THE world’s history needs to be rewritten once more. It has already been told in terms of politics, economics, geography, climate, sea power, war, race, sex, and of great men and heroes. It should next be written in terms of insects. This is not the age of man; this is the age of insects. What the yellow fever mosquito, for instance, or the cattle tick, or the tsetse fly has done to the human race is still largely unrecorded.
France has erected in the Midi three battle monuments to commemorate her victory over a single plant louse. This little insect, the phylloxera, was swiftly and surely destroying the grape industry of that grape-famous country. This bug, an American immigrant, was finally defeated by means of help from the United States. Roots of our wild grapevines, immune to this little plant louse, were taken to France, and French grapevines grafted on them. Since the insects attacked only the root, this meant defeat and death to this pest.
There are estimated to be over four million kinds of insects in the world, and all of them are of significance to mankind. Most of them are frankly and openly either our friends or our enemies; few are neutral. They are all our competitors — we are all bidders for the world’s limited food supply. Who shall finally inherit this earth, man or bug, will depend in the last analysis on which creature is most efficient in securing his daily ration.
When we remember the bug’s appetite for food, particularly for the green and growing plant, and the bug’s capacity to reproduce and multiply, we begin to feel uncertain about our own future survival. Consider, for instance, that little tiny green bug, the cabbage aphid. Under favorable conditions, there are thirty generations of these bugs in one year. Under somewhat unfavorable conditions in New York State, from a single pair will come twelve generations in one summer. Twelve or thirteen days are needed for one generation. The mother aphid who lays her eggs the first of April becomes the progenitor of twelve generations by the middle of August. She produces forty-one young in one generation. Therefore, by the middle of August, if all the mother-aphid descendants should live, there would be alive at one time some five hundred and sixty-four quadrillion aphids! Or to state it more exactly, we should have the astronomical number 564,087,257,509,154,652 aphids. A minute calculation of the weight of these aphids by Professor Glenn Herrick shows that they would weigh eight hundred and twenty-two million tons — that is, almost exactly eight times the weight of all the human inhabitants of this globe.
This shows rather strikingly what one mother aphid can do in four and one-half months, if she has plenty of food and no enemies. In a warmer climate, such as Texas, she would do much better than this. In this connection we must also remember the size of the insect’s appetite — especially when it is in the larva stage. Familiar examples of the larva are the maggots — children of the common house fly — and the unsightly caterpillars, grub worms, tomato worms, tobacco worms, and so on, children of the butterflies and moths which play like fairies in the sunlight or moonlight. In fact, most of our common ugly worms are the larvæ of these dainty winged creatures. For many of our insects go through the complete metamorphosis — the egg, the larva, the pupa, and the adult stage. The larva stage is devoted to growth, the sole business of a larva being to eat and grow.
We can get some idea of the appetite of the larva when we note the food consumption of the caterpillar of the common Polyphemus moth. When this worm is fully grown, that is, in about fifty-six days, he has actually consumed 86,000 times his original weight. This is rather a terrifying fact, on the face of it, but we are able to reflect that thus far, at least, we have held our own against these greedy competitors for our food. That is the situation; the balance has been maintained thus far, between man and bug, so that the bug has not yet deprived his human competitors of too much of their food supply except in those few cases of insect plagues. Obviously, it would be very easy to disturb this ‘balance of nature.’
Will man or bug inherit the earth? If it is a question of the survival of the fittest, then the argument is all in favor of the bug. The cockroach, for instance, was here a million years before man came; therefore, he will likely be here a million years after man has joined the dinosaur and the dodo. The cockroach came with the coal age. He is versatile enough to adjust himself to his environment. Living first in Asia, he traveled by ship to Holland, and later became at home all over Europe. While he prefers the warm climate, he is found in numbers among the Laplanders of the far north. He even destroys in some years great quantities of the dried fish put away for the winter by these northern settlers. More famous, however, are the cockroaches of Brazil. One traveler reports spending some time in a private home on the Upper Paraguay. Here were a dozen children, each with his eyelashes more or less eaten off by cockroaches. The eyelashes were bitten off irregularly, and in some places quite close to the eyelid. Since Brazilian children naturally have the beautiful drooping lashes of the Latin race, their appearance as defaced by the cockroaches was indeed strange. These same cockroaches also bite off bits of the toenails. Apparently they confine their depredations to children.
As the cockroach has migrated all over the world, so, too, many other insects are doing. Man’s scientific means of insect control, which is his main argument in favor of ultimate survival, is offset by the modern means of travel which the insect now uses. Sailing ships have given place to steamships; horse carts to automobiles; and finally comes the airplane. When Lindbergh finished his 46,000-mile flight in 1928, touching three continents and dozens of countries, think of the scores of new insects he picked up and brought back to the United States! One female insect — even one insect egg — is enough to start a new insect pest in the United States which may have most serious economic consequences. Polyembryony, they term it, when one female insect lays a single egg which hatches out into a large number of maggots.
Whence came our present insect pests? Most of them came from foreign countries. The cotton boll weevil is from Mexico; she came, the theory is, in the egg stage, in a dirty cotton mattress of a Mexican laborer. The corn borer is from Europe, the gypsy moth from Japan, the cottony scale from Australia; the Mediterranean fruit fly was landed in Florida by some bootleggers from the West Indies; and so on. And far more serious, they come here without their natural enemies. In this way the balance is disturbed, the disturbance being wholly in favor of the insect. An insect in his home land is often so harmless and obscure that his presence is not even noted. This is because his natural enemies keep him in his place. But transplant this little bug to America, give him plenty of rich food and no enemies, and he will show what the biological laws of reproduction mean, and what the mathematical formula of geometric progression looks like when put into practice.
We have had many examples of this kind. The best one is perhaps the white fluted or cottony scale which once threatened the complete, speedy, and absolute extinction of the orangeand lemon-growing industry of California. The adult female of this beautiful and dangerous insect has a body which is scalelike and dark orange-red in color.
It was in the seventies when this insect came by ship from Australia to California, and made its first appearance on some acacia trees in Menlo Park. The insect attacked apple trees, fig, quince, pomegranate, roses, and it soon developed a preference for orange and lemon trees. The trees attacked were ruined. Since this insect left all its enemies behind in Australia, it had a free field for action, until the counterattack by man himself began. Few jobs ever done by the United States Department of Agriculture in the field of entomology or elsewhere have been so spectacular and so immediately beneficial as was this fight on the cottony scale. Victory was secured by the introduction from Australia of a particular ladybug whose diet is this cottony scale, and whose appetite is for this insect only.
But to win this battle was not the work of one year, for it was not so simple as it looks in retrospect. First of all, the Department sent two men to California to study the life history of the scales. These entomologists spent one year in this study and came to the correct conclusion that the insect was a native of Australia, but was not a pest there because natural enemies were keeping it down. One of the more seasoned bug hunters of the Department was accordingly sent to Australia to spy out these insects, and to collect specimens of its enemies. This entomologist, Mr. Albert Koebele, was a skilled collector. He found a small fly laying its eggs on the cottony scale; these eggs hatched and the little maggots devoured the scale. But this was not the final solution. He also found a little ladybug, small, reddishbrown, with a voracious appetite for this one insect. His next job was to transport a number of these ladybugs alive from Australia to California, a very difficult feat, for ladybugs do not have the habit of crossing the equator and going on ten-thousand-mile voyages.
Koebele selected a large number of the ladybugs. He put them in tin boxes, with food. These he placed in the ice box of the steamer at Sydney. Upon arrival in California they were found to be alive and well. A test was immediately made in Los Angeles to determine whether or not the scientist had correctly reasoned out his problem. An infested orange tree was surrounded with a tent of gauze. It was a glorious triumph for the scientist. The Australian ladybugs fell upon the American cottony scales with avidity; indeed, their appetites seemed whetted by the long sea voyage. The results more than justified the most sanguine expectations. It was the ‘most perfect experiment ever made by the Department,’ said the Chief of the Bureau of Entomology.
There are distinct and peculiar reasons why this experiment was such an unqualified success. First, there is the rate of increase of the ladybugs. Each female lays on the average three hundred eggs, and each of these eggs hatches into a hungry larva. If we assume that one half of these larvæ produce female bugs, and maximum reproduction goes on for the summer, a simple calculation shows that in five months a single ladybug becomes the ancestor of seventy-five billions of other ladybugs, each capable of destroying many cottony scales. The ladybug breeds twice as fast as the cottony scale. The ladybug feeds upon the eggs of the cottony scale. And this particular type of ladybug has no enemies of its own, although our American ladybugs have many parasite enemies. Finally there is the very important military advantage in favor of the ladybug in its attack on the scale — the ladybug is a quick mover, while the scale is still. For these reasons the ladybug is almost a perfect remedy for the fluted cottony scale. There have been no failures in its introduction into any of the different countries to which it has been carried. No other insect tried in international work has had such perfect success, California’s greatest agricultural industry was thus saved from a complete destruction and one of our greatest and most delicious health foods rescued to us by introducing from a foreign country one small insect which restored the ‘balance of nature.’
Next to this achievement stands our success in saving the dominating industry of the Hawaiian Islands — cane sugar — from annihilation at the hands of another Australian insect. In this case it was the cane-leaf hopper. Its depredations ran up into many millions of dollars. The rise and decline of this insect may be sharply pictured by the statistics of sugar production on one big plantation: —
This diminuendo and crescendo marks the fight put on by the Department of Agriculture. The entomologist sent to Australia succeeded, finally, in finding and carrying to Hawaii the parasite which is the natural enemy of the cane-leaf hopper. The parasite multiplied rapidly. His rise marked the decline of the leaf hopper. C’est la guerre! There is no pity, no mercy, in this war. Like the battle of Kipling’s mongoose and the cobra, it ends only when one of the combatants is dead. When Chief L. O. Howard of the Bureau of Entomology visited Hawaii in 1915 he pronounced the situation with regard to the sugar-cane-leaf hopper as ‘almost perfect.'
These two brilliant successes in overcoming our insect enemies had one undesirable effect, and that was, they created a sense of false security in the minds of the general public. The feeling became general that for our defense in the war against the insect hordes we may look with confidence to the highly proficient professional entomologists in the Department of Agriculture and in the State colleges and experiment stations. The fact, remains that in only a few conspicuous cases have we won the battle against the bug. With most of the harmful insects in the United States, either the bug has definitely won the war or the fighting is still going on. We have already surrendered to the chestnut blight and these noble and useful trees are fast becoming extinct. Congress last year spent ten million dollars in the corn-borer campaign, and the total effect was to mitigate very slightly the ravages of this insect pest. Scientists on the job report that the slow westward march of the corn borer will not stop with Ohio and Michigan, but will inevitably continue until the whole corn belt is covered. We shall have to sign a truce with this bug and give him perpetual tribute in the form of a few million or a few hundred million bushels of corn a year. This pest has never been stopped yet in any country.
In this manner we have learned to live with the Hessian fly, who came over from Europe with the mercenary troops of the British army during the Revolutionary War. We have already paid him tribute to the extent of hundreds of millions of bushels of wheat, and shall keep on doing so indefinitely.
The Mediterranean fruit fly, one of the most dangerous insects known in the citrus industry, was discovered in Florida early in 1929. In a few months it had traveled westward as far as Dallas, Texas. It now definitely threatens the citrus industry of California.
The cotton boll weevil arrived at Brownsville, Texas, in 1892 from Mexico. By the year 1924 it had traversed the cotton belt and reached Virginia. Its original home is the plateau region of Central America and Mexico. Its only food is the cotton plant. This insect has definitely established himself in every cotton state except California. He is with us as a permanent boarder. The fight will continue against him, as against the corn borer, not to exterminate him, but to keep him within bounds. It would take several pages of this magazine just to list the harmful insects now definitely and permanently established in the United States, all of which are in real competition for our food supply, and all of which are capable of rapid reproduction.
We multiply our scientific means of overcoming these harmful insects. But as fast as one bug is destroyed we discover two new ones to take his place. Hence our worst pests to-day are bugs which our grandfathers never heard of. We may venture the prophecy, therefore, that our grandchildren will be struggling with new and more harmful insects than we now know. Even at the present moment entomologists estimate that we are acquainted with only one kind of insect out of eight or ten actually in existence. The biological methods of fighting insects — those which maintain the balance of nature — are far the most effective. By this we mean the work done by the birds, by insects themselves, and by those most tiny of all insects, the predacious parasites.
First of all, we ought to encourage the birds to come, and we ought to protect them in every way. This may involve getting rid of a large portion of the cats, particularly those night prowlers which destroy birds on their nests. The nuthatch, or the downy woodpecker which works up and down the limbs of trees in the wintertime, inspecting each nook and cranny with meticulous care, destroys the eggs of insects. We can calculate how large a quantity of insects would come from the eggs destroyed in a single day by a single bird if these eggs were left to hatch. Studies made of the food of birds show that from three hundred to five hundred insects are sometimes found in the stomach of one bird. Insects constitute 65 per cent of the food of the downy woodpecker, 95 per cent of the food of the house wren, and 96 per cent of the food of the flycatcher. Birds have their own peculiar habits in catching insects. The phœbe, the flycatcher, and swallows live upon flying insects; robins and meadow larks feed upon ground insects and grubs; cuckoos, orioles, warblers, and vireos catch leaf-eating insects; titmice, creepers, woodpeckers, nuthatches, and chickadees explore tree trunks and limbs for small insects and insect eggs.
It is when we turn to our insect friends, however, that we find the most efficacious means of fighting our insect enemies. Shakespeare makes Touchstone say, ’I will kill thee a hundred and fifty ways.’ But when it comes to various and sundry methods of killing, the insects have Touchstone beaten. In our boyhood days we became familiar with the thread-waisted wasp, known as mud dauber. This wasp is not merely a skilled engineer and mechanic, but she also has uncanny skill in the use of anæsthetics. At any rate, she lays by a stock of food for her unhatched larvæ, the food consisting of tender, juicy spiders put to sleep by an injection of the wasp’s powerful narcotic in exactly the right nerve centre. The relation of the spider to the fly is so well known that it has become a proverb; but the relation of certain flies to the spider is not appreciated. There is a group of hunchbacked, smallheaded flies which feed entirely on spiders. These carnivorous flies in the maggot or larva stage live within the bodies of the spiders or in their egg cases.
Some insects attack others openly, as do the dragon flies, and the praying mantid. The praying mantid is the only bug with a religious name, — Mantis religiosa, —and he gets this name because he folds his hands as if in prayer. At such moments he is ready to prey on the first insect that comes within his clutches.
Some insects catch other insects in snares, like the spider web, or in pits of ingenious construction, like the ant lion’s trap. These predacious insects, as they are called, account for a great many victims. But the great majority of insect-eating insects, when young, live within the bodies of their victims and eat their way out, or within their eggs. These are the true parasites; they are the real farmer’s friends. It is not an uncommon thing, especially in vineyards, to find a feeble caterpillar with its back covered with little white oblong bodies, which the casual observer usually takes for its own eggs. These are the cocoons of a little fly parasite known as the braconid parasite. Its larvæ eat and grow within the body of the caterpillar. Just before the caterpillar dies they leave it, and spin their silken cocoons upon its back.
Most of the caterpillars you see are already marked for an untimely death, for they have a sort of glorified form of tuberculosis. Within their own bodies are the maggots of these parasites, which must eat their way out.
One of the most interesting of these small braconid flies is called the aphidius, and she deposits her egg within the body of that troublesome plant louse, the aphid. The aphid is then doomed to a death more horrible than that of tuberculosis; the parasite in emerging from his host cuts a very regular circular lid in the top of his host’s abdomen. You can sometimes catch the mother aphidius in the act of depositing her eggs; she selects the plant louse, and stands with her head toward it. Bending her abdomen under her thorax, she darts her ovipositor forward into the body of the aphis. The dreadful end of the aphis is then a matter of a few days.
The tachinid fly, which looks like a house fly, is an enemy of many insects. Some tachinids lay their eggs on the back of the caterpillar so that the maggot can bore in and live a life of ease and gluttony as long as the caterpillar can carry on. When life departs from the caterpillar the maggot is ready to do likewise. Other tachinid flies deposit eggs on leaves of plants infested with caterpillars. The caterpillar devours the egg with the leaf, but without chewing or injuring the egg. In due season the egg hatches, and the maggot sets up housekeeping within his host. He feeds upon the body of the caterpillar till he destroys it. Still other flies have other means of attacking the nonresisting caterpillar; some deposit living maggots on the leaves, and these maggots attach themselves firmly to the first caterpillar that comes along, and complete their growth within the body of the luckless caterpillar; other flies deposit living maggots within the body of the caterpillar, which is, of course, then marked for death.
Predacious parasites have even more refined ways of killing. Some of the small flies, the parasitic Hymenoptera, lay their eggs within the eggs of other insects. Here the tiny parasites come to maturity. Dr. Grace Griswold has succeeded in finding parasite eggs within the parasite eggs within the insect eggs.
Great fleas have little fleas upon their backs to bite ’em,
And little fleas have lesser fleas, and so ad infinitum.
Our scientific and practical progress in entomology during the last fifty years has been enormous. To two people much of this success is due — Professor John Henry Comstock of Cornell, and his wife, Anna Botsford Comstock. Almost exactly fifty years ago (1879-1881) Professor Comstock was in charge of the work in entomology of the United States Department of Agriculture. He had three and a half workers under him. Forty years later there were 545 workers in the Bureau of Entomology. Another outstanding piece of pioneering done by Professor Comstock was the establishment of the first insectary in the world. Indeed, he coined the word himself, applying it to a building on the Cornell campus erected in the early eighties. ‘There should be a place,’he said, ‘where living plants can be kept with insects upon them, and all the conditions of growth of both plants and insects should be under control.’ So a building, named an insectary, was erected. After many years this building gave place to a modern structure. Breeding cages are used for insects. Subterranean insects are observed by means of root cages — boxes with glass sides. Now there are many such insectaries scattered over the world.