Effects of life stage, substrate, and crop position on the exposure and susceptibility of Aphidius rhopalosiphi DeStefani‐perez (Hymenoptera: Braconidae) to deltamethrin

Abstract

AbstractLaboratory bioassays were carried out to determine the toxicity of deltamethrin residues to the aphid parasitoid Aphidius rhopalosiphi DeStefani‐Perez (Hymenoptera: Braconidae) on glass and foliar substrates. Residual toxicities were compared by iterating sequences of lethal dose ratios between LD10 and LD90 from the dose–response statistics for each substrate. The calculated mean ratios were termed “toxicity factors” (Tf). The Tf values indicated that the toxicity of deltamethrin residues on glass plates was 1.3 times greater than on cereal flag leaves. Parasitoid mortality was reduced by lowering the deltamethrin concentration and parasitoid exposure period. Individuals exposed to field concentrations for less than 1 h recovered when exposure ceased. Parasitoid emergence from hosts was reduced by deltamethrin at field concentrations; however, the fecundity of surviving females did not differ from controls. Deltamethrin applied at half rate had no effect on parasitoid emergence. Spray deposition on aphid mummies within a mature winter wheat canopy varied significantly with leaf position. Mummies located on awns and adaxial surfaces of flag leaves received the highest deposition rates, with the lowest levels recorded for mummies on stems and abaxial leaf surfaces. The low volumes of spray solution impinging upon mummies in the field indicated that current laboratory bioassays may overestimate pesticidal effects on developing parasitoids by between 3.4‐ and 266‐fold.