Soybean aphid populations cycle between two hosts; a primary overwintering host, R. cathartica, and the secondary host, Glycine max L. (Merr). In this study, soybean aphid populations on R. cathartica (buckthorn) were observed throughout the spring of 2005 near Guelph, Ontario, Canada. The contribution of predation to aphid mortality was greater than that of parasitism; Coccinellidae (Coleoptera) and Cecidomyiidae (Diptera) were the most frequently observed predators in association with aphid populations. The lack of parasitoid activity prompted behavioural studies in which the influence of aphid population density on host and prey location was tested in two-way olfactometry trials using the parasitoid Aphidius colemani Viereck (Hymenoptera: Aphidiidae) and the predator Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae). H. axyridis appeared insensitive to volatiles produced by aphid feeding on R. cathartica, suggesting that physical, rather than olfactory cues, may be of greater importance in locating prey. A. colemani responded positively only at aphid densities exceeding those found naturally across our study site. Differences in host and prey-location behaviour by these species may have contributed to the differences in predator and parasitoid diversity observed in this study.