12 INSECTS INJURIOUS TO VEGETABLES 



should recognize their usefulness, but some are prone to ex- 

 pect too much from them in the subjugation of farm pests, 

 and opinions are so diverse that the grower is sometimes in 

 doubt as to whether the insects which are indicated as his 

 allies are not, in reality, pests. A little study is necessary in 

 many cases to discriminate between noxious and innoxious 

 species and those which are truly and exclusively beneficial. 



Organisms beneficial to agriculture may be variously classi- 

 fied, but fall naturally into four groups. The most important 



Fig. 18- — Fiery ground-beetle, a, Larva; 



Fig. 19. —A soldier-bug 

 (Milyas cinctus). (Riley, 



b, beetle. (From Riley) U. S- Dept. Agr.) 



forms are: (i) predaceous insects, consisting of those which 

 feed externally upon their prey; (2) predatory animals other 

 than insects, such as birds and mammals; (3) parasitic insects 

 which live in the bodies of their hosts; (4) fungi and diseases 

 of bacterial origin. 



The weather has quite as great effect in the control of in- 

 sects as in the yield of the crops themselves. Extremes of heat 

 or cold, excess of moisture or dryness have the same effect 

 on insect as on plant life. The results of severe rainstorms, 

 sudden cold snaps and prolonged drought on many insect pests 

 are well known. 



Of predaceous insects the most useful are undoubtedly the 



