CHAPTER V. INSECTS ATTACKING CROPS 
According to the Yeerbook of Agriculture for 1952, approximately 
10,000 species of insects in the United States are important enough 
to he called public enemies. About four-fifths of them are injuri- 
ous to crops. Some cause heavy losses each year, but many are 
limited in distribution and cause only occasional or minor damage. 
Estimates of the lesses caused by insects in this country have 
been assembled by different workers at various times, but there is 
no adequate basis available for estimeting the total loss by all 
destructive species. Such estimates as have been made concern only 
a few species, chiefly those of major economic importance, Persons 
attempting to make estimates soon find that they are faced with 
many variables and complicating factors which make the task a 
formidable one. For instance, the crop damage caused by one species 
differs from year to year and frem one area to another. Furthermore, 
most. insects of economic importance tend to appear in cycles of 
abundance. They may cause relatively little or fairly uniform 
damage for several years, followed by an upsurge of damage when 
they reach outbreak prorporticns. 
Insects cause losses in many ways. They reduce the yield of 
crops, lower the quality and ccntaminate the marketed product, and 
increase the ccst of producing, processing, or marketing it. They 
also transmit plant and animal diseases from infected to healthy 
ereps and livestock. This chapter covers estimates on only the 
direct losses to crops. Estimates on losses caused by insects 
directly attacking animals are in Chapter X; those relating to 
diseases transmitted by insects are included in Chapters IV (Plant 
Diseases) and X (Livestock and Poultry); grasshopper damage to 
rangeland is discussed in Chapter VI; storage insects are covered 
in Chapter VII; forest insects are discussed in Chapter IX; and 
estimates of the costs of insect control are given in Chapter XII. 
Today's insect losses to many crops differ greatly from those of a 
few vears ago. Some pests, particularly those not native to this 
country, have spread to new areas and have thus extended their damage. 
Examples are the European corn borer and the Japanese beetle. Other 
insects have modified their habits, and have hecome pests of economic 
importance on new crops. For example, the green peach aphid did not 
become an important problem on tobacco until about 1916. 
Changing agricultural conditions and practices also affect insect 
losses. Twenty years ago the tomato pinworm was a major pest of 
the tomato crop in southern California. In recent years, owing 
chiefly to chmges in cultural practices which involve destruction 
of crop remnants, this insect has been of minor importance, With 
the development of new insecticides, beginning in 195 when DDT was 
released to the public, the situation with respect to many, insects 
has changed appreciably. The chlorinated hydrocarbon and organic 
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