Why do pine processionary caterpillars Thaumetopoea pityocampa (Lepidoptera, Thaumetopoeidae) live in large groups? An experimental study

Optimal group size of gregarious larvae is the result of a trade-off between the costs and benefi ts undergone by individuals living in groups of different sizes. Thus, females should adjust their clutch size to an optimal-minimum group size. In this study, we experimentally manipulated the size of colonies of pine processionary caterpillars, a capital breeder species, to test the hypothesis that a large group size enhances larval growth and survival. We also explored whether this relationship fi ts a quadratic or an asymptotic curve and estimated an optimum or a minimal-optimum group size. The results showed signifi cant differences in the fi nal larval sizes in the various treatments, being greater in the larger groups. In addition, according to the existence of a minimal-optimum group size, we found that a Piecewise Linear Regression fi ts the above relationship better than does a linear regression. Groups larger than 32 individuals did not differ in growth or survival parameters. Although the number of dead larvae per group did not differ between experimental treatments, large experimental colonies suffered a lower percentage of mortality. Thus, the probability of reaching the pupal stage was greater for larvae from large colonies because of dilution effects. Our results demonstrated a minimum group size, above which group size did predict larval growth or mortality, thereby explaining why pine processionary caterpillars live in large groups.