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

Abstract

1. In gregarious insects, groups commonly originate from females laying eggs in masses and feeding groups are established as soon as larvae hatch. Some groupliving insect species may aggregate beyond the individual parent level, such that offspring from two or more egg masses develop within a common resource.2. Here we show that aggregative oviposition can vary with population density at oviposition and possibly be an important factor in outbreak dynamics of phytophagous insects.3. We analysed density data with respect to egg mass aggregation for two species of pine processionary moths, Thaumetopoea pinivora (in Sweden 2005-2019) and T. pityocampa (in Spain 1973(in Spain -1991. Both species lay their eggs in egg masses and feed in groups. During the study periods, insect population density for both species varied by at least an order of magnitude.4. The two species showed strikingly similar patterns of egg mass aggregation. Egg masses were overdispersed at high population density, with few trees showing a high load of egg masses. 5. Our data suggest that aggregative oviposition can be important in explaining the previously documented higher propensity for outbreaks in insects laying eggs in clusters, compared with those laying individual eggs.

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

“…In summer, each female T. pityocampa moth oviposits all her eggs in one batch (150-350 eggs) on the outer foliage of a pine tree, and the eggs hatch in a month [13]. The species has a univoltine lifecycle and is gregarious from egg to moth [19]. Unlike most defoliating insects of temperate regions, the caterpillars feed throughout the winter.…”

“…Thaumetopoea pityocampa caterpillars spend their life in a tent with their siblings and conspecifics from other colonies. In a species with such gregarious behaviour from egg to moth [19], it is expected that individuals have cooperative and synchronised activities to perform well as a colony. Tent construction is directly followed by foraging.…”

Ecological characteristics of pine processionary moth Thaumetopoea pityocampa (Lepidoptera: Notodontidae) larval nests in pine and cedar forests of Algeria

“…The pine processionary moth (PPM) Thaumetopoea pityocampa (Lepidoptera: Notodontidae) is the most important forest pest in the Mediterranean and other Southern European areas (Jacquet et al 2013;Li et al 2015;Sbay and Zas 2018;Campoa et al 2021;Camarero et al 2022;Battisti et al 2023). Thaumetopoea pityocampa is currently showing an expansion northward and higher altitude in Europe due to increased winter temperatures Robinet et al 2013).…”