Aggregative oviposition varies with density in processionary moths—Implications for insect outbreak propensity Battisti, Andrea Hodar Correa, José Antonio Egg mass Gregariousness Group living Lepidoptera Notodontidae Thaumetopoea 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 1973–1991). 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. 2022-11-28T08:57:21Z 2022-11-28T08:57:21Z 2022-11-01 journal article Battisti, A... [et al.] (2022) Aggregative oviposition varies with density in processionary moths—Implications for insect outbreak propensity. Ecological Entomology, 1– 10. Available from: [https://doi.org/10.1111/een.13205] https://hdl.handle.net/10481/78146 10.1111/een.13205 eng info:eu-repo/grantAgreement/EC/H2020/771271 http://creativecommons.org/licenses/by-nc/4.0/ open access Atribución-NoComercial 4.0 Internacional Wiley