Over the past ten years, mathematical models of infectious disease have elucidated the impact of natural selection upon pathogen virulence. Specifically, whether natural selection will lead to higher or lower virulence is thought to depend upon the effect of virulence upon disease transmissibility. The investigators propose to test this hypothesis by using two clones of the nuclear polythedrosis virus of gypsy moth, Lymantria dispar; these clones are of very different virulence and can be identified from field samples on the basis of restriction endonuclease maps. A combination of laboratory measurements and small-scale field experiments will be used to estimate different aspects of the transmissibility of the two clones. These estimates will be used to predict which clone will predominate in the field. This prediction will be tested by seeding 1 hectare plots with disinfected gypsy moth eggs, and simultaneously introducing both clones of the virus. Competitive superiority will be indicated by which clone infects more larvae over the course of an epizootic. Assumptions about the evolution of pathogen virulence have influenced research on disease ecology for decades, but have been largely untested. The work that is proposed here will greatly reduce the gap between theory and reality in the study of the evolutionary ecology of disease.
