Response of the egg parasitoids of the pine processionary moth to host density and forest cover at the southern edge of the range

Thaumetopoea pityocampa is the most important pine defoliator in the Mediterranean basin. Despite being attacked by a number of natural enemies, populations occur frequently at high density in several areas. Egg parasitism was studied in 27 pine and cedar forests in Algeria, in relation to the host density (tents per tree) and the proportion of forest cover in the landscape. Egg parasitism varied from 2% to 25%, accounted by two parasitoid species, the specialist Baryscapus servadeii and the generalist Ooencyrtus pityocampae. Tent density was negatively correlated with parasitism by B. servadeii but not with that of O. pityocampae. Conversely, parasitism by O. pityocampae increased with the proportion of forest and agricultural cover, but not in the case of B. servadeii. Maximum summer temperature showed no correlation with parasitism rates. Still, temperature frequently exceeded 40 °C during the period of adult parasitoid activity. The low performance of the egg parasitoids at the southern edge of the host range could be explained by the reduced fecundity of the host, climate effects, and phenological mismatching between the parasitoids and the egg development. These and other factors potentially involved the need to be further explored with a long-term study of population dynamics.