Cereal aphid and parasitoid survival in a logarithmically diluted deltamethrin spray transect in winter wheat: field-based assessment.

Abstract

Dose rates of the pyrethroid insecticide deltamethrin were optimized for aphid control and the conservation of beneficial parasitoids in winter wheat. A logarithmic dose sprayer was used to apply concentrations ranging from twice the recommended field rate to a concentration representing one-fiftieth of the field rate. Volumetric analysis of colored dye deposits quantified the partitioning of pesticide through the crop canopy and was used to calculate direct exposure rates of aphid colonies and residue concentration on different plant structures contacted by foraging parasitoids. Aphid parasitoids, from the genus Aphidius, were found to be more at risk from exposure to insecticide residues than were their aphid hosts under the given bioassay conditions. However, the toxicity of deltamethrin residues on flag leaves was found to dissipate over a 5-d period after application. Deltamethrin provided significant aphid control at dose rates greater than 1 g a.i./ha (or at approximately one-sixth of the recommended field rate). At low concentrations of deltamethrin, aphids survived in the lower crop strata within low toxicity refuges. The implications of these refugia and the spatial redistribution of prey and host populations following spray application for the success of biological control agents are discussed.