After the Decline: What Maintains Low Winter Moth Density after Successful Biological Control?

Abstract

Life-table data for winter moth are analysed for an 8-year period following introduction of parasitoids for biological control in British Columbia, Canada. Winter moth density declined in 1984, and has remained low since 1985 at an average density of approximately 1.20 adults per m 2 . Introduced parasitoids contributed to mortality during the decline, and have a weak, delayed density-dependent effect at the new low density. Mortality of unparasitized pupae in the soil (primarily by predation) is strongly, and directly density-dependent at the new low density, and is the factor which is most strongly regulatory. Analysis indicates that the strong regulation of winter moth numbers by generalist predators allows parasitism levels to vary greatly after suppression without the eruption of winter moth populations; if parasitism were absent, winter moth populations would erupt to pre-biocontrol levels