Effect of pirimicarb on the foraging behaviour of Diaeretiella rapae (Hymenoptera: Braconidae) on host-free and infested oilseed rape plants

The effects of the aphicide pirimicarb on the foraging behaviour of the aphid parasitoid Diaeretiella rapae were investigated in the laboratory, using aphid-infested and uninfested, young oilseed rape plants. Female parasitoids spent significantly less time foraging on plants sprayed with pirimicarb within the previous 24 h than on untreated plants, regardless of the presence of aphid hosts. Whilst foraging on sprayed plants, parasitoids spent relatively more time walking than they did when foraging on untreated plants. On sprayed plants foraging parasitoids tended to avoid leaf surfaces, thereby reducing the chances of encountering aphid hosts whilst on the plant. These sublethal effects of the aphicide on foraging parasitoids would considerably reduce their impact on aphid populations which survived in a crop after spraying. This has important implications for the control of aphids, such as Myzus persicae, which have developed pesticide resistant strains.