THEIR RELATION TO WEATHER 143 



During a wet spring certain species of plant lice 

 may become so abundant as to threaten a given crop, 

 with their natural enemies so far in the rear a^ to seem 

 hopelessly out of the running. A sudden change to 

 hot dry weather will change conditions so radically, 

 that within a week the lice arc gone, while ladybird 

 larva; and other plant louse destroyers are feeding 

 upon each other. 



No kind of insect is more sensitive to weather 

 changes than are the Aphids, and few of them are able 

 to resist a sudden change of temperature exceeding 

 30° in range; but an increase is not nearly so fatal to 

 most of them as a sudden drop. By the term sudden 

 I mean within an hour or two, because ranges of 30° or 

 over within twenty-four hours, are not uncommon in 

 most portions of the United States. 



The character of the winter has much to do with 

 the abundance of insects during the summer following. 

 It is not so much the hard or the mild winter as the 

 variable winter that is fatal to insect life. When arf 

 insect goes into hibernation in cither larval or adult 

 stage it becomes torpid and capable of resisting all 

 usual degrees of cold. Even if the cold is long continued 

 at its most intense point it makes little difference and, 

 in general, we may say that a continuously severe 

 winter is favorable to insect life. The insect simply 

 remains torpid and no change in condition occurs. On 

 the other hand if there are alternations of freezing and 

 thawing, the insect may become partially or altogether 

 active and again torpid, losing in vitality at every 

 change until it dies or reaches spring in such condition 

 as to be unable to complete its tranformations or to 

 reproduce its kind. Such alternations are particularly 

 hard on pupse and on larvae that winter underground 

 in cells. A thaw results in softening the ground and 



