Large Male Caterpillars Are the Primary Builders: Exploring Tent Construction and Foraging Behaviour in Gregarious Pine Processionary Caterpillar

SIMPLE SUMMARY: Thaumetopoea pityocampa caterpillars are a significant pest of conifer trees in Europe and live gregariously in a communal tent with siblings and conspecifics from other colonies. Despite their economic and medical importance, there has been a lack of quantitative information on the tent construction and foraging behaviour in T. pityocampa colonies. In this study, we observed the tent construction and foraging behaviour of T. pityocampa caterpillars in the field at Tregnago, Verona, Italy. At around sunset, large male caterpillars emerged from the tent first to construct the tent, while many female caterpillars emerged later in the night only to forage. As younger instar caterpillars moulted to older larval instars, the environmental temperature decreased, which consequently increased the duration of their foraging activities. The final tent structure constructed by later instar caterpillars was not isodiametric; more silk was applied on the southern side of the tent to receive maximum insolation during the winter months. This study demonstrated the importance of the winter tent and individual variation in tent construction and foraging behaviour of T. pityocampa caterpillars amongst sex, body size, and larval instar and lays the basis for further investigation in polyethism. ABSTRACT: As a social organism, living in a communal structure is one of the most important physical barriers against environmental elements and natural enemies. Thaumetopoea pityocampa (Notodontidae, Thaumetopoeinae) caterpillars are conifer pests that spend most of their larval stage in winter. Although T. pityocampa holds economic and medical significance, the tent construction and foraging behaviour are poorly understood. We observed the tent construction behaviour in autumn (October and November) when third- and fourth-instar T. pityocampa caterpillars build the ‘winter tent’ that can withstand winter conditions. Just before sunset, with no rain and temperatures over 12 °C, tent construction was undertaken by early active individuals, primarily larger male caterpillars. Early active caterpillars emerge from the tent first and spin silk on the tent for expansion and strength. Once temperatures dropped below 12 °C and twilight had passed, the early active caterpillars went out to forage and were later joined by the late active caterpillars, which were predominantly smaller females that had remained inside the tent. Foraging behaviour was continuously monitored for the first to fourth larval instars in the field. Foraging was more frequent in younger instars when environmental temperatures were warmer and became continuous and prolonged in later instar caterpillars as temperatures dropped. The final tent structure built by later instar caterpillars had the thickest layer of silk on the southern side of the tent compared to other orientations to receive maximum solar radiation during the winter. Our study provided additional insights into the collective nest building, foraging and social behaviours observed in Lepidoptera, as well as the roles of individuals within non-eusocial insect colonies.