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, Operophtera brumata (L.), occurred in Richmond, on the lower mainland of British Columbia. This was accompanied by the introduced parasitoid, Cyzenis albicans (Fallén). Populations were monitored at 2 birch woodlands and 2 blueberry plots between 1989 and 1993. Parasitism by C. albicans and predation by generalist predators were important mortality factors during the outbreak. Predation of moth pupae increased at all sites between 1989 and 1990. Parasitism by C. albicans caused significant mortality each year reaching a maximum in 1991 and remaining high through to 1993 at birch sites. The winter moth populations collapsed simultaneously in 1992 at all study sites, despite different levels of parasitism and 2 very different host plants. As the outbreak collapsed at Richmond, the moth continued to increase in numbers and cause severe defoliation of birch at new sites in southern Vancouver where parasitism remained low. C. albicans is unable to prevent the initial outbreak of winter moth even when the 2 species are introduced simultaneously. The parasitoid requires high density host populations before becoming well established, but may contribute sufficient additional mortality to prevent subsequent prolonged outbreaks. The control of high density winter moth populations in North America by C. albicans supports the hypothesis that natural enemies that are rare in their native habitat will be effective control agents when released into exotic habitats without competitors or their own natural enemies.