Cyzenis albicans (Diptera : Tachinidae) does not prevent the outbreak of winter moth (Lepidoptera : Geometridae) in birch stands and blueberry plots on the lower mainland of British Columbia

In the late 1980s, a new outbreak of the introduced winter moth, Operophter a brumata (L.), occurred in Richmond, on the lower mainland of British Colu mbia. This was accompanied Ly the introduced parasitoid, Cyzenis albicans ( Fallen). Populations were monitored at 2 birch woodlands and 2 blueberry pl ots between 1989 and 1993. Parasitism by C, albicans and predation by gener alist predators were important mortality factors during the outbreak. Preda tion of moth pupae increased at all sites between 1989 and 1990. Parasitism by C. albicans caused significant mortality each year reaching a maximum i n 1991 and remaining high through to 1993 at birch sites. The winter moth p opulations collapsed simultaneously in 1992 at all study sites, despite dif ferent levels of parasitism and 2 very different host plants. As the outbre ak collapsed at Richmond, the moth continued to increase in numbers and cau se severe defoliation of birch at new sites in southern Vancouver where par asitism remained low. C albicans is unable to prevent the initial outbreak of winter-moth even when the 2 species are introduced simultaneously. The p arasitoid requires high density host populations before becoming well estab lished, but may contribute sufficient additional mortality to prevent subse quent prolonged outbreaks. The control of high density winter moth populati ons in North America by C, albicans supports the hypothesis that natural en emies that are rare in their native habitat will be effective control agent s when released into exotic habitats without competitors or their own natur al enemies.