SOME INSECTS AT WORK ON FARM CROPS 



271 



Fig. H3. An aphid 

 skin with a hole 

 in its back, 

 whence 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. 111) 

 consume the 

 aphids bodily. 

 Lacewing fly lar- 

 vae (fig. 112) and 



Fig. 114. 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"). 



