SOME INSECTS AT WORK ON !FARM CROPS 271 



PlG. 113. An aphid 

 skin with a hole 

 in its back, 

 whenc e has 

 emerged a para- 

 site. 



to our shores along with imports of plant materials of various 



sorts. They have become established in our fields; but 

 fortunately they attack only a few of our 

 plants that are closely related to their own 

 native food-plants. Pests like the brown-tail 

 moth, having an unusually wide range of diet 

 (including in this example the leaves of most 

 of our deciduous trees), are unusually difficult 

 to control. 



Under natural conditions, there is an occa- 

 sional excessive increase of foraging insects. 

 Hordes of them suddenly appear, and 



destroy the foliage of one or two species of plants. For 



this evil, nature has her own methods of control. She 



uses carnivores and parasites to keep each species in check. 



In the midst of the 



aphid colony on a 



cabbage leaf, or on 



the curled tip of 



an aphid-infested 



apple spray, one 



may often see both 



predatory and 



parasitic foes of 



the aphids work- 

 ing side by side to 



keep down the 



colony. Ladybird 



beetles, and their 



larvae (fig. in) 



consume the 



aphids bodily. p IG .n 4 



Lacewing fly lar- 

 vae (fig. 112) and 



a 



A parasitized moth larva on a blue-grass top: 

 some of its parasites have spun their cocoons beside it, 

 others, on the grass-blade above, b, shows an easy 

 method of hatching out the adult parasites from the 

 cocoons. (From the author's "General Biology"). 



