302 INSECTS 



attack. It is impossible to fix an arbitrary date, for 

 that varies with latitude, and even in the same latitude 

 the time at which the flies appear on the wing is deter- 

 mined by weather conditions: a drought may delay 

 them until the very latest period for safe sowing has 

 passed, and in that case early and late sown are apt to 

 be equally infested. Sweet potato growers in regions 

 infested by flea beetles have learned that if they delay 

 setting out their plants until the middle of June they 

 have little to fear from the insects; but if plants are 

 set in May, they are almost certain to be seriously 

 injured. Other cases might be cited, but it is sufficient 

 to show that by a careful study of the habits of a species 

 we can often avoid injury without a direct fight. Wheat 

 harvested in July and left in shock is very likely to 

 become infested by the Angoumois grain moth. Carted 

 from shock to mow the infestation spreads until, in 

 September or October when threshing time comes round, 

 a large percentage of grain is defective and "flies" or 

 "moth" are numerous. If instead of being left in the 

 fields and then mowed, the grain had been at once 

 threshed and binned, there might have been a little 

 surface infestation, but there could have been no serious 

 spread in the bulked grain. 



Occasionally an insect can be diverted from a more 

 to a less valuable crop, as in the case of the squash 

 borer which prefers late squashes like the hubbard or 

 marrowfats when it can get them, but will accept sum- 

 mer varieties like the crook-neck if the others are not 

 present in equal attractiveness. The grower therefore 

 plants crook-necks early, and on the same ground puts 

 in the other varieties late. The early vigorous growers 

 attract the moths, the plants become infested but are 

 vigorous enough to produce a crop that pays for the 

 labor, before the late varieties need the ground and the 



