Have the insects got us licked?
There are two million kinds of them. They kill forests, eat crops and spread plague. We're fighting back with DDT and newer poisons— we've even enlisted radiation—but they're getting wilier and tougher
N. J. BERRILL
Early in 1939 millions of farmers, foresters, gardeners and ordinary people who just like sitting out on the front porch read what seemed to them to he the most important war communiqué in history.
The spectacular insect-killing properties of a wonderful chemical called DDT had just been discovered and man was about to win an unconditional surrender from his oldest and most implacable foe. To vast numbers of people to whom no war between man and man had brought direct suffering, this was more exciting news than Alamein or Normandy.
In the nineteen years since, DDT has indeed brought some victories for its human sponsors. So have other post-w'ar weapons and means of using them. But already some of these victories have proved to be only local and many others temporary. A few have actually rebounded and may in the end do the human race more harm than good. Scientists are still searching for a foolproof weapon. The newest, as yet used only in the laboratory, involves stealing the insects’ growth hormones and using them to upset the natural life cycle. But despite research like this, if the war ever ends the insects will almost certainly be the winners.
This is how we stand today.
If a visitor from Mars dropped in to have a look at us he would report that the land is covered with vegetation and mainly infested with human beings and insects.
According to the fossil record giant cockroaches were chewing up the old fern forests and giant dragonflies swooped around with a gleam in their eyes for many million years before the
first animal with a backbone crawled out of the water. Insects had a long head-start on our most remote land-living ancestors, and in a sense they have been waiting for us ever since. When man first appeared on the scene he was, from the viewpoint of insects, just a wonderful expanse of thin naked skin with tasty blood beneath. If he died from diseases carried by the insects, that was his bad luck.
Only since we settled down to farm and mine the land a few thousand years ago and began to look on the earth, with all its animals, minerals and vegetables, as exclusively our own, have we started to fight back. Our wits have been matched against the insect’s capacity to multiply and to change its nature.
The weapons our side is using in this era of chemical warfare sometimes backfire or go off in the wrong direction. For example, spruce budworm. which is the larva of a sawfly, threatens the existence of spruce and pine forests in both northeastern and northwestern America. In New Brunswick and southern Ouebec several million dollars are spent each year by lumber companies for aerial spraying of the forests with DDT. Each season the trees are for the most part saved for another year, and so long as winters continue to be so much milder and more congenial to the budworm than they used to be. such treatment may be necessary. The DDT kills the budworm grubs, and the chemical is washed by the rains into the soil and down to the streams and rivers. But the aquatic insects that form much of the food of fishes, and the fish themselves, are both susceptible to DDT. Since the New Brunswick
rivers are noted for their
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“Ten thousand kinds of insects in Canada and the U. S. should be listed as public enemies”
salmon there is, to put it mildly, a conflict of interests.
Insects, of course, are merely doing the best they can to survive and reproduce their kind. Some of them are useful to us. Obviously we cannot do without insect pollinators, particularly bees, which are responsible for the success of some fifty seed and fruit crops. Then there are hosts of insects that serve as food for the many fish and birds whose existence we appreciate. And there are insects that prey on, or parasitize, other insects and so help hold down the total number.
On the other hand about ten thousand different kinds of insects have been listed in Canada and the United States as being sufficiently harmful to our interests to be called public enemies, while many more from other parts of the world are simply awaiting transportation. With every ship and plane there is danger of a new outlaw insect landing.
One of the most alarming newcomers arrived in California two years ago. A melon fly from Hawaii, it was apparently a lonely newcomer that was seen, recognized, and caught. Once established on this continent, however, the fly could have destroyed nearly all our melons, squash, cucumbers, tomatoes and many other vegetables, as it did in Hawaii not so long ago.
An example of what can happen when a dangerous insect gets a foothold was the case of the malaria-carrying African mosquito, which appeared in Brazil about twenty years ago, coinciding with the first regular trans-Atlantic airplane flight in that region. In 1938 more than twenty thousand persons died in Brazil as a result, and only by a tremendous co-operative effort by the Rockefeller Foundation and the Brazilian government was this mosquito eradicated during the following two years. Otherwise much of South America might have become almost uninhabitable by human beings, just as large parts of Africa have been abandoned because of the tse-tse fly which carries the fatal sleeping sickness.
Once an insect has become fully established, it is nearly impossible to eliminate. If all the descendants of one pair of house flies survived and reproduced for one season from April to August, they would amount to nearly two hundred million, million, million. In nature, insufficient food and the presence of natural enemies cut the number down to a small fraction, but the potentiality is there, and in some insects such as the cabbage aphid it may even be a million times greater. In numbers of species—probably about two million—and in actual numbers, insects undoubtedly dominate the earth.
From our somewhat self-centred viewpoint they are hugely destructive. Between 1939 and 1949 the annual loss in the United States from the European corn borer rose from around $4 million to $350 million, while the total damage to crops and timber by insects on this continent now runs into several billion dollars annually. It is so serious that the manufacture of insecticides has itself become a major industry.
The course of human history, in fact,
has been changed for better or for worse by insects. The fall of the Roman Empire has been attributed at least in part to the undermining of health by malaria-carrying mosquitoes. Bubonic plague spread by fleas, carried by rats, killed more than half the population of England during the fourteenth century and about one quarter of the population of Europe.
Not that the threat of plague is a thing of the past: some time ago a rat with infected fleas escaped from a ship in San Francisco and managed to pass the disease on to the fleas of the western ground squirrels. Now ground squirrels throughout the west form a permanent reserve for the plague germ.
The Canadian north might have been exploited much sooner if mosquitoes and black flies had not been so abundant. Whether we will ever enjoy the north without them is doubtful. More likely we’ll go on putting up with them, just as people in the southeastern U. S. will probably have to suffer perpetual cohabitation with the vicious little red fire ant now spreading across the country like a wave, from Mobile, Ala., where it was accidentally introduced from South America about thirty years ago.
What can we do to control the attacks and devastations of such a multitudinous and resourceful enemy, particularly when some of our indispensable partners are mixed in their ranks?
The locust menace
There is no simple answer; there arc just too many insects. Even certain insects themselves seem to recognize overpopulation problems and periodically get rid of their surplus by migration. The large flocks of butterflies sometimes seen flying far out to sea off tropical coasts are generally expendable males on a voyage of no return.
Locust migrations, too, are brought on by a surplus-population problem. The most spectacular migrations of locusts on fhis continent were seen late in the last century when swarms of Rocky Mountain grasshoppers stripped much of Nebraska as bare as if the ground had been burned over. The invading swarms averaged half a mile in height, were one hundred miles wide and three hundred miles long. In one particular migration more than one hundred billion locusts were on the move. The threat persists and, on a lesser scale, has often been carried out.
Every device human ingenuity can think up has been brought to bear against outlaw insects, from direct attack to the most devious trickery. If there has been a turning point in our favor, however, it has come with the use of the airplane as a means of spraying or dusting large areas. The method actually has been used for a long time, particularly in efforts to control the boll weevil in the cotton fields of the U. S., but it is only since the airmindedness resulting from World War II that aerial control has really developed. One good-sized airplane can cover as much area with a single load of insecticide as a truck-borne spray rig would in four
The more or less successful attacks we are now making by air employ new chemical insecticides and aerosol, a nonpoisonous liquid gas, as a means of dispersing them. DDT is the best known and generally the most effective of the newer insecticides, although many others are in use and new ones are being made and tried out continually.
But there is a problem, for if an insecticide washes into the streams trouble arises for the fish, whereas if it remains in the soil, how much harm will be done to important soil organisms or to crops? DDT itself appears to be relatively harmless so far as plants are concerned, but arsenic residues from orchard and cotton spraying have spoiled large areas for growing crops of any kind for many years.
Other effects of insecticides are much more unexpected. Take the case of the house fly. When the war ended, the public purchased millions of pounds of DDT for controlling flies, with spectacular results. There was talk of exterminating flies altogether. Yet after only two years, at the end of 1947, reports came in that flies were no longer being killed by DDT, from places as far apart as California, Italy and Sweden. Apparently DDT killed only about ninety percent of the flies it reached, leaving the resistant ten percent to repopulate the fly kingdom. In the course of a number of generations, races of flies almost completely resistant to DDT evolved, so that we are more or less back where we started. Many species of mosquito in various parts of the world are now evolving strains resistant to DDT, although with less effectiveness than the flies.
Nor is DDT the only chemical to which insects are developing a built-in resistance. The red scale insect, a major pest on Californian citrus trees, has been kept under control for many years by fumigation with deadly hydrocyanic acid. But the insects are much harder to kill than they used to be, and other methods of control are being sought.
There is another unlooked-for effect of using DDT. The chemical is very effective against most leaf-feeding insects that attack apples and pears, such as tent caterpillars, fall web-worms, Japanese beetles, and leaf hoppers. But it is much less effective against many others. So that using DDT, or any other insecticide for that matter, upsets a balance of nature and may do much more than kill off a particular pest. Many orchard pests, for instance, are held in check not so much by chemicals as by parasitic or preying insects, predaceous mites and spiders. Most of these natural enemies, however, are highly susceptible to DDT. They are killed off at a much higher rate than some of the injurious pests they prey upon—spraying in these cases may make the pest worse than ever.
Bees are about as susceptible to DDT, arsenic or other insecticides as most insects. Yet how can you protect the bees when you poison the plants or trees they visit?
So we are in a quandary. The extensive use of insecticides on cultivated crops has driven beekeeping out of many areas. In cotton-growing areas in particular the beekeeping industry has been wiped out. Yet for other crops the bees are essential for pollination. The curious situation is now arising where orchardists have to pay rental fees to induce beekeepers into their fields during blossom time.
Chemical warfare is not the only kind now being waged against insects. Biological warfare is also well under way. It is a striking fact that many of the worst crop pests are not native insects of this continent but are importations from other
parts of the world, and that in the regions where they come from they usually do much less harm. In their homeland there is a natural balance between the pest insect and one or more parasitic insects. So that if we can discover the natural parasites of an obnoxious insect we have a chance of employing them as a biological weapon.
A good instance was the first case in which this system was used. The cottonycushion scale insect appeared in California in 1872 and attacked the citrus trees, threatening to destroy the industry. It was known that the scale originated in Australia. In Australia, however, it did comparatively little damage because a parasite seemed to hold it in check. A California scientist managed to get to Australia in 1888. He found and shipped back not only the tiny parasitic tly he was looking for but also a previously unknown beetle that feeds greedily on the eggs and larvae of the scale insect. Since then this particular scale has remained under complete control — except that DDT recently introduced to control another scale insect also kills the beetle that controls the cottony-cushion scale.
Methods have now been developed for mass production of imported parasites and predators, and the search goes on for parasites that will hold down a particular pest. Perhaps this is the most effective kind of control in the long run, but it is the most difficult to work out.
The meanest trick we are playing on insects involves both biology and radia-
tion. Screw-worms are the grubs of certain flies that lay their eggs in cuts and sores in the hide of livestock, if not controlled they can wipe out entire herds of cattle, sheep, hogs and goats. There are various forms of treatment but real control lies only in more or less exterminating the flies, not in treating livestock injuries.
The new procedure is to raise the maggots in the laboratory in enormous numbers and to expose them to strong radiation just before they are ready to become flying insects. This makes them sterile. Then they are distributed by airplane over the area to be controlled. The sterile males mate freely with wild fertile females and cause them in turn to lay infertile eggs. Conducted on a large scale and repeated a number of times, the sterilized males so outnumber the wild ones that the fly population dwindles and disappears. The method has already succeeded in eliminating the screw-worm from restricted places such as the Caribbean island of Curaçao where reinfestation from outside is difficult, and it is now being applied in the southern United States.
And so the war continues. One side or the other momentarily gets the upper hand. The insects are close to two million kinds to our one, but we have intelligence to pit against their instincts. Still, on the evidence, we are more likely to exterminate ourselves. than to do away with the multitude of six-legged creatures that plague us. ★