DDT and the Balance of Nature


» Will man’s extensive use of DDT upset the balance of nature in our forests, fields, arid streams?

FROM ancient times diseases conveyed by insects have been the main cause of casualties in military campaigns: the housefly as a carrier of dysentery and typhoid, the louse as the carrier of famine fever or epidemic typhus, the Anopheles mosquito as the carrier of malaria. There are many ways of combating these insects and breaking the chain of transmission of disease from man to insect and to man again. Among the methods is the use of insecticides.

At the outbreak of the war there was one insecticide which occupied a key position in relation to these insects of medical importance; and that was pyrethrum, the extract from the daisy flower Pyrethrum Cinerariaefolium, a substance exceedingly poisonous to insects but practically harmless to man. The world’s supply of pyrethrum came chiefly from Japan and from Kenya in East Africa. At first there was plenty of pyrethrum available for the British forces. But as the war extended into tropical areas, the supply could not keep pace with the demand. The war with Japan closed one main source of supply; the entry of the United States into the conflict enormously increased the demand, and the situation became acute.

In time of peace the United States was the chief consumer of pyrethrum, and it came as something of a shock to British entomologists to discover that virtually the whole of the available stocks of pyrethrum, grown by British planters in Kenya, had been bought up by America! This situation was met by negotiations which led to restriction in the use of pyrethrum for agricultural or other civil purposes, and to the sharing of the available supplies between the two countries according to their military needs. But these supplies were hopelessly inadequate to meet the rising demands, — skyrocketing from month to month, — and desperate efforts were made to find a sufficient substitute. It was against this background of anxiety that DDT appeared.

Pyrethrum is a contact insecticide: it kills insects when it comes in contact with the surface of their bodies. So is the other famous vegetable insecticide, derris. For many years chemists had been seeking to produce a synthetic contact insecticide, capable of being made in the laboratory and manufactured on a large scale in the chemical works, which would rival these vegetable products in effectiveness and surpass them in cheapness and availability. But until the war these hopes had been largely disappointed.

Arrival of DDT

In 1940 the Swiss firm of J. R. Geigy, A.G., of Basle discovered the insecticidal properties of a chemical that is known as 2,2-bis (parachlorophenyl) 1,1,1-trichloroethane or more familiarly as diehlorodiphenyltrichloroethane, later abbreviated to DDT. The firm took out patents to cover the manufacture of this chemical and its use as an insecticide, and early in 1942 the British and American branches of the firm brought these patents to the notice of the entomologists in the two countries, who were seeking a substitute for pyrethrum.

In England we read those patents and were frankly skeptical. It seemed to us that too much was claimed. The new insecticide appeared to be so exactly what we wanted that it looked too good to be true. But clearly the stuff should be tested. A 100-pound sample was obtained from Switzerland. A small laboratory sample was prepared in Manchester. Both had the same properties: they were colorless solids, nearly odorless or with a not unpleasant odor, highly poisonous to insects. The body louse was killed by an amount little greater than the minute dose of pyrethrum needed for this purpose.

Meanwhile the same thing was happening in America. The Swiss claims were fully substantiated. And immediately, on both sides of the Atlantic, entomologists got to work to discover how best to use the new material in the fight against the diseasecarrying insects. There was the closest coöperation throughout between the British and American scientists, and development went forward rapidly along parallel lines. New ways of using DDT were discovered, as it seemed, almost daily—until both British and American entomologists almost came to feel that they had discovered the stuff.

The chemists worked with equal speed. Improved methods of manufacture were devised, chemical plants were set up by the manufacturing firms in America and England, and soon after the requirements were defined the supplies were available. As the months went by, more uses were discovered; requirements went up and up; until, with an output of some scores of tons a month, the military demands could still scarcely be fully met.

DDT against the louse

Mixed as a dust and rubbed into the clothing, DDT is exceedingly effective in killing lice; and unlike the vegetable insecticides pyrethrum and derris, which soon lose their potency, DDT is extremely stable and will continue to kill lice for three or four weeks after it has been applied.

Early in 1944, an outbreak of typhus occurred in Naples. Judging by what has happened in the past when typhus has broken out among an overcrowded, dirty, and ill-fed population during the winter months, this might have been expected to become a great and spreading epidemic. Dusts, at first containing pyrethrum and later containing DDT, were puffed up the sleeves and trouser legs, down necks and into the waists of skirts and trousers, systematically throughout the affected population. The results were dramatic: the epidemic, instead of rising to a terrifying peak, just faded out.

On the western front, in World War I there was an efficient organization for destroying body lice, mainly by heat treatment of clothing, and by soaping and bathing of the men. But the methods employed had one great defect : they left the soldier’s clothing ready to become infested again as soon as he could pick up fresh lice from his comrades; thus, in spite of systematic delousing, the men in the trenches were always heavily infested and the louse-borne trench fever" was prevalent.

What was needed was some treatment that would leave the clothing proofed against lice for a long period. DDT has proved ideal for this purpose. It dissolves in oils but not in water. By dipping shirts and underclothes in suitable emulsions or solutions of DDT they can be so impregnated that lice which crawl over them are killed. The effect lasts several weeks even in clothing worn continuously. And some effect persists even after washing several times in hot soap and water. This impregnated clothing was highly successful in the prevention of lousiness in the armies that liberated Europe. The percentage of men infested with lice among the American and British forces was exceedingly low — in striking contrast with conditions in the German armies.

Mosquitoes and malaria

The full story of malaria in the war has yet to be written. But this much can be said. The maintenance and operation of large armies in the highly malarious jungles of Burma or New Guinea could never have been effected without steps being taken for the prevention of malaria. Furthermore, these preventive measures, as well as the medical treatment of malaria, were so much more efficient among the American, Australian, and British Armies (indeed preventive measures were almost nonexistent among the Japanese) that, in the war in the Far East, malaria was our most potent ally.

Perhaps the most important single measure in this fight against malaria was the regular use of drugs, quinine or atabrine (mepacrine), which suppress the symptoms of the disease. But measures directed against the Anopheles mosquitoes which carry the infection from man to man were certainly valuable, sometimes very valuable, and among these measures DDT figured prominently.

Mists containing DDT and a little pyrethrum, dispersed by means of ordinary hand spray guns, proved better for killing the adult winged mosquitoes than the standard pyrethrum sprays. The famous “aerosol bomb” devised by the U. S. Department of Agriculture, in which pyrethrum was blown out as a fine mist by freon kept liquefied by pressure, was later filled with DDT instead; and there were many other ingenious methods for producing a suspended cloud of DDT that would kill mosquitoes.

But there is another way of killing the winged mosquito. It was early noted by the Swiss workers that if a thin film of DDT was applied to the surface of a wall, houseflies settling on that surface were subsequently killed. This effect persisted for several weeks. Films of DDT act in the same way against the mosquito; the poison appears to be absorbed through the feet of the insect — and this happens without causing it any discomfort, so that the insect is not repelled from the treated surface but remains there until it has taken in the tiny dose that is needed to kill. Treatment of the surface of huts and tents has been one of the most successful means of controlling Anopheles mosquitoes.

Since the larval stages of the mosquito are spent in water, most of the classic methods of malaria control have been directed against the larvae in their breeding places. DDT proved valuable in this connection too. A mist of DDT in kerosene sprayed over the surface of the water, the addition of DDT to the petroleum oils commonly used to kill mosquito larvae, the spraying of an emulsion of DDT in water— these were some of the methods. It was found that as little as two ounces of DDT would kill all the larvae over an acre of water surface. But there was one disappointing feature. Whereas it had been hoped that the killing action would persist for long periods, it lasted in fact little more than a week.

It has long been the practice to control mosquito breeding in inaccessible places or over wide expanses of water by distributing Paris green from airplanes. DDT lent itself particularly well to this procedure. Great advances have been made in the regulated dispersion of oily solutions or emulsions of DDT from the air, and this practice has proved very successful in controlling mosquito larvae.

But it was soon discovered that these aerial sprays yielded a bonus: they also killed the adult mosquitoes. And not only did they kill the day-biting mosquitoes which were on the wing at the time of spraying: they killed the Anopheles mosquitoes which shelter in places beyond the reach of the spray during the daytime and come out to feed only at night. Apparently sufficient poison was deposited upon the surface of the vegetation for the mosquitoes to pick up a fatal dose when they alighted on the leaves during their nocturnal flights. For the rapid improvement of malarial conditions in areas newly occupied by troops, or even before the landing of invading armies, — that is, until the regular ground organization could get to work, — or for maintaining control in difficult or inaccessible country, this method has proved most valuable. It was used with great success in Italy, in India, and in Guadalcanal and many other regions in the Pacific zone.

Bedbugs too, which can be a troublesome pest in barracks, or in native houses, or on board ship, are killed by residual films of DDT — not so easily as houseflies or mosquitoes, but well enough for practical purposes. The result is that virtual extermination of bedbugs has been achieved in jails, barracks, and ships in many parts of the world. DDT was useful in air-raid shelters in London. A suitably prepared film will continue to kill bedbugs for at least three months and often much longer. Promising results have been obtained by the use of paints (water or oil paints) in which DDT has been incorporated.

For other insects, such as roaches or ants, which may trouble the soldier or the sailor, DDT has been good but not much better than materials already in use.

Use of DDT in peace

The control of malaria by preventing the breeding of mosquitoes has been possible since the end of the last century, when the role of the mosquito in transmitting the disease was first revealed. And it has been put into effect in Panama, in the wealthy Chinese and European settlements in Malaya, in the rich Dutch settlements in Java, and on tea plantations in India and elsewhere — to take a very few examples. This method of control has not been used in the past in the poor rural communities in India or Africa or in the southern part of the United States — because the cost was out of all proportion to the slender resources of people living at a bare level of subsistence.

There are, it is true, certain parts of the world where something has been done for rural malaria by improved water management in the rivers and streams, as in Ceylon and in the Tennessee Valley; or by improved agriculture, as in some of the ricegrowing areas in Java; but it is fair to say that where such methods have not been feasible, no direct attack on the mosquito has been possible in rural areas.

Has this state of affairs been materially changed by the advent of DDT? It is too early to be sure; but if DDT becomes available at a sufficiently low price, we may have, for the first time, in the “residual film” deposited on the walls of houses, a really effective method of control that could be applied at a cost within the reach of rural communities.

The control of rural malaria, so long the despair of the malariologist, may at last become a reality. Preliminary trials in West Africa and in India have been highly encouraging. This method figures largely in the “Extended Program” of the U. S. Public Health Service, in which the energetic campaign for the control of malaria in war areas is being enlarged to cover all the malarious regions in the United Slates; and in the control of rural malaria by the Health and Safety Division of the TVA. It is acceptable to the people; and it has the advantage that its action is concentrated automatically against those mosquitoes that come into houses — which are precisely the mosquitoes that are carrying the disease.

Nations are becoming increasingly aware of the risk they run of new malarial mosquitoes being introduced by airplane from other countries. The terrific epidemic of malaria in Brazil in the early thirties, which followed the introduction of Anopheles Gambiae from West Africa, has been an object lesson on what may occur. To meet this risk, international agreements have been made to ensure that airplanes are so treated as to kill any dangerous insects that may be on board.

Spraying with insecticidal mists has been the standard method employed; and sometimes this work is efficiently done. But it must be admitted that not infrequently the performance with the spray gun is a mere formality that can cause little inconvenience to any insects present. A residual film of DDT, thoroughly applied at suitable intervals to the whole interior of the airplane, would be a very great additional safeguard, and is likely to figure largely in the sanitation of commercial planes.

A residual film applied in houses will be of enormous value in the campaign against the bedbug and in helping to keep houses free from flies. Likewise dusts containing DDT will have their use in dealing with head lice in children and with epidemics of typhus and other louse-borne diseases in the countries in which they are likely to occur. But the real cure for all these troubles is an improved standard of hygiene. DDT is a valuable supplement to hygiene and cleanliness. It will not take their place.

Veterinary uses for DDT

Almost all the vast output of DDT during the war years was earmarked for service users. But enough has been made available for trials to prepare the way for what will soon be its main applications — against insects of veterinary importance, against the pests of growing crops, and perhaps against forest insects.

The swarms of flies in unsanitary cow barns constitute a veterinary problem, and the deposit of a film of DDT on the walls has had a spectacular effect upon their numbers, which at once impresses the observer with the powers of this insecticide. But when we look into the matter a little more closely, the limitations of the method become apparent. DDT as a residual film kills very slowly; it may take two or three hours before the flies resting on the film finally succumb.

However, these flies on the walls do not seriously trouble the herdman. The flies he is concerned about are those which are brought in by the cows when they come from the pasture to the barn for milking. It is these flies, often quite few in number, which irritate the animals and make them restless during milking; and it is sometimes observed that although the whole population of flies which remains behind in the milking shed is killed by DDT on the walls, the cows are still as restless as ever while they are milked.

Moreover, there are a good many sorts of flies on cattle, and they behave in different ways. The houseflies, which tickle the animals but do not bite, readily leave them and settle on the walls and are poisoned. The stable flies, which are biting, bloodsucking insects, stay on the cattle until they have succeeded in getting a meal of blood. They are a source of much annoyance, and a small number may cause a lot of stamping before they leave the cows and come into contact with the treated walls. The horn flies are little bloodsucking insects which never leave the body of the animal save for a few moments when it drops its dung, in which they lay their eggs. They are quite unaffected by DDT on the walls of the barn.

That suggests the possibility of applying DDT to the back of the animal as a spray or in a dip. The effect of this treatment on the stable fly is only shortlived; after a few days its toxicity is almost gone. But against the horn fly, which remains continuously on the animal, the results have been far more satisfactory. In the northern territories of Australia, in Texas, and in Florida, it has been found that spraying or dipping the cattle in a DDT preparation has kept them free from this troublesome insect for several weeks.

In England very promising results have been obtained against the fleece worm or blowfly in sheep by the same means. Instead of trying to kill the young maggots as they develop in the soiled fleece and skin, the wool is treated with DDT to poison the female fly when she is seeking to lay her eggs. Furthermore, it is possible to treat the wool in addition with materials which make the sheep extra-attractive to the egglaying female and so to turn the treated sheep into a veritable trap for blowflies.

It may be that something of the same sort will prove of value against the tsetse flies in tropical Africa, which carry a number of fatal diseases of stock as well as human sleeping sickness. These flies collect in large numbers on cloth screens or dummy animals, and there is evidence that they can be killed over longish periods if such screens are treated with DDT.

Lice on goats and cattle, and ticks on sheep, are among the pests of stock against which DDT is proving extremely successful. And it is effective against fleas on cats and dogs.

DDT in agriculture and forestry

In agriculture, DDT has been used with promising results against flea beetles and onion thrips. It has been good against caterpillars, but it has been disappointing against most sorts of aphid or green fly. As a measure for the control of the potato leaf hopper it has been so outstanding that some observers have suggested that its widespread use for this purpose in the United States will create the problem of an excessive production of potatoes unless the acreage devoted to this crop is markedly reduced! DDT is a promising insecticide against the potato beetle. It is without effect on the Mexican bean beetle, however, and it has not proved very successful against grasshoppers.

The codling moth, the Oriental fruit moth, the cotton bollworm, the boll weevil, the corn borer, the Japanese beetle — these are a few of the major pests of agriculture against which DDT is being widely tested. Will DDT make possible control of the corn borer or corn-ear worm on field corn? Will DDT save the United States from invasion by the pink bollworm which is threatening from Mexico and which so far has been kept at bay by quarantine measures? Has an answer at last been found to the problem of the Japanese beetle? These are a few of the questions that agricultural entomologists are asking and seeking to answer.

Most sorts of caterpillars are easily killed by DDT, either from eating vegetation that has been sprayed, or from crawling over a residual film of the chemical. The tent caterpillars which have been so conspicuous in the eastern part of the United States during the past season, although they are not a serious pest, are readily controlled by DDT. The young caterpillars die when they come out from the egg masses in the twigs and creep along the branches if these have been sprayed with DDT — even when the spraying has been done some weeks before.

The gypsy moth too is a ready victim. Although the method has not yet been fully tried out, the spraying of the trunks of trees and debris around the base may prove a useful way of dealing with the gypsy moth in residential areas. Likewise the cankerworm, which so often ruins the shade trees in cities, can be controlled by power sprayers using a variety of insecticides. DDT is proving equal or superior to the best of them.

For use on a really large scale in the forests, the spraying of DDT from the air is a method that has caught the imagination of the scientist and of the general public as holding out some hope of coping with the great outbreaks of the spruce budworm, the various sawffies, or the hemlock looper, which from time to time sweep through vast tracts of forest and defoliate or kill the trees over thousands of acres. Highly concentrated solutions of DDT, when discharged from airplanes as a fine rain, undoubtedly kill these insects.

Wider implications of DDT

So far I have been giving just a general and somewhat superficial account of what DDT appears capable of doing. What remains to be seen is whether the method is feasible economically and desirable biologically.

The organic thiocyanates, which were coming to the fore in the pre-war years, were a big advance in the search for synthetic chemicals which would rival as contact insecticides the natural vegetable products. DDT is another big step forward in that search. But it is quite certain that it is not the last word. DDT has unquestionably provided a tremendous stimulus to experimentation; even the insecticide manufacturers have been amazed at the size of the potential market for a really good insecticide. There are doubtless other chemicals on the way. One has already appeared — the British material gammexane, or 666, which rivals DDT and for certain purposes seems even to surpass it.

What the chemists have been seeking so far is a universal insecticide: a material that will kill every insect with which it comes in contact, and at the same time a material that does not injure the plants upon which it is spread or poison human beings when they eat t hose plants or when they come into contact with it in the course of manufacture or use. Has this aim been achieved in the discovery of the properties of DDT and is it an objective that is wholly desirable? Those are the questions which biologists are asking themselves today.

DDT seems to have the effect of poisoning some part of the nervous system. Insects which have walked on films of DDT soon begin to stagger in a drunken manner. Their legs twitch and they cannot coördinate their movements. Sometimes they remain for several days in this “drunken” state before dying. The poison appears to enter in minute traces through the feet and elsewhere on the surface of the insect.

In mammals such as rabbits, rats, mice, or guinea pigs, DDT has a somewhat similar effect. There are muscular tremors and spasms, incoördination, and finally paralysis and death. But for reasons that are not understood, a much larger dose of the chemical in proportion to their weight is required by mammals than by insects. Poisoning in the animals can be produced most easily if the DDT is dissolved in oil and then either eaten or rubbed into the skin. It seems harmless when eaten in the dry form and is certainly harmless when applied as powder to the skin.

Large amounts of DDT have been handled and experimented with during the past three years; yet only one case of DDT poisoning in man has been described, and this was in a laboratory worker who had deliberately rubbed into his skin very strong solutions of DDT in a way that could never happen in practice. The symptoms produced were like those in other animals, but when contact with DDT was stopped he gradually recovered completely.

It is therefore generally felt that, used with ordinary care, DDT is a safe insecticide so far as man and domestic or farm animals are concerned.

Tadpoles and fish are more susceptible to DDT. They are not affected by the very small quantities that are applied to the surface of the water to kill mosquito larvae. But when strong solutions, such as are used for spraying the forests from the air, fall on water, not only are all the aquatic insects — May flies, dragonflies, and others — killed, but trout and other fish are destroyed and the streams rendered practically devoid of animal life.

Plants, on the of her hand, are not much affected by DDT if it is suitably applied. Where they have been injured, the harm has generally been traced to the action of other chemicals with which the DDT has been mixed.

Harmful and beneficial insects

Man is the measure of all things. We judge the world by its impact upon ourselves. Insects which convey diseases to domestic animals or to man, or insects which feed upon the crops we wish to eat, are described as harmful. And these insects attract so much attention to themselves that we come to regard all insects as enemies of man. But among the teeming millions of insects there is incessant strife. Under favorable conditions insects can multiply with incredible rapidity, and in the course of several months may increase their numbers a millionfold or more. Their numbers are held in check by fluctuations in the climate, sudden cold or heat, drought or flooding; by the limitation of the available food supply; and by the biological pressure of competitors or of insects and other animals which prey upon them.

Insects, I have said, can multiply at a prodigious rate. But as a rule their numbers do not change very much. Although a female may lay perhaps three hundred eggs, the one generation may be only a little larger or a little smaller than the next. Of the three hundred offspring, two hundred and ninety-eight must have perished. In this colossal mortality other insects play a large part. There are “predaceous” insects which attack and devour their fellows; and there are “parasitic” insects which lay their eggs within the bodies of their victims so that their young may slowly devour them.

This process is commonly known as “biological control,” and many instances could be quoted in which insect parasites have berm introduced to deal with an insect pest and have done so to such good purpose that failing industries have been restored to prosperity. It is not surprising that the method has caught the imagination of the world. But aside from these spectacular achievements, our insect friends the insect predators and parasites are everywhere quietly working all the time, and any decrease of their activities is followed by an outbreak of some insect pest.

Insect pests in farm crops are on the whole much less severe in Britain than in America; and the chief reason for this lies in the better opportunity afforded to the insect friends and the birds in preying upon the pests. In Britain the fields are small, the crops are changed in rotation each year, there are hedgerows around the fields providing shelter for the birds and insects which serve in the defense. But in America the large areas under the same crops, grown perhaps for several years on the same land, provide ideal opport unities for the pests to build up vast populations which decimate the crops.

DDT and the balance of nature

Now let us look again at the action of DDT, bearing in mind that alongside the pests at which we are aiming is a complex assemblage of interacting organisms and that perhaps DDT is like a blunderbuss discharging shot in a manner so haphazard that friend and foe alike are killed.

It may be said at once that the various insects in the house are not limited in numbers by parasites and predators sharing their domestic haunts. Even though spiders kill a certain number of insects indoors, few housewives regard spiders as friendly inmates of the house. John Keats, with the sympathy of the poet, may write in kindly terms of the crickets as “little inmates” of the house, “full of mirth,” and call them to witness that the “ poetry of earth is never dead”; and the English naturalist of the eighteenth century, Gilbert White, writing in 1790, may refer to the detestable roach, which had then only just arrived in Britain, with something more than tolerance, and may express the hope that this interesting insect which had appeared in his friend’s house might soon visit his own. But today all insects in the house are regarded as unwanted aliens. If we can eradicate them all, so much the better. In so doing there is no risk of upsetting the balance of nature.

DDT is popular, when used in houses, precisely for the reason that it destroys insects of all kinds. In America I was told of the colored housewife who complained bitterly, after her house had been treated with DDT to kill malarial mosquitoes, that she was forever having to sweep up “dem damn dead flies”! But some people are never satisfied.

The point is that conditions in the house are such that there is no risk of killing friends along with the enemies. DDT used in this way seems to confer nothing but benefits.

The same is true when we consider the insects which live on the bodies of man and animals. There are no friendly insects which haunt our persons. When we can kill such insects without risk of poisoning their hosts (and we can with DDT) the result is wholly good.

When we turn to the control of mosquitoes in the open we are on less safe ground. The very small quantities of insecticide which need to be applied on water to kill mosquito larvae do not normally kill fish; but they kill a variable proportion of the other aquatic insects. This result may not matter very much. But it may have far-reaching effects which it is impossible at present to predict. The broadcasting of sprays from airplanes in order to kill the flying mosquito is still more prone to upset the balance of nature. These sprays settle on the vegetation and kill vast numbers of insects of all sorts. Without careful study it is impossible to guess what the ultimate results of this process may be.

Still more drastic are the effects of the highly concentrated sprays of DDT that are being used against the forest pests. These sprays kill other kinds of insects, in addition to the pests against which they are directed. They settle into the streams and kill the aquatic insects. Some fish, such as the speckled trout, are reported to have been killed when they fed on poisoned insects. Crayfish and tadpoles are killed either by contact with the poison or by eating contaminated food. These again are important sources of food for fish, which must suffer when they are destroyed.

There seems little doubt that the general insect population in the forests is greatly reduced and that the natural waters may be nearly sterilized and lifeless. It has even been suggested that nesting birds may suffer from the shortage of insects and be unable properly to feed their young. Only careful experiments can prove whether birds will suffer or not.

When DDT is applied to a small area of woodland it will not matter very much if almost complete destruction of insect life results: the area will soon be colonized again from the surrounding woods. But where really large areas of forest, extending over hundreds of acres, are treated, the effects may be more serious. That problem also must be resolved by experiment, and the results of the trials carried out this year in the United States and in Canada will be awaited with interest.

We already know a little more about the results of applying DDT to farm crops and orchards. We know that DDT sprayed on peach trees with the object of killing the caterpillars of the Oriental fruit moth is even more effective in killing the parasite that is being liberated to control this pest than it is in killing the pest itself. We know that DDT is not much good for killing the woolly aphis. But it is a wonderful insecticide against the Aphelinus parasite that normally keeps the numbers of woolly aphis down. Already it has been observed that orchards containing woolly aphis, when they have been sprayed with DDT, have developed outbreaks of this pest such as had never been seen before.

The same thing has happened with the fruit tree red spider. In orchards where this mite has normally been kept within reasonable bounds by the enemies that prey upon it, trees have been literally scarlet, with the red spiders after being sprayed with DDT.

DDT and honey bees

There is one insect that is generally regarded as the friend of man; and that is the honey bee. Honey is itself a useful article of commerce, but the making of honey is not the only service we receive from the bee. Unless flowers are pollinated they will not set seed. Bees are among the most valuable pollinators of flowers, and at the present time most beemen consider that the chief value of the bee to man lies in its activities in transferring pollen from one flower to another. In orchards or in the clover fields, the crop may depend upon the bee. In certain areas the yield of clover seed drops from year to year, and one reason is thought to be a growing shortage of bees.

Now there are pests of apple flowers, such as the apple-blossom weevil of Europe, and pests of clover or alfalfa, such as the lygus bugs, which are well controlled by DDT. But DDT kills honey bees. If bees are confined in cages with plants that have been sprayed with DDT they are soon killed. On the other hand, there is some evidence that the effect on bees visiting sprayed blossoms in the field may not be so serious as was feared. Entomologists are cautious as to what they say about the matter at the moment. But beekeepers are a vociferous race. Like the bees they care for, their more lovable qualities may become obscured when they are roused — and they do not take kindly to DDT.

This is no new problem. Beekeepers have suffered in past years from the use of arsenic in orchards during blossom time or from derris. Only careful and unbiased work can determine how and when DDT can be used so as not to impede the invaluable activities of the honey bee.

Chemists and insect pests

It is obvious enough that DDT is a two-edged sword. We can see how seriously it may upset the balance locally between insect enemies and friends.

Now this too is no new thing. The manufacture of chemical insecticides and fungicides for the use of the farmer has increased enormously during the past thirty years; yet more and more of these chemicals are called for. Time was when a couple of arsenical washes were adequate to control the codling moth on apples. Now, five, six, or even seven washes are needed in some orchards. As time goes on, the codling grub, for reasons that are not wholly understood, becomes more resistant to arsenic. Early in this century the California red scale on citrus was readily controlled by fumigation of the tree with hydrogen cyanide. But in recent years a “resistant race” has developed and it becomes increasingly difficult to keep this scale down with cyanide gas. Three or four years ago it was discovered that tartar emetic was the perfect control for the citrus thrips in California. But within two years a race of thrips resistant to tartar emetic had replaced the normal race and tartar emetic ceased to work.

It is the same story with the fungicides used in the orchards. One of the best chemicals for the control of apple scab is sulphur. Sulphur is good for killing the red spider. But it also kills other mites; and in some localities there are many sorts of mites which prey on the red spider, and in order to replace the activities of these mites, more and more chemicals have to be sprayed upon the trees.

All this is grand from the chemist’s point of view. The more chemicals are applied, the more they are needed, and the demand for chemical insecticides goes up and up. It is not so good from the grower’s point of view, and it is certainly bad from the standpoint of the biologist, who likes to feel that he is controlling nature in an intelligent manner, and not merely throwing grit into the works.

Future of chemical insecticides

Chemicals which upset the balance of nature have been known before. DDT is merely the latest and one of the most violent. It can bring about within a single year a disturbance that it would take other chemicals a good many years to produce. It has thus focused attention on the problem and has provided a great stimulus to entomologists to reflect upon what is happening.

One conclusion they have come to is that we know far too little about the interaction of pests with their physical environment and with the other insects around them. We need to know far more about their ecology— that is, about their natural history studied scientifically. When the ecology of an insect pest is fully known, it is often possible to modify the conditions in such a way that its world no longer suits it. The crop may be planted too early for it; the order of crops in the rotation may be changed; varieties of the plant that are unfavorable for the insect may be grown, and so forth.

But when all these so-called cultural or naturalistic methods of control have been developed, there remains a large residue of pests for which insecticides must be used. Here again more ecology is needed. We want to learn how to apply the insecticide at such a time or in such a way as to touch the pest and not its enemies. We want to choose insecticides which discriminate between friend and foe.

This is probably where the future of insecticides lies — in the development of materials with a selective action. It may well be that in the long run an insecticide which kills 50 per cent of the pest insect and none of its predators or parasites may be far more valuable than one which kills 95 per cent but at the same time eliminates its natural enemies. It has been found in England that in fumigating truck crops with nicotine it is possible to adjust the dose of fumigant to such a level that the aphids or green flies are killed but the insect enemies are spared. That is an example of the type of selective insecticide we need. The development of such materials will provide unlimited scope for the ingenuity of biologists and chemists.