The effects of pyrethroid insecticides upon parasitic hymenoptera in the cereal ecosystem

Abstract

Data are presented from a three year study to evaluate the risk posed by summer applications of the synthetic pyrethroid insecticide, deltamethrin, to parasitic Hymenoptera associated with aphids in temperate cereal crops. Such a study was required to aid short-term risk predictions for parasitoids in the field, derived from current laboratory and semi-field experiments. Standard laboratory bioassays were used to investigate lethal and sublethal effects of deltamethrin upon parasitoid life-stages. Modifications were made to these tests to improve realism and enable field risk predictions. Results indicated that the toxicity of deltamethrin residues was dependent upon concentration, parasitoid exposure time and substrate. Immature parasitoid stages within aphid hosts were shown to be protected from deltamethrin. Estimates of actual exposure of mummies to pesticides in the field suggested that current laboratory bioassays overestimate exposure and therefore possibly the toxic effects of insecticides. A quantification of the contribution made by the different routes of exposure (topical, residual and dietary) of parasitoid life-stages to insecticides was obtained with radio-labelled and radioinert diazinon. Uptake by these routes was shown to be dependent upon insecticide concentration and time of exposure. Laboratory behavioural studies with parasitoids were undertaken to investigate the degree of attraction/arrestment by honeydew deposits in the presence of deltamethrin, which has repellent properties. Insecticide residues altered foraging behaviour, with honeydew causing parasitoids to remain longer in insecticide-treated areas. The implications of this for foraging parasitoids in the field are discussed. Field studies in winter wheat investigated the short-term effects of deltamethrin applied at a series of reduced dose-rates on aphid and parasitoid populations. Aphid colonies were found to survive on the lower crop strata and toxicity of deltamethrin residues to parasitoids declined over subsequent days after treatment. Results suggested that concentrations as low as 1/6th of the current recommended field rate may be suitable for uniting chemical and parasitic control in future integrated pest management programmes. Larger field studies investigating the longer- term effects of aphid primary parasitoid and hyperparasitoid populations rapidly recovered in 4ha treated plots of winter wheat through a progressive reinvasion of individuals from surrounding untreated areas. deltamethrin applications indicated that Overall, the results from this study are discussed in terms of how toxicological and ecological data may be collected and used to improve risk predictions for parasitoid species exposed to insecticides. Recommendations are given for the design and analysis of future laboratory, semi- field and field studies of aphid parasitoids.

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