SYNCHRONY IN OUTBREAKS OF FOREST LEPIDOPTERA - A POSSIBLE EXAMPLE OF THE MORAN EFFECT

I compiled information from the literature on 140 population outbreaks of 26 species of forest Lepidoptera that occurred between the years 1 932 and 1992 in the Northern Hemisphere, including Asia, North America , and Europe. Considerable variation in the number of outbreaks occurr ed among years, with a tendency for outbreaks to be synchronous. Years of numerous outbreaks include 1948, 1956, 1964, the mid-1970s, and th e mid-1980s. Published summaries of weather in the Northern Hemisphere indicate the occurrence of above-normal precipitation and cool temper atures between 1954 and 1957, and a high proportion of populations wer e at peak density in 1954 and 1956. Cool springs in 1964, 1965, and 19 76 were also associated with outbreaks in a number of species of fores t Lepidoptera. Periods of synchrony of population outbreaks and cool t emperatures appear to be associated with troughs in the sunspot cycle. While the details of population dynamics of caterpillars are determin ed through endogenous processes that influence fecundity and mortality , exogenous forces, such as cool springs, may occasionally play a role in synchronizing populations in the Northern Hemisphere. Thus weather conditions may act as a Moran effect and synchronize fluctuating popu lations of forest caterpillars over large geographic areas.