Are inbreeding and sex determination a key factor of Diprion pini L. population dynamics ?
Résumé
Diprion pini defoliates Scots pine (Pinus sylvestris) stands during eruptive outbreaks in Europe. These sudden and spectacular pullulations are controlled by food, parasitism and diapause. On the other hand, the pre-outbreak phase is poorly known. Based on the results of crossing experiments in a Scots pine nursery in France and in the laboratory, and cytological investigations on laboratory and field collected populations (the latter in Fontainebleau Forest, France), this paper proposes that a genetic determination in diapause, and a complementary sex determination leading to diploid male production in association with inbreeding, limits population growth when populations are low. Karyotypic observations, sib matings and crosses between insects from various populations supported this hypothesis. The existence of diploid males in this arrhenotokous species was demonstrated and their occurrence in a laboratory strain and in a residual population was ascertained. Moreover, it is suggested that diapause was really dependent on genetic factors and interacted with consanguinity. Such phenomena are able to modify the population genetic structure during outbreaks. The level of diploid male production in natural populations and their possible effect on population dynamics are discussed.