Predicting the risk posed by deltamethrin to beneficial invertebrates in temperate cereal crops

Abstract

Data are presented from a three year study to evaluate the risk posed by summer applications of the synthetic pyrethroid insecticide deltamethrin to a range of polyphagous and aphid-specific predators that inhabit temperate cereal crops. The need for such a study has become evident from the difficulty in interpreting data from large-scale field trials and the general lack of quantitative data concerning the mechanisms of susceptibility and exposure that mediate the short-term effects of pesticides on these predators in cereal crops. Initially, laboratory topical bioassays established a 300 fold range of predator susceptibility to deltamethrin, with a linyphiid spider being the most susceptible species tested. In addition, residual bioassays indicated that fresh deltamethrin residues on cereal plant foliage may be 50 to 60 times more toxic to predators than residues on a sandy loam soil. Dietary uptake studies, with a single polyphagous predator, showed that predator mortality may also result from consumption of deltamethrin contaminated prey. Following this, in situ bioassays determined the toxic risk posed by direct contact and exposure to realistic concentrations of deltamethrin in the field. Results indicated that low levels of mortality may occur for nocturnal ground-active predators, such as large carabid beetles, however for some plant-active diurnal predators, such as coccinellids, it may be necessary to reduce the recommended dose rate of deltamethrin by three quarters to preserve approximately 50% of the population in the crop during the 10 days after spraying. The results are discussed in terms of how toxicological and ecological criteria can be used to predict which species are most at risk from a summer spray application and how these predicitions may be used to aid the interpretation of results from large-scale field trials. The experimental framework developed in this study may be adapted to evaluate pesticide side-effects in other crops.

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