10 



THE RELATION OF SPARKOWS TO AGRICULTURE. 



Fig. 



5. —Ichneumon fly (after Howard; 

 Division of Entomology). 



loaned, by 



and although some (of the genus Lasius), and perhaps others, pos- 

 sess certain injurious traits, while a few may have traits that are 

 beneficial, 3^et the effects in au}^ event are of minor importance; 

 so that ants as a whole may safely be classed as neutral. Spiders, 

 which for purposes of convenience are here classed with insects, are 



carnivorous, but their i^rey 

 seems to include about as 

 many beneficial insects as 

 pests. The damage done 'by 

 weevils, grasshoppers, and 

 smooth caterpillars is noto- 

 rious. Cutworms and army 

 worms often do an immense 

 amount of harm, and grass- 

 hoppers frequentl}^ occur in 

 such voracious hosts that 

 they sweep away ever}^ ves- 

 tige of green vegetation be- 

 fore them. On the other 

 hand, carnivorous ground- 

 beetles (Carabid^e, see fig. 

 G) kill multitudes of insect 

 pests, and certain parasitic wasp-like hymenopterous insects of the 

 families Braconid?e, Chalcididse, and Ichneumonidfe destro}^ great 

 numbers of caterpillars. One of these parasitic insects will deposit 

 in the back of a caterpillar from 20 to 2,000 eggs, which soon hatch 

 into grub-like larvae that feed upon the fatty tissues and exhaust 

 the caterpillar so that it is not able to transform 

 into a perfect insect. 



The fact that birds do not discriminate between 

 insects that aid the farmer, such as parasitic Hy- 

 menoptera and carnivorous ground-beetles, and 

 those that are harmful to his interests, led the 

 entomologist, Benjamin D. Walsh, to denj^ their 

 usefulness as insect destroyers. He asserts that 

 the good done b}^ the consumption of insect pests 

 is more than counterbalanced by the destruction 

 of useful species. His argument is that there are 

 thirty times as many individual insect pests as 

 there are insect enemies which subsist upon them, 

 and that therefore no insectivorous bird can be considered a ' public 

 benefactor ' until it can be shown to destroy at least thirtj^ times as 

 many injurious as beneficial insects. ^ Applied to the destruction by 

 birds of highly effective parasites of important pests which annually 

 or at intervals cause a large loss to staple crops, Walsh's statement 



Fig. 6.— Ground-beetle 

 (after Riley; loaned 

 by Division of Ento- 

 mology). 



Practical Entomologist, Vol. II, No. 4, p. 47, 1867. 



