GYPSY-MOTH POPULATION SIZES AND THE WINDOW OF OPPORTUNITY IN SPRING

Many outbreaking species of forest Lepidoptera emerge from winter diapause at the time of host budburst. I tested the hypothesis that variable synchrony between insect emergence and tree budburst influences variability in population size of the gypsy moth (Lymantria dispar L.). Since long-term records of insect and plant phenology are not available for testing this hypothesis I constructed models to estimate the timing of insect and leaf emergence in spring. A model of the rate of starvation of newly hatched larvae was also created, to estimate the time between egg hatch and death from starvation. The proportion of larvae alive on days with weather suitable for seeking hosts and on which host leaves were available was estimated for each of 23 years using these models. Increased opportunity for seeking hosts in spring was associated with higher population density in the subsequent autumn, but it was not related to year-to-year changes in gypsy moth density. The phenological models indicated that weather after hatch was more important in determining host-seeking opportunities than was asynchrony of egg hatch with budburst. While spring weather has a potential modifying effect on L. dispar population dynamics, it does not drive the population fluctuations.