Thermal ecology and behaviour of the nomadic social forager Malacosoma disstria

The present study examines whether the nomadic social caterpillar Malacosoma disstria Hübner (Lepidoptera: Lasiocampidae) can thermoregulate despite the lack of a tent, and evaluates the role of thermoregulation in directing the colony's behaviour. The presence of a radiant heat and light source (i.e. a lamp in the laboratory experiments and the sun in the field observations) enables caterpillar colonies to increase body temperature by basking (remaining still under a heat source) and this is only effective when caterpillars cluster in groups. Body temperatures achieved when basking in a group coincide with the temperatures at which the development rate is maximal for this species. Indeed, in the laboratory experiments, the presence of a lamp results in higher growth rates, confirming that thermoregulation is an advantage to group living. When a radiant heat/light source is provided at a distance from the food in the laboratory, caterpillars behave to maximize thermal gains: colonies move away from the food to bivouac (i.e. group together and remain still on a silk mat) under the lamp, spend more time on the bivouac and cluster in a more cohesive group. Thermal needs thus influence habitat selection and colony aggregation. Malacosoma disstria relies on developing rapidly, despite low seasonal temperatures, aiming to benefit from springtime high food quality and low predation rates; however, unlike others in its genus, it does not build a tent but instead exhibits collective nomadic foraging (i.e. the whole colony moves together between temporary resting and feeding sites). In this species, collective thermoregulatory behaviour is not only possible and advantageous, but also drives much of the colony's behaviour, in large part dictating the temporal and spatial patterns of movement. These findings suggest that thermoregulation may be an important selection pressure keeping colonies together.