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Full text of "Some notable instances of the distribution of injurious insects by artificial means"

Cornell University Library 

SB 933.T38 

.notable instances of the d»^ 





S>tate College of Slgrfculture 

Sit Cornell Sntiiersttp 

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SOME NOTABLE INSTANCES 

OF THE DKTRIBUTIQN OF 

INJURIOUS INSECTS BY 

ARTIFICIAL MEANS 



BY FRED V. THEOBALD, M,A. 

, VICE-PRINCIPAL OF THE SOUTH-EASTERN AGRICULTURAL 
COLLEGE 1 PRESIDENT Or THE ASSOaATION OF-^CON^MIC 
Bl(jLC)GlSTis OF BRITAIN , ' FOREIGN MEMBER "dV THE 
ASSOCIATION, OP- ECONOMIC ENTOMOLOGISTS^ U.S.A. ; ETC. 



[from "SCIENCE PROGRESS,". Na. I, JULY 1906] 





Cornell University 
Library 



The original of this book is in 
the Cornell University Library. 

There are no known copyright restrictions in 
the United States on the use of the text. 



http://www.archive.org/details/cu31924018345912 



SOME NOTABLE INSTANCES OF THE DIS- 
TRIBUTION OF INJURIOUS INSECTS 
BY ARTIFICIAL MEANS 

,Ey FRED. V. THEOBALD, M.A. 

Vice-Principal of the Soutk-Eastem Agricultural College; President of the 

Association of Economic Biologists of Britain ; Foreign Member of 

the Association of Economic Entomologists, U.S.A. ; etc. 

One of the most noticeable phenomena amongst the insects 
injurious to fruit, farm and garden crops, stores, man and his 
domesticated animals, is the almost world-wide distribution ot 
certain of these pests. That this distribution is not a natural 
one we may safely infer from what we know of the general 
range of insects. Some species spread over a very wide area 
naturally, such as the Cotton Boll Worm {Heliothis obsoleta, 
Fabricius), which is found in Europe, Africa, and America ; the 
Army Worm {Leucania unipunda, Haworth), which is common 
to both hemispheres ; and the Migratory Butterfly (Danais 
archippus). Such instances are comparatively few in number. 
In spite of these, we can say definitely that there is no insect 
really cosmopolitan by nature. Just as with birds and other 
animals, so with insects — each species has a definite area of 
distribution. Many may increase this area by natural means, as 
we see has taken place with the Mexican Boll Weevil {Antho- 
\3tomus grandis, Boheman), and the Colorado Beetle {Doryphora 
decemlineata, Say.), but only within certain limits. Hence when 
we find some insects that attack plants, man, or animals almost 
world-wide in distribution, we may be sure that their range is 
due to some artificial cause or causes. On studying this subject 
we can at once see how easy it is for certain pests to be carried 
over the face of the earth by man's agency. 

\1 This distribution takes place by means of boats and trains 
and all other ways of intercourse. The more rapid these means 
of communication become, the more likely we are to see a 
concomitant increase and spread of many injurious insects, 
unless checked by stringent regulations. This dispersal has 



2 SCIENCE PROGRESS 

taken place mostly from north and south towards the Equator. 
We find that many temperate-climate insects will live and 
flourish in sub-tropical and tropical climates, but the reverse 
only applies within certain narrow limits according to each 
species. 

It is extremely unlikely that many tropical pests would 
live and flourish in the warmer climates of Europe, although 
we have an instance of such in the Yellow-fever Mosquito 
{Stegomyia fasciata, Fabricius). On the other hand, sub-tropical 
species may do so, and even penetrate into still more temperate 
regions — as, for instance, the San Jose Scale {Aspidiotus perni- 
ciosus, Comstock), which is found spreading as far north 
as Canada. The Yellow-fever Mosquito {Stegomyia fasciata, 
Fabricius), however, does not seem to occur farther north and 
south of the Equator than 48°. It has evidently spread outwards 
from the Central American States. We also see that the 
Brown Spotted-Mosquito (Theobaldinella spathipalpis, Rondani) 
has spread from Europe to the Sudan, and also far into the 
Cape. 

Of wider distribution still are some insects which attack 
stored goods. We find the Corn and Rice Weevils {Calandra 
granaria and Calandra oryzce, Linnaeus) now in almost all 
countries from the Equator to Norway and New Zealand, 
because they are so easily carried in grain. 

The manner in which an introduced insect may behave in a 
new country cannot be foretold. It may increase very rapidly, 
such as did in California, the Cottony Cushion Scale (Jcerya 
purchasi, Comstock), which came from Australia. On the other 
hand, it may die out sooner or later, as happened with the 
Tasmanian Lady-birds {Leis conformis), which I introduced into 
this country. The Cushion Scale found a comfortable home 
and there being none of its natural enemies to prey upon it, the 
Scale increased at an enormous rate. The Tasmanian Lady- 
birds, although they survived two seasons, found the climatic 
conditions unsuited to them, and consequently died out. 

Many insects from the tropics and sub-tropics may be 
imported to temperate climates, such as our own, and will 
flourish under glass, where they find congenial heat and 
moisture. Scale insects, or Coccidce, are particularly prone to 
do so. Most of the palm- and hot-house Scale insects we have in 
Britain are foreign importations. Only recently a Mealy Bug 



DISTRIBUTION OF INJURIOUS INSECTS 3 

{Dactylopius nipce, Maskell) new to Britain has been sent to me 
on palms from Blackheath. It was originally described from 
Demerara, and has also occurred at Bournemouth in Hampshire. 
The introduction of injurious insects can only be stopped by 
legislation. We find that most ot the countries of the world 
have regulations protecting them from foreign importations, 
such as " An Act to prevent the introduction and provide for 



R K A N S A S 




Mexico 



Fig. I.— Area in Louisiana infested by the 
Cotton Boll Weevil in December 1904 
(after Newell). 



'MEXICO 



Fig. 2. — ^Area in Louisiana infested by the 
Cotton Boll Weevil in November 1905 
(after Newell). 



the eradication of disease affecting orchards and gardens" in 
New Zealand. 

We may also get greatly increased local distribution by 
natural methods. As an instance of rapid natural distribution, 
we may mention the Cotton Boll Weevil. The beetles migrate 
in definite directions, and at certain times of the year. How 
great this distribution may be we can best judge by a glance 
at the figures showing the range of this serious cotton pest in 
Louisiana in 1904 and 1905 (figs, i and 2). 



4 SCIENCE PROGRESS 

First, we will consider insects that attack truit-trees in 
orchards and gardens, which may easily be carried from one 
country to another over sea, or from one state to another over 
land. The following are some of the most important in this 
category : ^ 

The Codling Moth {Carpocapsa pomonella)* ; the Bud Moth 
{Tmetocera ocellana)] ; the Pith Moth {Laverna atra); Pistol and 
Cigar Case-Bearers {Coleophoridce spp.) ; Bagworms (Thyrido- 
pteryx, etc.) ; Lackey Moths {Clissiocampidce spp.) ; the Peach 
Borer {JEgeria exitiosa, Say.) ; and Currant Borer {^geria 
Hpuliformis, Linn.) ; the Shot-borer Beetles {Xyleborus dispar, 
etc.); Bark Beetles {Scolytus rugulosus and S. destructor, etc.)t ; 
Currant Sawflies {Nematus spp.) ; the Pear Slugworm {Erio- 
campa limacina). Various fruit flies, such as the Apple Fruit 
Y\y {Trypeta pomonella)] ; the Mediterranean ¥vmt ¥\y {Ceratitts 
capitatd) ; the Australian Fruit Flies [Dacus tyroni, etc.) ; and 
the Indian Fruit Fly {Dacus ferrugineus). Psyllidae, such as 
the Apple Sucker {Psylla tnalt) of Europe and the Pear Sucker 
(Psylla pyri) of America ; the Woolly Aphis {Schizoneura 
lanigera)] ; the Currant-root Louse {Schizoneura fodiens) ; the 
Phylloxera or Vine Louse {Phylloxera vastatrix). Various 
aphides of all kinds, such as the Cherry Aphis {Myzus cerasi) ; 
Apple Aphides {Aphis pomi, A. fitchii, and A. sorbi). Of scale 
insects or Coccidae, the most important are the Mussel Scale 
(Mytilaspis pomorum)* ; the San Jose Scale {Aspidiotus pemi- 
ciosus) ; the Japanese Cherry Scale {Diaspis amygdali)] ; 
the Peach Scale {Aspidiotus persicce) ; the Cottony and Egyptian 
Cushion Scales {Icerya purchasi and /. cegyptica) ; and very 
many Citrus Scales, such as Aspidiotus aurantice and Mytilaspis 
cttricola. 

Besides true insects we get the ova of Red Spiders {Bryobia 
pretiosd) ; various gall-mites or Eriophyidas, such as the Currant- 
bud Mite {Eriophyes ribis) and the Pear-leaf Blister Mite {Erio- 
phyes pyri). 

These various pests may be carried in three different ways — 
namely (i) in and on fruits and seeds ; (2) on living plants ; and (3) 
in the cases and packages in which fruits and plants are sent. It 
will at once be said that the first and last are of little importance, 
as such cases of fruits are taken to large towns and markets. 

' Those marked with an asterisk come in abundance to Britain ; those with a 
dagger may do so now and again. 



DISTRIBUTION OF INJURIOUS INSECTS 5 

But they are very important, for these cases are distributed 
from the central markets to stores in small towns and villages. 
The stores, especially of the latter, are frequently close to 
gardens and even large orchards. Thus the pests that come 
over with the fruit may easily reach trees and bushes near by, 
either by flying or crawling, or by being carried by birds, other 
insects, or by man. Where the pests in the fruit are active 
insects, they often crawl to the baskets, that home-grown fruit 
is sent to the market in, for shelter. In this way many insects 
are carried back to our plantations, and many insect enemies 
have thus been spread over the earth. Nursery stock has also 
played, and does still, an important part in distributing diseases. 
The most important insect spread by means of fruit is the 
Codling Moth {Carpocapsa pomonelld). Originally this apple 
pest seems to have come from Europe. Its life-history is too 
well known to recapitulate fully here. Suffice it to point out that 
the larvae, or maggots, occur in the fruit at all stages. When 
they are quite young they do not show any very marked 
symptoms of attack ; but a careful examination will always 
reveal the presence of a small quantity of brown " frass " 
around the " eye," a sure sign of the maggots' presence. The 
larvae always leave the fruit when full grown to pupate in 
convenient shelter, which is normally found under the bark of 
apple trees. They leave the fruit just the same when it has 
been picked, and then seek shelter in store-rooms and in the 
cases and barrels in which the apples are dispatched. Failing 
this, they will leave the barrels in large numbers when opened 
and seek shelter elsewhere, such as in market baskets near 
by. In the latter way they are conveyed to the country. Quite 
exceptionally the maggots may be carried with nursery stock, 
they having spun their cocoons in some fork of the branches. 
That the Codling Moth is sent in numbers from country to 
country any one can verify for himself by examining apples 
imported into this country when the barrels are opened in the 
markets. It must not be imagined that all apples come thus 
infested. I have not detected any in Tasmanian apples, but 
have very frequently in American and Canadian. Perhaps most 
occur in the shipments of Portuguese and Madeira fruits. The 
result of this means of transit has been that we now have the 
Codling Moth not only all over Europe, but in America, Canada, 
Madeira, Teneriffe, Cape Colony, Australia, and New Zealand. 



6 SCIENCE PROGRESS 

The amount of damage done by it must be millions of pounds 
a year in the collective countries. 

Fortunately, many of our colonies are alive to the importance 
of this subject. The Codling Moth Act of Tasmania, passed in 
1884 and amended in 1891, has been so far successful that the 
colony is nov^r almost free of this pest. All possible steps are 
taken to prevent its importation. 

Natal has also legislated concerning the importation of this 
and other fruit pests. An Act (Law 15) was passed in 1881 " To 
regulate the introduction in this country of plants or cuttings 
which by reason of disease or otherwise might be injurious to 
the interests thereof." The benefit of this legislation we see in 
the case of the Codling Moth. Natal is one of the few countries 
where apples are grown in which the Codling Moth does not 
occur. It has been said to do so, but I am informed that 
these statements are erroneous. Mr. Fuller, the Government 
Entomologist, has shown that the diseased apples have been 
attacked by Fruit Flies. 

Why is this ? Simply because, under the Act referred to, 
a proclamation (79, 1897) was made prohibiting the introduction 
of all plants, portions of plants, cuttings, and anything taken 
off or from apple trees in the island of Madeira. Later, all 
diseased apples were prohibited coming from any region. Under 
these powers we find that in September 1903 seventy-five cases 
of Portuguese apples were destroyed ; in September 1904 
thirteen baskets of apples from Madeira were immediately 
reshipped on account of this pest. As a result, the colony is 
kept free from one of the most insidious apple and pear enemies 
the grower has to put up with. 

The same is done in New Zealand, where 2,257 cases of 
apples were destroyed in 1901. In 1904 we find that the 
authorities in the Hawaiian Islands, acting under legal powers, 
destroyed all infested apples sent there. 

It is almost useless to deal with this insect in a country 
unless protection is given from invasion from without. 

The other important insects distributed in fruit are the 
numerous species of Fruit Flies {Trypetidce). Different species 
attack a great variety of fruits, such as apples, pears, peaches, 
citrus fruits, guavas, bananas, etc. There are three genera that 
are destructive as Fruit Flies — namely, Dacus, Ceratitis, and 
Trypeta. The former has clear wings with one or two dark 



DISTRIBUTION OF INJURIOUS INSECTS 7 

lines, and has been wrongly referred (so I am informed by Mr. 
Austen, Dipterologist at the British Museum, by Mr. Froggatt 
and others in Australasia) to the genus Tephritis; Ceratitis has 
ornamented wings with a dense fine network of veins at the 
base; whilst the last named has mottled wings with normal 
venation. These Fruit Flies lay their eggs in both sound and 
rotting fruits; the maggots live in the pulp, and can at once 
be told from Codling Maggots by being apodal. Numbers are 
frequently found in a single struck fruit. 

The great importance of these pests cannot be overesti- 
mated, as we have no remedy for them. All that can be done 
to guard against Fruit Flies is to protect the trees by means of 
muslin tents, an operation too costly to be carried out in most 
places. 

That these insects have been and are still imported with the 
fruit we know quite well. The Mediterranean Fruit Fly {Cera- 
titis capitata) has thus been distributed into Australia and Cape 
Colony and elsewhere, and has become a serious pest in its 
new homes. The various Australian Fruit Flies {Dacus tryoni, 
Froggatt) and others are found in numbers in exported fruit. 
As a result. New Zealand prohibits all fruit infested with Fruit 
Flies from being landed. We find in the official records that 
3,700 cases of fruit were destroyed in 1901 on account of the 
presence of these enemies. 

Similarly, St. Helena has passed an Ordinance (1904) prohibit- 
ing the importation of all fruits from South Africa, Mauritius, 
Cape Verde, and Malta, and for the extermination of the Peach 
Fly {Ceratitis capitata). 

Neither Ceratitis nor Dacus are likely to flourish here, if 
imported ; but they may do so in many warm countries where 
they are not yet known as pests, and might even exist in hot- 
houses here. 

We find these insects also in Mexico and many other coun- 
tries. During 1904 many cases of fruits were destroyed at the 
Hawaiian Islands on account of the presence of Mexican Fruit 
Flies. Recently they have been found attacking melons in the 
Sudan {Dacus sp. ?). 

One species may easily be imported into Britain — namely, 
the Apple Fruit Fly of America {Trypeta pomonella, Walsh). I 
feel confident some apples sent me from the Isle of Thanet were 
attacked by this pest ; but, unfortunately, the dipterous l&rvEe 



8 SCIENCE PROGRESS 

did not hatch out. Should it appear in this country, it would 
prove another serious enemy in our already much-smitten 
orchards, and we have no law to prevent any such incursion. 

Scale Insects, or Coccidce, are also largely distributed on 
fruit, the majority on citrus fruits, but many on other kinds. 
We find amongst these the Red Scale (Aspidiotus aurantice), the 
San Jose Scale {Aspidiotus perniciosus), and the Mussel Scale 
{Mytilaspis pomorum). 

The Mussel Scale occurs most abundantly on apple and 
pear, but also on many other plants, both cultivated and wild, 
such as thorns, in this country. By means of imported stock 
and fruits this insect has become almost world-wide in dis- 
tribution. It is now found in Cape Colony, Natal, Egypt, 
all over America, Australia, and New Zealand, as well as in 
Europe, which is probably its original home. This Coccid is 
found on trunks, boughs, leaves, and fruit. It is not so often 
seen on the fruit in this country, but it comes over from 
abroad in large numbers in this way on apples, pears, etc. 
The great difficulty of keeping the Mussel Scale in check is 
well known, as it lives on wild as well as cultivated plants. 
Moreover, it is difficult to kill, except with strong paraffin 
emulsion ; hence it is very important to keep it from entering 
a country or district where it does not occur. This can only 
be done by absolutely prohibiting any plants infested with it. 
Fumigation with hydrocyanic acid gas, I have found, has no 
effect upon the egg stage. When trees are lifted for removal or 
export, the scale is usually found in this condition. I do not 
think it is yet found in Central, Eastern, or Western Africa, so 
these areas should guard against its introduction in no uncertain 
manner. 

The danger of introducing a scale insect is best seen in the 
case of the White Cushion Scale {Icerya purchasi, Comstock). 
This insect was imported into America from Australia. Its 
rapid increase, with such disastrous results to fruit-growers, is 
now a matter of history; and also its subsequent check by 
bringing over its natural enemy from Australia — the Lady-bird 
{Vedalia cardinalis). We now have this Icerya working in other 
places, notably in Egypt, in company with an allied species 
{Icerya cegyptica), and also in Portugal. 

The San Jose Scale {Aspidiotus perniciosus, Comstock) is 
perhaps most feared of all Coccidse. Its original home cannot 



DISTRIBUTION OF INJURIOUS INSECTS 9 

be said to be definitely known. Most probably it came from 
Northern China, where it has been recognised so long that there 
is no record of its origin. It did not call for much comment 
there, because it was held in check by its natural enemies. The 
introduction of one of these enemies (a lady-bird beetle) into 
America has not had the same good results as the Australian 
Vedalia, partly because the beetles themselves were attacked by 
a parasite. 

The actual introduction of the San Jose Scale into America 
seems to have been made from Japan, on some Japanese plums 
sent to California with the hope they would prove Cuculio- 
proof From California it was sent with nursery stock to Penn- 
sylvania in 1890 and 1891, and also to Virginia and a few other 
Eastern States. From that time it has gradually spread over 
America until it now does millions of dollars' worth of damage 
every year. So important is this pest that not only most of 
the American States, but most of the European countries, have 
laws safeguarding them against its introduction — even Turkey. 
Britain stands alone in not fearing the advent of this pest! 
There is no reason why it should not flourish in the West of 
England just as it does in Canada. It occurs also in Australia. 
Not only may it be spread by means of nursery stock, but also 
on fruit, especially apples and pears.^ It also infests countless 
hardy plants, trees, shrubs, and vines. 

Thus in three ways we see how insects have been spread 
from one country to another by artificial means — namely, on 
stems of plants, in and on fruit, and in packages. There is yet 
a fourth way — namely, on the roots. 

The Woolly Aphis {Schizoneura lanigerd), or, as it is wrongly 
called, the American Blight, not only lives on the trunk and 
twigs of apple and pear, but also on the roots, where it produces 
galls of a similar form to those above ground. This woolly aphis 
lives during winter in two ways — (i) as active insects hidden 
in crevices in the bark, both above and below ground, and (2) 
as eggs, which are few in number and which are placed close 
to the base of the stem. It is extremely difficult to see a few 
of these insects or ova during the winter, when they are hidden 
away. As no steps were taken formerly to check insect intro- 
duction into our colonies and America, it is not surprising to 
find this Schizoneura wherever apples are grown. It abounds 

' This is doubted by many of the chief authorities in America. 



10 SCIENCE PROGRESS 

in the United States and Canada, it is very common in Australia 
and in New Zealand, and we find it in Natal and in Cape 
Colony, as well as all over Europe, from north to south. 

The Phylloxera is another root-form which has been spread 
artificially over the face of the earth, carrying ruin with it 
throughout the vineyards. The Vine Louse also seems to 
be an European insect, and has been spread to America, 
Australia, the Cape, etc., with vine plants and cuttings. With 
this vine pest we get, as in the Woolly Aphis, a subterranean 
and an aerial race, and it is probably on the roots that it has been 
distributed so widely. Every country save our own has some 
Act or Ordinance forbidding the entry of vines, cuttings, etc., 
from foreign countries, or else limiting the introduction in 
certain drastic ways. 

Another European Phylloxera {Phylloxera corticalis) which 
attacks oaks in Europe has recently been found in South Africa 
doing considerable damage. It has doubtless been introduced 
with seedling oaks. 

Many Aphides have been spread on fruit trees and plants. 
One that has the widest distribution is the Black Fly of the 
Cherry (Myzus cerast), which is now found in America, Australia, 
New Zealand, and South Africa, as well as all over Europe. 
The small black eggs of this aphis are not at all easy to detect 
on young cherry trees. Thus they are easily passed unnoticed 
with nursery stock into new areas. Living aphides may also 
be imported from some distance. A consignment of strawberries 
from England were examined (as all imported stock is) on 
arrival at Durban, and all the plants were found to have many 
living aphides upon them. Had there been no fumigating 
regulations in vogue, a serious pest would have been introduced 
into the colony which previously did not exist. The Apple 
Aphides {Aphis malt, A. sorbi, and A.fitchii) common to Europe 
and America doubtless were imported from one country to 
another with nursery stock many years ago. 

The earth around the roots of plants must also bear its quota 
of insect enemies, for we find the Pear Midge {Diplosis pyrivora) 
in a few localities in America. It cannot have entered in any 
other way. We know that the larval midges fall from the 
fruitlets when they are mature and pupate in the soil. These 
puparia are very small, and are easily overlooked. Mixed up 
with particles of earth that stick to the roots, they may easily 



DISTRIBUTION OF INJURIOUS INSECTS ii 

be carried over land and sea for any distance. It is surprising 
that this pest has not made its way to other parts than America 
before now. 

In a similar way we can account for the wide distribution of 
the Pear and Cherry Sawfly, or Slugworm {Eriocampa limacina), 
which exists in America and Cape Colony. The Slugworms 
pass the winter in small earthen cocoons in the soil. They are 
often very abundant on nursery stock, and some cocoons are 
sure to be lifted with the stock and may remain attached to the 
roots with small clots of earth. Taken to a fresh country, they 
will hatch out in due course, unless the roots are previously 
treated. 

As there is no doubt about the dispersal of such pests on 
rootage, it is very important that all imported stock should 
have the roots well cleansed before being planted. This is, I 
fancy, a point generally overlooked, but one of great importance 
in the artificial distribution of insect enemies. 

Just as with hexapods, so with Eriophyid mites. These 
minute acari, formerly known as Phytoptidce, live either in buds 
alone, as we see happens with the Big Bud Mite of the Currant 
{Eriophyes ribis, Nalepa), or in galls formed on leafage and 
blossom, as in the Pear-leaf Blister Mite {E. pyri, Sch.) and 
the Cotton Mite {E. gossyperi) respectively. In the pear pest 
the winter is passed in the buds, and the same applies to those 
that gall the leaves of the plum. It is extremely difficult to 
detect the presence in winter buds of the leaf-living forms, 
as the invaded buds do not swell. On the other hand, we all 
know the well-marked, swollen appearance of the " Big Bud " 
attack in black currants. It is not surprising, therefore, to find 
that the pear phytopt has a wide distribution. The Pear-leaf 
Blister Mite {Eriophyes pyri) is now found in America, Canada, 
and in Cape Colony. There is no doubt that it has been imported 
into all three countries with nursery stock, grafts, etc. ; but we 
have no evidence showing when this took place. In some of 
its new environments this mite seems much more destructive 
than it is in its original home. 

The Big Bud Mite of the Currant {Eriophyes ribis) so far does 
not occur outside Europe, but its distribution there has increased 
very rapidly. No natural causes can account for it. On the 
other hand, we can do so by the distribution of infested stock. 
That this has taken place in a most persistent manner for some 



12 SCIENCE PROGRESS 

years we know. Some growers at one time even maintained 
that the " Big Buds " were signs of increased vitality. The 
importation of such diseased plants into any new district has 
always been followed by the disease spreading out around the 
infected plantation by natural means. The minute acari are 
carried about by bees and birds. They are also spread artifi- 
cially around an infested plantation by means of the baskets in 
which fruit is picked, by the clothing of the pickers, and even 
with the mud on people's boots. 

The Colorado Beetle {Doryphora decemlineata, Say.) has 
been distributed artificially to this country, and there is not 
the least doubt that if it had not been rigorously stamped out 
by the authorities of the Board of Agriculture, acting under 
the Colorado Beetle Order of 1877, it would have spread to 
a disastrous extent. First it was found breeding luxuriously in 
Tilbury Dockyard in 1901, and continued until 1902, in spite 
of most drastic measures. In the latter year it was effectually 
stamped out. 

The spread of this beetle by natural means has been remark- 
able. Originally it lived on wild solanaceae in the Rocky 
Mountains ; but as settlers pushed forward with their patches 
of potatoes, the beetles left the wild plants for the cultivated, 
and so spread farther and farther afield until now it is found 
flying in the neighbourhood of New York, and has even been 
seen in the city. How it was conveyed to England we do not 
know, but probably some specimens flew on to a ship and left 
it again on arrival at Tilbury. 

The Hessian Fly {Cecidomyia destructor, Say.) is another 
instance showing the great harm caused by the introduction 
by artificial means of an injurious insect into a new country. 
There is plenty of evidence to show that this corn pest is 
European in origin. It attacks not only cultivated graminaceae, 
but also wild kinds. We hear, for instance, of it devastating 
" twitch " or " couch " grass in Siberia. We know it occurs 
on the same and on other grasses in Britain. One heard so 
much of the Hessian Fly in the papers in 1891 that one was 
brought to believe that it had been imported into this country 
from America. In reality the insects were imported into 
America with straw by the hired Hessian troops, and first took 
up their abode at Long Island, and have gradually spread over 
the greater part of the wheat-growing areas of North America. 



DISTRIBUTION OF INJURIOUS INSECTS 13 

Thus again we find great damage done by an introduced insect. 
In England it never will be a very serious pest, because the 
chief damage it does is to autumn-sown corn, which in America 
is sufficiently up to allow of attack by the second brood, but with 
us a second brood would die off before the seed had germinated. 
This pest may be disseminated in two ways — one on the straw, 
as we see happened in regard to America, and another with seed 
com, for some of the puparia may be found in it, although most 
come away in the " tailings." 

Here again we find that somehow the natural enemies were 
not imported with the pest into America. For with us and in 
Russia the puparia are frequently found parasitised by chalcid 
flies to such an extent that one breeds out often more parasites 
than flies in this country ; hence it is kept in check with us. 

The Wheat Midge {Diplosis tritici) of Europe, also found in 
North America, doubtless owes its origin there to similar factors. 

Of the pests of animals we may still more expect to find a 
wide distribution due to artificial causes. 

The Sheep Scab Mite {Psoroptes communis, var. ovis, Fusten- 
berg) has undoubtedly been spread to all sheep-farming countries 
with the imported stock. Thorough quarantining would have 
prevented this, such as the regulations which are in force in 
most countries now concerning the introduction of animals. 
We can see similar results in regard to the insect enemies of 
stock. The Sheep Nasal Fly {(Estrus ovis), which lives in its 
maggot state in the nasal cavities and sinus of the sheep, has 
been spread with the sheep, and we now find it in America and 
Austraha. The same has happened with the Ked (Melophagus 
ovinus). 

Of all human injurious insects the Mosquitoes, or Culicidce, 
stand foremost in this subject of artificial distribution. This is 
not so much so on account of the great importance they are to 
man as annoying agents and as the means of conveying such 
diseases as yellow fever, malaria, filariasis, and dengue fever, as 
on account of the wide knowledge we have of their distribution, 
and the means by which they have been and still are spread over 
the globe. It is only natural to expect that those with a wide 
distribution are intimately connected with man ; such is what 
we find to be the case. It is those which we may class as 
domesticated mosquitoes which we find have a wide range. The 
three most noticeable are the Yellow Fever Carrier {Stegomyia 



14 



SCIENCE PROGRESS 



fasciata, Fabricius), the Household Brown Mosquito {Culex 
fatigans, Wiedemann), and the Brown Spotted-Mosquito {Theo- 
baldinella spathipalpis, Rondani). 

The Yellow Fever Carrier {Stegomyia fasciata) is commonly 
called the " Banded " or Tiger Mosquito. It occurs in the fol- 
lowing countries : Europe — in Spain, Portugal (south). Southern 
Italy, Greece, the Mediterranean Islands; Asia — in India, the 
Malay ports, China and Japan; Africa — in Natal, Transvaal 
and Orange River Colonies, in Rhodesia, Uganda, on both east 
and west coasts, in Egypt and up the Nile past Khartoum, 
in Algeria and Tunis ; North America, in the central and 
southern States, and, as we proceed south, still more abundantly 




Fig. 3.— The Distribution of the Yellow Fever Mosquito (Stegomyia fasciata Fab.). 



in Central and South America and the West Indies, which 
regions seem to be its original home. It is abundant in Australia, 
and passes up the East Indies to join the others at Malaya. 
Most oceanic islands also have this insect in their fauna. There 
is little doubt that the Central American States and West Indies 
are the home of Stegomyia fasciata, and that it has spread out 
from this area by artificial means. This is easily explained by 
the fact that we frequently find the Tiger Mosquito on board 
ship, and often in considerable numbers. In the old days they 
probably bred in the ship's tanks, just as we find them doing 
now on the Nile steamers. Coming to port, a certain number 
may fly to land, and so set up a new colony. In this respect 



DISTRIBUTION OF INJURIOUS INSECTS 15 

we may note that in some countries — Malaya, for instance — 
they so far only occur in the ports and along the littoral. 
Their further advance inland is more gradual. This takes place 
mainly along the river courses by boats and along the rail tracts. 
In both river steamers and trains we often find this mosquito 
in numbers. Not only by artificial means do we get a most 
annoying insect spread, but there is the concomitant danger of 
yellow fever as long as we have the insect, that carries it, present. 
Luckily this mosquito is found breeding almost exclusively in 
and around houses and dwelling-places, and so can easily be 
destroyed. 

In a similar way the Brown Household Mosquito {Culex 
fatigans, Wiedemann) seems to have been distributed. Skuse 
tells us that its advance inland in Australia has followed the 
opening of the railways. We know, as with the former insect, 
that it is often a fellow-passenger on board ship. Both these 
insects have nevertheless their finality of distribution, and we 
find that they will not live if they reach farther than somewhere 
near 48° north and south of the Equator (under normal con- 
ditions). They have both spread outwards from the warmer 
regions. 

On the other hand, by similar artificial means the Brown 
Spotted-Mosquito {Theobaldinella spathipalpis, Rondani) of 
Southern Europe has spread in the reverse way, and we now 
find it at Khartoum and at the Cape. We know of no records 
until the last few years of it at the Cape, its advent being 
probably due to the large number of transports running there 
during the recent war. This species is found in abundance at 
Teneriff'e and other islands on the way to Africa. 

The instances quoted here are merely a few which show from 
recent observations how insect enemies may and have been 
distributed from country to country, and how they may or may 
not increase to such an extent that they even out-rival the 
damage they do in their native lands. To repeat once more, 
"One can never prophesy how an introduced insect may act 
in its new home." It is therefore essential to the well-being of 
mankind that this insect dispersal by artificial means should 
be dealt with universally, in regard to those pests which attack 
farm and garden produce, stores, stock, and man, to save further 
loss and danger.