Preoutbreak dynamics of a recently established invasive herbivore: roles of natural enemies and habitat structure in stage-specific performance of gypsy moth (Lepidoptera: Lymantriidae) populations in northeastern Wisconsin.

A major challenge to addressing biological invasions is that the need for emergency responses often precludes opportunities to analyze the dynamics between initial establishment and population eruption. Thus, a broader understanding of underlying processes and management opportunities is often lacking. We examined the effects of habitat structure and natural enemies on recently established preeruptive gypsy moth, Lymantria dispar L., populations over 4 yr in northeastern Wisconsin. Forty-five sites were established across a range of habitat structures in oak-dominated northern hardwood forests. The number of egg masses was positively related to percent composition of oaks and other favored species. Other life stages were not related to habitat structure variables. Abundance of each life stage can predict the subsequent life stage with variable degrees of accuracy, but male moth densities were only a weak predictor of egg mass or larval densities the following year. The parasitic fly Compsilura concinnata (Meigen), an introduced generalist with deleterious nontarget effects, caused the highest mortality to larvae. The specialist pathogens Entomophaga maimaiga and nucleopolyhedrosis virus were widely distributed but caused less mortality than reported in the northeastern United States, where gypsy moth has been established much longer. Small mammals are the major predators of pupae as elsewhere, but invertebrates seem less important along the western than southern advancing front of gypsy moth. Overall habitat structure did not influence natural enemy populations. These results suggest that the pre-eruptive phase is distinct from the pre-establishment phase by high mating success and from the eruptive phase by the prominent role of generalist natural enemies. Improved understanding of these dynamics can help guide silvicultural and biocontrol strategies in newly invaded regions of the Midwest and provide general insight into invasive forest defoliators.

Forest structure and light regimes following moderate wind storms: implications for multi-cohort management.

Moderate-severity disturbances appear to be common throughout much of North America, but they have received relatively little detailed study compared to catastrophic disturbances and small gap dynamics. In this study, we examined the immediate impact of moderate-intensity wind storms on stand structure, opening sizes, and light regimes in three hemlock-hardwood forests of northeastern Wisconsin. These were compared to three stands managed by single-tree and group selection, the predominant forest management system for northern hardwoods in the region. Wind storms removed an average of 41% of the stand basal area, compared to 27% removed by uneven-aged harvests, but both disturbances removed trees from a wide range of size classes. The removal of nearly half of the large trees by wind in two old-growth stands caused partial retrogression to mature forest structure, which has been hypothesized to be a major disturbance pathway in the region. Wind storms resulted in residual stand conditions that were much more heterogeneous than in managed stands. Gap sizes ranged from less than 10 m2 up to 5000 m2 in wind-disturbed stands, whereas the largest opening observed in managed stands was only 200 m2. Wind-disturbed stands had, on average, double the available solar radiation at the forest floor compared to managed stands. Solar radiation levels were also more heterogeneous in wind-disturbed stands, with six times more variability at small scales (0.1225 ha) and 15 times more variability at the whole-stand level. Modification of uneven-aged management regimes to include occasional harvests of variable intensity and spatial pattern may help avoid the decline in species diversity that tends to occur after many decades of conventional uneven-aged management. At the same time, a multi-cohort system with these properties would retain a high degree of average crown cover, promote structural heterogeneity typical of old-growth forests, and maintain dominance by late-successional species.