Bats in relation to the production of guano and the destruction of insects

Historic, archived document

Do not assume content reflects current
scientific knowledge, policies, or practices.

TED STATES DEPARTMENT OF AGRICULTURE

Washington, D. C. March, 1926

BATS IN RELATION TO THE PRODUCTION OF GUANO
AND THE DESTRUCTION OF INSECTS

By Epwarp W. NELSON, Chief, Bureau of Biological Survey

CONTENTS

Page Page
Economic relations of bats____-~_~_ i foG MANO. ;GCPOSitt 2 — 2 te se 6
The Mexican free-tailed bat __---__ 2 | Artificial roosts for bats __--__.--~_- 7
gre XO ee eee 4 muildinges occupied. see. 9
General habits of the species_____~ 4 The Florida free-tailed bat ________ 10
ES REDS OOM ep) ee oe oe 5 Malarial control by bats2=———- _- = __ 10
beet: 1324 bis ee eee 6 SEN a ars Se SPREE i ee ea es fet

ECONOMIC RELATIONS OF BATS

Much has appeared in the public press in recent years about bats,
their valuable deposits of guano, their alleged destruction of malarial
and other mosquitoes, and the possibilities of increasing their use-
fulness to man by building artificial roosts for them, and many
requests for information on these subjects have come to the depart-
ment. L. O. Howard, chief of the Bureau of Entomology, in
a paper on “Mosquitoes and Bats” read before the meeting of the
New Jersey Mosquito Extermination Association in 1916+ discussed
the subject chiefly from the viewpoint of the alleged destruction of
Anopheles in the vicinity of a bat roost near San Antonio, Tex.
Further definite information on the life history and general habits
of bats is still in demand from entomologists and officials in charge
of health administration and general education.

The available information on the bats of North America would
fill a large volume, for scattered from Panama and the Antilles to
Alaska and Labrador there are about 260 species and subspecies be-
longing to 77 genera and 8 different families. Some of the tropical
species are blood-sucking vampires and others are fruit-eaters, but
nearly all the bats of the United States and farther north are
Insectivorous. Still the habits of the different species often differ
as widely as do their structure, appearance, and range, and the useful

* Reprinted also in Public Health Reports, vol. 35, no. 31, pp. 1789-1795, July 30, 1920
(Reprint No. 715).

71736°—26 1

2 BULLETIN 1395, U. S. DEPARTMENT OF AGRICULTURE

habits of one species may not apply to others of the same or distant
localities. Generalizations from one species can not be safely ap- |
plied to others without a full knowledge of their habits. Certain
bats of highly colonial habits are found in the Tropics and across
the southern United States, limited mainly to the southern parts
of these States, but these colonial habits do not apply generally
to the more northern bats and by no means to all species in the
South. This bulletin discusses the relation of colonial bats to the
production of guano and the destruction of insects.?

THE MEXICAN FREE-TAILED BAT

Most of the sensational reports of discoveries of a great commer-
cial value of bats as well as of sanitary benefits from their presence
have been based on one species occurring in southern Texas, the
Mexican free-tailed bat, Zadarida mexicana, long known in litera-
ture as Vyctinomus mexicanus, and belonging to the mainly tropical
family Molossidae. In tropical and subtropical America, the West
Indies, southern Europe, eastern Africa, southern Asia, Australia,

Fic. 1.—Mexican free-tailed bat (Tadarida mevicana), a small sooty-brown mam-
mal with a wing spread of about 12 inches; habitant of Mexico and the arid
Southwest, and the only species in the United States known to produce guano
in commercial quantity

and many islands of the Pacific there are about 40 species of the
venus Tadarida, but only 4 of these reach northward into southern
United States, and only 1 is known from southern Texas. _

The range of bats of the species Tadarida mexicana 1s n eneral
restricted to the arid and semiarid sections of the Lower ustral
Zone in the United States from Texas to California, and extends
also throughout most of the warmer parts of Mexico. The north-
ward limit of their range is apparently fixed by temperature or
suitable food supply, and the eastward perhaps by the presence
of a related species. Tadarida eynocephala, with very similar habits
but occupying the humid division of the Lower Austral Zone from
southern Louisiana to Florida and South Carolina. Two closely
related forms Zadarida femorosacca and Tadarida depressa, are of
rare occurrence in the Southwestern States.

The Mexican free-tailed bats are small, sooty-brown mammals
spreading about 12 inches across extended wings, with projecting -

j i i j harles A. R. Campbell,
2 While this bulletin was in press a volume just published, by C : }
ee oust Mosquitoes, and Dollars,” was received. The book does not in any way alter

the conclusions as set forth in these pages.

BATS IN RELATION TO GUANO AND INSECTS 3

tails, short, wide ears pointing forward over the eyes like a hat rim,
and short velvety fur. (Figs.1and2.) Individually these bats are
cleanly animals, but they have the extremely strong musky odor
peculiar to the group. This permeates the air of every cave or
house which they occupy in any numbers. In some houses where
they are present in large numbers it becomes almost unbearably
strong, and its offensiveness is increased by the added pungent odor

Fig. 2.—Mexican free-tailed bat (Tadarida mericana), natural size (see also
. : figure 1

of ammonia from their excrement. In many houses where bats are
found among the tiles of the roofs of porches and in®crevices over
the lintels of doorways and similar places, the odor pervading the
premises often advertises their presence to anyone approaching.

In the numerous reports on bat guanos and their chemical com-
ponents and value as fertilizer, samples have been listed from
the caves of Africa, India, China, South America, the West Indies,

4 BULLETIN 1395, U. S. DEPARTMENT OF AGRICULTURE t

yi +
Texas, and New Mexico. In Porto Rico 110 caves have been listed,
containing from 12 to 3,144 tons of guano, and many others exam- |
ined containing little or no guano.* In no case is the species or
genus of bat responsible for the guano deposits mentioned, but in
every case the records of deposits lie within the range of some
species of the genus 7'adarida. This does not mean that these are
the only bats producing guano of high fertilizer value in commercial
quantities, but so far as known they are the only bats in America
sufficiently gregarious to do so.

BAT CAVES

The most strikingly characteristic habit of Mexican free-tailed
bats is that of gathering in large colonies in caves and buildings.
This habit renders them valuable through the production of large
quantities of guano, and possibly through the consumption of many
kinds of injurious insects. They are especially numerous in south-
ern Texas and New Mexico, where for centuries they must have
occupied great numbers of the natural caves which afford ideal
refuges for them. Many of the larger caves contain hundreds of
thousands during the whole or a part of each year.

Some of the caves, such as the great Carlsbad Cavern, N. Mex.,
are mainly wintering places, where the bats hang in fall, and sleep
until the'warm days of spring call them out again to seek the most
satisfactory feeding grounds, and to feast on their winged prey.
Where the caves are in proximity to an abundance of insect life the
bats remain throughout the summer, occupying the caves as roosting
places during the day, and swarming out over the country in search
of food at night. One such cave is about 19 miles north of San
Antonio, Tex., on Cibolo Creek; and, although not very extensive,
it annually yields a large quantity of guano. This and other caves
west of San Antonio are occupied by great numbers of these bats,
as are still others in western Texas and across southern New Mexico.
Apparently this whole cave region is stocked with bats to its carry-
ing capacity, besides providing wintering shelter for those of out-
lying areas, where no caves occur.

GENERAL HABITS OF THE SPECIES

Like most other bats, the Mexican free-tailed bats are almost
entirely nocturnal in habits and apparently continue active through-
out the night. Emerging from their roosting places after sunset,
they seek their feeding grounds for pursuit of the flying insects
which are their food. They are strong, rapid flyers and will even
breast a stiff breeze, but in feeding they keep mainly in the vicinity
of trees, buildings, or cliffs, where the air is teeming with insect life.
Their progress while feeding is in quick and erratic zigzags as the
insects are rapidly snapped up and devoured without a moment’s
pause.

During summer evenings at San Antonio, Tex., soon after sun-
down it is common to see great numbers of these bats flying over

2Gile, P. L., and J. O. Carrero, “ The Bat Guanos of Porto Rico and Their Fertilizing
Value”: Bul. No. 25, Porto Rico Agr, Exp. Sta., pp. 66, 1918.

BATS IN RELATION TO GUANO AND INSECTS 5

the town, practically all headed in one direction. They fly several
hundred feet above the ground, on their way from their roost to
favorite hunting grounds, often in the vicinity of water.

Bats often come into open rooms while seeking their prey and
fly about with great skill, avoiding all objects and feasting on such
insects as also enter through the open doors or windows, “alighting
for a moment on the walls or moldings to rest or devour a “moth,
but always careful to keep out of reach “of human occupants. All of
our native bats are absolutely harmless. The fear which they
sometimes inspire is wholly baseless and has its origin in fictitious
stories told to children and passed on from ceneration to generation.

At early dawn the bats return to their regular sleeping places in
eaves, dark rooms, under tiles or other openings in roofs, in holes
and crevices among rocks, or in hollow walls of buildings. They
are usually suspended by their sharp, curved hind claws ‘and sleep
until the next evening, where conditions for such a position are
favorable. In other places they may be found packed in deep
crevices, frequently too narrow even to admit the hand. In the
roofs of small caves in the rockhills near the suburbs of Mexico City
there are many bats in crevices of this kind. They commonly take
possession of deserted attics and similar places in old churches or
other buildings and in deserted storerooms.

Mexican free-tailed bats, like many others, have the habit of
hanging up part of the night in convenient places near their roosts,
as under the roofs of verandas or similar shelter, and the fioor
beneath is covered each morning with many scattered pellets of ex-

¢crement.

_ Exceedingly gregarious, these bats sleep in large numbers closely

packed, sometimes hundreds, or even thousands, in a solid mass,
clinging to one another in such density that it is difficult to under-
stand How those in the middle of the mass can avoid suffocation.
At night, when they leave large caves or buildings where great
numbers of them are living, they pour out through their chosen
exits in such a swarm that in the dusk of ear ly evening it has almost
the appearance of a cloud of smoke.

Observations made in Mexico indicated it to be a common habit
for certain bats to fly a number of miles from their roosts to their
feeding grounds every evening, but there are no observations as to
the distance the Mexican free-tailed bats may go for this purpose.
Undoubtedly, however, they comb many square miles of territory
about their great roosting places.

To what extent these bats migrate to take advantage of favorite
winter and summer climates, or to find favorable wintering caves,
is not known, but like many other bats they undoubtedly move about
to seek the best seasonal conditions. At times a few wander in sum-
mer far from their regular range, as shown by records of scattered
occurrences as far north as Colorado and Kansas. Extensive migra-
tions, however, such as occur among some other more northern
species of buts, are not known in this group.

HIBERNATION

In winter the Mexican free-tailed bats seek caves or buildings
where the temperature is approximately constant and ues too cold,

..

6 BULLETIN 1395, U. S. DEPARTMENT OF AGRICULTURE

{~~
A

and with the first freezing nights outside they hang up for the long
winter’s sleep. So far as known a temperature of 50 to 55° F. in |
moderately dry air suits their needs for hibernation, but the limits
of variation which they can endure have not been fully ascertained.
Before entering hibernation they become very fat, storing up inside
of their skins a rich supply of oily food material sufficient to support
the greatly reduced vital processes during their torpor of winter.

Before they are fully torpid their stomachs and alimentary canals
become entirely empty, their temperature gradually falls to approxi-
mately that of the surrounding air, and their circulation and respira-
tion become greatly reduced. When fully in the embrace of the
winter sleep their bodies are cold and motionless, and they are ap-
parently dead. Thus they hang often four or five months until the
warmer air of spring penetrates to their chambers and stirs their
circulation to renewed activity.

In the Tropics all species appear to be active throughout the year.

FOOD HABITS

So far as known the food of the free-tailed bats consists wholly
of insects, almost entirely of night-flying species captured on the
wing. Moths and beetles seem generally to form the great bulk of
the food, but many other insects also are eaten, and in case of an
unusual abundance of any nocturnal species, these might be expected
to figure largely in the food.

These bats are gluttonous feeders, and in some species 20 minutes
after their appearance in the evening the stomachs have been found
distended with food, the contents averaging one-quarter the weight
of the animal. This would imply a capacity for at least half their
weight in insects every night, and even a possibility of their actually
eating their weight in them every 24 hours. Such estimates are
merely suggestive and must not be used in any conclusive sense until
more careful tests can be made of the food actually consumed by each
species under varying conditions.

Many other bats not colonial in habits probably have a similar
capacity, and over parts of the country where they exist in sufficient
numbers may well have an economic value comparable to that of in-
sectivorous birds.

If there were more caves attractive to bats and a greater food
supply there would undoubtedly be more bats, whereas any consider-
able diminution of either factor would tend to reduce their num-
bers. If, however, the bats should become so numerous as to destroy
most of the insect life, there would necessarily result a correspond-
ing decrease in the bat population. The fact that bats of this species
each produce only one young a year suggests a long-established and
conservative balance between the food supply and the increase of
the species.

GUANO DEPOSITS

Bat droppings composed entirely of insect remains and well
moistened with bat urine accumulate under the roosting places,
often in such large quantity as to be of value as fertilizer. When
neither too wet nor too dry it is rich in nitrogen, phosphoric acid,

a

BATS IN RELATION TO GUANO AND INSECTS aq

and other important ingredients and has a commerical value in
some cases, as shown by chemical analysis, of $30 to $40 a ton.*

- The rate of accumulation of guano varies greatly in different
places, being slight in the caves where the bats merely spend the
winters, but much more rapid where they live in great numbers all
the year, or through the period of summer activity. In the Cibolo
Creek Cave, north of San Antonio, Tex., on March 5, 1924, about
an inch of fresh deposit was found on the floor where the guano
had been removed during the winter. The bats had been active only

‘a short time, and insect life had but recently become common.

From this not very extensive cave there are taken out each year
about 60 or 70 tons of guano, said to bring $30 a ton.

The Cibolo Creek Cave consists of a great tunnel sloping gently
downward for 200 or 300 yards through the limestone formation
toward the creek valley below, and widening out at the lower part
in a great room some 75 feet high and 150 feet wide. Here the
bats gather in vast numbers, possibly by millions, hanging to the
high arched roof, and rain down their little pellets over the entire
floor of the cave throughout their season of activity.

The bat roost in this cave has not only been a source of income to
the owner since 1896, but also perhaps has been of importance to the
community in the destruction of enough insects within the nightly
range of the bats to maintain this enormous host. Nevertheless
mosquitoes are said to be troublesome at times in the vicinity of
the cave, and no scarcity of any kind of insect has been noticed by the
residents.

In another cave on the Frio River, not far from Uvalde, Tex.,
about the same quantity of guano had been taken out for many
years, until a fire in the guano drove out the bats and for the time
ended the deposition. Other caves in the region west of San
Antonio yield a somewhat smaller output of guano, and many in
western Texas and southern New Mexico and Arizona have yielded
it in commercial quantities.

In the great Carlsbad Cavern of southeastern New Mexico the
quantity of guano removed during the 20 years from 1901 to 1921 is
estimated roughly at 100,000 tons. This was the accumulation of
hundreds or perhaps thousands of years, but as vast numbers of bats
congregate here in fall to use the cave largely as a wintering place
and as many leave again in spring for lower country and a better
food supply, the guano accumulation is relatively slow, apparently
not more than 1 inch a year. Many years must elapse before the

“

deposit can again become of commercial value.
ARTIFICIAL ROOSTS FOR BATS

‘Interesting experiments in building roosts for the purpose of
colonizing guano-producing bats (Zadarida mexicana) have been
carried on for many years by C. A. R. Campbell, of San Antonio,
Tex. An excellent description of one of these buildings (fig. 3)
at Lake Mitchell is given by Doctor Howard,t from careful notes

?Gile and Carrero, op. cit.
* Howard, L. O., op. cit., pp. 1792-1793.

8 BULLETIN 1395, U. S. DEPARTMENT OF AGRICULTURE

f™
A

by F. C. Bishopp, in charge of the field station of the Bureau
of Entomology, at Dallas, Tex., as follows:

The Campbell bat roost consists of a sort of tower set on four posts about 10
feet above the ground. According to Doctor Campbell, the size of the roost
may be varied considerably. As I recall them, the dimensions of the roost at
Mitchell Lake are about as follows: Twelve feet square at the bottom, the walls
slanting inward toward the top, which is about 6 feet square. Height, about 20
feet. On the outside, the building is covered with drop siding with tar paper
beneath. The roof is shingled and projects over the edges. It is slightly ele-
vated so as to permit of the entrance and exit of the bats. Additional entry
space is allowed entirely down one side of the building. This opening, which ©
is about 24% feet wide, is provided with boards slanting upward so as to

exclude light to some extent but allows

the bats to enter between them. The
central portion of the house from the
side provided with the entrance to the
opposite side is unobstructed from top
to bottom, thus leaving an air space
about 2% feet wide. On each side of
the shaft, and running to the two other
sides of the building, is a series of
shelves made of matched flooring.
These shelves slant upward and out-
ward at an angle of about 30°. In the
first house constructed these shelves
were about 5 inches apart, but I believe
in the later model they are closer to-
gether. Wire-netting is tacked on top
of each of the shelves so as to provide
places for the bats to hang. The slant
is given so as to allow the guano to
. roll down and drop into the center of
the bottom structure, which is provided
with trapdoors opening downward.

This is to permit of the emptying of the

manure into a wagon which is placed

under the roost.

Fic. 8.—Bat roost at Mitchell Lake, Tex., This roost when visited by Ver-
oaaupiod by a colony of Mexican fee non Bailey, of the Biological Sur-
fo nunbel 10,000, and vieding about vey, in company with F.C.
graph by Charles H. Gable) Bishopp and Charles H. Gable, of

the Bureau of Entomology, on

March 4, 1924, was occupied by a large number of bats, all of the

species Zadarida mexicana. During the evening it was estimated

that about 10,000 bats flew from the roost in the half hour between

6.45 and 7.15 o’clock. They left so rapidly from both sides of the

tower that they could not be counted after the first five minutes. It

seemed, probable that fully 10,000 were using the roost at that time.

Mr. Gable says that they were much more numerous in midsummer,

but none were seen there in winter, as, according to Doctor Campbell,

most of the bats disappear then and return early i in spring, the first
sometimes appearing about the middle of February.

About two tons of guano are taken from this roost every eden
part of which is pac ked in 10- pound sacks and sold at $1 a sack to
local florists and individuals for use on house plants, and part of
it in larger lots to local gardeners at lower prices. This roost ap-
parently pays good div idends on the original cost and is considered
a valuable investment. It is the only one of five, however, con-
structed in and about San Antonio that has ever been extensively

BATS IN RELATION TO GUANO AND INSECTS 9

occupied by bats, and others built in Florida and Georgia had failed
to attract bats up to March, 1924. So far as can be learned, the bat
-roost at Mitchell Lake, near San Antonio, is the only one of the
eight built that has been occupied by bats, but this does not mean
that others in favorable situations may not attract them.

There would be many advantages in being able to colonize bats
successfully in certain localities either in artificial roosts or artifi-
cial caves and tunnels, but so far none of the experiments can be
considered entirely successful. Doctor Campbell’s experiment is
interesting as far as it reaches, but
so far it has succeeded in only one
case and with only one species
of bat.

The choice of roosting places by
bats is evidently dependent upon
factors not always readily under-
stood. In many cave regions where
bats are abundant, only certain of
the numerous caves available and
apparently suitable are actually oc-
cupied. Although bats may in
some instances be attracted to arti-
ficially prepared roosts, prospective
builders should be informed, before
deciding to risk what may be a
needless expenditure of funds, that
there is no assurance that such
structures will be used (fig. 4).

BUILDINGS OCCUPIED

The walls or attics of many old

Del Rio. and other communities in ular subscription in 1920 at a cost
of $2,000, for mosquito control, but

western Texas are occupied by never occupied. In only one of eight

Bat roost near Military Acad-
., built by pop-

such structures, from Florida to
Texas, have bats taken up their abode

these bats. In 1923 two wagon-
loads of bat guano were taken out
of the attic of the courthouse at Austin, and the openings carefully
closed to prevent the return of the bats. The strong odor of the bats
and guano rendered the building unendurable for human beings, as
it has in the case of many private dwellings where the bats “have
taken up their abode in hollow walls or attics. To prevent theiy
return, however, it is only necessary to close all openings in the
buildings at night when the bats are out.

In the vicinity of Patzcuaro, Michoacan, at an altitude of 7.500
feet, the writer found Zadarida mexicana the most common species
of bat, although the summer climate here was much cooler than in
the localities most frequented by them. Here they were sometimes
found in caves, but were more numerous among the tiles and other
crevices in the roofs of the houses, and in openings in larger build-
ings as well. In an old building in the middle of Patzcuaro, where
these bats were clinging in great numbers to the ceilings of two
rooms, the owner made a regular business of gathering the guano

——_ ae

10 BULLETIN 1395, U. S. DEPARTMENT OF AGRICULTURE

and selling it at the rate of 3 cents a pound, Mexican money. At
the time of the writer’s visit in July thousands of bats were hanging
from the roof and several inches of excrement covered the floor,
giving off an excessively strong odor of ammonia. The owner
stated that the bats migrate when the weather becomes cold in fall
and return in spring. Soon after they return they have their young.

At another place in the same town a man built a small detached
room in the yard back of his house for the purpose of smoking bacon
and other meat, but before he began to use it bats of this species
took possession in such numbers and produced so much guano that
he abandoned his original purpose and was making a good return
on his investment through the sale of guano. The writer heard also
of bat guano being taken from a cave on the southern slope of Mount
Popocatepetl.

THE FLORIDA FREE-TAILED BAT

The following extract from a letter from Hiram Byrd, State
Health Officer of Florida, to L. O. Howard, Chief of the Bureau of
Entomology, under date of June 26, 1912, relative to bats taking up
their abode in an uncompleted opera house begun in 1895, at Tavares,
Fla., undoubtedly refers to the Florida free-tailed bat, Tadarida
cynocephala, and if so shows that this species has the same colonial
habits as the one found in Texas:

The doors and windows of the lower floor of this opera house were securely
fastened up to keep intruders out, but the upper windows were only closed by
loose boards, which svon dropped out, making it easily accessible to bats.
They took advantage of it, and in the course of a few years were there in
countless thousands. I know of no way of estimating the number. ... The
only time I was ever there at the right hour was on a trip to Eustis. The
train stopped at Tavares one-half hour before sunset and remained there
something like 45 minutes. I took advantage of the occasion to see the bats
emerge from the building. I had only been watching a few minutes when
they began, first a single one, then two or three together, and as if the rustle
started them, then they began seriously flying out of the window with in-
credible swiftness. There must have been at least half a hundred a second.
I watched this stream of bats pouring out for half an hour or so, and was
told by some of the residents of Tavares that it would continue until something
like half an hour after dark, making probably two hours altogether.

About two years after the opera house had been cleaned out and
converted into a packing house, Doctor Byrd made inquiries of
citizens in the vicinity of Tavares and Eustis, Fla., as to whether
they had experienced any appreciable difference in the number of
mosquitoes since the time the bat roost in the building was at its
height, and as a result of these inquiries stated that he was convinced
that if there was any difference it was not noticeable.

MALARIAL CONTROL BY BATS

During many years of study of the mammals of Mexico, the
writer lived a large part of the time in places where Mexican free-
tailed bats were extremely abundant. Their presence in no case
appeared to have the slightest influence on the prevalence of ma-
laria. In many Mexican villages and ranches, where nearly every
inhabitant was infected with malaria and where malarial mosquitoes

|
|
;
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BATS IN RELATION TO GUANO AND INSECTS 11

were swarming about the houses, these bats were living in the roofs
in great numbers, apparently without having the slightest influence
on the numbers of the mosquitoes. At one large cave, water in the
entrance afforded a breeding place for mosquitoes, which were pres-
ent in such numbers as to cause great annoyance in efforts to collect
some of the bats for scientific specimens.

The assertions of Doctor Campbell that bats feed very extensively
on mosquitoes, practically eliminating them in the vicinity of bat
roosts and thus effectively preventing malaria, have been followed
with interest by entomologists of the Department of Agriculture,
and all evidence carefully weighed by Doctor Howard? in his paper
on the subject. He has written the following two paragraphs for
insertion in this bulletin:

“After a prolonged effort I have been unable to substantiate the
claims made by Doctor Campbell as to the value of bat roosts in
the great reduction of the mosquito population of a given locality,
even in Texas. Bats obviously prefer other and larger insects.
They undoubtedly swallow mosquitoes when they encounter them in
flight, but only incidentally. Observations by trained men on the
eround in Texas deny his claims both as to marked relief from
mosquitoes or relief from malaria. One of the experts of the Bureau
of Entomology stationed at San Antonio informs me (May, 1925)
that of the four bat roosts at San Antonio only one is inhabited by
bats, and that in endeavoring to watch the flight of the bats from
this roost in the evening jhe was so annoyed by mosquitoes that he
was obliged to abandon his observations. He further tells me that a
Mexican, resident 300 yards from the roost, states that the mos-
quitoes are very bad at his house.

“As to other parts of the world, I am told by no less authority
than Professor Grassi that in Italy the most malarious regions are
precisely those where bats are most abundant.”

SUMMARY

Nearly all the bats of North America north of the Tropics con-
sume vast quantities of insects, but apparently do not exterminate
any. In evaluating their services as insect destroyers it is to be
borne in mind that they feed almost entirely on night-flying species.

The Mexican free-tailed bat, ranging in the United States in
southern Texas, New Mexico, Arizona, and California, is extremely
colonial in habits, occupying numerous caves and some buildings, and
producing in places sufficient quantities of guano to be used com-
mercially as fertilizer.

The possibility of colonizing these bats by building suitable roosts
has been demonstrated in the United States, but in only one instance.
Many difficulties are likely to be encountered in establishing colonies.
Elaborate and expensive structures built outside the range of a
colonial species or in places where the bats of the locality find other
quarters preferable may not be occupied. Unless the bats can be
attracted in large numbers, there is little hope of establishing a

5 Op. cit.

12 BULLETIN 1395, U. 8S. DEPARTMENT OF AGRICULTURE
on

worth-while colony, because of the slow rate of reproduction, there
being only one at a birth.

In the single colony of bats successfully established there seems
to be a profitable yield of guano. Other attempts have failed, and
anyone contemplating the construction of bat roosts for commercial
gain should be advised that the returns may be disappointing and
wholly out of proportion to funds expended.

Mosquitoes have been found abundant in and about bat caves,
and in the single case known where colonial bats have been artifi-
cially established there has been no appreciable diminution in the
insect life or in the local abundance of mosquitoes. The assertions
that bats will eradicate or even noticeably reduce the numbers of
mosquitoes, and with them malaria, are shown by studies of their
food and general life habits to be misleading and without foundation.

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