96 THE GYPSY MOTH. 



able for the moths' increase. The season of 1894 and that 

 of 1895 appear also to have furnished conditions especially 

 favorable for an abnormal mnltiplication of the insect. 



The operation of the causes of these sudden outbreaks 

 is not fully understood. It is evident, however, that the 

 warm, pleasant spring weather of the past two years (1894 

 and 1895) hastened the development of the caterpillars, 

 thereby shortening their term of life. The lengih of life of 

 the caterpillar varies from six to twelve weeks. During cold, 

 rainy weather the caterpillars eat little and grow slowly. 

 During warm, dry weather they consume much more food 

 and grow with great rapidity. In the unusually warm 

 spring and early summer of 1895 many of the caterpillars 

 moulted a less number of times than usual, and their length 

 of life did not exceed six or seven weeks. Under these 

 conditions they proved more quickly injurious to foliage 

 than in a more normal season, and were more completely 

 destructive within any given area in which their numbers 

 were great. And they were not so long exposed to the 

 attacks of their enemies. While it may be true that the 

 parasitic enemies of the moth will also develop rapidly 

 under conditions that hasten the growth of their host, birds 

 and other vertebrate enemies will secure fewer of the moths 

 in six or seven weeks than in ten or twelve. This would 

 probably be true of many predaceous insects. It is believed 

 that dry weather is unfavorable for vegetable parasites of 

 insects, but to what extent the caterpillars are affected by 

 them in a humid season it is impossible to say. 



The past two years have been ' ' canker worm years " in 

 the infested region. Many of the birds which habitually 

 feed on the caterpillars of the gypsy moth have been largely 

 occupied during May and the early part of June in catch- 

 ing canker worms, which they seem to prefer, turning their 

 attention to the gypsy-moth caterpillars in the latter part 

 of June and July, when the canker worms have disappeared. 

 The birds, therefore, have not been as useful in checking the 

 increase of the gypsy moth as in years when the canker worms 

 were less numerous. 



A few of the restraining influences which have been less 

 active than usual during the past two years have been men- 



