Climate and outbreaks of the forest tent caterpillar

Some have suggested that the periodic outbreaks of the forest tent caterpillar, Malacosoma disstria are triggered by weather: the temperature at the time of early larval feeding, and overwintering temperatures. To assess the role of these factors, defoliation maps, compiled annually from flight surveys for the province of Ontario, were compared to similarly scaled temperature records. An analysis of the year to year variation shows no relationship between the pattern of increases or declines in defoliation and either temperatures during early larval development, or overwintering temperatures. Four periods of defoliation by forest tent caterpillars were observed in Ontario over 41 yr, but at individual sites extensive defoliation did not occur for each of the outbreak periods. Defoliation was less severe in regions with low overwintering temperatures, but was not related to the average number of de ree days in the early spring. Outbreaks were most common in areas where deciduous forests were extensive, and the mean overwintering temperatures were above -40 degrees C. While these weather variables do not apparently explain the details of population dynamics of forest tent caterpillars, extreme weather conditions might synchronize populations.