THE INSECTS: HEXAPODA 99 



Economic Importance of Insects. According to the report 

 of the Secretary of Agriculture for 1925, about 30 per cent 

 of our entire population now live on farms. The gross in- 

 come from' farm products in 1925 was over twelve billions 

 of dollars. When it is considered that our crops are attacked 

 not by one but often by many different insects, and that, 

 according to the estimate of one of the state entomologists 

 of New York, there is no crop cultivated which infesting 

 insects do not diminish by at least one tenth, it is plain that 

 the economic relations of insects to agriculture are ex- 

 tremely important. Nearly every order has its injurious 

 forms. Thus the Orthoptera has its locusts ; the Hemiptera, 

 its plant bugs ; the Homoptera, its aphids ; the Coleoptera, 

 its wireworms and beetles ; the Diptera, various flies ; and 

 almost the whole army of the larvae of the Lepidoptera feed 

 on plants. 



Control of Crop Pests. In the preceding chapters frequent 

 mention has been made of insects injurious to native and 

 cultivated vegetation. Regardless of whether the damage 

 is by adult or immature insects it assumes one or the other 

 of two forms, depending upon the type of mouth parts pos- 

 sessed by the attacking insect. Chewing insects devour the 

 tissues of the plants, and sucking insects rob them of their 

 sap, so that the plants wilt and become weakened or die 

 (Fig. 62). In either case the old proverb of "an ounce of pre- 

 vention is worth a pound of cure," holds true. The United 

 States Department of Agriculture, state departments of agri- 

 culture and individual entomologists have long studied the 

 various insect pests of importance to agriculture and have 

 worked out many methods of control and prevention of 

 injury. For destruction of chewing insects, poison sprays 

 and powders are applied to the plants to be protected. The 

 insects die upon eating these poisons. Such treatment is 

 wholly ineffective for sucking insects. They are controlled 



