Impact of insecticide drift on aphids and their parasitoids: residual toxicity, persistence and recolonisation

The effects of the insecticide Lannate® 20 L on target and non-target organisms were studied in sprayed and drift areas of kohlrabi (Brassica oleracea). The study comprised eight field plots (each 12.5 m×6 m), providing four replicates of a water control and the application with Lannate® 20 L at 0.45 kg/ha. Each plot comprised a 4 m wide sprayed area and a 2 m wide "drift" area, left unsprayed. The parasitoid Aphidius colemani (Hymenoptera; Braconidae) and its host aphid Myzus persicae (Hemiptera; Aphididae) served as indicator organisms. The residual toxicity of Lannate® 20 L to A. colemani and M. persicae was quantified in an exposure experiment. Plants were taken from the field at different time intervals following the insecticide treatment to expose test organisms to plant surfaces in the laboratory. Mortality rates of A. colemani were 94% on plants removed on the day of the application from the treated area and 79 and 69% on plants 0.6 and 2 m downwind from the treated area. No toxic effects on parasitoids were observed 4 days after treatment (a.t.). M. persicae were less susceptible to Lannate® 20 L residues compared to A. colemani. M. persicae mortality was 46% on plants collected from the sprayed area on the day of the application and 5 and 8% on plants collected from the drift area. In addition, densities of aphids and aphid mummies in the treated and drift areas of field plots were assessed from 1 day pre-treatment to 14 days post-insecticide application. Spraying significantly decreased aphid densities, but the population built up again. Compared to the control, no effect was detected on aphid population densities in the drift area of field plots. Densities of mummies in the insecticide treated area of plots were lower compared to densities in the control from 1 day post-application. There was no effect on mummy densities in the drift area. Recolonisation of field plots following the insecticide treatment was estimated by the release of A. colemani. Since A. colemani is not indigenous in northern Germany it was possible to determine their dispersal into the crop by mummy collections. Recovery of A. colemani mummies was low.