20 CIVIC BIOLOGY 



screen to keep flies and mosquitoes out of houses amounts to 

 at least §12,500,000 annually. Nor does the above estimate 

 take account of the depreciation of property, loss of time, and 

 cost of diseases like malaria or yellow fever, known to be car- 

 ried by certain mosquitoes, infantile paralysis, carried by the 

 stable fly, and typhoid, cholera infantum, dysentery, and many 

 other filth infections which are transmitted by common house 

 flies. With tliese additions it is safe to say that insects annu- 

 ally levy a tax of not less than $1,500,000,000 on the people 

 of this country, and this in addition to the annoyance and 

 sufferino- which thev cause to human and animal life. 



Work for control of insects. Shice insects possess such power 

 of rapid incrcLise, and since this increase is limited mainly by 

 food supply, natural enemies, and artificial means of destruc- 

 tion, any relaxation of natural or artificial checks tends to 

 permit insects to increase up to the limit of food supply. AVith 

 these checks entirely removed, insects would take practically 

 the entire agricultural product in an incrediljly short time. 



The relative efliciencv of natural and artificial checks is 

 well exemplified by a number of cases in which an insect has 

 been accidentally introduced from some other continent with- 

 out bringmg the natural enemies of the species. The cottony 

 cushion scale of Australia swept over the orange groves of 

 California Hke a consunnno- fire until its natural enemv, the 

 Vedalia lady ])eetle, was imported. The gyp^y and brown- 

 tailed moths in Massachusetts show even more clearly how 

 ineffectual human effort is when pitted aganist such forces of 

 nature. After expenditure of several millions of dollars and 

 twenty years of futile effort we are l)rought to realize that our 

 best hope of permanent success lies in the importation of 

 natural insect enemies. The San Jose scale, accidentally in- 

 troduced from Chma, is now rapidly exterminating fruit 

 orchards and ornamental trees over almost the entire country. 

 In 1901, Dr. Marlatt succeeded in importing a Chinese lady 



