Egg parasitoids select for large clutch sizes and covering layers in pine processionary moths (Thaumetopoea pityocampa)

Female insects have a limited energy budget to invest in reproduction. Clutch size and egg size are two traits typically involved in energy budget trade-offs, and an optimum clutch size is generally predicted. This trade-off, however, is influenced by many factors including the probability of egg parasitism. We studied this possibility in the pine processionary moth (Thaumetopoea pityocampa), which frequently suffers from parasitoids. We found support for a trade-off between clutch size and egg size in this species, but the intermediate clutch size did not correspond to higher hatching success. We suggest that the lack of an optimum clutch size in relation to hatching success was mediated by parasitoids: parasitoids preferentially selected small clutches containing larger eggs. In addition, we experimentally tested whether the scales that coat egg batches reduce the effect of parasitism. Egg batches with experimentally removed scales showed a significantly higher parasitism rate than control batches, which demonstrates that scales protect egg batches from parasitoids. We discuss three possible explanations for our results.