THE WAR ON INSECTS 301 



web-worms occur and, as a rule, old sod is better fall- 

 plowed if insect injury to the next following crop is to 

 be avoided. Most wire-w^orms and white grubs require 

 two entire years to come to their full growth, pupating 

 or even changing to an adult in the late fall of the 

 second year, and coming out as adults during the 

 spring of the third. Fall plowing at the end of the sec- 

 ond year will expose these pupae or recent adults and 

 kill them, w^hereas if delayed until the land is fit the 

 spring following, the beetles would be sufficiently mature 

 to survive. 



The corn-worm and many similar pests also pupate 

 in the fall, making cells in which they lie safely in all or- 

 dinary winters. Fall-plow^ing breaks up those cells and 

 brings the soil into direct contact with the pups, which 

 are killed by the contractions and expansions of the 

 soil about them, under the influence of frost and thaw. 

 Of course fall-plowing is not always good farm prac- 

 tice from other points of view, and the grower must 

 decide what he had better do after balancing all the 

 factors of his problem. But in the control of under- 

 ground pests this practice is important. Sometimes, 

 indeed, our efforts are indirect, as when in late fall we 

 plow land infested with root-lice very deeply, to destroy 

 the nests of the ants that shelter them or their eggs 

 during the winter. 



Frequently the time of planting or the time or 

 manner of harvesting determines the question of in- 

 jury. We have learned that in regions subject to Hes- 

 sian fly attacks, late-sown wheat may be almost en- 

 tirely free from infestation, while that sown early may 

 be almost totally destroyed. This is because the early 

 fall rains bring the adult flies to maturity and they lay 

 their eggs on wheat or other grasses very soon there- 

 after. Anything that comes up later is exempt from 



