Mayis, I9IS Wilt of Gipsy- Moth Caterpillars 103 



visited in Maine, 15 in New Hampshire, 90 in IMassachusetts, and 

 3 in Rhode Island. That the number of infested places in Massachusetts 

 given above exceeds those of New Hampshire is probably owing to the 

 fact that 75 more places were studied in the former than in the latter 

 State and that the number in New Hampshire exceeds that in Maine is 

 due to a similar reason, for 1 1 more localities were visited in New Hamp- 

 shire than in Maine. In short, the disease was found wherever close and 

 continuous observations were made, with the possible exception of one 

 or two places, but even these were doubtfully healthy. 



The epidemiology of wilt is not noticeably different from that of 

 Wipfelkrankheit, for the writer was able to confirm most of the field ob- 

 servations of Wahl (i8),Tubeuf (14, 15), and others who have studied the 

 nun-moth disease. When a territory becomes heavily infested, an 

 epidemic occurs sooner or later, for these larvae defoliate all the 

 trees and later many congregate in masses on the trunks. Naturally 

 when the disease breaks out in such a mass most of the caterpillars 

 become infected, and since they are everywhere abundant and are 

 crawling around in search of food, infected individuals rapidly spread the 

 disease. The lack of food, which is necessarily brought on by defoliation, 

 furthermore, causes caterpillars to lose their vitality, producing greater 

 susceptibility to the disease. Gipsy-moth caterpillars mature in July, 

 when it is usually very hot, and after having stripped a tract of woodland 

 of its leaves, they are almost entirely exposed to the sun's rays. Ksche- 

 rich and Miyajima (4) have shown experimentally that sunlight can 

 convert the chronic into the acute form of wilt, and one can readily 

 become convinced of the accuracy of this observation by visiting a heavily 

 infested, stripped piece of woodland during a hot spell. Thousands of 

 gipsy-moth caterpillars that have died of this disease will be found 

 hanging to limbs and tree trunks (PI. XI). There will be an enormous 

 reduction in the number of adult moths and consequently in the number 

 of egg clusters, but a complete extermination does not take place, owing 

 in part to the immunity of certain individuals (p. 124). 



In a lightly infested woodland the conditions are different. Here the 

 caterpillars are much more widely separated and an epidemic is not pro- 

 duced. There is sufficient food throughout the season, and the trees 

 are never completely defoliated; hence, caterpillars can always find cool 

 places in which to rest during the midday heat. Yet even in such a 

 favorable locaUty a few caterpillars will die, but the mortality as a whole 

 is small in proportion to the number of individuals; many apparently 

 escape infection, and most of the next generation will escape likewise the 

 following year, unless the increase of caterpillars produces an epidemic. 



Wilt is more prevalent among the older caterpillars for the reasons 

 given above, but younger caterpillars also die of the disease, as is shown 

 by field observations during the season of 191 3, when a few typical 

 cases were found as early as May 27, at which time the temperature 



