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 shelterbuilding Lepidoptera, egg batches of the pine processionary moth, Thaumetopoea pityocampa Denis & Schiffermüller (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.

Occupational Exposure of Forest Workers to the Urticating Setae of the Pine Processionary Moth Thaumetopoea pityocampa

“…Chainsaw workers were largely affected because of the proximity to the source of urticating setae, i.e., the branches where the larvae feed during the night and the nest where they rest during the day [ 17 ]. As long as the setae are thought to be a defense against vertebrate predators of the larvae [ 15 ], they are shed by the larvae in case of an attack, which is unlikely to happen during the tree harvest operations.…”

“…The infestation level by the pine processionary moth level was moderate, with one nest per tree on average. In the period of the measures, the larvae were typically feeding at night-time and returning to the nest before sunrise [ 17 ]. The workers could be thus exposed to setae released by the larvae during the night feeding, e.g., if they were disturbed by a predator or shed by the nest during the tree felling and debranching operations.…”

Aggregative oviposition varies with density in processionary moths—Implications for insect outbreak propensity

“…In order to maintain a functional group size, colonies may merge and the total number of colonies will decrease. Such merging can occur during the first instar in T. pinivora (Ronnås et al, 2010) but becomes more common during later instars in T. pityocampa (Uemura et al, 2021). When food is not limited, larger colonies may have positive effects on survival, as discussed in the previous paragraph.…”

“…The short‐lived females lay all their eggs in one mass on pine needles, and the larvae hatch in about 1 month. They immediately start to feed on pine needles and build a few silk ‘tents’ during the autumn, to spin the final winter tent in the apical parts of the tree (Uemura et al, 2021). From these conspicuous silk tents larvae have nightly forays to forage during winter whenever temperature conditions are met (Battisti et al, 2005).…”

Selection Forces Driving Herding of Herbivorous Insect Larvae

“…A few notable well-documented biogeographical outliers include species that are active during winter in cool regions (Thaumatopoea pityocampa (Notodontidae) in the Mediterranean and Eucheira socialis (Pieridae) in Mexico) and cooperate to build tents to maximize solar radiation (Fitzgerald and Underwood, 2000;Uemura et al, 2020). As in the early-spring feeders above, these caterpillars are active at low, even below-freezing, temperatures, when cooperative thermoregulation is most advantageous.…”