406 



and marketing methods, and the use of aircraft for spraying and seed- 

 ing, became enormously productive of food in relation to the man- 

 hours of labor required. This scientific and technological agriculture 

 provided the basis for a great increase in urban population, dependent 

 for food on a small and dwindling number of farmers. 



Essentiality of pesticides in single-crop fanning 



Cost/ejffectiveness considerations, however, reduced the number of 

 different crops, and the number of different varieties within each crop. 

 Single-crop acreages replaced the natural variety of wild growth in 

 forest or prairie. Great acreages of land were occupied by unrelieved 

 stretches of identical crops, all planted, ripening, and harvested over 

 the same annual growing schedule. Simultaneously, this change re- 

 duced the protective shelter available for insect-eating birds, frogs, 

 toads, and predator insects; and increased the food supply available 

 for those insects that were specially attracted to it — wheat, corn, cot- 

 ton, potatoes, tomatoes, and so on. Given an abundance of food, and 

 an absence of natural enemies, the insects (or other pests, including 

 rodents, injurious fungi, and even "pest" birds — like blackbirds and 

 crows) multiplied prodigiously. 



To overcome the menace of freely multiplying pests, farmers learned 

 to use various poisonous materials. A new industry sprang up, to pro- 

 duce and market such commercial poisons as Bordeaux mixture (h;^- 

 drated lime and copper sulfate) , lead arsenate, Paris green (an arseni- 

 cal copper acetate) , and other poisonous salts. The new science of syn- 

 thetic orsfanic chemistry ^ began to create new families of pesticides 

 with a wide array of special properties. During and immediately after 

 World War II, these new pesticides came into enormous and world- 

 wide application. For example, in 1946 the U.S. Army Air Corps 

 sprayed 500,000 pounds of DDT solution (dichlorodiphenyltrichloro- 

 ethane) in a single operation to curb an insect-borne epidemic disease 

 in Egypt. Millions of pounds were applied in Europe. 



Generous application of these effective new poisons served to control 

 populations of pests. They may also have been safer than the extremely 

 dangerous arsenical s they replaced. However, as time went on, reports 

 began to appear of insect species that had developed immunity to one 

 or another of the poisons, so that increased dosages — or new combina- 

 tions of toxic agents— were needed for effectiveness. The chemical in- 

 dustry was kept active, searching for additional organic compounds 

 with higher toxicities or special properties in this war against the insect 

 world, and the legion of other pests in the environment. 



Alternatives to chemical control of pests 



Although the primary means of controlling pests was by the appli- 

 cation of chemicals designed specifically to destroy them, a number of 

 other techniques were selectively applied, with success, in pest control. 

 Three general concepts evolved. These were: (1) devising a physical 

 environment or environmental change effectively hostile to the targest 

 pest at a crucial point in its development; (2) encouraging the multi- 

 plication of otherwise harmless natural enemies of the target pest; 

 (8) interference with the reproduction processes of the target pest. 

 Examples of these three methods of control are: (1) lowering and 



"See ch. 14 (thalidomide), pp. 359-360. 



