UNIT X SECTION 4 LARGE-SCALE CHEMICAL WARFARE IS IN COMBATING INSECTS EMPLOYED Introduction. L. 0. Howard, former chief of the United States Bureau of Entomology, has stated that: "If human beings are to continue to exist, they must first gain mastery over insects. Life may develop into a struggle between man and insects. Fig. 317. Rodolia cardinalis (Vedalia beetle). A. Larvae feeding on egg mass on cottony cushion scale. B. Pupa of beetle. C. Adult. D. Infected twig showing eggs, larvae, and adult beetles among scales (cottony cushion). (Courtesy of the U. S. Department of Agriculture, Bureau of Entomology and Plant Quarantine.) . . . Insects are better equipped to occupy the earth than humans, having been on the earth for 50,000,000 years, while the human race is but 500,000 years old." Insects seriously interfere with man's activities in the very fertile tropical regions, and in only a few places has man won from them any significant victories, such as the eradication of mosquitoes in the Panama Canal Zone. In the temperate zones, some important victories have been won in the battle against such insects as the Japanese beetle, the corn borer, and the cotton-boll weevil. Many of these insects are invaders of the United States, brought from foreign lands. Such invaders include the Hessian fly of wheat, the codling moth that puts worms in apples, the gypsy moth, the boll weevil, and the corn borer. Nearly 10,000 of the 80,000 different kinds of insects that inhabit North America are pests, and at least 200 of these species of insects are enemies of mankind. The annual damage done by insects in the 750
CHEMICAL WARFARE USED TO COMBAT INSECTS 751 United States alone amounts to more than $1,500,000,000. The Federal Government appropriated somewhat more than $2,182,532 for the fiscal year 1941 for research work for the Bureau of Entomology. More than $200,000,000 is spent each year in the United States for insecticides and fumigants. Armies of insect allies have been drafted to combat insect enemies. For example, a gnat-sized wasplet has helped a great deal in abating the ravages of the codling moth. Space will not permit the discussion of many other examples where beneficial insects are used to control injurious insects. The immediate onslaughts of enemy insects must usually be met by chemicals of various kinds, the nature of which is determined by the insects being combated. The majority of insects are harmless, and many of them are of definite economic importance. Insects Are Man's Chief Competitors for Foods. In the grim struggle between man and insects, the latter possess the advantage of small size, which enables them to live on small quantities of food and multiply enormously when plenty of food is available. A large oak tree may support as many as one million insects, while forest loam may contain as many as fifty million insects per acre. Insects also possess the advantage of rapid reproduction in large numbers. For example, a single pair of ladybird beetles could multiply to 22,000,000,000,000 beetles in six months if conditions were favorable. The progeny of a single pair of aphids, if they all lived, would be sufficient at the end of one year to fill up the Atlantic Ocean. The warfare against insects is never finished because new legions can replace their fallen brothers and sisters in a short time. Insects are superior to man in that they carry lighter weights and do their work with a smaller expenditure of energy than is possible for man. There are insects which can live under almost any conditions. For example, certain insects live in saturated salt water; others live on deserts or in hot springs. Insects Transmit Many Important Diseases. The role that mosquitoes play in spreading malaria, yellow fever, and elephantiasis is well known. Human lice and rat fleas spread typhus fever, tsetse flies disseminate sleeping sickness, and houseflies carry typhoid fever and cholera germs. A long list of diseases carried by insects could be added to the few given above. Insects are also carriers of many plant diseases.