Impact of botfly parasitism on Microtus townsendii populations

Abstract

The impact of botfly (Cuterebra grisea) parasitism on populations of Microtus townsendii was studied on four areas near Vancouver, British Columbia, from 1971 to 1978. Botfly larvae were found in voles during the period from late June to late November, with a peak occurring from mid-August to mid-September. There was a significant negative relationship between vole density and the proportion of the population infested. The percentage of voles infested changed in the following manner in the three weight classes: juveniles (18.5%) > adults (16.5%) > subadults (13.0%), with the sexes showing no differences between them. Overall, noninfested voles always survived better than infested ones did in the three classes. In both adults and juveniles, survival declined as botfly larvae number increased; subadults did not show this relationship. Infested adult male voles survived more poorly at high densities (r = 0.62); adult females did not. Infested large adult males (≥ 55 g) survived better than small adults did (40–54 g), but the two size classes in females showed no survival differences. Infested adult voles had significantly lower rates of reproduction: 8% fewer males were in breeding condition, 10% fewer females were lactating, and 4% fewer females were pregnant. Parasitism significantly depressed growth rates in all weight classes above 40 g. We conclude that botflies had a significant detrimental influence on vole survival, reproduction, and growth.