Winter foraging patterns and voluntary hypothermia in the social caterpillar Eucheira socialis

1. Analysis of 28 years of weather data for the Sierra Madre Occidentals of Mexico showed that while flight, mating, and oviposition of the social caterpillar Eucheira socialis (Lepidoptera: Pieridae) occurred in the warmest and wettest months, much of the caterpillar's feeding and growth occurred in the winter when nocturnal temperatures often fell below 0 degrees C. 2. Although daytime temperatures at the study site in midwinter were markedly warmer than overnight temperatures, colonies remained sequestered in their bolsas by day. Caterpillars initiated activity shortly after the onset of darkness and foraged overnight at temperatures as low as -2 degrees C. The remarkably low chill-coma temperature recorded for this species has been reported previously only for a sub-Antarctic caterpillar. 3. Temperature measurements on sunny days showed the interiors of bolsas to be thermally heterogeneous, with an average differential of 12 degrees C between the warmest and coolest regions of the structure. Although caterpillars clustered within the bolsas had body temperatures significantly greater than ambient, they exhibited voluntary hypothermia by day, seeking out and resting in the coolest pockets of the bolsas. 4. Voluntary hypothermia may influence growth rate adaptively and prevent acclimatisation to daytime temperatures that would have a negative effect on the caterpillar's ability to locomote at low overnight temperatures.