Behavioural plasticity and tree architecture shapes tent and foraging locations of pine processionary larval colonies

Abstract

Establishing in the right location is particularly important for larval insects. Lepidopteran females are generally selective when choosing oviposition sites to ensure the best survival for their offspring. Unlike most social and shelter-building Lepidoptera, egg batches of the pine processionary moth, Thaumetopoea pityocampa Denis & Schiffermuller (1775) (Lepidoptera: Notodontidae), are oviposited randomly on the host tree. Thaumetopoea pityocampa is gregarious throughout all larval instars and live in silken tents, spun repeatedly and maintained by the colony during larval development. In this study, a single T. pityocampa egg batch was transplanted on Pinus nigra trees free from natural egg batches in one of four compass orientations: north, east, south or west. The orientation of transplanted egg batches had no significant effect on the final survival of the larvae. Thaumetopoea pityocampa larvae were behavioural thermoregulators from first to final instar and tended to position their tent in a southerly orientation for maximum sun exposure. Thermoregulation was the utmost priority for T. pityocampa larvae as they feed through the winter. Feeding behaviour and number of tents built by early instars changed as larvae became older, and this could be explained by natural enemy avoidance and/or evasion of plant defence. The results can help predict larval movement and assist in pest management strategies for T. pityocampa at a microhabitat level. This study is the first to investigate detailed movement behaviour of all T. pityocampa larval instars on a host tree, describing the transition of larvae starting as patch-restricted foragers to central place foragers.