Synchrony in outbreaks of forest Lepidoptera: A possible example of the Moran effect

Abstract

I compiled information from the literature on 140 population outbreaks of 26 species of forest Lepidoptera that occurred between the years 1932 and 1992 in the Northern Hemisphere, including Asia, North America, and Europe. Considerable variation in the number of outbreaks occurred among years, with a tendency for outbreaks to be synchronous. Years of numerous outbreaks include 1948, 1956, 1964, the mid-1970s, and the mid-1980s. Published summaries of weather in the Northern Hemisphere indicate the occurrence of above-normal precipitation and cool temperatures between 1954 and 1957, and a high proportion of populations were at peak density in 1954 and 1956. Cool springs in 1964, 1965, and 1976 were also associated with outbreaks in a number of species of forest Lepidoptera. Periods of synchrony of population outbreaks and cool temperatures appear to be associated with troughs in the sunspot cycle. While the details of population dynamics of caterpillars are determined 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 populations of forest caterpillars over large geographic areas.