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Primary pathogens and their role in the development of an epizootic in the gypsy moth


Journal of Invertebrate Pathology 15(1): 21-33
Primary pathogens and their role in the development of an epizootic in the gypsy moth
The development of an epizootic was studied in a dense population of larvae of the gypsy moth, Porthetria dispar. The two pathogens involved were a nuclear-polyhedrosis virus and a variant of Streptococcus faecalis. It was known that there was approximately 10% acute infection from trans-ovum transmission of the nuclear-polyhedrosis virus. The origin of the initial inoculum of the variant of S. faecalis is unknown but it became common in the second, and later instars. The behavior of the larvae increased the relative density of the population and enhanced the rate of larva-to-larva spread of the pathogens. Larvae in the first four instars fed most heavily in the tops of trees and dead larvae accumulated on the upper surfaces of the leaves. These cadavers disintegrated and adhered firmly to the leaves, forming an abundant source of inoculum for feeding larvae. This occurred early enough to account for the later, massive increase in disease. Results from counts in the field and collections of larvae reared in the laboratory indicated that there was an increasing rate of infection and mortality that reached a climax when larvae were in the last instars. The threatened defoliation did not occur and the population declined sharply. Although the nuclear-polyhedrosis virus appeared to be most important, up to 50% of the larvae in some collections were killed by the variant of S. faecalis. Observations indicated that the epizootic was density-dependent and that the rapid spread of pathogens in the susceptible population was enhanced by the behavior of the larvae during the early instars.

Accession: 006184004

DOI: 10.1016/0022-2011(70)90094-7

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