4 THE GIPSY MOTH AS A FOREST INSECT. 



the contrary, if conditions are otherwise favorable an infected cater- 

 pillar will live, complete its transformations, and (it is believed) 

 transmit the germs of the disease directly to its offspring. Under 

 these circumstances generation could follow generation, and in the 

 course of time the race of gipsy moths might gradually be purged of 

 the disease, so that eventually only a few individuals would carry it. 



This would all be changed were the infected caterpillars to become 

 weakened through any other cause. Under such circumstances an 

 apparently healthy individual will sicken and die, and in a surpris- 

 ingly short time the entire contents of its body will be resolved into 

 a black liquid containing countless myriads of the germs of disease 

 where before there were but few. Death is particularly likely to 

 ensue upon the topmost twig of the tree, and the disintegrated body 

 of the victim, breaking of its own weight, permits the black poison 

 to defile the foliage below. Another caterpillar feeding upon this 

 foliage contracts the disease and, provided it also be weakened 

 through any other cause, it quickly dies, and the process is repeated. 



It is veiy evident that the more abundant the caterpillars chance 

 to be in a given forest the greater the chance that the disease will be 

 thus transmitted; the more these caterpillars chance to be weakened 

 through a lack of suitable food the more likely they are quickly to 

 succumb and transmit the malady to their fellows. It thus results 

 that when a forest is threatened with defoliatioii by infected cater- 

 pillars the disease becomes epidemic and spreads with astounding 

 rapidity. 



A yet more important cause for the development of the "wilt " than 

 partial defoliation is to be found in unfavorable food. It used to be 

 easy to rear caterpillars in the laboratory upon lettuce, for example, 

 when they were free from the taint of the disease, but it is practically 

 impossible to do so to-day, if American eggs are used. With foreign 

 eggs, collected from a locality where the wilt is not prevalent, this 

 tlid not prove to be the case in the course of experiments recently 

 conducted at the Gipsy Moth Parasite Laboratory, which it is pro- 

 posed to repeat the present year. Nor is lettuce an exception among 

 foods. The same may be said of practically all herbaceous plants, 

 and, fortunately, of a considerable variety of trees and shrubs. 



Herein lies the most potent cause for the less destructive character 

 of the gipsy moth in recent years, according to the opinion of the 

 writer — an opinion which it is expected wall be abundantly confirmed 

 in the course of the coming summer. And herein lies the real secret 

 of the practical resistance of certain species of trees to gipsy-moth 

 attack. 



