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Factors affecting the mortality of winter moth in the lower mainland of British Columbia Horgan, Finbarr G.

Abstract

Populations of winter moth, Operophtera brumata (L.), were monitored at four sites in Richmond, British Columbia, between 1989 and 1992. Populations peaked in 1990 on both blueberry and birch and declined with an eventual population crash in 1992.Parasitism by Cyzenis albicans (Fall.) fluctuated between years at each site. Parasitism reached its highest levels on both birch (ca. 55%) and blueberry (ca. 35%) in 1991. Pupa predation was the most important stage specific mortality factor throughout the four years. In 1992, the year of population crash, larval mortality was high. Trends in "death of pupae due to unknown causes" were linked to larval mortality and are suggested to result mainly from poor foliage quality. An unusually early spring in 1992, may have led to the observed increases in these two mortality factors. The incidence of viral or other diseases among the populations are low. Generally, pupal predation peaked in 1990 (ca. 90%) and then declined. Pterostichus spp., Amara spp., Harpalus affinus and subsoil beetle larvae are implicated as important predators. The levels and trends in predation were similar at sites with very different assemblages and abundances of beetles. There were no differences in the abundances of beetles at each site between 1991 and 1992. However, many of the important predatory species declined in 1992. An examination of the possible interactions between C. albi cans and generalist predators is made. The increased range of pupal sizes in the soil, due to the presence of C. albi cans may be a mechanism for inducing a numerical response among generalist predators. However, simultaneous population declines at sites with low levels of C. albicans indicate that winter moth outbreak and decline in North America may be induced by a number of different factors.

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