SOME INSECTS AT WORK ON FARM CROPS 271 
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 
Fic. 113. An aphid 
skin with a hole to control, 
in its back 
whence tae Under natural conditions, there is an occa- 
emerged a para- 
Sita 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 ona 
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 
Fic. 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. 6, shows an easy 
method of hatching out the adult parasites from the 
cocoons. (From the author’s “General Biology’’). 
