I04 Journal of Agricultural Research voi. iv, no. 2 



ranged between 51° and 69° F. Of course, it has been known for a 

 long time that first and second stage caterpillars will die of the wilt when 

 kept in a warm laboratory under unfavorable conditions, but the fore- 

 going observations demonstrated that sometimes these caterpillars will 

 succumb in the field even under what seem to be most favorable condi- 

 tions. Many small caterpillars die of starvation, especially on coniferous 

 trees, and many undoubtedly meet death after exposure on a cold night; 

 therefore no diagnosis will be valid unless made microscopically for the 

 polyhedra described below. 



In some localities studied during the summer of 191 3 wilt did not 

 appear until very late in the season, when most of the caterpillars were 

 full grown and pupating. In an infestation near Provincetown on Cape 

 Cod, Mass., no indications of the disease were found till the very last 

 of July, when the caterpillars were beginning to pupate. The colonies at 

 Provincetown are isolated from the remaining infestations in Massa- 

 chusetts and it seemed likely that the disease had not spread to this 

 locality, but visits during the latter part of the season demonstrated the 

 existence of true cases of wilt. Provincetown faces the Atlantic, and the 

 rather cool climate may have kept diseased individuals in a chronic con- 

 dition for a long time. That is the reason wilt was not noticed earlier. 

 This example is cited simply to show how careful one must be in pro- 

 nouncing a locality healthy. 



PATHOLOGY OF WILT 



When a caterpillar dies of wilt, all of its tissues are in a state of dis- 

 organization. The intestine is the last internal organ to disintegrate, 

 owing to the fact, perhaps, that it is capable of resisting the fermenta- 

 tive or toxic character of the virus longer than the remaining tissues. 

 If a smear of the brov/n liquid from a dead caterpillar is examined 

 microscopically with a high-power dry or oil-immersion lens, it will be 

 found to contain, besides the elements of disorganized tissues, myriads 

 of polyhedral bodies of various sizes. (PI. XII, fig. i, and text fig. i.) 

 The average polyhedron measures from i to 6/i in diameter, and the 

 individual faces of such a single body vary also. Certain polyhedra 

 have been found to measure ^^/j. and less, while still others reach the 

 size of 1 5/i. If there is plenty of liquid on the slide, air currents will 

 cause the polyhedral bodies to turn over and over, so that one can obtain 

 excellent views of all their faces. Their shape varies as much as their 

 size, but in general the form is that of a polyhedron, with more or less 

 rounded angles. They never assume the shape of a perfect sphere, and 

 an actual geometric outline has never been observed, as is the case with 

 the silkworm polyhedra, which are almost perfect octahedra. (PI. XIII, 

 fig. I.) In general appearance the polyhedra of the gipsy moth are more 

 like those of the nun moth than those of the silkworm. 



