a1 Agriculture and Agri-Food Canada, Lethbridge Research Centre, PO Box 3000, Alberta, T1J 4B1, Canada
Common house flies, Musca domestica Linnaeus, were reared to pupation with exposure to ivermectin, washed, and then exposed to parasitism by Muscidifurax zaraptor Kogan & Legner. Pupae exposed to ≥ 0.25 ppm ivermectin produced 63% fewer parasitoids than did control pupae. Pupae exposed to 0.01 ppm ivermectin produced 23% more parasitoids. Exposure to 0.10 ppm ivermectin had no detectable effect. Treatments did not affect the developmental time of male or female F1parasitoids. Treatments did not affect the production of adult F2 progeny by F1 females, nor the sex ratios of these offspring. This study documents an indirect effect of ivermectin residues across trophic levels. It identifies altered host quality as another mechanism by which faecal residues of ivermectin may affect insect activity in dung of treated cattle. The importance of this phenomenon under field conditions is undetermined.
(Accepted March 14 1999)