Epsilon Open Archive

Abstract

During outbreaks, Microdiprion pallipes (Fall.) is the most destructive of the pine sawflies in Sweden. Its distribution includes most provinces, but damaging outbreaks have until recently occurred only in two inland areas in northern Sweden. These areas are characterised by high elevation, a harsh climate, and slow tree growth. The four recorded outbreak periods showed a 10 year periodicity. Outside these areas, a lesser outbreak occurred in 1988 to 1990, on the east coast (province of Uppland). Outbreak patterns, life history variation, and mortality factors were studied. Factors that may explain the distribution of outbreaks and the population patterns were identified. Experimental and observational evidence on the potential of various factors to influence fecundity, dispersal, and survival was evaluated. In the outbreak areas, there were few major population factors. Parasitism by Rhorus substitutor (Thunb.) was the largest cause of larval mortality and the only important density dependent mortality factor. The different diapause strategies of M. pallipes and R. substitutor may contribute to stabilise this system. Different flight periods of the host and the parasitoid may explain a possible correlation between weather and outbreaks. Elsewhere in Sweden, where low population densities prevail, there may be similarities in population processes between M. pallipes and the other widely distributed diprionids with solitary larvae, which never have attained outbreak densities in Sweden. Interactions with other diprionids through shared natural enemies may be an important population process and may influence the distribution of outbreaks.