Invasion of eastern forests by the non-native insect, gypsy moth (Lymantria dispar L.), has resulted in widespread defoliation and subsequent tree mortality. Silvicultural treatments can be used prior to gypsy moth defoliation to minimize gypsy moth effects on tree mortality. Defoliation during outbreaks has totaled as much as 12 million acres in one year. Mortality that results from the sequence of decline events initiated by the defoliation peaks one to three years after defoliation ends. Disturbance from these factors varies widely across the landscape; some stands have little or no mortality while other stands have almost complete mortality. Catastrophic levels of mortality (exceeding 50 percent of stand basal area) occur on about 20 to 30 percent of the defoliated area and mortality rates of 20 to 50 percent of stand basal area occur on about half of the defoliated area resulting in average mortality rates of 25 to 35 percent of stand basal area. Silvicultural treatments have been proposed as an alternative to insect suppression treatments to minimize gypsy moth effects. There are two main approaches to creating more resilient forests to gypsy moth: to reduce the susceptibility of the stand to gypsy moth defoliation and to reduce the vulnerability of the stand to gypsy moth-related mortality. Stand susceptibility is reduced by: changing species composition to hosts less preferred by gypsy moth (creating healthy, mixed stands), removing structural features that are hiding places for gypsy moth larvae and pupae, and promoting predator and parasite habitat. Stand vulnerability is reduced by: removing high-risk trees and stands, increasing tree and stand vigor, and converting high-risk stands to mixed stands.

An experimental test was conducted to evaluate the effectiveness of two silvicultural treatments (presalvage and sanitation thinnings) in minimizing gypsy moth effects on forests. Sanitation thinnings have as their primary objective to reduce the susceptibility of the stand to gypsy moth defoliation. The thinning treatment achieves this objective through manipulation of the species composition, reducing the preferred host composition of mixed stands to 20 percent or less of the basal area. Presalvage thinnings have as their primary objective to reduce the vulnerability of the stand to gypsy moth-related mortality. The thinning treatment achieves this objective by removing trees with higher probabilities of mortality if defoliated (low crown vigor trees) and retaining trees with lower probabilities of mortality if defoliated (high crown vigor trees). Four replicates of each thinning and adjacent unthinned treatment stands were installed prior to gypsy moth defoliation. Six of sixteen stands were defoliated for two years by gypsy moth. Three years after defoliation ended, mortality was evaluated. Host preference class had a significant effect on defoliation patterns but thinning did not. Mortality was strongly influenced by defoliation patterns and by thinning. Thinning and defoliation had a significant interaction: in undefoliated stands, thinning had no effect on mortality, but in defoliated stands, it reduced mortality. Defoliated sanitation thinnings did not have a significant effect on either defoliation or mortality, but thinned stands did have lower mortality rates. Defoliated presalvage thinnings had significantly lower mortality rates than unthinned stands. These results support the use of silvicultural treatments prior to gypsy moth defoliation to minimize gypsy moth effects on tree mortality.