The influence of forest management on defoliator populations: A case study with Neodiprion abietis in precommercially thinned and natural forest stands

This dissertation examines the factors influencing outbreak and declining populations of Neodiprion abietis (Harris) (Hymenoptera: Diprionidae) in natural and precommercially thinned forest stands of western Newfoundland. This was accomplished through analysis of the age-specific mortality of natural populations complemented with manipulative field experiments that specifically investigated the interplay between the first and second trophic levels. At each study site, N. abietis populations remained at high densities for two to four years before collapsing. In the increasing phase of outbreaks, immigration, high fecundity, female-biased sex ratios, and high survival of old-instar larvae and cocoons permitted populations to build up rapidly. Then, the baculovirus NeabNPV apparently initialized the collapse of N. abietis populations in all precommercially thinned and natural stands by causing high mortality among old-instar larvae and moderate levels of mortality during the cocoon stage. The cocoon parasitoid Mesopolobus verditer (Norton) (Hymenoptera: Chalcidoidea) sometimes accelerated the decline of N. abietis populations induced by NeabNPV. Shifts from female to male-biased sex ratios and reductions in fecundity were associated with increasing levels of defoliation, with both factors also contributing to population decline. Defoliation negatively affected N. abietis performance because it reduced the availability of differently-aged foliage, which is required for maximum fitness of N. abietis, and because it reduced foliage quality. However, less of a decline was observed in N. abietis performance after defoliation in thinned than in natural stands, indicating that thinning mitigated the negative effects of previous defoliation on N. abietis performance. Accordingly, defoliation and the amplitude of population fluctuations were higher in thinned than in natural forests. This suggests that thinning increases the carrying capacity of stands for N. abietis due to a reduction of host-plant inducible responses following defoliation. Precommercial thinning treatments are therefore suspected to be responsible, in part, for the increased severity of current outbreaks of N. abietis occurring in western Newfoundland. The present study is the first to suggest how changes in tree density may alter the processes involved in population fluctuations of an insect defoliator. This dissertation explicitly argues against an often sought single explanation for population fluctuations.