Decline in Winter Moth Populations in North America: Direct Versus Indirect Effect of Introduced Parasites

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Abstract
(1) Re-analysis of winter moth data from Nova Scotia [Canada] indicates that pupal mortality, not parasitism, was the principal proximate cause of population decline. (2) Data from a recent introduction in British Columbia also implicate pupal mortality as the major cause of population decline, not introduced parasites. (3) Predation by beetle larvae is the major cause of pupal mortality in British Columbia. (4) In both populations, pupal mortality does not rise until after parasitoid introduction, this rise probably resulting from the interaction of parasitism and predation on pupae. (5) The indirect effect of parasitism, through its interaction with predation, may be far more important in successful control of winter moth than is the direct effect of adding mortality.