Heat for the Masses: Thermal Ecology of the Western Tent Caterpillar

A unique feature of some gregarious, colonial insects is their ability to create external structures that alter environmental conditions for the entire (often family) group. A combination of physical alteration of local microhabitats and behavioral thermoregulation allows many of these animals to actively control their body temperatures, which allows them to regulate energy use and metabolism in variable thermal environments. Here I describe mechanisms of microhabitat modification and thermal regulation in the western tent caterpillar, Malacosoma californicum pluviale. Tent caterpillars build communal silk tents, whose temperatures can rise substantially above ambient air temperature. I experimentally manipulated colony sizes and examined effects on tent temperatures and on rates of larval development and mortality. I predicted that large colonies would construct warmer tents, allowing larvae to grow faster in cool spring conditions. I found that temperatures in tents of large colonies (100-150 individuals) were up to 22 °C higher than ambient air temperature, and on average 15 °C higher than the mean temperature of tent temperatures of smaller colonies (10 individuals). As a consequence, larvae from larger colonies reached the final instar 30% faster than those from smaller colonies, and only larvae from large colonies survived to pupation. I then tested three hypotheses regarding excess temperatures observed in for tents of larger caterpillar colonies: (1) The tent collects heat like a greenhouse, (2) aggregating caterpillars act biophysically as a larger organism, reducing heat loss and increasing insulation to trap heat, and (3) the caterpillars warm the tent with their own metabolic heat. Using infrared imaging, I recorded daytime body temperatures of caterpillars in naturally occurring colonies, alone or in groups, either on or off their tents. I found that grouped caterpillars located off their tent had slightly higher body temperatures than solitary ones (about 1-2 °C), a result that was replicated using operative temperature models, where no metabolic heat is generated. However, caterpillars grouped on their tent reached significantly higher temperatures than those off the tent (4-5 °C), suggesting that both a warm tent and behavioral thermal regulation by grouping contribute significantly to observed excesses in caterpillar habitat and body temperature. With the ability to both physically alter their local microhabitats and behaviorally thermoregulate, one might predict that this species may be less vulnerable to climate change than other temperate insects