Some have suggested that the periodic outbreaks of the forest tent cat
erpillar, Malacosoma disstria are triggered by weather: the temperatur
e at the time of early larval feeding, and overwintering temperatures.
To assess the role of these factors, defoliation maps, compiled annua
lly 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 de
clines in defoliation and either temperatures during early larval deve
lopment, 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 o
utbreak periods. Defoliation was less severe in regions with low overw
intering temperatures, but was not related to the average number of de
ree days in the early spring. Outbreaks were most common in areas whe
re deciduous forests were extensive, and the mean overwintering temper
atures were above -40 degrees C. While these weather variables do not
apparently explain the details of population dynamics of forest tent c
aterpillars, extreme weather conditions might synchronize populations.