a vast amount of
valuable material for clothing and other purposes, but has
added immeasurably to the glitter of personal adornment.
Quite possibly in the future the product of the silkworm may
be supplanted by chemically treated wood-fibre and other
inferior materials of similar appearance.

Shellac is also the production of an insect, of considerable
commercial importance, and something for which no accept-
able substitute has so far been evolved. It does not seem
improbable, however, that the lac-insect may become obso-

xii INTRODUCTION

lete, a fate which overtook the cochineal insect. The latter
was formerly far more important as a source for carmine and
the red dyes made therefrom, but has been replaced by in-
ferior, though more cheaply produced chemical colors.

Few other insects are directly useful to an extent that
makes them of general economic importance, and so much
has been written of the species just mentioned that they are
omitted from the following account. It is confined to those
insects which bear a less pleasant relation to man and to
those which are secondarily beneficial in so far as they may
regulate the abundance of the primarily destructive species.

For convenience we may group insects in their relation to
human welfare, into several categories as has been done in
the following chapters. In their association with various
diseases of man and of the higher animals, insects are of very
direct importance and this aspect stands more or less clearly
apart from the others. As enemies of various cultivated
plants, the depredations of insects are of importance in so far
as they interfere with agricultural production, decreasing
the supply of foods and of other less necessary although
valuable materials. Their influence upon the forests is similar
to the last in many ways, but as will appear on closer exami-
nation presents different problems from the practical stand-
point.

Many insects, some of them of far less real importance than
some of their less sociable fellows, will be considered under
the caption of household insects. These creatures, of many
diverse sorts, have taken up their abode with man and are
continually intruding themselves upon his attention. Some
are dealt with elsewhere as they are related to diseases or
have otherwise striking economic relations; the remainder,
no worse than nuisances, show many interesting and sur-
prising adaptations.

INSECTS AND HUMAN WELFARE

CHAPTER I

INSECTS AND THE PUBLIC HEALTH

SCARCELY more than two decades have elapsed since the
scientific world entertained its first suspicion that certain
human diseases might be spread through the agency of in-
sects. Eighteen years have gone by since that suspicion be-
came an established fact, and in this short space of time
so much has been learned concerning the pernicious activities
of these small animals in disseminating disease-causing or-
ganisms among man and the higher animals, that the science
of preventive medicine can now be applied to many impor-
tant diseases which were before utterly beyond its reach.
Every year brings forth fresh evidence that insects are im-
portant factors in relation to public health, and adds to the
list of diseases that are partially or entirely dependent upon
certain insects for their spread.

A brief statement of the nature of communicable diseases
and of the general habits of the kinds of insects that are impli-
cated in carrying disease will serve to define roughly the field
of medical investigation which is open to the entomologist.
Communicable diseases are invariably due to parasitic or-
ganisms in the body which are capable of inducing similar
symptoms in other persons or animals if transferred to
healthy individuals from diseased ones. Many conditions
modify the transfer of communicable diseases ; some individ-
uals are more easily infected than others; some may be
immune as the result of a previous attack; and, on the other
hand, the virulence of pathogenic organisms often varies
greatly in accordance with conditions to which they have
been subjected previously. A simple method of spread occurs
with many diseases, for example typhoid fever and pulmon-
ary tuberculosis. With the former, the Bacillus typhosux

4 INSECTS AND HUMAN WELFARE

which is the disease-producing organism, is present in the
dejecta of an infected person and may find its way from
these to food, carried by flies or otherwise; ingested by a
healthy person, it may quite likely multiply and induce a
second case of typhoid. With tuberculosis, the tubercle
bacillus from desiccated sputum may enter the lungs of a
healthy person with dust and there reproduce the disease.
As we shall see later, certain insects are commonly associated
with the spread of diseases of this type, although from the
very nature of such diseases, insects are not exclusive factors,
and may be referred to as contaminative carriers.

A second type of communicable diseases differs from the
one just mentioned in that the organism which causes the
disease must live for a time in the body of some other animal
to undergo certain definite changes before it can again induce
the disease in another individual. The most- important in-
sect-borne diseases belong to this type, for in the case of man
and domestic animals, certain insects and ticks act as the
secondary host animals for the organisms of many diseases.
Thus, yellow fever is spread only through the agency of a
certain mosquito, for in its body alone can the yellow fever
organism live and undergo the changes that are necessary
before it can be introduced into another patient by the bite
of an infected mosquito. Malaria belongs to the same cate-
gory, for it spreads only through the bite of certain mos-
quitoes that obtain the organisms with their meal of blood,
and then afterwards inject into the blood of another person,
a later stage of the malarial parasite which has developed
meanwhile within the mosquito.

The importance of insects as detrimental to public health
is well known to professional zoologists, medical men, and
laymen alike, but is usually emphasized only under the stress
of particular circumstances, such as the safety of soldiers in
the recent war, or of unusual outbreaks of diseases for which
insects are directly responsible.

Insect-borne diseases present a constant menace to the
world, and aside from the actual toll of lives which they exact,

PUBLIC HEALTH 5

they impair its efficiency by enfeebling the health of its hu-
man population. Their direful influence is more pronounced
in the tropics, whence it has been most commonly pro-
claimed, but our own country is by no means exempt, al-
though its cooler climate causes it to be less severely affected.
No other insects can compete with the mosquitoes as per-
sistent annoyers of man, and none, with the possible excep-
tion of the rat-flea, hold over him such p_ower_fpreviL. Practi-
cally no part of the globe that can serve for human existence
is free from mosquitoes, and large areas from the tropics to
the arctic are periodically invaded by them in varying abun-
dance. Even where irrigation has made the " desert blossom
like the rose" it has often also produced a crop of mosquitoes
to annoy or even afflict with disease the inhabitants of the
garden.

On account of their phlebotomic habits, and particularly
their fondness for man, mosquitoes have always been heartily
detested, even by the entomologist, and only their known
association with human diseases has brought them to the
serious attention of zoologists. With this incentive, however,
a vast amount of work has been done by entomologists and
medical men and an enormous mass of literature has been
produced in less than two decades, bearing on every con-
ceivable aspect of the subject. We now know that mosquitoes
are responsible for many deaths, much human misery and
great economic loss through then* activity as disseminators
of jnalarial fevers, yellow fever, dengue fever, filariasis, etc.
In all of this, several of the more important relations of
mosquitoes to public health stand out very clearly. They are:
,^41) Some very important diseases of man are transmitted by
certain specific mosquitoes, the latter being absolutely
necessary for the continued existence of these diseases. (
The disease-bearing mosquitoes are most widely distributed
in the tropics, whence they extend into portions of the tem-
perate zones. ^3) The range of mosquito-borne diseases is
not necessarily coextensive with the distribution of their
insect carriers, but is dependent upon other factors as well.

6 INSECTS AND HUMAN WELFARE

Mosquito-borne disease may be combated either by the
elimination of the mosquito responsible; by the protection
of the population from its bites; by the careful screening of
human patients to prevent them from infecting mosquitoes;
or by a medical prophylaxis or immunization of the suscept-
ible population^(5) Remedial measures are preferably ap-
plied against only the specific mosquitoes responsible, not
against mosquitoes in general. The last is primarily a matter
of economy, that the most vital needs of the community be

FIG. 1. Distribution of Malaria in the United States. (After Trask.)

first fulfilled; often more willingness on the part of the com-
munity is evinced to cooperate in fighting the most annoying
or abundant species of mosquitoes rather than the ones most
deleterious to the public health.

Of these methods, the first has proved the most generally
applicable, preferably combined with the second and third.
The last has not proved generally suitable even with malaria,
where quinine is a specific remedy, or with plague, where
immunization is possible.

One of the best known insect-borne diseases, and one which
is of great importance in many parts of our own country is
malarial fever, variously termed ague, chills and fever, etc.
This was the first human disease traced directly to insect
carriers and gave the impetus which has led to the unraveling
of the facts connected with other insect-borne diseases.

PUBLIC HEALTH 7

The protozoan Mood parasites that cause malaria were first
demonstrated many years ago, in 1880, by a French surgeon,
Laveran, who discovered them in the blood of persons suffer-
ing from malaria. Five years later an Italian, Golgi, dis-
tinguished three kinds, each associated with one of the more
familiar types of malaria. They were found to go through a
regular life cycle in the red blood corpuscles and, from anal-
ogy with other known Protozoa, it was suspected that in
addition to their ixon-sexual generations in the human blood
there must be a sexual development in some cold-blooded
animal. Manson was led to suspect that some insect might
be the secondary host and, working on this hypothesis, Ross
in India first found the malarial parasites in a certain kind of
mosquito in 1898. He had worked for nearly three years on
a common mosquito belonging to the genus Culex without
result, but finally in a mosquito of the genus ^TWph^s was
able to trace the development of the parasite. His epoch-
making discovery has been since amply confirmed and ex-
tended by experimental proof till we now know that the
various types of malarial blood parasites complete their life-
cycles in anopheline mosquitoes, the latter acting as the
sole carriers of the disease.

The details of growth and development of these parasites,
which belong to the Protozoan genus Plasmodium, are ex-
tremely interesting, but far too complicated to discuss briefly.
In general it may be said that the blood of persons suffering
from malaria contains the parasitic organisms, and that these,
on being taken into the stomach of the proper kind of mos-
quito, undergo certain changes and later penetrate the wall
of the stomach to form vesicular swellings. Within these
they multiply, and finally on the bursting of the nodule are
set free in the body cavity and find their way to the salivary
glands. After becoming infected, a period of twelve to twenty
days is required for these changes in the mosquito. Then for
a period of several weeks the virulent organisms remain in
the salivary glands and if the mosquito bites a second person
the parasites are introduced with the salivary secretion,

8

INSECTS AND HUMAN WELFARE

through the puncture, into the circulation. Here they mul-
tiply and produce another case of human malaria, which
develops from ten days to three weeks after inoculation.

As previously stated, only certain mosquitoes can transmit
malaria, for when the parasites are ingested by other species
of mosquitoes they do not continue their development, but
die without passing through the stomach into the salivary
glands.

At the present time malaria in its several forms is the veri-
table scourge of the tropics. It also extends generally into the

Fio. 2. Distribution of Malarial Mosquitoes in the United States.

subtropics and warmer temperate regions and is prevalent
over a considerable part of the southern United States. In
these areas, as appears from investigations of the U. S. Pub-
lic Health Service, its range is roughly coincident with the
moist austral zones east of the 100th meridian as defined by
Merriam. Aside from this main area, there is a small one in
southern New England and another in central California;
there are also a few isolated localities scattered through the
country where malaria is thought to be endemic (Fig. 1).
At least three species of Anopheles are known to act as car-
riers of malaria in the United States and, from data given by
Howard, Dyar and Knab (Fig. 2) the distribution of these
taken together corresponds closely with that of malaria, al-
though slightly more extensive, especially along the Atlantic

PUBLIC HEALTH 9

seaboard. As these malarious regions include a population
of about 40,000,000 people, it will be seen that the importance
of malaria from the standpoint of public health is very great
indeed. It must be remembered, however, that the incidence
of malaria varies widely, being greatest in the large south-
eastern area, and very much less in the more densely popu-
lated northern district. Thus in Mississippi about 80 cases
of malaria per 1,000 of population were reported during the
last year, or 158,000 for the entire state. Other southern
states do not report the disease so thoroughly and it is diffi-
cult to estimate to what extent they are affected. It would
seem, however, that one million cases each year would be a
conservative estimate, especially as von Ezdorf found in a
portion of one mill town in the endemic area that over 13 per
cent, or one person in seven, harbored the malarial parasite
in the blood, while 233 out of 500, or nearly 50 per cent, re-
ported having had chills and fever during the summer pre-
ceding his examination. Although the death rate from
malaria outside the tropics is not very great (9 per 1,000
calculated on the data for Mississippi cited above) it is by no
means inconsiderable in the mass. On the other hand, the
economic loss is enormous, due to inability to work during
the acute attack of the fever and due to a loss of efficiency
during prolonged periods following. That malaria responds
quickly to anti-mosquito work and quininization is shown
by the result following an application of these measures to
the mill town mentioned. Referring to von Ezdorf's report,
Trask says: "Measures were inaugurated to get rid of mos-
quito-breeding places and the use of quinine was encouraged.
A year later the town was again visited and the blood of 780
persons examined. Of these only 35, or 4.5 per cent, showed
infection. The health officer reported at this time that his
visits among the mill employees for several months had aver-
aged not over one a day, and that many of these were un-
doubtedly for old infections lasting over from previous years.
The malaria rate had continuously decreased during the
months when it was usually at its worst. The health officer

10 INSECTS AND HUMAN WELFARE

of the town in his report for 1914 stated that while during the
summer of 1913, prior to antimalarial work, the mills were
constantly short of help on account of the large numbers of
employees sick with malaria, during the summer of 1914
there had not been a day when the mills did not have suffi-
cient help. The manager of one mill stated that the improve-
ment in the regularity and efficiency of the employees had
been such that the amount ($1,000) which the mill had con-
tributed to the fund for anti-mosquito work was more than
regained in one month's operation."

While these conditions are those of one of the most severely
affected districts, they are nevertheless repeated very gener-
ally throughout the entire area, more especially in the low-
lands, for the hilly or mountainous and better-drained
sections suffer less.

As mentioned above, several species of Anopheles mos-
quitoes are concerned in the transmission of malaria in the
United States. Till very recently only one species, Anopheles
quadrimaculatus, has been thought to be of much importance.
Recent studies by Mitzmain and others show that this mos-
quito is the most important, as it may harbor the parasites
of all three types of malaria, but Anopheles punctipennis
(Fig. 3), a common, widespread species and Anopheles cru-
cians, a species abundant along the eastern coast of the
United States, may serve both as hosts for the tertian and
estivo-autumnal or subtertian forms. The ease and fre-
quency with which A. punctipennis may become infected
seems to vary greatly under different conditions and its
importance at least in northern districts is by no means
proved.

All three species readily enter houses and are persistent
biters, although no more so than some other mosquitoes.
Both quadrimaculatus and punctipennis breed in stagnant
water, usually that of permanent nature containing algae or
other plant growth. They commonly occur near together,
with punctipennis usually more abundant. Larvae of the
latter species occur also rarely in temporary puddles and both

PUBLIC HEALTH 11

are occasionally to be found in the growth along the sides
of slowly flowing streams. The breeding grounds of A. cru-
cians are mainly restricted to regions adjoining the salt and
brackish marshes along the coast, although the larvse are
most abundant in fresh water. The two Anopheles occurring
on the Pacific coast region, A. pseudopunctipennis and A.
occidentalis, have habits similar to those of quadrimaculatus,

FIG. 3. Anopheles punclipennis, female.

although the second species breeds in brackish waters also.
As is the general habit among adults of Anopheles, these
mosquitoes feed mainly at twilight and malaria is acquired
only by persons who expose themselves to their bites after
nightfall. Occasionally they bite during the daytime, but as
malaria appears never to follow such bites it seems probable
that only newly emerged females and consequently non-in-
fected ones bite at this time, as is the case with the yellow-
fever mosquito.

12 INSECTS AND HUMAN WELFARE

Like other insect-borne diseases, malaria shows in its
seasonal prevalence a close relation to the seasons (Fig. 4),
undergoing a period of quiescence during the winter and at-
taining a sudden maximum in late summer, after which it
rapidly declines. This is in response to the increasing abun-
dance of Anopheles during the summer, coupled with a

JAN FEB MCH APL MAY JUN JUL AUG SEP OCT NOV DEC

FIG. 4. Seasonal Prevalence of Malaria in the United States.

relatively high temperature, favorable for activity on the
part of the mosquito and for the development of the malarial
parasites in its body. It has recently been clearly shown by
Mitzmain that under the conditions of temperature prevail-
ing in the United States the malarial parasites do not persist
through the winter in hibernating mosquitoes, but winter
over in the human host from whence the Anopheles secure
them the following season.

The larval or preparatory stages of anopheline, and of
practically all other mosquitoes, are passed in the water of
small quiet ponds, puddles, exposed vessels containing water,
rain barrels, etc., and it is during this period that they are
most easily controlled. This is accomplished by oiling the
water with either crude or refined petroleum or with some
miscible oil. The petroleum forms a film over the surface of

PUBLIC HEALTH 13

the water through which the larvae can not extrude their
breathing tubes and they are thus suffocated. The applica-
tion of miscible oils is efficacious, but attended with some
danger, since it destroys fish and predatory insects which are
themselves some of the most important natural enemies of
mosquitoes. Very frequently even oiling is not necessary,
as much swamp land may be permanently freed from mos-
quitoes by very simple systems of drainage ditches which
prevent the accumulation of the stagnant water in which the
larvse occur.

As pointed out, the reduction of malaria in communities is
permanently accomplished most readily by the application
of anti-mosquito measures, aimed mainly at the breeding-
places of Anopheles mosquitoes. Such work is being exten-
sively carried on by the TJ. S. Public Health Service, by other
federal bureaus, by many state boards and commissions, and
by certain private or semi-private institutions in widely
scattered parts of the country. Through their individual
and collective efforts an enormous amount has been accom-
plished, although painfully little in comparison with what
could well be spent upon the problem, which is without
question one of the great public health problems in the
United States at the present time.

Another mosquito-borne disease which has aroused more
interest in America on account of its spectacular appearance
and higher mortality is yellow fever. This is due to a filter-
able virus, concerning the nature of which we can only specu-
ulate at the present time, although enough has been ascer-
tained through experimental work to demonstrate that the
virus is a living organism which undergoes a development of
definite periodicity in mosquitoes of a single species known as
Stegomyiafasciata. This mosquito enjoys a very wide distri-
bution in many parts of the world, mainly in the tropics, but
also extends into the warmer temperate regions. Yellow fever
is not so extensively distributed, being absent in many places
where Stegomyia occurs, but it is nevertheless present in
many parts of the tropics in both hemispheres and all that is

14 INSECTS AND HUMAN WELFARE

necessary for the development of a possible epidemic in a
region where Stegomyia occurs is the introduction of a human
case in the early stages of the fever.

The larval habits of Stegomyia are in quite marked con-
trast to those of Anopheles. The adults are strictly domestic
mosquitoes and occur almost entirely in the neighborhood of
human habitations. Their larvse occur in the same places,
breeding preferably in vessels containing small amounts of
water, rain barrels, cisterns, stray tin cans filled with rain
water, etc. On this account, extermination work against the
yellow-fever mosquito resolves itself mainly into the exami-
nation and treatment of cities, towns, and the immediate
environs of smaller settlements.

A Stegomyia feeding upon- the blood of a person suffering
from yellow fever becomes infected only during the first three
or four days after the onset of the fever; later than this mos-
quitoes do not obtain the virus. An incubation period of at
least twelve or fourteen days in the mosquito is now necessary
before the mosquito can infect a second person, after which
the Stegomyia remains infectious for a long period and may
be responsible for a series of new cases. These facts were first
discovered during the summer of 1900 by a Yellow Fever
Commission consisting of Drs. Reed, Carroll, Lazear, and
Agramonte, of the U. S. Army. Two of these men, Carroll
and Lazear, allowed themselves to be bitten by infected
mosquitoes. Lazear died from a severe case of fever con-
tracted during the course of the experiments, and Carroll
nearly succumbed to a severe case following the bite of an
experimentally infected mosquito.

Little further has since been learned of the etiology of
yellow fever, but wonderful strides have been made in the
application of these simple facts for its eradication. In Cuba,
where the commission conducted their investigations, the
first attempts were made, and in 1902 yellow fever had been
entirely eliminated in Havana. Other West Indian islands
were formerly badly infested with yellow fever, but at the
present time there is little more danger from this disease

PUBLIC HEALTH 15

there than in the United States. Rio de Janeiro was once a
hot-bed for yellow fever, but it too yielded to the destruction
of mosquitoes and the screening of patients, till after a six
years' fight, the fever vanished. Still more remarkable are
the results accomplished in the Panama Canal Zone under
the direction of Colonel Gorgas. Here the warfare against
yellow fever has gone hand in hand with anti-malarial work

FIG. 5. Distribution of the Yellow-fever Mosquito in the United States.

and the isthmus has been transformed from a veritable
death-trap to a condition which compares favorably with
that of any region on earth.

Our own country has suffered from yellow fever in the
past, mainly in the south, but extending to southern Illinois
in 1878, to Philadelphia in the terrible epidemic of 1793, and
even to Boston and into interior New England towns in the
earlier days.

Yellow fever no longer causes serious concern to residents
of any part of the United States, or, for that matter, to
those of most parts of the American tropics. Most of us can
recall very clearly in the not far distant past, however, the
terror and demoralization which accompanied its periodical
appearance in our southern ports. The yellow-fever mos-
quito is still abundant and widely distributed throughout
the southeastern states (Fig. 5) and sometimes becomes
temporarily established further north during the summer.

16 INSECTS AND HUMAN WELFARE

There is no yellow fever, except an occasional stray case from
the tropics, which does not get beyond the keen eyes of the
Public Health Service, and consequently our population of
yellow-fever mosquitoes remain free from the dread disease.
In this case several factors have combined to make possible
the elimination of the disease without more than temporary
and local eradication of the mosquito. In our southern states
the disease does not easily survive the winter and chronic
human carriers do not exist, so that past outbreaks have been
due to fresh introductions and have been terminated by cold
weather. During the last epidemic that occurred in New
Orleans, in 1905, vigorous anti-mosquito measures were
necessary, but, due to the greater severity of the disease, the
consequently greater ease with which it is recognized, the
limited area to be dealt with, and the absence of chronic hu-
man carriers, the eradication of yellow fever without perma-
nent mosquito repression has been easy in comparison with
the control of malaria. The success of this campaign has
undoubtedly sounded the death knell of the yellow-fever
epidemic and panic in the United States, for New Orleans
has amply demonstrated what may be accomplished in the
control of an epidemic by an efficient group of workers,
backed by a sympathetic public and supplied with reasonable
funds. Even in parts of the tropics where it persists through-
out the year, it is being rapidly and permanently eliminated.
Indeed it bids fair to be the first disease actually to become
extinct as a direct result of human discovery and applied
science. What a refreshing contrast to the fate of the Ameri-
can pigeon and the forlorn remnants of the American bison!
Another tropicopolitan, semi-domesticated mosquito ex-
tends quite widely into the warmer parts of the United States
(Fig. 6). This is Culex fatigans, now known as Culex quinque-
fasciatus (Fig. 7), responsible for the transmission of dengue
fever or "break-bone fever." Dengue is a mild (i.e., non-
fatal) disease which causes great distress and temporary
disability. It is therefore a factor contributing to lack of
efficiency and goes hand in hand with malaria in this respect.

PUBLIC HEALTH 17

Like malaria, dengue is tropicopolitan in range, and extends
only into the warmer parts of the south. Here it sometimes
appears in extensive epidemics, but in a much more erratic
way than malaria, which has the well-deserved reputation
of appearing year in and year out in the same districts. This
difference is probably due to the absence of chronic human

FIG. 6. Distribution of Culex quinqiiefasciatus in the United States.

carriers and the fact that Culex quinquefasciatus does not
breed in permanent water, but is an almost truly domesti-
cated species which breeds in temporary water near human
habitations, and under the climatic and other conditions of
our country does not find anything like uniform opportunities
for breeding from one season to another. In consequence of
their separate breeding grounds, measures designed to con-
trol malarial mosquitoes have no effect or practically none
upon the dengue mosquito. It must be dealt with mainly
by education leading to individual effort and cooperation in
communities. Aside from its pathogenic possibilities, this
species is a rather persistent biter, which is another argument
for its control.

The same Culex quinquefasciatus has been shown to be at
least partly responsible for the transmission of a parasitic
disease of the tropics known as filariasis. The direct cause
is a nematode worm belonging to the genus Filaria which is
present in the circulation and lymphatics of the infected

18

INSECTS AND HUMAN WELFARE

person. In the late stages of the disease the microscopic
larval worms occur abundantly in the blood. For some unex-
plained reason they remain in the deep-seated blood vessels
during the day, but usually appear more abundantly in the
peripheral circulation during the night. Here they are readily
obtained by mosquitoes with their meal of blood. In the

FIG. 7. Culex qidnquefascialus, female.

alimentary canal of the mosquito the larval Filaria discards
a sheath-like envelope which has previously invested it, and
works its way through the wall of the stomach into the tho-
racic muscles where it increases greatly in size and finally
migrates to the base of the proboscis. From two to three
weeks are necessary for this metamorphosis, and for some
time longer the Filaria may remain in the proboscis awaiting
its opportunity to enter another person through the wound
occasioned by the mosquito's beak. Once they have been
transferred to their human host, the parasites enter the
lymphatics where they attain sexual maturity and give rise
to the abundant microscopic larval Filarias that reenter the
circulation to await ingestion by another mosquito.

PUBLIC HEALTH 19

Filariasis is most common in equatorial regions, but ex-
tends less commonly into the subtropics. The parasites
themselves do not ordinarily cause great inconvenience, but
their presence in the lymphatics may clog these vessels and
large swellings developing in the limbs or other parts of the
body are thought to be sequelae.

Less directly detrimental to public health are other mos-
quitoes not associated with any human disease, but making
life miserable at some season of the year for human beings
in practically all parts of the world. Although the LTnited
States supports an extensive mosquito fauna, a very few
species aside from those already mentioned make up the bulk
of those annoying man. Two in particular are widespread,
abundant, and on account of their strikingly different habits,
perhaps worthy of mention in this connection. The first of
these is the house mosquito, Culex pipiens, a palearctic
species, now common throughout the eastern states, that
breeds in rain barrels, cesspools, sewer catch-basins, puddles,
or practically anywhere, no matter how foul the water. The
other, Aedes sollicitans, the salt-marsh or "Jersey" mos-
quito, breeds only along the coast in salt marshes. Broods
of this mosquito follow the lunar calendar, developing after
the high tides flood the meadows and fill the pools in which
the larvae live. The eggs of this form are laid on the mud and
hatch quickly when submerged in water. It is generally be-
lieved that all the eggs laid by this mosquito must pass the
winter before hatching and that the successive broods are
only installments of eggs induced suddenly to hatch in turn
by successive wettings. This is a true migratory mosquito,
which invades the country for many miles adjacent to the
salt marshes. Such incursions follow the appearance of each
brood.

Much attention has been given to the control of this mos-
quito in New Jersey and the territory surrounding Long
Island Sound, and its numbers have been marvelously les-
sened through the drainage of marshes by ditching. In the
case of this species reforms have been easier than with the

20 INSECTS AND HUMAN WELFARE

malarial mosquitoes, as an expectation of relief from the
great personal discomfort of myriad mosquito bites exerts a
stronger appeal to the average person than the much more
important health problem of malaria. In the public mind,
the latter is unfortunately not usually regarded as so im-
mediately personal, as the fever and the bite are not co-
incident .

At some time during the insect season, usually in the
spring, many districts are visited by swarms of small hump-
backed flies which viciously bite man and animals alike. On
account of their dark color these have been called black flies.
They pass their developmental stages almost entirely in
swiftly moving brooks and streams, where the larvae and
pupse are attached to stones and other objects in the water.
Wherever there are suitable streams in which they can breed,
these pests appear abundantly, and may be occasionally
present far from streams, where they would not be expected.
They are not known to be concerned in the transmission of
any disease.

Minute flies, somewhat like mosquitoes, which are vicious
bloodsuckers, often appear in great abundance, particularly
in the cooler parts of the United States. These insects belong
to several genera, developing from aquatic larvae inhabiting
fresh water and also brackish water along the seacoast.
They are generally crepuscular, biting most abundantly at
dusk, and are persistent at that time, causing a stinging sen-
sation out of all proportion to their almost microscopic size.
None of our species are known to be disease carriers.

The regions surrounding the Mediterranean Sea are the
centers of distribution for a very interesting, but not danger-
ous insect-borne disease known as phlebotomus fever. In
this case the carriers are minute gnat-like flies of the family
Psychodidae known as Phlebotomus. These insects are semi-
aquatic in the larval condition, occurring in damp situations,
drains, cellars, etc., where they feed on plant matter. The
adult is a vicious biter in spite of the fact that it is scarcely
over one millimeter in length. It rarely bites except at night,

PUBLIC HEALTH 21

following the habits of certain mosquitoes in this respect.
The specific cause of phlebotomus fever is not known, but
it has been shown to be an invisible virus. At the present
time it is impossible to state whether other insects may play
a part in its transmission, although such does not seem prob-
able. We have at least one species of Phlebotomus in the
United States and it is possible that it might act as a vector
should the disease be introduced into our country, although
it would seem that such a possibility would have been real-
ized already if it were likely to occur, for cases of this common
European fever must undoubtedly have been imported.

The common housefly (Musca domestica) is a common
agent in the spread of certain infections which are often
grouped under the term of fly-borne diseases. Of these it
may be urged that, strictly speaking, there are none, at least
in the sense of mosquito-borne diseases. The housefly is not
known to be wholly responsible for the transmission of any
disease and its relative importance in disseminating several
infections of man is still a moot question. By some it is
strongly urged as the main means of transmission for several
enteric diseases in certain communities; by others it is cast
aside without reasonable consideration as a sort of entomol-
ogist's nightmare. I can not believe that either course is
justified; each seems to be based on prejudice due to lack of
knowledge, either respecting the fly or relating to other
channels of infection.

The housefly (Fig. 8) is more truly domesticated than any
other insect; it lives and flourishes wherever man establishes
himself, but does not thrive elsewhere. It has evidently been
associated with him from the remotest antiquity, but has by
no means failed to adapt itself to changed conditions. It
still develops in the animal and vegetable refuse which ac-
cumulates about his habitations and still invades his dwell-
ings to partake of his food. In short, it is practically ever-
present, for its preferred larval food, horse manure, is usually
to be found, and, if not, substitutes are available in greater
or less abundance.

INSECTS AND HUMAN WELFARE

Several other species of flies appear regularly in houses,
but in far lesser numbers, and none exhibit to such a marked
degree the peculiar tastes of the housefly, which wanders
back and forth from filth to food, feeding on each in turn.
In this method of feeding lies the danger of infection by
houseflies ; they are equally fond of clean and filthy materials,
and their frequent migrations from one to the other multiply

FIG. 8. Housefly, showing method of feeding.

their opportunities to pick up pathogenic organisms that may
be later deposited upon foods. The frequency with which
this actually happens is of course the vital point, and it is
upon this that it is very difficult to obtain concise data.

It has been shown rather conclusively that adult flies do
not retain in the alimentary tract bacteria which they may
have ingested as larvae that have developed and fed in ma-
terial containing, for example, the bacillus of typhoid fever.
On the other hand, adult flies readily obtain this bacillus from
contaminated substances and may retain and later deposit it
in a living condition on food designed for immediate human
consumption. There can be no question but that this occurs
commonly under many circumstances, particularly in com-
munities where there is no adequate system of sewage dis-

PUBLIC HEALTH

posal. That these bacteria should be more attenuated than
those occurring in drinking water does not seem probable.
Many facts show that flies are a very important factor in the
dissemination of typhoid fever. The greater frequency with
which persons on country vacations contract the disease is
very striking, although this may, of course, be attributed to
bad water supply. Other opportunities for infection, aside

JAN

FEB

MCH

APL

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

•i

1 •

I •

/ ''.

f

i

\

A

/

• >

\

•_ /
./

\

A

x\

\

\

Q

s

\

/

'/

\

\v

\
\

•

J

%

\

\
\

f

/

\

\>,

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"I
f"

\

V

-~.

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^

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t

/

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FIG. 9. Seasonal Prevalence of Typhoid Fever in Several States; solid line,
New York; dotted line, Alabama; dashed line, Washington.

from the fly, are, however, no greater there than in the city.
In other parts of the world where the water supply is reason-
ably good, e.g., certain South American cities, typhoid flour-
ishes to an alarming extent, due undoubtedly to excessive
soil pollution, where flies can almost instantly transfer ma-
terial from typhoid carriers to food, while the latter is abun-
dantly exposed on the streets for sale to be eaten on the spot.
In our own country the seasonal incidence of typhoid fever
corresponds to some extent with fly prevalence, and still
more significant is its greater summer prevalence in regions
where systems for sewage disposal are not generally installed.
This disparity is shown on the accompanying chart (Fig. 9,)
which gives data for two of our eastern states, New York and

24 INSECTS AND HUMAN WELFARE

Alabama, and one western state, Washington. The greater
uniformity throughout the year in New York, where the
opportunities for fly-borne infection are curtailed, is very
marked. Another way in which the housefly can aid in the
spread of typhoid is through infecting milk on dairy farms
where carriers are present and offer the flies a chance to be-
come infected.

Flies are also responsible, and to a much greater extent,
for the prevalence of infantile diarrhea or summer complaint,
and here their relation is verv easilv seen.

«/ «/

Other activities of houseflies detrimental to public health
are of far less importance, but by no means negligible. They
can carry the eggs of parasitic worms as well as many bacteria
and other microorganisms present in the several types of un-
savory food upon which they feed indiscriminately.

Recently much progress has been made in methods of
abating the housefly nuisance. It has been found by workers
in the Federal Department of Agriculture that certain sub-
stances, notably borax, hellebore, and a fertilizer consisting
of calcium cyanamid, acid phosphate and kainit, are highly
destructive to fly-larvae in horse manure (wThence the great
majority of our houseflies come) and that these substances
do not ruin the manure for agricultural purposes. Practical
traps whereby fly-larvae in stored manure may be caught and
destroyed before transformation have also been devised.
Richardson has shown that housefly larvae can develop only
in alkaline material, and some substances may thus be acidi-
fied to eliminate them as larval food.

The people of the United States spend great sums of money
for fly-screens, fly-paper, fly-swatters and fly-traps and suf-
fer much sickness and death as a result of the ubiquitous
housefly. As yet no great reduction of houseflies has been
accomplished, but the public regards them less and less as
harmless creatures, and should soon be in the proper mental
state to launch a decisive campaign against them.

Cattle, horses, and other domestic animals, and more rarely
man himself, are troubled in nearly all parts of the world by a

PUBLIC HEALTH 25

small blood-sucking fly resembling the housefly in size and
general color (Fig. 10). On account of its great abundance
about horses and cattle it has been termed the stable-fly,
although its larva0 breed • mainly in fermenting vegetable
material rather than in manure. The adult flies readily bite
human beings, particularly in damp weather, and this habit
has given rise to the popular idea that houseflies bite before

FIG. 10. Stable-fly (Stomoxys calcitrans}.

a shower. The stable-fly is most important as a pest of ani-
mals, as it has not been definitely proved to be more than an
accidental carrier of any disease affecting man. It was at one
time thought to be a carrier of poliomyelitis (infantile paraly-
sis), but it now seems probable that such is not the case.

One of the most important insect-borne human diseases
which does not exist in the new world is African sleeping
sickness. In recent years this malady has decimated the
native population in certain parts of eastern equatorial
Africa and any extension of its range would be most serious.
It seems very unlikely that America will ever have to face an
epidemic, for the introduction of sleeping sickness together
with its carriers is not at all probable, and the possibility of
its becoming established, even after introduction, is still

INSECTS AND HUMAN WELFARE

more remote. As is well known, sleeping sickness depends
for its spread entirely upon certain biting flies known as

FIG. 11. Tsetse-flies. Above, Glossina palpalis, the carrier of human sleeping sick-
ness; below aiiother species of Glossina with a more enlarged view of the head and
blood-sucking mouth parts.

tsetse-flies belonging to the same family as our common house-
fly and stable-fly. The genus Glossina (Fig. 11) in which

PUBLIC HEALTH 27

these flies are included, is restricted to the African continent,
but is there represented by a number of species, several of
which have been shown to act as carriers for trypanosome
diseases in animals. One only, Glossina palpalis, carries the
common trypanosome of human sleeping sickness, Trypano-
soma gambiense. The disease appears to have been originally
endemic only in West Africa, but was found in eastern equa-
torial Africa something over fifteen years ago, and it is in
this latter region that its ravages have been so pronounced.
Owing to certain peculiarities in the habits of the tsetse-flies,
the distribution of sleeping sickness is limited to very definite
areas in the region where it occurs. The fly, which has a
sharp needle-like beak for sucking blood, resembles our own
stable-fly (Stomoxys calcitrans) in general appearance but is
considerably larger, measuring about half an inch in length.
It is found only in the dense brush which grows along the
edges of streams, ponds and lakes. In such places persons
and animals may be bitten by the flies and it is exclusively
through such bites that these insects may obtain virus of
sleeping sickness from the blood of a person or animal suffer-
ing with the disease. Should the fly obtain a meal of blood
containing trypanosomes, these may multiply in the body of
the fly, although not always, for only about one in twenty of
such flies becomes infectious. A considerable period must
now elapse before the infected fly is in condition to inoculate
a new patient, usually thirty or forty days, but after this for
at least seventy-five days it remains infectious, and may in-
troduce the trypanosomes into the blood of any animal upon
which it feeds during this period.

The tsetse-flies develop in a very different way from most
insects. The female does not deposit her eggs, but a single
one develops to the fully grown larval condition before being
deposited. This larva soon pupates in the shade beneath the
brush bordering the water where it has been dropped by the
parent fly, and later emerges in the winged adult condition.
The pupae require such moist shade, and it is apparently for
this reason alone that the flies never occur away from the

28 INSECTS AND HUMAN WELFARE

immediate vicinity of the water. As a result of their method
of development, the tsetse-flies do not multiply rapidly, and
under favorable conditions only one larva is produced in a
ten-day period.

The trypanosome of sleeping sickness was discovered by
Bruce in 1902 and a year later the role of Glossina palpdlis in
its transmission was proved. Since then much energy has
been expended in attempting to stamp out the disease by
every possible means. It was thought at first, that by mov-
ing all the natives back from the edges of the water the flies
thus left without opportunities for reinfection, would become
free from trypanosomes, and that by isolating and treating
cases of the disease in fly-free areas it would be possible to
eliminate it entirely. In conjunction with this, the cutting of
brush, especially about boat landings and watering places,
has been practised as far as possible. Contrary to expecta-
tions, it has been found that even after three or four years,
infected flies still occur along the uninhabited shores. This
led to experimentation upon animals and it is now known
that various wild antelopes as well as certain domestic ani-
mals may act as reservoirs for the virus of sleeping sickness
which may thus persist in the complete absence of any hu-
man subjects. As a result of this discovery the great difficul-
ties of combating the disease among the ignorant African
natives have been vastly increased.

Louse-borne diseases have come into prominence during
the recent war in Europe on account of the prevalence of
typhus fever in Serbia and certain parts of the eastern war-
front. Typhus fever has been known to be conveyed by lice
for a number of years, but the association of these insects
with trench fever was proved only during the latter part of
the war, and their role as carriers of a certain type of relaps-
ing fever is also a quite recent discovery.

Human lice, or cooties, to use their most recent nickname,
belong to three species, two known respectively as the head
and body louse (Fig. 12) (Pediculus humanus and P. corporis),
and the third as the crab louse (Phthirius pubis). All three

PUBLIC HEALTH

are strictly human parasites and do not occur upon any other
animals. Only the body louse appears to act as a carrier of
the louse-borne diseases.

Typhus fever has been well known for many years and re-
garded as a disease characteristic of filthy surroundings.

FIG. 12., The body louse (Pedicuhis corporis).

During our own civil war it claimed many victims among
the inmates of army prisons, and has been endemic though not
very prevalent in many parts of the world in times of peace
(Figs. 13a & 13b). Through the researches of Ricketts and
others we know that typhus is spread by the body louse and
its epidemiology is at once made clear. When it broke out in
Serbia in 1915 in severe epidemic form, a knowledge of the
method of its transmission made control possible, even under
extremely difficult and unfavorable circumstances.

Trench fever has attracted notice in the European war
zones, to which it appears to be restricted so far as present
knowledge extends. That it is a new disease is, however,

30

INSECTS AND HUMAN WELFARE

without question an utterly unwarranted assumption for
it has undoubtedly been brought to Europe from some little-
known quarter of the globe, unless it may have previously
existed in Europe which does not appear probable. During
the later part of the war it was successively recognized as a
distinct disease, suspected of association with the louse, and
soon proved actually to be louse-borne. We now know that

FIG. 13a. Map illustrating the distribution of typhus fever in the old world. It is a common disease
in the regions marked with black, less common in those marked with cross-hatched lines and rare
in those marked with parallel lines. (After Byam, et al.)

the disease is due to a living microorganism probably of such
small size that it cannot be recognized under the microscope.
This virus is obtained by the lice with their meal of blood
taken from an infected person. At least five days must elapse
before the louse becomes capable of transmitting the disease,
indicating that the organism must undergo a development of
definite periodicity in the insect. If it is transferred to
another person its bite is not or only rarely infectious, but
its excrement contains the virus and if scratched into the
skin, trench fever develops. Typhus fever is a very danger-

PUBLIC HEALTH

31

ous disease with a quite high death-rate, but trench fever is
non-fatal and its importance in the war zones has been due

FIG. 13b. Map illustrating the distribution of typhus fever in the new world. It is a common
disease in the regions marked with black, less common in those marked with cross-hatched lines
and rare in those marked with parallel lines. (After Byam, et al.)

to its great prevalence and the fact that persons affected with
it are incapacitated for extended periods.

The louse is also responsible for the spread of relapsing
fever in Eurasia, North America, and northern Africa. This

INSECTS AND HUMAN WELFARE

disease is endemic over very extensive areas and is a common
malady in the Old World. In Africa a different form of re-
lapsing fever is carried by a tick (Ornithodoros moubata) and
ticks are also thought to convey it in South America. For
a long time ticks were believed to harbor both types of re-
lapsing fever, but recent researches have disproved this idea.
Rocky Mountain spotted fever is the only definitely
characterized tick-borne human disease that occurs within

FIG. 14. Rocky Mountain Spotted-fever Tick, male at left.
Unengorged female at right.

the confines of the United States. In 1902 Wilson and
Chowning suggested that ticks might carry this disease,
and four years later Ricketts definitely proved such to be the
case. Rocky Mountain spotted fever is restricted to the far
western and northwestern states, whence 290 cases were re-
ported during the year 1916. Over half of these occurred in
Idaho, although the disease extends into the neighboring
states of Montana, WTyoming, Colorado, Utah, Nevada,
California, Oregon, and Washington, as well as northward
into Canada. As shown by the fatality rate, the disease is
most virulent in western Montana and northern Idaho, where
the mortality is said to reach 70 or 80 per cent. A single
species of tick, Dermacentor venustus (Fig. 14), common in
these regions is known to act as the vector. The Dermacen-
tor ticks occur abundantly on various small wild mammals

PUBLIC HEALTH 33

in the younger stages and as adults on domesticated animals,
such as cattle, and from these become transferred to man.
It has been experimentally shown that certain rodents are
susceptible to the disease, and a tick thus infected in the
nymphal stage can retain the disease organism till it becomes
adult. It may then reach its human host through the medium
of domesticated animals such as cattle. It appears that this
is the ordinary way in which human cases have their origin,
i.e., through the bites of adult ticks, although the newly
hatched "seed ticks" derived from eggs laid by infected
mother ticks are known to contain the organism also.

Although Rocky Mountain fever is of minor importance at
present, it is feared that it may increase its range at any time,
since other ticks of wider distribution are apparently capable
of acting as carriers.

Whether this may happen is by no means certain, however,
and the vigorous measures already undertaken to reduce the
abundance of ticks on domesticated animals will undoubtedly
bear fruit in the gradual reduction of this locally much-
dreaded disease.

The flea is another domestic insect which was looked upon
only as a nuisance until it was shown that certain kinds of
fleas are agents in spreading bubonic plague. The most
terrible epidemics of which we have any historical record
have been those of plague, or "black death." One swept
from Egypt in the sixth century before the Christian era and
invaded Europe and Asia, where it raged for sixty years. A
similar one spread through the whole known world in the
fourteenth century and is thought to have caused over
twenty-five million deaths before it subsided. In 1898 Si-
mond suspected fleas as agents in the spread of plague and
his suspicions were later proved to be true through the re-
searches of Verjbitski and the Indian Plague Commission.
Plague is primarily a disease of rats and certain other ro-
dents, and is usually carried to man by the bites of fleas which
have become infected from plague-stricken rats. From 300,-
000 to 400,000 cases of this disease are reported from India

34 INSECTS AND HUMAN WELFARE

every year, over half of which terminate fatally. At the pres-
ent time it is most widespread and abundant in tropical
countries, although by no means confined to them, and is
excluded from our own country only by dint of repressive
measures administered with the greatest thoroughness.
Within the past few years it has appeared only sparingly in
the United States, but at several times has given rise to a
temporary apprehension lest it pass beyond control. That it
has not done so shows the probability of future danger is re-
mote.

Nevertheless there are other good reasons why we should
spare no efforts in reducing the number of rats. They are
said by Nelson to destroy annually $200,000,000 worth of
our foodstuffs and other property; they constitute a fire-
menace, and besides, they can hardly be considered as de-
serving our hospitality from an esthetic standpoint. In short,
war against rats is important for many reasons, one of which
is the security against plague which it entails, and gradual
repression through individual effort is much easier than in-
tensive campaigns necessitated by the advent of plague in a
community.

The relation of the flea to the transmission of plague is due
to the fact that rats are regularly infested by fleas that may
become infected with the bacillus of plague, if it be present
in the blood of the host upon which they are feeding. These
bacilli remain in a viable condition for some time in the gut
of the flea and may be transferred to a human subject bitten
by an infected flea. Thus, when a rat dies of plague, its fleas
leave it to search for a new host; if they attach themselves to
a rat, that animal is liable to infection, or if they feed upon a
human being, as they frequently do, the disease may become
transferred to man. Two species of fleas are commonly con-
cerned in the transfer, one in tropical and subtropical regions
and another in temperate regions. The tropical rat-flea,
Xenopsylla cheopis (Fig. 15) is thus of greatest importance in
the warm countries where it is most abundant, and the other,
Ceratophyllus fasciatus (Fig. 16), in cooler countries. Both

PUBLIC HEALTH

35

occur, in the United States, neither specifically associated
with plague except as previously outlined; other fleas may

FIG. 15. Tropical Rat-flea (Xenopsylla ckeopis).

act as carriers equally well, but are not so abundant on rats
and do not bite persons so frequently.

The plague bacilli (Bacillus pestis) appear only in fleas
which have bitten affected persons or rats twelve to twenty -

FIG. 16. Rat-flea of Temperate Regions
(Ceratophyllus Jasciatus.)

six hours previous to death, for after this time the bacilli do
not occur in the blood. The vitality and virulence of the

36

INSECTS AND HUMAN WELFARE

bacilli are preserved for nearly a week at least and sometimes
fully a month; and there is actually an increase in their num-
ber during the first few days. Infection from these insects
may then occur through their bites, if they contain extremely
virulent bacilli, but probably occurs more commonly by the
insects being crushed in silu after they have punctured the
skin.

There are many reasons for believing that infantile paralysis
may be an insect-borne disease, and a few words in regard to

JAN FEBMCHAPL MAY JUN JUL AUG SEP OCT NOV DEC

FIG. 17. Seasonal Incidence of Infantile Paralysis in the United States During 1916

this baffling disease may not be amiss. Its summer preva-
lence (Fig. 17) is well known and its general distribution and
occurrence are similar to those of insect-borne diseases.
That it may be proved to be spread by the rat-flea is not
improbable, and if so, would be another strong argument for
the reduction of our rat population.

No account of insect-borne diseases, however brief, could
be complete without some reference to animal diseases. A
few of these have already been referred to incidentally as
affecting both man and animals, and it is quite likely that
other human diseases whose etiology is at present obscure,
will in the future be shown to bear some relation to those of

PUBLIC HEALTH 37

animals. Apart from this, the economic loss occasioned by
such affections of domestic animals is enormous, although it
is in great part preventable.

A wide-spread disease of cattle in the southern part of the
United States, known as splentic fever, or "Texas fever," is
the most important insect-borne animal disease that occurs
in this country, and is particularly interesting since it was the
first disease of any kind shown to be carried exclusively by
insects or ticks. It occurs very generally throughout the gulf
states as far north as the thirty-sixth parallel of latitude and
is the cause of immense pecuniary loss to this region, not only
on account of the cattle lost, but as a result of the greatly
weakened condition of the animals in general. Southern
cattle are usually immunized by an attack at an early age,
but northern animals die in large numbers when exposed to
the disease.

Smith and Kilborne showed in 1893, that the protozoan
blood-parasite, Babesia bigemina, which Smith had discovered
several years earlier to be the cause of the disease, is carried
by ticks. The common cattle-tick of the southern United
States, Margaropus annulalus, acts as the exclusive vector,
becoming infected during its period of engorgement when
feeding on the blood of a diseased animal and then transmit-
ting the Babesia through its eggs to the young ticks of the
next generation. These may feed on healthy animals the
next season, conveying to them the parasites that have been
handed down from the mother tick.

Several similar diseases of cattle occur in other parts of the
world. In Africa, related forms of Piroplasma carried by ticks
are the cause of redwater, East Coast fever, Rhodesian fever,
and in various parts of the world other piroplasmoses have
been observed in many animals.

Spirochsetosis in animals, due to organisms similar to those
producing relapsing fever, is well known. The most fa-
miliar example is probably a disease of fowls which is car-
ried by Argas miniatus, a common tick which infests these
birds.

38 INSECTS AND HUMAN WELFARE

Trypanosomiasis is a general term for diseases like sleeping
sickness due to trypanosomes and there are many diseases of
this type, among which may be mentioned an old-world
affection of horses known as Surra; an African one, Nagana,
that attacks other domestic animals as well; and a South
American type termed Mai de Caderas. Flies are the insects
implicated in the transmission of these diseases, mainly the
large tabanid horse flies and the smaller stable-flies of the
genus Stomoxys. Surra has been introduced into the United
States, but was successfully stamped out before it had be-
come established.

Among bacterial diseases of animals, anthrax may be men-
tioned as one which is sometimes transmitted by biting flies,
the insects acting as mechanical or contaminative carriers
only.

The foregoing enumeration of insect-borne diseases is by
no means complete. Indeed, it would be well-nigh impossible
to make it so, in view of the rapid strides which are being
made at the present time toward a knowledge of these many
problems which bear on the question of public health. New
discoveries are being rapidly announced in all parts of the
world, and while it is difficult to see how the fundamentally
important revelations of the past twenty years can be
equalled in the near future, we should be very unwise to pre-
dict that they will not be exceeded.

CHAPTER II

INSECTS AND THE FOOD SUPPLY

IT is a widely recognized fact that insects consume much food
material that might otherwise serve for human consumption.
The truth of this assertion has been repeatedly brought home
to all who have attempted in a modest way to supply their
own household needs by means of vegetable gardens, as well
as to those engaged primarily in agricultural pursuits. Fol-
lowing the spirit of the times, the home vegetable garden has
become successively tjie war-garden and the conservation-
garden, and agricultural production in America has grown
apace. These immediate changes have not affected the
fundamental relations existing between insects and food-
plants, nor have they influenced their economic expression
to any noticeable extent. They have, however, served to
impress upon the entomologist his responsibility as an inter-
preter of insect activities in so far as these relate to the pro-
duction of human foodstuffs.

The matter is far less simple than might appear at first
sight. In the first place, it depends upon many of the factors
which determine the so-called "balance of nature," and sec-
ondly it involves the abnormal and rapidly changing environ-
ment which has resulted from agricultural development.

The extent of the loss occasioned by insects to growing
agricultural crops, and to edible products in storage, can be
estimated with some degree of accuracy by persons familiar
with agricultural practice and with insect depredations over
wide sections of the country. Several times the Federal Bu-
reau of Entomology has gathered together statistics which
give an adequate idea of the proportion of vegetable food
products actually lost through insect injury, and the conse-
quent monetary loss to the people of the United States. From
their estimates it appears that fully 10 per cent of our agri-

39

40 INSECTS AND HUMAN WELFARE

cultural production is annually destroyed by insects, or in
other words, that our food supply from this source is only 90
per cent of that which would be available if insects were not a
factor in limiting the yield. I think all entomologists will
agree that this 10 per cent reduction is an under-estimate
rather than an exaggeration and as the value of the products
in question is roughly $10,000,000,000 it follows that agri-
cultural insect pests rob the country of approximately a
billion dollars annually. This omits, of course, such impor-
tant crops as cotton, tobacco, etc., wrhich have no food value,
or of which the food value is secondary, as is the case with
cotton which is grown primarily for the lint although the
seed forms an important part of the rations of food animals
like cattle, which are in turn utilized for human food. It
includes on the other hand the loss of products in storage
such as flours, meals, rice, beans, etc. The table below
differs somewhat from others of similar nature that have
been published, but is essentially similar, and is, I believe, as
close an approach to accuracy as can reasonably be expected.

TABLE SHOWING ESTIMATED DAMAGE TO VARIOUS

PRODUCTS CAUSED BY INSECTS, BASED MAINLY

ON PRICES CURRENT IN 191G

Product Value Loss

Cereals $4,241,000,000 $424,100,000

Hay 1,008,000,000 100,800,000

Cotton 1,066,000,000 106,600,000

Tobacco 169,000,000 16,900,000

Truck Crops 512,000,000 76,800,000

Sugars 130,000,000 13,000,000

Fruits 262,000,000 39,300,000

Farm Forests 110,000,000 11,000,000

Misc. Crops 97,000,000 9,700,000

Animal Products 4,338,000,000 433,800,000

Stored Products 219,000,000 21,900,000

Forest Products 100,000,000 10,000,000

Total ; . $12,2.52 000,000 $1,263,900,000

Food Total ' $9,723,000,000 $1,021,000,000

1 Omitting tobacco, farm forests, forest product entirely, and cotton and animal products in
part.

FOOD SUPPLY

41

It does not take into consideration the present greatly ad-
vanced prices which are, no doubt, temporary exacerbations

48.2

PHYTOPHAGOUS

16

PREDATORY

11.3

SAPROPHAGOUS

12

ENTOMOPHAGOUS

2.4

EPIZOIC

1

ANTHOPHILOUS

2.1

UNIMPORTANT

FIG. 18. Diagram illustrating the comparative abundance of various types of food-habits in
insects. The figures at the left indicate the percentage of species exhibiting the several types.
This chart is based on the recorded fauna of a limited area (New Jersey) which includes about
10,000 species. (Modified from Weiss.)

destined to subside, like many of the invasions of insects
detailed in one of the following chapters.

When we consider that insects are, with the exception of
the Protozoa and certain marine invertebrates, the most
abundant animals on the earth, it is not surprising that no
plants appear to be free from insect enemies. I have usually
assumed that about one-half of the 450,000 described, living

42 INSECTS AND HUMAN WELFARE

species of insects were phytophagous in habits, feeding di-
rectly upon the tissues of various plants. This proportion is
probably not far amiss as Weiss has recently tabulated the
food habits of the 10,000 insects of New Jersey enumerated
in Smith's list. He finds (Fig. 18) that 48.2 per cent of these
are phytophagous; that 16 per cent are predatory, feeding
mainly on other insects; that 17.3 per cent are saprophagous,
living as scavengers; that 12 per cent are entomophagous
parasites, developing in the bodies of other living insects;
and that of the small residuum of 6.5 per cent, 2.4 per cent are
epizoic parasites of vertebrates. Only 2.1 per cent appear to
be of no human importance in so far as we can judge at the
present time.

Of course, the great majority of phytophagous insects af-
fect plants never or rarely utilized by man and bear no direct
relation to food production. However, agricultural plants
show no immunity to insect attack, but quite decidedly the
opposite condition prevails, that they are more susceptible;
and this is undoubtedly due to several causes which I shall
attempt to enumerate.

The species of plants suitable for cultivation as sources of
food, are naturally those which produce more than average
amounts of food material in some part of the plant body.
This may develop in the roots, as in carrots, in the stem as in
sugar cane, in the foliage as in spinach, in the seeds as in
beans, in the tissue enveloping the seeds as in many fruits,
or more rarely in special organs. On this account alone, such
plants are unusually acceptable to insects as they are to our-
selves. Another reason for the great susceptibility of agri-
cultural plants to insect injury depends upon the removal of
certain barriers to insect multiplication which follow neces-
sarily as a result of all agricultural development. Under
natural conditions, plants almost always occur in complex
associations of numerous species, of which one or several may
be noticeably dominant, and many of these are capable of
assuming a temporary dominance when for any reason the
more abundant species decline. Such a stable, gradually

FOOD SUPPLY

43

changing, or temporarily shifting condition is reached
through the struggle for existence among the plants, and the
balance of nature maintained by the action of adverse con-
ditions in their environment, of which the insect population
is one factor. So far as the insects dependent upon plants are
concerned, the innovations of agricultural development
represent a cataclysm in their environment. When an ex-

FIG. 19. Map showing the spread of the Mexican Cotton-boll Weevil (Anthonomus grandis),
introduced into the United States from Mexico in 1892. After about thirty years it had spread over
the most productive cotton growing areas of the United States; since 1913 its range has been
still further extended. The limits of the cotton belt are indicated by the heavy line. (After the
Bureau of Entomology, U. S. Dept. of Agriculture.)

tensive area is planted to some particular crop, three alter-
natives present themselves to the original insect inhabitants
of the area. With their natural food plant eliminated more
or less completely, they may become suddenly extinct or
nearly so; this will be the fate of the great proportion and it is
of practically no human concern that it is so. Others (in very
rare cases, but nevertheless important ones) may turn their
attention to the newly arrived plant, and find it a satisfactory
substitute for their former diet. Still a third class, fortu-
nately a very small one, will find the crop plant closely
allied to, or even identical with their original source of food,
and consequently acceptable to their appetites.

44

INSECTS AND HUMAN WELFARE

The proportion of these three classes of insects determine
to some degree the extent of the damage which the farmer
will find has been inflicted upon his crop by the end of the
season.

In order to understand more fully the relations of these
classes, it is necessary to inquire rather closely into the degree

FIG. 20. The European cabbage butterfly (Pontia rap/z) and (be-
low) its American congener (Pontia philodice). Both have the same
food plants and the native butterfly has become more scarce as the
naturalized species has increased in abundance.

of association exhibited by plant-eating insects to their host
plants. A few insects avail themselves of very many plants
as food, and such polyphagous forms like the locusts or grass-
hoppers, are a constant menace, whose varying abundance
depends upon factors not yet mentioned. Numerous other
forms which may be called oligophagous, depend upon several,

FOOD SUPPLY

45

usually related, species of plants for their sustenance. Still
others appear to be monophagous, or restricted to a single
food plant, like the Mexican cotton-boll weevil and many
others of our important agricultural insect pests.

From the agricultural standpoint, monophagous insects
feeding upon the crop plants are usually the most destructive.
If present in the locality where cultivation takes place, they

\

FIG. 21. Map showing the spread of the European white cabbage butterfly (Potitia rapce),
following its first introduction, indicated by white spot, near Quebec in 1860. This insect which
spreads with great rapidity feeds in the larval state upon cabbage and other closely related plants.
(In part after Scudder.)

seek out their host plant with unerring accuracy to find enor-
mous opportunities for development and multiplication, a
condition which, like the vacuum, would be abhorred by na-
ture, but which is a corollary of all agricultural development.
Further than this, the same crop is planted and carefully
cultivated during succeeding years, to give never ending op-
portunities for increasing hordes of insects. The economic
entomologist has no criticism to make of such a method of
agricultural procedure, as he cannot, except perhaps within
very narrow limits, propose a better one. He cannot, how-
ever, hold himself guilty when it is pointed out to him that
insect ravages are constantly increasing in spite of his feeble
efforts to reduce them.

46 INSECTS AND HUMAN WELFARE

The situation is still further aggravated by the constantly
increasing number of foreign insects which find their way to
our shores and become established to spread gradually
throughout those parts of our agricultural regions that enjoy
a suitable climate. Such insects, of which a very consider-
able number could be enumerated, are our most destructive
pests. They are, of all species of animals, the most fortu-
nate. They are suddenly dropped in the midst of plenty;
they are unhampered by the enemies and parasites of their
native land. That they immediately make the best of their
opportunities and proceed to exploit the country is seen by
the phenomenal injury done by the Mexican cotton-boll
weevil or Picudo (Fig. 19) to the cotton crop, the gipsy moth
or Grosse Schwammspinner to deciduous trees, the Hessian
fly to the wheat crop, the cabbage butterfly (Figs. 20, 21)
to various cruciferous crops, and many other naturalized
insects to various other cultivated plants.

The explanation for the abnormally rapid multiplication of
imported insects is not far to seek, and depends upon prin-
ciples well recognized by all biologists. As was previously
pointed out, approximately one-sixth of our insects are ento-
mophagous parasites (Fig. 22), passing their developmental
stages within the bodies of other living insects. Some of these
parasites prey upon other parasites, but a goodly proportion
affect phytophagous insects which they destroy at the time
they complete their growth. Like the plant-eating forms
they are restricted to definite hosts, generally few in number,
with which they show a very definite association. In fact, so
fixed are their instincts to restrict themselves to the same
host that they ordinarily avoid any insects which they do not
normally parasitize. With this in view, if we consider for a
moment the conditions surrounding an insect pest introduced
from another part of the world, we see that it is no longer at
the mercy of the entomophagous parasites that would affect
it in its native land, and that the parasites with which it is
now associated will leave it unmolested. Since many of our.
common native insects have a dozen or more well-known

FOOD SUPPLY

47

parasites which may frequently produce a death rate of from
50-90 per cent in a single season, the importance of this factor
in fostering imported pests is easily realized.

A parasitic insect of this sort may be defined as one which
passes a part of its developmental stages (usually the larval

FIG. 22. Two species of Parasitic Hymenoptera. Above, Dino-
tomus, a common parasite of butterfly caterpillars. Below, Spilo-
cryptus; in this as in many other insects of this group, the female
has a needle-like ovipositor which she thrusts into her victim to
deposit her eggs within its body.

and often the egg or pupal stage also) upon or within the
body of another insect or in its eggs, from which it derives
its entire food supply and which it almost invariably kills
before attaining its full growth. There are a great many
species of parasitic insects, and there are probably very few
or perhaps almost no predacious or phytophagous insects
which are not subject to their attack. Even parasitic insects
themselves are quite frequently destroyed by other species

48 INSECTS AND HUMAN WELFARE

known as secondary parasites, and in at least a few cases the
secondary parasites are known to be infested with others
known as tertiary parasites.

A parasite stands in marked contrast to a predacious
species since it does not feed indiscriminately upon whatever
suitable prey it may discover, but has a certain host species
or series of hosts which it always selects to the exclusion of
others. From generation to generation these same hosts are
always chosen with most extraordinary persistence, for rea-
sons which are at present in most cases obscure. Nevertheless
such appears to be almost invariably the case, although some
species of parasites have been found to attack a much larger
series of hosts than others, and a very few appear to be some-
what indiscriminate in their tastes.

Parasitic species belonging to a number of the different
orders of insects are known, and from our knowledge of the
relationships of the different groups it is evident that the
phenomenon of parasitism has arisen independently in each
group, but by far the most important from an economic
standpoint are those included in the Hymenoptera and Dip-
tera. The parasitic Hymenoptera have the greater signifi-
cance, and include a vast complex of species of the most va-
ried habits, included in a long series of families. Of these the
Ichneumonidse and Braconida? attack the larval or more
rarely the pupal stages, the female ordinarily depositing her
eggs within the body of the insect and the larva feeding upon
the blood and tissues of its host, which it finally leaves for
pupation, or destroys entirely by consuming the vital organs.
In the former case, an affected larva may perhaps survive
under exceptional circumstances, but ordinarily succumbs to
the attack. According to the comparative size of the host
and parasite, one or a series of parasites may be nourished by
a single host. The members of a series of families belonging
to the superfamily Chalcidoidea, which includes a vast
number of small or minute species, are parasitic in habits,
some of them attacking their hosts like the previously men-
tioned families (Fig. 23) . Others present a most remarkable

FOOD SUPPLY

49

phenomenon of psedogenetic, or precocious multiplication
known as polyembryony in which a single egg of the parasite

FIG. 23. A Hymenopterous parasite (Spalangia museidarum) of the housefly
and other related flies. The female Spalangia is shown thrusting her ovipositor
into the puparium of the fly. Below are shown the larva and pupa which de-
velop within the puparium. (After Pinkus.)

deposited in the host egg gives rise to a series of larvae (often
several hundred) which develop within the host larva. Still

50

INSECTS AND HUMAN WELFARE

other very minute species are egg parasites, completing their
entire growth within the eggs of larger insects, within which
the female deposits her own minute eggs. The superfamily
Proctotrypoidea includes several families of minute species
which are egg parasites, developing in much the same way
within the eggs of other larger insects. To this group belong

FIG. 24. A parasitic Tachina fly (Compsiltira concinnata),
an enemy of the gipsy moth.

also some other families with habits more like those of the
Ichneumonidse, which attack the later preparatory stages of
their hosts.

Second in importance are a large series of Diptera belonging
to the family Tachinidse, parasitic during their larval devel-
opment within the bodies of other insects, mainly the cater-
pillars of various Lepidoptera, although they by no means
restrict their attacks to insects of this order. The Tachina
flies (Fig. 24) are in some ways less specialized in their habits
and as a rule do not have such definite host relations as the
parasitic Hymenoptera, most species attacking a larger and
more varied series of hosts. On the other hand, the methods
by which the larva? gain access to the host show highly de-
veloped adaptations. Some deposit large, oval white eggs
directly upon the caterpillars or other insects within which
they may develop, and the larvae on hatching bore through
the cuticle and gain entrance to the visceral cavity. Others
thrust maggots directly through the skin of their victims,

FOOD SUPPLY 51

while still other types deposit large numbers of minute eggs
or larvae on the foliage upon which the host insects are feed-
ing. In the latter cases the eggs or maggots are ingested with
the food, whereupon the eggs hatch and the larvae, perforat-
ing the wall of the alimentary tract, reach the visceral cavity.
One or many larvae may live at the expense of a single host
according to their relative sizes, and on attaining full growth,
the larvae usually quit the host through orifices they make in
its body wall, and enter the soil for pupation, later to emerge
as adult flies.

It seems probable, at least in regard to the Tachina flies,
that the parasitic habit has been derived as a modification of
the habit of feeding upon recently dead insects, which has
been transferred to living ones and later developed its most
remarkable adaptations as seen in the group at the present
time. With the parasitic Hymenoptera the derivation of
the parasitic habit is far more obscure, and doubtless of
much more ancient origin.

Ever since scientific methods of investigation were applied
to the problems of economic entomology the importance of
parasites has been recognized as a factor involved in the
natural control of insect pests, but it is only within com-
paratively recent times that their full significance has been
realized. With the recent introduction of several injurious
insects into our own country a minute comparison of the
controlling factors here and in the country of origin has
brought clearly to light the prime importance of insect para-
sites and attempts have been made to colonize the parasitic
enemies of several pests in the United States. Similar ex-
periments have been undertaken by foreign entomologists
also, and one of the most extensive yet well under way is that
aimed at the control of the gipsy moth which is discussed on
a later page. The same condition appears to prevail gener-
ally in regard to the diseases of insects caused by fungi (Fig.
26), bacteria and other microorganisms, but this matter has
been carefully investigated in only a few cases and it would
seem inadvisable to make very definite statements at the
present time.

INSECTS AND HUMAN WELFARE

Predatory insects also play an important part in reducing
the abundance of phytophagous insects. In habits they are
directly comparable to the beasts of prey among the higher
animals, which destroy the weaker and more timid mammals.
They do not show the same close correlation to their prey
exhibited by insect parasites and cannot usually be depended

•

*v

FlQ. 25. A common beetle (Helops micans) killed by a parasitic fungus.

upon to restrict their depredations to any particular insect
pest. Quite frequently, however, they are a great aid in
combination with parasites and several exotic ones intro-
duced into the United States have aided in the control of the
gipsy moth, scale insects, etc.

Another factor which must not be overlooked, relates to
the provenience of our agricultural crop plants. These are
the result of selection during the course of many centuries,
and have been gathered together from the most diverse parts
of the world. Thus most of them are grown to a very great

FOOD SUPPLY 53

extent outside of their natural habitat and consequently find
themselves frequently and to varying extents, free from at
least some of their original insect pests. With the gradual
growth of world commerce and the great increase in the ease
and rapidity of transportation, insects are more easily carried
over natural barriers and the aggregation of agricultural pests
is continually becoming more homogeneous.

As was mentioned before, cultivated plants are often at-
tacked by insects which have previously fed on related plants
and such occurrences, although uncommon, are by no means
rare. Many of our most important food plants belong to a
very few of the natural families of plants and it is noticeable
also that these plant groups are usually very widely distrib-
uted ones with many representatives. Thus the grasses or
Graminese furnish us with wheat, Indian corn, oats, rye,
barley and sugar cane; the Leguminosse with beans, soy
beans, chick peas, peas and cowpeas; the Solanacese with the
potato, tomato, sweet pepper and egg plant, the Umbelliferse
with carrots, parsley, celery and parsnips; the Cucurbitacese
with the pumpkin, squash, vegetable marrow, cucumber and
melons; the Cruciferse with cabbage, cauliflower, Brussels
sprouts, kohl-rabi, mustard, turnips and radishes; the Com-
positse with salsify, lettuce, and the Jerusalem artichoke; the
Polygonacese with buckwheat and rhubarb; the Liliacese
with asparagus, onions, leeks, garlic, etc.

There are, of course, very numerous exceptions to this as is
the case in food plants like the beet, sweet potato, etc., but
these only modify the general principle that the utilization
of numerous related plants is the common practice. It is
noticeable also that a good many of these plants are of
tropical or semitropical origin, and that they are commonly
cultivated in climates far more rigorous than those in which
they have originated. This is true of a large variety of well-
known food plants like corn, potatoes, tomatoes, pump-
kins, etc.

As might naturally be expected from the foregoing, we
actually find that some of our pests of wheat are species

54

INSECTS AND HUMAN WELFARE

occurring on, and probably original enemies of native Ameri-
can grasses, several of which are very closely related to the
cultivated wheat plant. A still more interesting case is that
of the Colorado potato beetle (Fig. 26), a well-known and
very destructive enemy of the potato (Solanum tubewsiim) .
We have good evidence that this is a neotropical insect of
Mexican or Central American origin which extended its
range within historic times into the middle west as far as

FIG. 26. The Colorado potato beetle (Leptinotarsa decemlineata), originally
a wanderer from Mexico, now firmly established in practically every potato
patch. The larva is shown at the left.

Colorado, following the probable northward migration of its
original food plant, Solanum rostratum, which is a common
roadside weed. In the late sixties of the past century it was
first noticed feeding on the foliage of cultivated potatoes and
within twenty-five years had encompassed the entire United
States east of the Rockies, breeding upon potato plants
everywhere, and even invading Europe where its incipient
colonies were wiped out.

This same group of plants, the Solanums, also illustrates
nicely the vagaries of numerous insects in relation to food
plants. Thus we find that the Colorado potato beetle now
feeds upon Solanum rostratum far less commonly than upon
potatoes and that it does not occur at all on the tomato or

FOOD SUPPLY 55

egg plant, both of which are species of the same genus, al-
though it occasionally affects tobacco. On the other hand,

FIG. 27. Heliothis obsoleta, variously known as the cotton boll-worm, corn ear-worm,
tobacco bud-worm, etc. Moth is shown above; below, the larva feeding in a cotton boll.

the tomato is frequently injured by a large caterpillar known
as the tomato fruit-worm (Heliothis obsoleta} (Fig. 27) which

56 INSECTS AND HUMAN WELFARE

does not occur on the egg plant or potato, although it is the
common enemy of tobacco known as the bud-worm, and is
moreover a very important enemy of both corn and cotton,
two totally unrelated plants. On these latter it has been
called the corn ear- worm and the cotton boll-wTorm, but it is
identical and feeds on these several plants without much
apparent choice.

Insects of this type point out clearly the fallacy of any
statement that related plants always have the same or re-
lated insect pests. That they quite generally have the same
enemies or nearly related ones writh similar life history and
habits is, however, ordinarily quite true.

How can this damage be abated or lessened is the natural
inquiry from the non-academic mind. We have seen that a
return to natural conditions would speedily reduce insect
injury and bring into play the forces of nature which would
maintain a more or less stable condition. Such a return
would not take place, however, if human influence were
simply removed; it is true that a stable condition would re-
sult, but it would be very different from that which existed

t/

in the original flora and fauna. Indeed human ingenuity
could not again make things as they were, even if it were de-
sired to do so at the cost of discarding all agricultural prog-
ress. It is, however, possible by an additional disturbance
to approach in some respects to a natural biological associa-
tion without disturbing agriculture in the least degree.

This partial return to the native environment can be
achieved to some extent by the introduction of parasitic in-
sects and other organisms, and this biological method of
combating imported insect pests has proved itself of value
in many cases. Up to the present time the importation, colo-
nization and distribution of such enemies has been accom-
plished in this country with the parasites of the gipsy moth
and to a less degree with those of certain other insect pests.
The method gives great future promise, but there are many
technical difficulties to be overcome. Its great advantage
lies in the fact that it insures permanent relief once it has

FOOD SUPPLY 57

been started, and that its limitations are in a broad way de-
termined primarily by the insect species against which it is
directed. Aside from the repeated application of poisons and
other insect-destroying materials, it is the only feasible means
except those which entail changes in agricultural procedure.

Of the latter methods, there are several which can be em-
ployed without seriously disturbing agriculture, and at least
one of them is, in fact, only a slight modification of the prin-
ciple of the rotation of crops which had its origin not in rela-
tion to insects, but as a practice to delay or prevent the loss
of soil fertility.

We have already seen how the continued presence of the
same crop plant in the same place from year to year serves
to augment insect depredations. Since many different crops
have insect enemies in common, it follows that when such
crops follow one another on the same area, the opportunities
for insect multiplication are not conspicuously less than if
there were no rotation. Similarly the planting of newly
plowed grass-land to corn, wheat or other grasses courts dis-
aster in the same way. Rotation practised with insect injury
in mind, however, where the crop plants are without impor-
tant enemies in common, serves to reduce insect injury to at
least some extent, dependent mainly upon the ability of the
insects concerned to migrate, and it need in no way impair
the other advantages derived from the process. Agricultural
entomology has now reached such a condition that reliable
advice with reference to many insects can be given in this
matter.

A concrete example of the utilization of this method in the
control of certain insects that are difficult to combat other-
wise has recently been demonstrated. In this case the insects
are the large grubs of May-beetles which feed on the roots of
many plants. They are particularly injurious to corn in the
northern middle states, but do not injure clover nor do they
deposit their eggs in soil that is covered by a stand of clover.
In this region corn is commonly planted in rotation with
timothy, oats or barley, but if clover is grown the year before

58 INSECTS AND HUMAN WELFARE

a field is to be planted to corn, few grubs will be present in
the cornfield. As the beetles require two years for their life
cycle, appearing most abundantly in alternate years, the
crop rotation can be planned in relation to the insects.

The growing of substitute crops is practically this method
applied in a somewhat different way, and has the further
advantage that the original crop is not removed to a distance,
but is more or less completely eliminated. This is a rather
fundamental agricultural change and it must be taken for
granted also that a substitute is something less desirable. As
some of us have become rather enthusiastic over substitute
foods during the past few years, it may be of interest to point
out that two potato substitutes, the dasheen in the South
and edible sunflower root or "Jerusalem artichoke" in the
North are plants which appear to be, for the present at least,
not severely affected by insects, and for this reason at least are
worthy of encouragement .

Aside from the factors already referred to as regulating the
abundance and destructiveness of agricultural insects, va-
rious poisons are in very general use for the control of such
pests. Under present conditions these are the most direct,
and probably the most efficient means, that can be employed
for the protection of many crops, although with others there
are apparently insuperable difficulties to the satisfactory use
of such insecticides.

At the time the Colorado potato beetle began its invasion
of the Eastern United States agriculturists were in despair, as
it appeared that the cultivation of this important crop would
have to be abandoned almost entirely over large sections
of our country. However, as a result of the discovery that
Paris Green dusted or sprayed upon the foliage would destroy
the beetle grubs without injuring the plants, the potato crop
was harvested in spite of the beetle, and spraying for potato
bugs has become a commonplace occupation for all who at-
tempt the culture of potatoes. The substitute of other less
soluble arsenical compounds such as arsenate of lead has
rendered spraying a more effective and safer procedure, and

FOOD SUPPLY

59

careful studies of the life history and habits of specific insects
have greatly extended the field of usefulness of the arsenical
insecticides. This is well illustrated by studies made on the
codling moth by Melander. This insect is a very important
enemy of the apple, within the fruit of which its develop-
mental stages are passed. It is usually controlled by spray-

Side -wormy

Vermorel SPRAYED 40 p. c. Calyx -wormy

Straight Bordeaux 25 p. c. Calyx • wormy

Bordeaux & Crook I 5 p. c. Calyx • wormy

Bord., Crook & Tower I 0 p. c. Calyx • wormy

Calyx -wormy

TOTAL GOOD 40 p. c.

OHM

TOTAL GOOD 85 p.c.
TOTAL GOOD 92 p. c.
TOTAL GOOD 97 p. c.
TOTAL GOOD 99 p. c

FIG. 28. Chart illustrating the effects of spraying with lead arsenate by different methods for the
control of the codling moth (Carpocapsa pomonella), a widespread and serious insect enemy of the apple.
In this case unsprayed trees yielded only 40 per cent of good fruit, while of the wormy fruit, 80 per cent
became wormy from larvae entering the fruit at the calyx cup; when sprayed with a Vermorel nozzle
giving a mist spray, 85 per cent of the fruit was good, while of the wormy fruit 40 per cent became
wormy from larvae entering at the calyx cup; when sprayed with a Bordeaux nozzle giving a forcible,
penetrating spray, 82 per cent of the fruit was good, while of the wormy fruit 25 per cent became wormy
from larvae entering at the calyx cup; when sprayed with a Bordeaux nozzle set at the proper angle by
means of a crook, 97 per cent of the fruit was good, while of the wormy fruit 5 per cent became wormy
from larvae entering at the calyx cup; finally, when the latter spraying was done from a movable tower,
99 per cent of good fruit was obtained and no apples became wormy from larvae entering at the calyx
cup. (After Melander.)

ing the young fruit with an arsenical, but the advantage
gained varies within very wide limits depending upon the
exact time at which the spray is applied, the method of appli-
cation, and the type of nozzle used. Slight modifications in
the form of calyx cup of the young apple, due apparently to
different climatic conditions, have in turn a profound influ-
ence upon the efficacy of the spray, and in some regions,
several sprayings must be substituted for the single one
found to be effective at other places (Fig. 28) .

60 INSECTS AND HUMAN WELFARE

Thus, in spite of their simplicity of action, arsenical sprays
can be prescribed with confidence only after a careful and
painstaking diagnosis has been made.

The arsenical insecticides are usually referred to as stom-
ach poisons as their toxic action is accomplished through the
medium of the alimentary tract. As we have seen, they are
readily ingested by chewing insects when applied to the
plants upon which such insects are feeding. Many of our
most destructive insects do not feed by chewing however,
but subsist upon juices which they extract from their food
plant by means of a piercing, sucking beak which is thrust
into the tissues of the plant. Such haustellate insects mani-
festly will not take into the mouth any arsenical poison pres-
ent upon the surface of the plant, and no successful means
have been devised for introducing poisonous materials into
the sap with which they might be taken into the body of
the insect.

For the destruction of such insects, other materials, known
as contact insecticides are in general use. These consist of va-
rious substances, such as nicotine, certain sulphides and oil
emulsions, etc., which kill through contact, by actual poison-
ing, suffocation or otherwise.

Tliese are often used in combination with the arsenicals
and with a fungicide such as the copper compound known as
Bordeaux mixture. Thus when several insects of different
types and fungous diseases are to be dealt with upon the
same plants, specific combinations may be prescribed. Some
of the methods of treatment available for application to va-
rious fruits are shown in the accompanying diagram (Fig. 29).

After foods are harvested they are commonly stored for
considerable periods before they are finally utilized, espe-
cially in the case of staple crops such as cereals, beans, etc.
During this time they are by no means free from insect in-
jury, in fact, their deterioration is undoubtedly hastened
more by insect activity than by any other agency. The in-
sects concerned are a most cosmopolitan lot, gathered from
all quarters of the globe, for their world-wide dissemination

FOOD SUPPLY

61

has been most readily accomplished in the extensive ship-
ments of such products. The Orient is represented by several
Chinese bean-weevils which affect the stored seeds of legumi-
nous plants; southern Europe by the Mediterranean flour

FIG. 29. Diagram illustrating the various combinations in which several arsenical insecticides may be
combined with contact insecticides and fungicides for the protection of certain orchard fruits from in-
sects and fungous diseases. In the figure, the sections between the heavy black lines represent the com-
binations suitable with three different arsenicals, while the smaller sectors represent suitable treatment
for several types of injury to the particular plants that are indicated about the periphery of the diagram.
(Slightly modified from Quaintance.)

moth, Mexico by the Mexican grain beetle, etc., while the
majority of the more common ones are so widely scattered
that it is almost impossible to ascertain their original home.
Owing to the non-living character of the material in which
these insects live, it is possible easily to control them by
means of simple physical or chemical means such as heating

62 INSECTS AND HUMAN WELFARE

to a temperature that will destroy insect life, or fumigation by
some insecticide such as carbon bisulphide, carbon tetra-
chloride or hydrocyanic acid gas, after which they may be
marketed in sealed containers. In spite of the ease with
which these insects can be controlled, they entail an immense
economic loss which is probably in the neighborhood of $20,-
000,000 annually in the United States. Then* origin as
enemies of stored products which are maintained under
purely unnatural conditions, may at first sight seem a trifle
obscure, but they all undoubtedly represent species originally
enjoying very limited opportunities for multiplication, and
subject to great vicissitudes. Like agricultural pests they
now flourish with the barriers removed.

CHAPTER III

FOREST INSECTS

THE necessity for the conservation of forests and the rational
utilization of their products has long been recognized by all
who have given attention to such matters. Changes in pri-
vate and public policy have come slowly, however, and until
recently but little attempt has been made to manage forests
with regard to both their present and future value. As a
consequence, forestry has lagged far behind agriculture and
although "forest conservation" has now become a byword
with the American public, its history in our own country is
very brief. Despite the great value and vast importance of
forest products, it is easy to find a reason for the more rapid
application of scientific ideas to agriculture, for there the re-
sults of improved methods manifest themselves within a
single season. On the other hand, timber trees are of slow
growth, and unlike agricultural crops, they have existed in
enormous quantities ready for man's use without any effort
on his part to further their growth. So long as primeval for-
ests were available in sufficient amount and in convenient
locations, little care was taken to provide for a future supply
of lumber. In fact it was tacitly assumed that nature would
provide a continuous supply of trees, and she was left to her
own devices to replace what human agency had removed.

Under the conditions now prevailing, she has consistently
failed to replace forests that have been removed, mainly on
account of fire and other disturbing elements resulting from
human activity. Thus to one section of the world after
another, has come the realization that forestry must be put
on a permanent basis if forest products like agricultural crops,
continue to be necessary for human existence. There appears
to be no promise of a future which will lessen its demands

63

64 INSECTS AND HUMAN WELFARE

upon the forests and there is every probability that they will
be called upon in rapidly increasing measure.

The problems entailed in maintaining forests in a produc-
tive state and those to be dealt with in rehabilitating de-
pleted ones, or in fostering the development of new ones, are
numerous and intricate, but manifestly they have much in
common with those encountered by the agriculturist. One
of the most striking differences is the element of time, due to
the slowness with which the lumber crop matures in com-
parison with the rapid development of the common annual
agricultural crops. This element of time at once exerts a
profound influence on the commercial aspect of the matter,
since the returns on the money invested in forests and that
spent in their maintenance are so delayed that they dwindle
in amount when compared with those which may be obtained
from investments of other kinds. Added to this, is a psy-
chological aspect due to the deep-seated human aversion for
rewards in the too distant future. These matters could be
dwelt upon at great length, but they are far from the present
discussion, except that tnev explain in great measure the
past and present relations between the human race and the
forests. One point which must be borne in mind, is that the
element of time bears also a very important biological re-
lation to the welfare of forests. This is least noticeable in
many primeval forests of mixed character, but becomes in-
creasingly significant in those of pure growth, and those
which are reproducing themselves after depletion through
extensive cutting, while it is of prime importance under the
conditions prevailing where extensive reforestation is under-
taken.

We have pointed out, in connection with the insect enemies
of agricultural crops, the general relationship which exists
between these plants and insects. In the case of forest in-
sects, it will soon be seen how greatly the long develop-
mental period of trees affects the character of the depreda-
tions of insects that live at their expense. Another difference
which influences damage by insects, depends upon the devel-

FOREST INSECTS 65

opment of woody tissue, which is practically absent in all of
the agricultural crop plants, and which serves as food for
quite a considerable variety of insects. On the other hand,
the large fleshy roots so characteristic of many food plants
are not developed in trees, which are consequently not in-
jured by insects of the type that are so destructive to vege-
tables like the carrot, radish, etc. With the horticultural trees
like the apple and peach, forest trees have more types of in-
sect enemies in common, but with the exception of the nut
trees, palms and a few others the fruit of forest trees is of
little consequence so long as sufficient seed is produced to
allow for reproduction. In exceptional cases, certain insects
actually do prevent the development of seeds to such a degree
as to menace seriously the reproduction of certain species of
coniferous trees, but ordinarily their influence upon seed
production is not of any great importance.

Aside from the fundamental differences in material compo-
sition between the individual tree and herbaceous agricul-
tural plant, and not to consider for the moment, the slow
development of the tree, we find another very striking differ-
ence in the environment encountered by the insects which
feed upon the two types of plants.

In the previous chapter on agricultural insects, attention
has been called to the way in which associations of very di-
verse plants have been replaced by large masses of crop
plants belonging to single species. This change has upset
entirely the balance which exists under natural conditions
between the plants and their insect enemies. Many forests
are comparable to the miscellaneous association of plants
which have given place to cultivated fields, and in fact, much
of the present agricultural land was once covered by forests.
On the other hand many forests, e.g., those consisting of co-
niferous trees, are frequently composed of single species grow-
ing to the practical exclusion of all others over very extensive
areas. When such is the case, the situation is very similar to
that presented by agricultural plants. It is, however, funda-
mentally different in at least one respect: these trees are

66 INSECTS AND HUMAN WELFARE

growing under natural conditions, and with their slow
growth, could never have reached their present age if they
were regularly subject to damage comparable to that inflicted
upon agricultural plants growing continuously in pure stand.
These coniferous forests present peculiar conditions in rela-
tion to insect damage which are of great importance at the
present time, and they afford interesting material for specu-
lation as to the damage which may be caused by insects in
the future, to any pure forests of other kinds of trees that may
be planted for specific purposes.

A characteristic of the insect damage to coniferous forests
is the great fluctuation which it exhibits both in extent and
severity from year to year. The appearance in great abun-
dance of many of the insect enemies of conifers often shows a
striking similarity to the epidemics of certain human diseases,
although the reason for their later disappearance is un-
doubtedly due to very different causes. One common insect
of Europe is of especial interest since it is a close relative of
the gipsy moth, already introduced into the United States.
This is the nun moth (Fig. 30), feeding in the larval stages
upon the foliage of various conifers. When the caterpillars
are abundant, they may completely defoliate trees over wide
areas and cause them to die in large numbers, as coniferous
trees very frequently fail to survive complete defoliation.
The more important and extensive "epidemics" of the nun
moth experienced in recent times occurred in 1795, 1839 and
1890, or approximately fifty years apart. Since this is about
the period required for the individual trees to attain a mature
size, the appearance of the insects shows a very interesting
parallel to the more or less regular appearance from year to
year of many insects destructive to annual agricultural
plants. In the case of trees of this sort, it is evident that a
more frequent occurrence of such outbreaks would seriously
menace the continued existence of the host-tree species. The
earlier outbreak cited above was suddenly terminated during
the third summer as the result of the appearance of a fatal
infectious disease of the caterpillars, and such disease, as well

FOREST INSECTS

67

as parasitic insects, in combination with a scarcity of food,
are undoubtedly responsible for the curbing of many in-
vasions of this kind.

In both Europe and North America the larch tree is af-
fected by the leaf-feeding, caterpillar-like larva of the larch
sawfly, an insect common to both continents. Like many

FIG. 30. The nun moth (Psilura monachal, an important enemy of
coniferous trees in Europe. This insect was once found in Brooklyn,
N. Y., but was stamped out before it had opportunity to spread.

other forest insects, this species has in a number of instances,
appeared suddenly in great abundance in localities where it
had previously been hardly noticed for many years. Since
the middle of the last century, there have been about half a
dozen outbreaks of this kind, culminating in a very wide-
spread one in 1903-1907 which involved a considerable part
of the area in the eastern part of America where the larch is
native, and caused the death of a large proportion of the ma-
ture larches in this region.

68 INSECTS AND HUMAN WELFARE

During the past decade certain parts of eastern Canada
and a considerable portion of the State of Maine have suf-
fered from an invasion of the spruce bud-worm (Tortrix fumi-
ferana), an insect which defoliates the balsam fir and spruce.
This insect has moved like the incoming tide and extended
its destructive range from year to year. The spruce woods of
northern Maine that furnish much of the pulpwood from
which some of our better class papers are produced have
suffered severely for several years. During this period certain
areas have in succession lost a goodly part of their tree popu-
lation of spruce and balsam fir, after which the insects have
suddenly disappeared. If we can judge by the past history
of this species, it will not again reappear in destructive abun-
dance for many years. As a result of its ravages, however,
there is grave danger that these forests may be replaced by
more rapidly growing and less valuable hardwood trees which
would not under natural conditions again give way to spruce
and fir for perhaps two or three hundred years. In the his-
tory of the world this is a mere incident, but it is a very se-
vere blow to one of our important present-day industries.

In the northwestern states and western Canada there is a
small, pale yellowish-white butterfly, quite similar to the
common European cabbage butterfly now so abundant in the
United States. It feeds during its larval stages upon the fo-
liage of several kinds of western conifers, and over the short
period during which it has been observed, has several times
suddenly appeared in incredible numbers in rather definitely
circumscribed areas. At such times, extensive portions of the
forests are defoliated, and the trees succumb as a result of the
injury.

This insect and others that produce the general defoliation
of coniferous trees may not always cause the immediate death
of the trees, but in cases where the weakened trees might
otherwise recover, they open the way for depredations on
the part of another group of destructive insects known as the
bark-beetles. The latter are small, brownish or blackish,
hard-bodied insects, represented by numerous species that

FOREST INSECTS 69

live at the expense of trees of almost every description. Some
of the larger ones, belonging to the genus Dendroctonus
(Fig. 31), attack perfectly robust, healthy trees, but the ma-
jority live in weakened, moribund, or dead trees. Those of the
first type commonly appear in the form of invasions or epi-
demics, of which many have been noticed in our own coun-
try, from time to time. The others find great opportunities

A Ui

FIG. 31. A. bark-beetle (DendToctonus monticoltr); adult beetle, larva and pupa. (After Swaine.)

for multiplication following sporadic outbreaks of other in-
sects like those mentioned above, and may complete the ruin
of considerable areas of forests that might eventually have
recovered from the effects of partial, or even complete defolia-
tion.

Fortunately the bark-beetles that kill healthy trees are
few in number, but they are rather widespread in our own
country, and our coniferous forests have suffered greatly from
them in numerous instances. Destruction in these cases also,
has usually been due to sudden outbreaks of limited duration.
Thus, at several times during the last century, spruces in the
northeastern states have been killed over rather definite areas

70 INSECTS AND HUMAN WELFARE

by the spruce bark-beetle (Dendroctonus piceaperda). With
this, as with other bark-beetles, the parent beetles excavate
galleries in the inner bark or cambium, along which they
deposit their eggs. When these eggs hatch, the larvae burrow
off to the sides where they feed upon the cambium layer until
they attain full growth, after which they bore their way
through the outer bark, and leave the tree. Extensive injury
to the cambium interferes with the metabolism of the tree
and kills it after the manner of the girdler's axe (Fig. 32) .

More spectacular than the work of the spruce bark-beetle,
has been that of the Black Hills beetle (Dendroctonus ponder-
osce. Following its sudden appearance in large numbers in
the Black Hills National Forest of South Dakota, during the
season of 1897, its ravages assumed the proportions of an
epidemic there, and Hopkins has estimated that trees con-
taining nearly a billion feet of timber were killed before the
epidemic subsided several years later.

As mentioned on a previous page, mixed forests present a
somewhat different problem to the entomologist from that
just discussed in connection with the unmixed, or pure
growths of coniferous forests. Our mixed forests are com-
posed mainly of deciduous, or broad-leaved trees, and these
ordinarily contain a well-mixed assortment of tree-species
belonging to at least several of the natural families of plants,
combined with much shrubby and herbaceous growth. They
at once exhibit a great diversity of plants, and support a
correspondingly large and varied insect fauna. A consider-
able proportion of these insects are species dependent upon
the plants other than trees; some live upon the rotting wood
or bark of fallen trees, others upon fungi, many are predatory
or parasitic upon the foregoing, till only a small number re-
maining, are actually destructive forest pests. These in turn,
are each almost entirely restricted to one or several related
trees as food plants, so that the inter-relationship of trees
and insects becomes a very complicated one. It would seem
that such an environment should tend to equalize the damage
done by insects to specific kinds of trees, from year to year,

FOREST INSECTS

71

and to inhibit the development of extensive outbreaks. Such
a conclusion appears to be borne out by experience, except in

FIG. 32. Work of the Black Hills bark-beetle (Dendroctonus -ponderosce) on the inner
surface of the living bark. The vertical galleries are excavated by the parent beetles
and the smaller lateral ones are the food burrows of the larvse. The entrance holes
(a) are at the lower end of the primary gallery, which is ventilated by other holes,
indicated as white spots. (Slightly modified from Hopkins.)

a few cases, and there the explanation must be sought for
along other lines.

It might appear from such a brief comparison of insect
damage to pure coniferous forests (i.e., those which practi-

72 INSECTS AND HUMAN WELFARE

cally inhibit the development of undergrowth on the forest
floor) with mixed forests of deciduous trees, that we may have
overlooked some fundamental principle regulating insect
damage to trees of the two types. That such is not the case
is shown by the less noticeable, and more restricted out-
breaks of certain species that occur in deciduous forests where
one species of tree may greatly predominate, or in cases of
insects that readily attack a considerably extensive series of
trees. That the excessive importance of forest insects in our
own country may in part be due also to the great amount of
waste, uncared-for land suggests another factor which may
perhaps modify these conclusions.

The striped maple caterpillar (Anisota rubicunda) is an
example of the first type mentioned above. This is a defoliat-
ing species, of rather general distribution in the cooler por-
tions of eastern North America. It is naturally always more
abundant where maples are numerous, but occasionally
appears in localized outbreaks of quite definite duration.
An example of the other type is the European gipsy moth
(Porthetria dispar, Fig. 33), now prevalent in a part of New
England where it became established as a result of accidental
introduction, through the agency of man.

The gipsy moth lives as a caterpillar on the foliage of a
large number of deciduous trees, and will, when pressed for
food, devour with evident relish many shrubs. Even the
foliage of pines and other conifers will serve as food for the
caterpillars after they have become partly grown. Although
certain trees such as the oaks are its favorite food it is a truly
polyphagous insect and a forest of deciduous trees contains
few species which it will not readily attack. It finds com-
monly, therefore, conditions very similar to those en-
countered by the enemies of coniferous trees. Consequently,
we find that its history in Europe, as far back as it has been
recorded with accuracy, includes a series of sudden outbreaks.
Thus in Europe in 1731, 1761, 1794, 1837, 1871, and 1909,
certain localities were visited by hordes of these caterpillars
which defoliated everything in the vicinity. It is very re-

FOREST INSECTS

73

markable to note the great regularity with which these out-
breaks have occurred (respectively 30, 33, 43, 34 and 38 years
apart), particularly when it is remembered that they have in
no way been coincident geographically. It must be remem-
bered, of course, that at many intermediate dates there have
been subsidiary outbreaks of lesser severity, and more re-
stricted range, but the periodicity of this insect, and that of

FIG. 33. The gipsy moth (Porthetria dispar). The female moth is shown above
and at the left. The dark colored specimen is the male.

the nun moth mentioned on a previous page, are undoubtedly
manifestations of some principle which influences the de-
structiveness of many forest insects.

In connection with agricultural insects we have already
called attention to the importance of entomophagous para-
sites, and the regulatory effect exercised by these is fully as
great in the case of forest insects. In fact some of our most
accurate knowledge of the economic value of parasitic insects
has been gained through a study of the parasites of the gipsy
moth, which was made on an elaborate scale by the Federal
Bureau of Entomology in its well-directed efforts toward
preventing the spread, and curbing the destructive activities

74 INSECTS AND HUMAN WELFARE

of the gipsy moth in New England. Since this insect bids
fair to become one of the worst forest insects over a consid-
erable portion of North America, it may be worth while to
review its status, in regard to natural control by parasites,
in both Europe and America. It was first introduced into the
United States about 1868, in a little town near Boston where
it did not become sufficiently abundant to attract attention
for nearly twenty years. Since then it has spread very slowly,
but persistently, in spite of the various repressive measures
undertaken to prevent its multiplication and dissemination
(Fig. 34) . At the present time it is the most abundant cater-
pillar feeding upon trees over a large part of its range, and
has caused unbelievable havoc in the woodlands of eastern
Massachusetts and contiguous territory.

The slow rate of spread of the gipsy moth is in striking
contrast to that of the European cabbage butterfly which has
been cited on another page, and aside from the vigorous re-
pressive measures undertaken to delay its dispersion may
perhaps be due to the inability of the adult female moths to
fly. On this account, aside from accidental transportation,
the species must depend upon the larva for migration, and,
when newly hatched from the egg, these are commonly blown
to considerable distances by the wind. Nevertheless, a
closely related European moth, the brown-tail moth (Euproc-
tis chrysorrhoeci) first detected in 1897, at almost the same
place as the gipsy moth, has in twenty years encompassed
but little more territory. It is now distributed over a narrow
coastal strip scarcely over 100 miles in width from Cape Cod
northward through Maine in Canada. The slow movement
of these pests is gratifying from an economic standpoint, but
is of only temporary significance, as both are undoubtedly
destined to cross the entire continent, and to assume nation-
wide importance. The cause of the slow spread of the brown-
tail moth is not easily to be pointed out, as this species flies
readily and finds its favorite food plants widely distributed.
Several factors are undoubtedly concerned, relating to me-
teorological conditions, as well as the nomadic instincts of the
moths themselves.

FOREST INSECTS

75

In Europe the gipsy moth has a number of insect parasites
and these serve to reduce its numbers greatly below those

Naotucket

Fio. 34. Map of the New England states showing the spread of the gipsy moth (Porthetria dispart.
In 1900, twenty years after its introduction near Boston, it had spread over the area marked in
black: the cross-hatched lines indicate its distribution in 1905, and the slanting lines its range in
1918.

present in the infested American districts, although as we
have already seen, severe outbreaks in Europe are by no
means unknown. In Japan, the species is still less destruc-

76

INSECTS AND HUMAN WELFARE

live, presumably on account of more efficient control by para-
sites. Since parasites capable of attacking the moth were
practically absent in the American fauna, an extensive at-
tempt has been made to colonize the Eurasian parasites on
this continent. This work has been well under way for about
ten years, and after numerous technical difficulties, a series

PARASITES

EGG

LARVAL STAGES

PUPA

ADULT

NEW
10
DAYS

OLD
280
DAYS

1st.

7
DAYS

2nd.

3rd.

4th.

5th.

6th.

7th.

PRE-
PUPA
2
DAYS

NEW
3
DAYS

OLD
7

DAYS

DAYS

DAYS

DAYS

DAYS

DAYS

DAYS

Anastatus bilasciatus
Schedius kuwanae

Apanteles melanoscelus
Apanteles Itcteicolor

Blepharipa sculellata
Crossocosmia sericariae
Compsilura concinnata
Zygobothria nidicola
Carcelia gnava
Parasetigena segregata
Tricholyga grandis
Tachina larvarum
Tachinajaponlca
Chalcis llavipes
Chalcis obscurata
Monodontomerus aereus

First

Jenera

lion
Se

1
:ondG

inerati

in

—

s—

— •

—

i

mm

i

FIG. 35. Chart illustrating the way in which parasitic insects aid in the control of an insect pest.
The sixteen species mentioned are some of the most important parasites of the gipsy moth in Eu-
rope and Japan where this insect is native. Of the large series introduced into the United States,
they are the most promising enemies of the gipsy moth as they appear to have become most readily
acclimated and to have multiplied most satisfactorily. Two species attack the moth eggs, eleven
attack the caterpillars and three the chrysalids. The light lines indicate the stages of the host during
which the parasites attack it and deposit their eggs, while the heavy lines indicate the ages of the
host during which the parasitic insects may live within its body. A series or sequence of parasites
acts as a much more efficient hindrance to the multiplication of an insect pest than any single
parasite, no matter how abundant. (Modified from Howard.)

of the parasites were successfully imported and liberated
(Fig. 35). Some of these bid fair to become of great value,
and they have a great advantage over arsenical sprays and
other methods of control, in that they maintain themselves
when once established, and do not require repeated human
effort with a lavish expenditure of money from year to year.
Efforts are also being made to foster certain diseases of
the gipsy moth which cause a high mortality among the

FOREST INSECTS 77

caterpillars. One disease is now very prevalent, and attempts
are being made with good prospect of success to introduce
another one which occurs in Japan. Also a large predatory
European beetle (Calosoma sycophantci) (Fig. 36) which feeds
upon the caterpillars has been liberated in America, and has
multiplied with astonishing rapidity.

This method of control is particularly adaptable to forest
insects, since the principle of annual rotation as applied to

FIG. 36. Calosoma sycophanta, a Euro-
pean beetle introduced into the United
States to combat the gipsy moth. Both
the larval and adult beetles feed upon the
caterpillars of the moth.

agricultural crops is impossible in the case of trees, and al-
though long term rotation is possible, it does not bear the
same relation to insect abundance. Spraying also is un-
profitable, and not at all practicable except under exceptional
circumstances. It appears, therefore, that control of forest
insects cannot be undertaken along as many lines as that of
agricultural insects, and from this point of view at least,
forest insects present much the more difficult problem.

The relation of parasites to the abundance of the gipsy
moth in America cannot, of course, be regarded as entirely
similar to that existing in Europe where the moth is native,
and an inquiry into the parasitic control of this moth in its
native home, or of any of our native insects will reveal much

78 INSECTS AND HUMAN WELFARE

more accurately the nature of the regulatory effect of para-
sites in the case of most of our forest insects. One common
native insect enemy of deciduous trees, closely related to the
gipsy moth, is the white marked tussock moth (Hemerocampa
leucostigma) . This species is especially destructive to shade
trees in cities. The caterpillars, which are familiar to nearly
everyone, are beautiful objects, adorned with pencils of black
and scarlet hairs, and with dense tufts of buff-colored hairs
on the back. They are periodically abundant in most of our
eastern cities, usually increasing in numbers for several years,
and then rather suddenly almost disappearing, gradually to
regain their previous abundance.

Some years ago Howard showed that the preparatory
stages of the tussock moth are subject to attack by at least
twenty-one primary parasites, belonging to several different
families of insects. These in turn fall prey to fourteen hyper-
parasites which attack the primary ones and thus tend to
reduce the numbers of the latter, although they do not pre-
vent the destruction of the parasitized caterpillars in any
case. There are two or three broods of the tussock moth
each season and, as has been noticed in the case of other in-
sects having more than one brood each year, the first is less
heavily parasitized than the second, or later ones. This is
probably due to a greater mortality during hibernation
among the parasites than with the host, so that the host
species usually shows a lesser degree of parasitism early in the
season, but due to a greater prolificacy and rapidity of mul-
tiplication, the parasites gain the upper hand later in the
season, only to lose most of their advantage during the
hibernating months. However, when the host has been
especially abundant one year, and thus permitted an unusual
increase among its parasites, it may be very highly para-
sitized early the next season. Such a condition was noticed
with the tussock moth by Howard in Washington, in 1895
and 1896. During the season of 1895 this pest had shown an
enormous increase, and threatened the destruction of shade
trees throughout the city. The third or autumn brood was

FOREST INSECTS 79

very heavily parasitized, and on examining the pupae of the
first brood during 1896, it was found that from 624 cocoons
collected only 12 moths emerged, while from those remain-
ing (excluding a few which had died of disease) no less than
916 specimens of parasitic insects emerged, indicating a
mortality of over 98 per cent. This explains very satis-
factorily the great fluctuation in numbers noticed with this
species, as also with many other pests which gradually be-
come more and more abundant from season to season for a
number of years, and then suddenly suffer a set-back, are
greatly decimated, and become relatively unimportant for a
number of years, until they can regain their lost ground.
When again very abundant, their parasites enjoy unusual
opportunities for multiplication, and there is another abrupt
decrease in the numbers of the host species.

The condition just described is similar in a way to the out-
breaks of other forest insects discussed earlier in the present
chapter, but although the depredations terminate suddenly
in both cases, their appearance is gradual in one instance, and
quite without warning in the other. Possibly entomologists
have only failed to read the signs aright, but it seems more
likely that most extensive and unexpected invasions are due
to other causes.

One interesting case of an insect enemy of deciduous trees,
which closely parallels the condition seen with agricultural
insects, is that of the locust-borer (Cyllene robinice), an
enemy of the yellow locust (Robinia pseudacacia) . The larvae
of the locust-borer feed within the sapwood of living trees
where they tunnel in the trunk and branches. This tree is
native over a very restricted range along the Appalachian
Mountains from Pennsylvania to northern Georgia, but has
been planted over wide areas throughout the eastern states,
on account of its rapid growth and valuable timber. The
locust-borer has spread with its host tree, and the thickly
planted groves of locusts have offered such opportunities for
the multiplication of the insect that the propagation of these
trees has been abandoned in many localities where they
might otherwise have been grown very profitably.

80 INSECTS AND HUMAN WELFARE

The age of individual trees may exercise a considerable
influence upon the amount of damage caused by insects. It
may be stated, with few reservations, that where old or well-
matured trees occur among very healthy and vigorous young
specimens, that the former are almost always more liable to
attack. Thus from an entomological standpoint, the felling
and removal of the older and more mature trees in a forest
that contains individuals of mixed ages, is a very wise and
satisfactory practice. Even though the younger trees are less
readily killed by insects, their growth may be temporarily
checked, and a part of the increment to the forest by rapid
growth may be lost. However, if the fully matured trees had
borne the brunt of the attack, they would quite possibly have
succumbed altogether, and have served, in addition, as breed-
ing places for a much larger brood of insects to attack the
remaining trees the next season. The relation which this
bears to proper forest management is at once evident, for the
utilization of mature trees, or those which have ceased to grow
at a satisfactory rate, tends to eliminate the ones which
should be removed solely upon entomological grounds. In
this way the application of methods devised with little or no
reference to insects, is a procedure which the entomologist
can heartily recommend.

Aside from the general preference exhibited by insects for
the older trees in a forest, there are some species which select
distinctly vigorous trees, saplings, or even seedlings, and
these are not amenable to even partial control by the ordi-
nary cultural methods. One of these, the white pine weevil
(Pissodes strobi), attacks saplings or small trees, and owing to
its habit of killing the leaders or terminal shoots produces a
deformation of the otherwise straight trunk of the white pine.
A number of the weevil eggs are placed by the parent beetle
within the tender bark near the tip of the tree, and the result-
ing colony of larvae work their way downwards beneath the
bark. They usually pass the first lateral shoots before be-
coming fully grown, and destroy the part through which they
have burrowed. Consequently, one of the lower side branches

FOREST INSECTS 81

must assume the place of the leader, and when it does, the
axis of the tree is displaced to one side. Commonly the same
tree may be attacked several times in succession, and its
value to the lumberman greatly lessened.

At the present time in New England much attention is
being given to the propagation of white pine, owing to the
lessened supply of this valuable wood, and to the fact that it
appears to be the species most suitable to replace much of the
hardwood timber that is being decimated by the gipsy moth
and other insects.

The white pine, both as seedlings in natural reproduction,
and in areas where small nursery-grown trees have been set
out for reforestation, is often severely injured and extensively
killed by a small beetle, Hylobius pales, the adults of which
eat the succulent bark from the tiny trees. The preparatory
stages of the Hylobius are passed in the stumps of the cut-
over areas, where their feeding causes no harm, but the vo-
racious adults kill a large proportion of the seedling trees in
the vicinity of their breeding places, and are thus a very im-
portant factor affecting the growth of the incipient white pine
forest.

The saplings of certain deciduous trees also have insect
pests which select them in preference to their older compan-
ions in the forests. The aspen and other poplars are fre-
quently killed in considerable numbers by a wood-boring
beetle of the genus Saperda. A single Saperda larva develops
within the stem of small poplars, producing a gall-like swell-
ing which usually spells death for the young tree.

Thus far we have considered the forest, or the individual
trees, as living plants whose successful growth, or even life,
may be endangered by the presence of destructive insects.
From a purely economic standpoint, however, the value of a
forest is measured by the quantity and quality of timber
which it produces, and it is perfectly true that the lumber in
mature trees killed by caterpillars or bark-beetles, is not ap-
preciably decreased in commercial value. From an esthetic
standpoint there is a vast difference between a growing or a

82 INSECTS AND HUMAN WELFARE

lifeless shade tree, but a forest tree killed by such insects as
do not affect the wood, is per se, at the time of its death as
valuable for timber as one felled for that particular purpose.
There are two factors which modify such conditions. In the
first place, such trees as are accessible and marketable are not
necessarily those that are affected by insects, and the real loss
depends upon what immediate use can be made of the timber.
Following severe epidemics or invasions of insect pests, it is
usually impossible to make prompt use of the killed trees and
they begin rapidly to decrease in value. A case in point is the
recent killing of spruce and fir by the spruce bud-worm, re-
ferred to on a previous page. The location of the areas af-
fected is such that the utilization of the dead trees before thev

i/

have become worthless is utterly impossible due to the elab-
orate preparations which have to be made for logging on an
extensive scale. This is, unfortunately, almost always true.
Secondly, the deterioration of the timber in killed trees is due
in great measure to the activities of many insects which will
not attack living trees, but which follow in the track of va-
rious primary destructive agencies, and reduce or utterly
ruin, the value of the trees as timber. There are many species
of insects of this type present in comparatively small numbers
in all forests, where they find a limited food supply in the
scattered trees that have recently succumbed to old age,
overcrowding, lightning, windstorms, etc. As these causes of
death remain tolerably constant from one year to another,
there is ordinarily no great variation in the insect population,
which unites with saprophytic fungi in hastening the disinte-
gration of the dead or moribund trees.

Any sudden destruction of trees furnishes extraordinary
opportunities for an increase of these insects, and they conse-
quently appear in a secondary wave following outbreaks of
insects like the Dendroctonus bark-beetles mentioned on a
previous page, or in the wake of severe windstorms, brush
fires and the like. For example, the pine forests of the gulf
states are periodically swept by violent windstorms, embody-
ing the unspent energy of West Indian hurricanes that have

FOREST INSECTS 83

strayed to our coast. In such an event many trees are laid
flat over extensive areas. A common large longicorn beetle
(Monohammus) which is widely distributed in this region
finds these trees excellent material, and its work soon be-
comes apparent. The beetles deposit their eggs in little pits
which they excavate in the bark, and their large succulent
larvae tunnel the wood with their food burrows until it is unfit
for most purposes. Outbreaks of this kind can, of course, be
foreseen much more readily than some of the other type
which we have referred to previously, but the damage they
cause is no less severe and the application of remedial meas-
ures is frequently very difficult.

The distinction between those insects that affect the wood
or bark of living trees and those occurring in ones recently
dead is not to be closely drawn, and when much dead or dying
timber is present in a forest, it may furnish such favorable
opportunities for the development of some species, that the
whole forest becomes endangered ; for when thus produced in
large numbers, many wood-boring or bark-beetles that live
normally in dead wood, may migrate to living trees and injure
or destroy them. Such dead or decaying timber in a forest is
thus a menace to the living tree population through the part
which it takes in the multiplication of these forms that may
on occasion affect living trees. Fire-scorched trees may in
the same way be attacked by bark-beetles and succumb
where uninjured trees would not have suffered, and vice versa,
insect-killed trees, especially in the case of pure coniferous
forests, may constitute a great fire menace due to the more
combustible nature of the lifeless trees after the bark-beetles
or other primary enemies have left them. There appears to
be good evidence that some of the destructive forest fires
which have occurred in the great coniferous forests of the
western United States have been furthered by insects in this
way.

An especially interesting example of a series of insect
enemies that may follow one another in a tree as it passes
from robust health into a gradual decline that ends in death,

84

INSECTS AND HUMAN WELFARE

is presented by several insects that affect the elm in New
England. Since the history of the insects concerned is so
well-known and the sequence of events so clearly marked, I
cannot refrain from considering the matter in some detail.

In the late thirties of the last century there appeared in
America near the city of Baltimore, a small European beetle
known as the elm leaf-beetle (Galerucella luleold) which feeds

FIG. 37. The leopard moth (Zeuzera pyrina), an imported shade tree pest.
Work of the larva in a branch from which the bark has been removed; above,
the larva.

upon the foliage of various elms. The adult beetles feed on
the leaves of the elm and are especially attracted to the Amer-
ican elm. Later they deposit their eggs upon the leaves which
serve as food for the larvae. Two or three successive broods
develop in a single season and greatly weaken the elms
affected. The elm leaf-beetle has gradually spread till it is
now present generally throughout the more temperate sec-
tions of eastern North America.

Many years later, in 1890, there was noticed on Long Is-
land another European insect of very different appearance
and habits, the leopard moth (Zeuzera pyrina) (Fig. 37) . This
insect is a large moth which feeds in the larval stages in the

FOREST INSECTS 85

branches and trunk of a great variety of trees and shrubs.
The parent moth deposits great numbers of eggs in the crev-
ices of the bark, and the larvae on hatching migrate to the
tips of the branches where they enter small twigs. In these
they construct a small circular burrow till they have in-
creased too much in size to remain within the twig. They
then leave it to enter a more commodious small branch,
and may thus move into larger quarters several times before
completing their growth. This requires two or three seasons
and the large caterpillars often partially girdle large branches
or the trunks of small trees, by tunnelling transversely
through the superficial sapwood and inner living bark. The
leopard moth slowly spread along the Atlantic seaboard after
its establishment on Long Island, and has proved itself to be
practically omnivorous, as one entomologist found it attack-
ing over 150 different kinds of shrubs and trees. Although
commonly found in many healthy and rapidly growing trees,
it seems to be particularly attracted to elms that are not in
prime condition. As a consequence it has found trees weak-
ened by the elm leaf-beetle greatly to its liking and has fol-
lowed the beetle very generally, reducing the vitality of the
elms to a still lower ebb. At the present time the leopard
moth occurs in America in only a very limited region from
New Jersey to Massachusetts, but it is undoubtedly destined
to cross the entire continent in the future.

Still more recently a third European insect enemy of the
elm has been noticed in this country. This is a small bark-
beetle (Eccoptog aster multistriata) , which made its appearance
in the vicinity of Boston about 1906. Its habits are similar
to those of the Dendroctonus beetles so destructive to conif-
erous trees, but the elm bark-beetle attacks only sickly trees,
or the dying branches of more prosperous ones. After the
manner of many other bark-beetles, large numbers of individ-
uals commonly center their activities upon selected trees
which have their inner bark riddled by the burrows of the
beetles. Trees weakened by the leopard moth are very sus-
ceptible to their attacks and succumb rapidly. Where the
beetles are not checked by preventive measures they may

86 INSECTS AND HUMAN WELFARE

then destroy more healthy trees, especially small ones, which
cannot withstand the onslaughts made by swarms of bark-
beetles. The elm bark-beetle has not yet spread far beyond
the point of its introduction, but like the leopard moth will
unquestionably extend its range by slow steps until it occurs
generally throughout the United States.

From the preceding account it can be readily seen how the
elm leaf-beetle, the leopard moth and the elm bark-beetle
naturally follow one another and how they affect the condi-
tions of the elms growing where they are prevalent. At the
present time the only part of the country where all three
occur is included withing a few townships in eastern Massa-
chusetts, but there the havoc they have wrought is painfully
evident. In Cambridge, Boston and many surrounding
towns that once boasted of many magnificent elms, only
pitiful remnants remain.

A condition of this sort is of course an extraordinary in-
stance, as all three insects are introduced species freed from
many of the handicaps that hamper their multiplication in
their native land, but it illustrates most beautifully in an
exaggerated form what is constantly taking place in our
forests. Agricultural crops do not lay themselves open to
attack by insects in this way, as they are replaced by a new
generation each succeeding year, and consequently their re-
lations to insects are in this respect at least, less complex.

Lastly, there is a type of injury by forest insects which
closely parallels that mentioned in connection with agricul-
tural products in storage. Stored lumber, and even that
which has been fashioned into tools or furniture is frequently
damaged by insects which are capable of developing in ex-
tremely dry wood. These are, in the main, certain beetles
characteristic of arid or desert regions that find a close ap-
proach to their natural environment in stored forest products.
Their originally very limited distribution has been greatly
extended, and they are becoming more or less cosmopolitan
in range, establishing themselves as opportunity offers in
places far from their native homes.

CHAPTER IV

HOUSEHOLD INSECTS

IN the chapter on Insects and the Food Supply, a brief
reference has already been made to certain species of plant-
feeding insects that occur in stored food products. Insects of
this kind form a transition between those living at the ex-
pense of growing plants and another series which may be
conveniently referred to as household insects.

As a highly social animal, man has lived in communities
since before the dawn of historic times. His relations with his
fellows have continued to grow more complex, but the asso-
ciation of families into more or less permanent households is
of such long and continuous standing that it has offered an
opportunity for many insects to take up their abode with him.
This is by no means strange, for it finds parallels among other
social animals like the ants and termites. These gregarious
insects almost invariably harbor in their nests, various non-
social insects, known as myrmecophiles and termitophiles.
These latter insects are highly modified species showing pe-
culiar adaptations for life with their social insect hosts. The
household insects associated with man are a very common-
place lot compared to the myrmecophiles and termitophiles,
a condition no doubt due to the keener senses of the ants and
termites in comparison with those of the human species, as
well as the comparatively smaller size of all insects in relation
to man. Like the less highly adapted myrmecophiles, the
tiny denizens of the human household live as tolerated or
persecuted guests, for hardly any would be considered as wel-
come additions by the ordinary person, no matter how fond
he might be of dogs, cats, canaries, or other household pets.

Probably the most widespread and persistent insect visitor
in houses is the housefly, named Musca domestica by Linnaeus

87

88

INSECTS AND HUMAN WELFARE

in recognition of its then, well-known habits. The housefly is
probably a native of India from whence it has followed man
to the most remote regions of the world. It invariably occurs
where human communities become established, but it is so
wedded to human society that it is never to be found else-
where. Originally a native of the tropics it can successfully

FIG. 38. A housefly killed by a parasitic fungus (Ernpusa muscae).
This fungus, which destroys countless flies, is very generally prevalent
during the autumn months.

overwinter in cold regions under the conditions brought
about by civilization, for it finds places of sufficient warmth
to enable it to withstand the rigorous winters of very cold
climates. In cool regions it is never abundant in the spring,
but due to its rapid development becomes increasingly nu-
merous as the season progresses, again to disappear rapidly
in the autumn (Fig. 38). On account of its importance to
public health, the habits of the housefly have already been
considered elsewhere.

Many of us are familiar with the old saying that houseflies
bite before a storm. Although responsible for many things,
the housefly cannot bite, but the stable-fly (Stomoxys calci-
trans), a related fly of similar size and color, with a sharp

HOUSEHOLD INSECTS 89

beak, is frequently mistaken for it. The stable fly originated
in Africa from whence it has spread almost as extensively as
the housefly. It is a blood-sucking species, confining itself
mainly to the larger domestic animals which it torments dur-
ing the later months of the summer season. It does not enter
houses so commonly as the housefly, nor does it very fre-
quently bite human beings. It bites more commonly in damp
sticky weather and has thus given rise to the adage of the bit-
ing housefly.

Several of the larger, noisy, metallic green or blue flies
that invade houses are species which live in the larval stages
upon decaying animal material. They can hardly be classed
as typical household insects. Our common species occur also
in Europe, but are probably native to both continents.

Another very interesting fly which almost invariably ap-
pears on windows in the early spring of our temperate regions
is the cluster-fly (Pollenia rudis) . They are somewhat larger
than the housefly and of lazy, awkward flight. These flies
often occur in large clusters within window casings, entering
for hibernation and later seeking to escape through the glazed
windows, on the approach of spring. Although a very com-
mon insect, its larval habits remain unknown as there are no
authentic records of the flies having been reared.

A very abundant and annoying, tiny fly makes its appear-
ance regularly in the latter part of the summer in most
households, and frequently persists far into the winter.
These little flies (Drosophila) live as larvae in decaying fruit,
with which they may easily be introduced. Our common
forms appear to have been brought hither in fruits from the
tropics, but their larvae will develop readily in the decaying
portions of most fruits or even the less strongly acid types of
pickles. The flies are very small and appear to come from
nowhere almost instantaneously when overripe fruit is ex-
posed. From a biological standpoint they are extremely in-
teresting as some of our most important recent discoveries
concerning the inheritance of color and other characters have
come from experimental studies of these tiny creatures.

90 INSECTS AND HUMAN WELFARE

Mosquitoes are very important from the standpoint of
disease and have been referred to in considerable detail in a
previous chapter. A few species may be regarded as truly
domestic or household insects. Thus the house mosquito,
Culex pipiens, native to Europe, but now widespread in the
temperate northern regions of both hemispheres almost al-
ways breeds in close proximity to dwellings. The larvae occur
abundantly in rain barrels, cisterns and similar receptacles,
and the adults enter houses with unusual persistence. In trop-
ical countries, the same is true of the yellow-fever mosquito
(Stegomyia fasci-ata) and the Filaria mosquito (Culex quin-
quefasciatus) , both of which are truly domestic forms. Many
other kinds of mosquitoes, especially the malarial forms
readily venture indoors, but this habit is much more pro-
nounced and constant in the case of the ones first mentioned.

Several species of cockroaches or roaches have become
closely associated with man and are now widely distributed
throughout the world. These insects are particularly inter-
esting in this respect as they belong to a group of insects of
very primitive character which has existed with little change
over long geological periods. One would not, therefore, ex-
pect to find the numerous present-day cockroaches very
adaptive or versatile in their behavior. Nevertheless, several
species have cast their lot with man, since his advent, and
have followed him in his wanderings. One of these, the small
so-called German roach (Blatella germanica), is especially
widespread and abundant in the cooler parts of the northern
hemisphere. In spite of its name, its association with man-
kind probably antedates the race to which it has been re-
ferred. From its probable home in the far-east it has spread
westward and reached America only at a very recent date.
It was first noticed by the residents of New York City at the
time the Croton aqueduct was put into operation, and was
called by them the Croton bug on the belief that it had come
with the water supply. This was a natural supposition as
these warmth-loving insects congregate in proximity to hot-
water pipes or in warm or humid kitchens. The Croton bug

HOUSEHOLD INSECTS

91

is the smallest of the domestic cockroaches and is even more
agile than its larger relatives. Like them it is a very general
feeder, consuming all sorts of animal matter, cereal products,
paste, glue, bread, etc. All of our domestic roaches show
their tropical origin in their inability to withstand cold and
their occurrence in cooler climates is restricted to heated
dwellings.

Three of our larger roaches are Blatta (Fig. 39), orientalis,
the oriental cockroach, Periplaneta australasios, the Aus-

FIG. 30. One of the larger domesticated cockroaches (Periplaneta). The male, shown at the
left, is more slender than the female. Neither of them often remains still long enough for one
to observe them closely.

tralian cockroach and Periplaneta americana, the American
cockroach. They are thought to have originated in the
orient, in Australia, and in the American tropics respectively,
but have been widely distributed for so long that their pro-
venience cannot be stated with any great assurance. All are
much larger than the Croton bug, especially the Australian
and American species, and they do not occur in such abun-
dance in cooler regions. They find especially salubrious con-

92 INSECTS AND HUMAN WELFARE

ditions on shipboard where they are almost always numerous
in spite of attempts to eradicate them. Such being the case,
it is easy to see how they have found migration easy. Abun-
dant in almost every shop and house in cities of the tropics,
in cooler places they appear most commonly in places that
are kept warm and moist. As they are especially fond of

FIG. 40. A common cricket. The male is wt-11 known
on account of his chirping noise.

starchy and saccharine materials, bakeries, breweries and
sugar refineries frequently harbor them in large numbers
where they are a great nuisance. The repulsive cockroach
odor and flavor which is often imparted to food soon becomes
familiar to those who live or travel in tropical countries, and
may often be recognized in sugar from northern refineries.

Not so far removed zoologically, from the cockroaches,
but of very different habits is the familiar and cheerful
cricket (Fig. 40). These insects enter houses more commonly
in the autumn and are far more regular visitors in the country
than in cities. Their shrill chirping is an accomplishment of
the male sex only. It is produced like most noises made by

HOUSEHOLD INSECTS 93

insects, by rubbing together parts of the body with a rapid
motion. In the case of the cricket each fore wing has a finely
grained, rasping surface; when the two are opposed a suc-
cession of jerky motions made by the cricket evokes the shrill
notes of far more penetrating quality than might be expected
from such a small creature. Crickets remain torpid in cool
weather but their activities readily respond to warmth so
that the appellation of the Cricket on the Hearth is quite
appropriate.

The erratic, darting flight of several tiny species of moths
that periodically appear in dwellings is always a signal of
alarm to the provident housekeeper, as these clothes moths
are well-known to every one both as winged insects and as
the cause of moth-eaten garments. Quite contrary to most
moths, these forms feed in the larval state not upon foliage or
vegetable material, but upon wool, fur, feathers, and material
of animal origin. Their eggs are deposited very generally
upon articles of clothing made from such materials and the
tiny larvae may construct cases for themselves or ill-defined
burrows, lined with loose silk which they spin about them.
There are several species, all rather closely similar, and prob-
ably of Eurasian origin. They are now widespread and
closely associated with man. Undoubtedly under natural
conditions the clothes moths were not abundant and lived a
rather precarious existence, but they were well known to the
ancients, whose crude garments suffered as does the highly
tailored and garnished clothing of the present day.

The larvae of several small beetles cause damage similar to
that due to the clothes moths. They belong to a family of
beetles known as the Dermestida? which feed very generally
upon dry animal matter of various kinds. The entomologist
can speak with great feeling upon the subject of these pests,
as they show a great fondness for his collections of pinned
insects upon which they feed with great readiness. In
households, carpets, rugs, and heavy furs frequently suffer.
Two of the smaller species belong to the genus Anthrenus
and are minute beetles of mottled color. The larvse are sue-

94

INSECTS AND HUMAN WELFARE

culent, bristly grubs (Fig. 41), very resistent to extreme dry-
ness and otherwise very tenacious of life. Their origin and
habits are somewhat shrouded in mystery ; at least one of our
species is of European origin and the other is probably a
native American insect. Their depredations have been j artic-
ularly noted in households of only certain of our cities and as
the adult beetles are often abundant upon flowers far from
dwellings, the larvae undoubtedly breed generally in the open.

FIG. 41. Larvse of the carpet beetle. These bristly little insects devour furs, woolens, and
all sorts of materials of animal origin, even the dried bodies of insects of other kinds. On
account of this last habit, the entomologist frequently finds them feasting upon his mounted
specimens.

The beetles commonly appear on window panes in the spring
in company with the somewhat larger, brown Attagenus pi-
ceus, a related widespread insect of similar habits.

One of our most generalized and primitive wingless insects,
known as Lepisma is a common household insect. It is a long
tapering, scaly insect with three slender thread-like append-
ages at the tip of the body, rather suggestive of a fish on
account of its form and glistening coat and hence commonly
called the :' silver-fish." The silver-fish feeds upon glue,
paste or starch and will gnaw into wall-paper, starched
clothing or even bookbindings to feed. It studiously avoids

HOUSEHOLD INSECTS 95

the light and is rarely seen till some object is moved, when it
will dart toward shelter in a most agile fashion. At the
present time these insects are widespread in Europe and
North America, but when or how they became companions
of man remains rather doubtful.

Certain powder-post beetles referred to previously as de-
structive to stored wood products, occasionally make their
presence in houses known in a very spectacular fashion
through the collapse of a chair or other article of furniture in
which they may have been present for a long time. Feeding
in thoroughly dried wood, successive generations of these
insects will thoroughly riddle the interior of wooden articles,
avoiding the surface layer, and thus giving no evidence of
their activities till the material literally crumbles into dust
when it becomes too weak to support the burdens imposed
upon it. The so-called * 'death- watch " is a related insect
whose deliberate, and audible gastronomic process is re-
sponsible for the intermittent "ticks" upon which so much
superstition has been based.

A few insects and spiders are attracted to houses, not by
the human inhabitants, but by other domestic insects. These
are much misunderstood creatures whose presence is usually
incorrectly interpreted. Spiders are highly predatory, and
although their webs may be unsightly, they serve greatly to
reduce the numbers of flies. Other jumping-spiders, that do
not spin webs, are also adept at catching flies and exhibit no
habits that could render them other than pleasant com-
panions.

Perhaps the most maligned visitors to our summer cot-
tages, are certain large hornets which construct the intricate
and wonderfully fashioned paper nests, so familiar to most of
us in our early youth as objects of awe and hatred. These
hornets (Fig. 42), feed their young with flies which they
capture for the purpose, and their errands frequently bring
them to our dwellings into which they occasionally enter.
Contrary to much human tradition they do not make use of
their powerful stings unless grossly annoyed and their peace-

96

INSECTS AND HUMAN WELFARE

ful mission should never be doubted. Like all social insects
they defend their nests and brood with great vigor and are no
respecters of persons when their home is threatened. The
hornets, belonging to the genus Vespa are largely replaced in
our southern states by other paper wasps known as Polistes.
The larvae of Polistes are fed by the adults upon caterpillars

FIG. 42. The White-faced Hornet (Vespa maculala), a frequent visitor to
our porches in the summer, where she comes in search of flies.

of various kinds which are sought for and captured by the
wasps, to be chewed up and distributed to their brood.

Many insects which overwinter in the adult condition find
that they can avoid the rigors of the hibernating season by
seeking shelter in houses. The majority of these are beetles.
A variety of these creatures appear upon window panes in
their search for a means of egress in the spring, and con-
tinuous observation will readily reveal the fact that some
kinds are quite regular visitors.

Through his slow and tedious emergence from savagery, and
undoubtedly from the much more remote stages of his evolu-

HOUSEHOLD INSECTS 97

tionary development, man has retained a small number of
epizoic or external body parasites, belonging to several groups
of insects. The number of such insects affecting the human
species appears to be generally less than that of those affect-
ing other mammals and this may perhaps be due to the
more cleanly personal habits which he has been able to
acquire. If we carry the matter further back, however,
to the stage of our as yet undetermined forebears, we find
that the monkeys and anthropoid apes are also compara-
tively free from bodily insect parasites. It is possible, there-
fore, that we may. have our much despised anthropoid ances-
tors to thank for this at least. It is known that monkeys
actively catch and destroy fleas and other parasites, but, if
the whole tale must be told - - they are eaten to insure their
destruction.

Several of the groups of insects which include human par-
asites have already been mentioned as carriers of certain
diseases. From this account it will be remembered that it is
rather difficult closely to draw the line between true human
parasites and other blood-sucking species that only rarely
attack man. There are all sorts of intergrades between
species like the stable-fly or the common green-head fly of our
seashore resorts, to whom human blood serves only for an
occasional meal, to the body louse which passes all stages of
its existence on the body upon which it is entirely dependent
for food. Thus the females of certain mosquitoes suck human
blood almost exclusively but are free-living creatures and dur-
ing their preparatory stages are aquatic. The human flea
(Pulex irritans) is generally restricted to man as a host dur-
ing its adult life, but its preparatory stages occur in accumu-
lations of dry organic matter of other kinds. While this is the
only flea peculiar to the human species, our domesticated dog
and cat have fleas peculiar to themselves, which have been
widely distributed with these animals, and are common house-
hold pests in most parts of the world. Indeed, the fleas most
annoying to persons are usually of the kinds regularly present
on dogs, cats, or even rats, rather than the human flea.

98 INSECTS AND HUMAN WELFARE

.

Most closely dependent upon man are a couple of species
of Cimex, known as bed-bugs. Although wingless like the flea
and dependent upon human blood for food, during its pre-
paratory stages as well as in the full-grown condition, the
bed-bug is free-living. Strange to say, this insect appears not
to be responsible for the transmission of any specific disease.
It has been under suspicion at various times but has not so
far been definitely proven to be a disease-carrier.

No reference to household insects could be complete with-
out some mention of the numerous species that gain entrance
to dwellings through the medium of food and remain there in
materials kept for future use. Some of these have already
been referred to in a previous chapter as enemies of stored
food products. As the household pantry is only a miniature
storage place for varied food products, spices, miscellaneous
drugs, and sundry other materials of plant and animal origin
its insect fauna is restricted only by the substances on hand
and the rapidity with which those infested by insects are
removed.

Certain ants are very annoying visitors to pantries into
which they wander from their nearby nests to feed upon and
carry home the saccharine substances of which they are very
fond. These ants are usually very small species and are es-
pecially abundant in warmer countries. They are extremely
persistent in their search for food and not easily discouraged.

Dry cereals of all kinds are particularly susceptible to in-
jury by insects and it is difficult to keep even small supplies
free from such pests. Several species of small moths live as
caterpillars in flour, meal, rolled oats and natural breakfast
foods, although they do not appear able to subsist upon the
patent varieties. During the process of feeding they spin loose
strands of silk which web the material together. If once ad-
mitted to receptacles containing suitable food, they pass
through successive generations until the contents are com-
pletely destroyed. A great variety of beetles have similar
habits, some preferring coarse materials, others the finer
flours, or rice, etc.

HOUSEHOLD INSECTS 99

Animal foods, such as hams, dried meats, and cheese are
frequently damaged by the larvae of certain beetles and flies
which normally occur in carrion, skins, and similar substances.
One of these, the cheese-skipper, is the larva of a small fly,
Piophila casei, quite thoroughly domesticated as an inhabi-
tant of cheese. Its name is derived from the power of the
larva to skip or jump by bending and then suddenly straight-
ening the body. If these larvae are accidentally swallowed
they are able to withstand the action of the digestive fluids
and may cause very distressing symptoms. The larvae of
quite a number of other insects, mainly flies of various kinds
are also capable of remaining alive in the alimentary tract,
and others develop regularly beneath the skin of animals or
hi their alimentary tracts. Species known to affect man be-
neath the skin are very rare and restricted to the tropics but
two common species affect cattle in this way in Europe and
North America and are the cause of considerable loss. Af-
fections of this sort are known by the general name of myiasis
and human cases are fortunately of uncommon occurrence.

One of the most versatile of the pantry insects is the so-
called "drug-store beetle'' (Sitodrepa panicea), formerly of
common occurrence in apothecary shops where it fed upon
the stores of spices, aromatic roots, red pepper and ill-smell-
ing or strongly flavored plant materials. The fondness shown
by the grubs of this beetle for ginger and cayenne pepper is
quite remarkable for such substances would seem to be rather
pungent for a regular diet. Even dry tobacco forms the fa-
vorite food of the related cigarette beetle (Lasioderma serri-
corne), the larvae of which sometimes riddle cigars with their
food burrows.

This is only a very small part of the varied series of insect
guests which we house and nourish, and which add to the
complexity of modern life. However annoying they may be,
we must admire them for the persistence with which they
force themselves upon us.

CHAPTER V

THE OUTLOOK FOR THE FUTURE

FROM even a most cursory examination of the activities of
insects as they affect directly the human species, it is very
evident that insects are as a group highly injurious to man,
with their influence in this direction slightly tempered by a
small number of species that are directly beneficial.

This is, however, a very unfortunate way in which to re-
gard the matter, although it is a viewpoint that has been
impressed upon the minds of many of us in early childhood.
Starting with the assumption that all animals were created
with a view toward their usefulness to man, the old-fashioned
pedagogue tried to fathom the depths of the divine mind, and
to ascertain in what way many obviously noxious ones might
fit into this scheme. Less orthodox, but still unfortunate from
the standpoint of biological understanding, is the still too
common attempt to regard all living things as either useful or
injurious to man.

Nevertheless it would seem that for reasons of expediency,
the economic entomologist must regard quite a considerable
number of insects as falling into one or the other of these
mutually exclusive categories, regardless of any considera-
tion of the interdependence of different species, which he
cannot formulate in rather exact terms. Following this a
little further, it is clear that our practical dealings with in-
sects are necessarily confined almost entirely to the eradica-
tion or reduction of numerous species which are detrimental.
These include primarily the great variety that spread diseases
of man or of domesticated animals, those annoying to our-
selves or to domestic animals, those destructive to agricul-
tural crops and to useful wild plants, those detrimental to
forests, and those injurious to stored products.

100

THE OUTLOOK FOR THE FUTURE 101

As is also clear from the foregoing chapters, scarcely any
two insects may be dealt with most satisfactorily in exactly
the same way. This at once imposes upon the entomologist,
the task of discovering with as great detail as possible, the
exact life history and economic relations of a vast series of
insects, in order that energy may not be lost in the misappli-
cation of control measures. The last few years have seen a
great improvement in our knowledge of this kind, which has
already borne fruit in the rapidly increasing efficiency of
practical work. We can be sure that the future will see con-
tinuous improvement along these lines.

The now widespread custom of spraying plants for the
destruction of insects has been made far more efficient
through careful attention to the physical and chemical prop-
erties of the materials and their physiological effects upon
specific insects. In the same way fumigation with poisonous
gases has been made more dependable. Various more or less
fundamentally new methods of combating various insects are
also continually coming into general use, usually through
some ingenious application of physical or chemical knowledge
to specific insects whose habits or physiological requirements
at some stage of their life-cycle lay them open to attack.
Thus mosquito larvae may be eliminated by oiling the surface
of the water in which they are developing, and housefly larvae
may be prevented from completing their growth in manure,
by storing it in specially constructed pits. In the same way
the chinch-bug, cotton-boll weevil, alfalfa weevil, certain
grasshoppers, cutworms, and a great variety of other pests
are commonly controlled in rather unique fashion. In fact,
the most efficient means of combating many insects are not
at all conventional, even to the entomologist.

The biological method of reducing the numbers of injurious
insects offers at the present time, the most promising field in
which to speculate concerning the future development of
entomological practice. Much has already been said of the
principles and details concerned in the introduction and fur-
thering of diseases and of predatory and parasitic enemies in

102 INSECTS AND HUMAN WELFARE

the induced natural control of insect pests. This method was
devised as a direct result of an understanding of the biologi-
cal environment of insects, and has been applied with the
greatest success to the control of imported pests. With cer-
tain modifications it has also proved of use in dealing with
native insects. Its possibilities have been by no means fully
explored, although it is continually receiving more attention.
There is no doubt that this work will continue to increase, in
our attempt to modify permanently the conditions that are
otherwise met by insect species imported into new regions.

The rapid and continual influx of new insect pests into
countries like our own, that enjoy extensive commercial inter-
course with other parts of the globe, will undoubtedly con-
tinue in the future and the specific problems of the ento-
mologist will become more international in their nature.

Practically all of the barriers to the dispersal of small
animals like insects, that existed when living things reached
their present state on the earth, have either been removed or
made less certain in their action. Mountain ranges and
deserts have been bridged by highways over which a con-
tinuous stream of humanity and materials is passing, bringing
with it everything that can make the comfortable journey.
Oceans, the most effective natural barriers, are crossed in all
directions with cargoes containing many things not listed on
the purser's books. In short, there is endless opportunity for
small living things to migrate to all quarters of the globe.
Even tropical insects find in the human communities of cooler
lands, opportunity to live under climatic conditions which
they could not otherwise withstand.

Such migrations of insect life have, of course, been in prog-
ress for many years, and their economic significance has
long been realized. Gradually, in various parts of the world,
legislative measures have been enacted, aiming at a restric-
tion of the opportunity for noxious insects, weeds, and for
animal and plant diseases to gain entrance to new regions.
These are naturally modeled after the quarantine measures
taken to prevent the spread of human diseases. They have

THE OUTLOOK FOR THE FUTURE 103

been effective in great measure in preventing the rapid spread
of insects, although in the nature of the case, they could never
be absolute without abolishing all commerce. In proportion
to the actual efficiency of the quarantine service, and the
difficulty of inspection, which naturally varies within very
wide limits, the spread of insects has been impeded. In many
cases the importation of certain plants and products has been
prohibited. This has been done on account of the fact that
the most general method of spread for agricultural, horti-
cultural and forest insects is upon the host plant, and one of
the more probable dangers is thus removed in so far as it is
possible to exclude the specific materials. Whether the
wholesale exclusion of food plants or their adequate inspec-
tion is to be preferred from a practical point, appears to be
rather a question of opinion. So far as the ultimate result is
concerned it matters little, for the immigration of undesirable
insects will take place only with greater difficulty and at a
slower rate, as human ingenuity is pitted against a seemingly
hopeless problem.

What surprises the entomologist more than the introduc-
tion of so many insect pests into our own country, is the fact
that more have not established themselves. It is evident
that many insects do not become naturalized so easily as
might be expected, and that when once well established, some
of them spread very slowly. This is evidenced by the long
time required by the gipsy moth to become abundant at the
point of its introduction in New England and the subse-
quently deliberate extension of its range, even before adequate
control measures were inaugurated. Quite probably it now
occurs in many isolated places far outside its recognized
range,1 but the spread of this species and that of the brown-
tail moth has been extremely slow in comparison with other
imported species like the cabbage butterfly and the Colorado
potato beetle. Such differences are partially explained by
the naturally migratory habits of certain species, but not

1 Since this was written it has been found in two other states, New Jersey and
Pennsylvania .

104 INSECTS AND HUMAN WELFARE

entirely, and they may be dependent also upon the ease and
rapidity with which some species become established in iso-
lated localities.

So far as the interchange of insect pests between Europe
and America is concerned, the New World has decidedly
played the losing game. Whether this has a biological basis
is doubtful, for undoubtedly the more thrifty and painstak-
ing European peoples have scrutinized more carefully the
things which have been received into their domain. On the
other hand it is quite possible that the fauna of the palaearctic
region may impose a more severe struggle for existence upon
newcomers from our region than occurs in the inverse di-
rection. At any rate America seems so far to have received
more undesirable insects from abroad than has Europe.

The most promising outlook which the entomologist can
enjoy is the decreasing prevalence of several of the more im-
portant insect-borne diseases. Through his efforts, coupled
with those of medical investigators and sanitarians, really
wonderful progress has been made in the reduction and re-
striction of malaria, yellow fever, plague, and typhus fever.
The future will undoubtedly see a still greater activity along
this line. One must not suppose, however, that the medical
entomologist need not expect to receive set-backs at any
time, or to have new problems thrust before him. Certain
disease-bearing insects may be expected to extend their
range in the same way that other insects are migrating. On
account of their close association with man, many of the in-
sect carriers of human diseases are already very widespread,
but many unpleasant surprises are doubtless now hidden in
little known parts of the world, from whence they may spread
without warning.

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