392 APPLIED BIOLOGY 



self (as by infecting with malaria), are conveniently grouped 

 as injurious insects. The truth is that a very large pro- 

 portion of insect species tend to be injurious, but usually 

 do not attract attention unless they become excessively 

 numerous, or when man develops a special interest in a par- 

 ticular animal or plant. For example, probably more than 

 eight hundred species of insects attack oak trees ; but it is 

 rare that enough appear on any one tree to do any noticeable 

 damage, and as long as there are plenty of oak trees no one 

 cares how many kinds of insects live on them. 



A few statistical estimates will give some idea of the damage 

 which a single species of insect can do. The grasshoppers 

 (Rocky Mountain locusts) destroyed crops to the value of 

 $200,000,000 in Iowa, Missouri, Kansas, and Nebraska in four 

 years, 1874-1877. Special pamphlets of the United States De- 

 partment of Agriculture record the enormous damage done by 

 chinch-bug on cereal plants, by Hessian fly on wheat, by scale- 

 bugs on fruit trees, by gypsy moth on forest and fruit trees, by 

 cotton-boll weevil, and by numerous others which do great 

 but less damage then those mentioned. Famous entomolo- 

 gists have estimated that insects damage farm crops in 

 United States annually to the extent of $300,000,000. In- 

 sect damage to valuable forest trees is on good authority 

 estimated at $100,000,000, yearly. Add to these figures the 

 enormous loss of animals through disease caused directly or 

 indirectly by insects ; the destruction of clothing, foods, and 

 other useful articles ; the value of the working time and ex- 

 pense of treatment of people who are ill through disease 

 caused by insects ( 329, 330) ; and the total annual cost of 

 insect damage in this country is probably more than the 

 combined cost of the army and navy and public-school 

 system. Such general estimates suggest the immensity of 

 the problem of dealing with the injurious insects. 



And yet such statistics must not be taken as a declaration 

 of war against insects indiscriminately, but they simply mean 



