UBC Theses and Dissertations
Success and failure of Cyzenis albicans in controlling its host the winter moth Roland, Jens
Situations in which the parasitic fly Cyzenis albicans appears to control numbers of its host the winter moth, were compared with situations where it does not. Comparisons were made by field observation and experimentation during the first four years of winter moth decline following introduction of C. albicans and Agirypon flaveolatum in Victoria, British Columbia, Canada. Successful control of winter moth on oaks was compared with failure on apple trees. Phenology of winter moth development differs on the two tree species, giving rise to potential asynchrony between host and parasite on apple trees. Difference in the degree of asynchrony on the two tree species does not, however, result in a difference in level of parasitism of winter moth. Aggregative attack by Cyzenis, in response to host density, occurs when winter moth are on oaks, but is absent when winter moth are on apple. Difference in pattern of attack did not result in difference in mortality from parasitism at high density but did so at low density. The manipulation of predatory beetle abundance by as much as 100-fold did not affect the magnitude of mortality of winter moth, and affected mortality of the parasitoid only slightly. Large, density-dependent mortality of Cyzenis in the soil did not preclude their increase to high density, and hence would not affect the impact of Cyzenis on winter moth densities. The magnitude and degree of density-dependence of winter moth pupal mortality in the soil (regardless of beetle abundance) suggests that mortality of this stage, rather than parasitism of larvae by introduced parasites, has caused population decline in both British Columbia and Nova Scotia. The factor causing most mortality in the soil in British Columbia appears to be pathogens. Suggested mechanism for the increase in soil mortality during the four years of study are: (1) introduction of and/or increased pathogen transmission via Cyzenis or (2) activation of latent infection by density-induced stress of the host. This study implicates factors other than parasitism by introduced parasitoids in the suppression of winter moth numbers. Correlation between degree of aggregation in response to host density and degree of successful control, however, supports the claim that parasitoids can stabilize host populations at low density.