The dietary toxicity of deltamethrin to the carabid, Nebria brevicollis (F.).

Abstract

Batches of adult Nebria brevicollis (F.) (Coleoptera: Carabidae) were provided with freeze-killed aphids, Metopolophium dirhodum (Walk.) (Homoptera: Aphididae), that had been treated topically with 30 ng a.i. deltamethrin. Beetles in different hunger states were found to consume contaminated aphids, but they consumed significantly fewer aphids than control beetles. The mean numbers of contaminated aphids eaten increased with hunger-level, from 2.5 aphids per beetle for those provided with food 24 h before exposure, to 4.5 aphids per beetle for those provided with food 120 h prior to exposure. The highest mortality that occurred over the following six days after consumption of contaminated aphids was 27% for the beetles that had been fed 120 h before exposure and the lowest was 13% for the beetles that had been fed 24 and 48 h prior to exposure. A regurgitation response was observed in 53-80% of the beetles that had consumed deltamethrin-treated aphids. The beetles that survived consumption of treated aphids showed a short-term reduction in mean numbers of untreated aphids consumed relative to the control beetles. Dietary and topical dose-response data for deltamethrin and N. brevicollis were compared by probit analysis and maximum likelihood procedures. The dietary exposure probit line had a significantly steeper slope than the topical exposure probit line but the two lines occupied similar positions along the dose axis. The steepness of the dietary exposure probit line may be partly explained by the regurgitation response shown by the beetles, which causes water loss and may amplify toxic effects. The results indicated that the consumption of deltamethrin-contaminated prey may be an important cause of predator mortality soon after pesticide application.