February, 'IG] ALLEN: INSECT SPREAD OF WILT 233 



NOTES ON THE RELATION OF INSECTS TO THE SPREAD OF 

 THE WILT DISEASE! 



By H. W. Allen, Gipsy Moth Laboratory, Melrose Highlands, Mass. 



During the caterpillar stages, vast numbers of the gipsy moth die 

 of a disease known as the "wilt," This affection is typical of a group 

 of insect diseases termed "wilt" or "polyhedral" diseases and 

 which are characterized by extensive breaking down of the body 

 tissue, and the formation of microscopic, angular bodies known as 

 "polyhedra." Polyhedra invariably occur in large numbers in dis- 

 eased insects and are considered to be reaction products. The disease 

 is believed to be produced by infection of the alimentary canal and 

 to be caused by a filterable virus, presumably an organism and very 

 much smaller than the polyhedra. 



Victims of the disease reach an advanced stage of disintegration 

 soon after death. Dead caterpillars often occur in great abundance, 

 and, as would be expected, attract many insect scavengers. Sarcopha- 

 gida), in particular, are attracted in abundance and breed freely in 

 the dead larvae and pupae resulting from the disease. 



Very little is known of how the infection causing the disease is spread. 

 Experiments by Glaser and Chapman ^ have indicated that it is not 

 essentially a wind-borne disease. The presence of scavengers in con- 

 nection with the disease suggested that they might be carriers of the 

 infection and agents in the spread of the disease. It was beheved 

 that if these insects could be shown to be carriers of polyhedra, it would 

 indicate that they might also be carriers of the virus and hence in all 

 probability distributors of the disease. Accordingly, an effort was 

 made to determine this in connection with other field work on the 

 wilt disease conducted at Lunenburg, Mass., during the past summer. 

 Although the work was prehminary, and not conclusive in indicating 

 that the disease is insect-borne, it has shown that insects commonly 

 act as carriers of polyhedral bodies and suggests that they also in 

 all probability act as carriers and distributors of the disease. 



In securing the record, specimens observed to have been in direct 

 contact with gipsy moth larvae and pupae that had recently died as a 

 result of the wilt disease, were collected in the field. These were 

 sent to Messrs. Glaser and Chapman, of this Bureau, for examination 

 and we are indebted to them for the findings in regard to polyhedra. 

 Only such specimens were taken as were observed to have been 



1 Published by permission of the Chief of the U. S. Bureau of Entomology. 



2 Jour, of Economic Ent., Vol. VI, No. 6, pp. 479-488. 



