The population dynamics of the gypsy moth, Porthetria dispar (L.), are being studied on 10 sites in the Town of Glenville, New York. This paper discusses the role of disease and a condition here termed “desiccation” in the dynamics of these populations during a 4-year period, 1958-1961 inclusive.

The term “desiccation” refers to dead pre-pupae that appeared shriveled, and were stiff to the touch, and which had a solid mass of food in their gut. The incidence of desiccation among pre-pupae was closely related to the number of eggs per egg mass produced at the end of the generation (a measure of relative insect density).

Disease incidence among larval gypsy moth populations was directly related to insect density. Disease incidence was also related to site conditions, with higher mortality occurring in wet sites.

When larval populations reached high densities, they always declined from the dense level within a few generations. These declines ranged from a sudden drastic reduction to a much more gradual decline. The former was preceded by virtual food exhaustion, while the latter was not usually preceded by exhaustion of the food supply.

Disease and desiccation were primary factors in producing the sudden type of population reduction noted above. Pathogens may also play an important part in the more gradual type of decline, but this point remains to be clarified.