4i8 



ELEMENTARY BIOLOGY 



agents and commissions, designed to counteract the injuries 

 done by insects to valuable plants. It is estimated that the 

 damage done to our crops by the activities of insects amounts 

 to from six hundred million to seven hundred million dollars 

 every year. To this must be added the injury to forests and 

 forest products, and the injury to animals. 



The locusts and many other species of insects will eat almost 

 every kind of plant ; but many insects confine their attentions 



Fig. 219. Cotton-ho\l weevil (AniAonom7(s £i-randh). (Greatly enlarged) 



This animal feeds only upon the cotton plant and could probably be completely exter- 

 minated if the planting of cotton were suspended for a year or two. This was the advice 

 of the government experts to the growers of Texas about twenty years ago ; but it was 

 unheeded, with the result that millions of dollars' worth of cotton have been destroyed 

 each year since. Rotation of crops was finally forced upon many of the farmers, with 

 beneficial results, a, larva ; l>, larva in mature boll ; c, pupa ; d, pupa in boll ; e, adult 



to one or a limited number of food plants. The damage done 

 by such insects is accordingly confined to special kinds of crops. 



The Colorado potato beetle is perhaps the best known of the 

 special-crop insects {Fig. 218). It is kept in check by the use 

 of poisonous sprays or powders applied to the growing plants. 



The cotton-boll zveevil has spread over a large part of our 

 cotton area in recent years and has ruined many a crop 

 (Fig. 219), This animal is very susceptible to extremes of 

 temperature and has many natural enemies. Planting early- 

 ripening varieties in wide rows, and then burning the stalks 

 and rubbish after the harvest, will do much to keep the pest 

 under control. 



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