As early as the 12th century, Chinese and Javanese gardeners piled 
brush between their citrus trees to form bridges for predatory ants to 
attack caterpillars in the trees. A century and a half ago Erasmus Darwin 
recommended using syrphid flies against aphids, and English entomologists 
found the lady beetle ideal for this purpose. Three-quarters of a century 
ago a tiny wasp, Apanteles glomeratus, was brought into the United States 
to combat the imported cabbageworm. 
At about that time our entomologists began searching foreign lands, 
where many of our most troubiesome pests originated, for effective biotic 
agents. American entomologist Alfred Koebele's introduction of the 
vedalia, a lady beetle from Australia, into Californiain 1888 quickly brought 
the cottony-cushion scale under control. 
Since then many parts of the world have been searched for parasites 
and predators, and a wide range of insects has been screened for parasitic 
and predatory species. Many insect families, genera, and species have been 
systematically tested against certain pests. 
Over 650 species of insect parasites and predators have been collected 
and brought into the United States, and at least 100 of them have become 
established. Although the control of the cottony-cushion scale by a lady 
beetle has been perhaps the most sensational, parasites and predators 
imported into this country are giving outstanding controlof 20 of our serious 
insect pests. 
The search for biological control agents is continuing. 
Growing resistant varieties is an ideal way to protect crops from 
damage by insects. For the past 50 years entomologists and agronomists 
have been cooperating in a search for insect resistance in various crops 
and in the development of varieties that can withstand insect attack. Asa 
result, insect resistance has been found in more than 100 species of plants. 
The practical importance of this type of insect control is demonstrated in 
strains of corn resistant to the European corn borer, corn earworm, and 
rice weevil; in varieties of alfalfa resistant to the pea aphid, leafhoppers, 
and spotted alfalfa aphid; and in varieties of wheat resistant to the hessian 
fly and wheat stem sawfly. 
Some recent scientific innovations offer additional ways to achieve 
insect control. These are chemicals which can be used to lure certain 
species to their doom and sexual-sterilization techniques to destroy the 
reproductive potential of a species. 
CHEMICALS AND INSECTS 
Insecticides are tremendously important in improving the efficiency of 
agricultural production and in protecting the health and comfort of man, but 
their use has created special problems. One of the problems is that some 
70 species of insects in the United States have displayed resistance to cer- 
tain of the chemicals used against them. 
Another problem is that some chemicals may destroy beneficial insects, 
birds, and other wildlife, and if applied directly or carried into lakes, 
ponds, and streams in sufficient amounts certain insecticides will kill fish. 
The use of chemicals is responsible for the buildup of certain insect pests 
through the destruction of their parasites and predators. 
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