AFTER THE DECLINE - WHAT MAINTAINS LOW WINTER MOTH DENSITY AFTER SUCCESSFUL BIOLOGICAL-CONTROL

1. Life-table data for winter moth are analysed for an 8-year period f ollowing introduction of parasitoids for biological control in British Columbia, Canada. 2. Winter moth density declined in 1984, and has re mained low since 1985 at an average density of approximately 1.20 adul ts per m2. 3. Introduced parasitoids contributed to mortality during t he decline, and have a weak, delayed density-dependent effect at the n ew low density. 4. Mortality of unparasitized pupae in the soil (prima rily by predation) is strongly, and directly density-dependent at the new low density, and is the factor which is most strongly regulatory. 5. Analysis indicates that the strong regulation of winter moth number s by generalist predators allows parasitism levels to vary greatly aft er suppression without the eruption of winter moth populations; if par asitism were absent, winter moth populations would erupt to pre-biocon trol levels.