The toxicity of dimethoate to predatory Coleoptera: developing an approach to risk analysis for broad-spectrum pesticides.
Topical toxicity bioassays were undertaken with the organophosphate compound dimethoate, O, O-dimethyl S-methylcarbamoylmethyl phosphorodithioate, against six species of predatory Coleoptera that are recognized predators of cereal aphids: the staphylinid Tachyporus hypnorum, the coccinellid Coccinella septempunctata and the carabids Demetrias atricapillus, Trechus quadristriatus, Bembidion obtusum and Nebria brevicollis. LD50 values for formulated dimethoate, diluted in water, varied between 3.4 and 98.8 ng a.i. insect-1 and 1.45 and 18.20 µg a.i. g-1 body weight. These values were similar to those obtained in another study by Wiles and Jepson (1992) for the pyrethroid insecticide deltamethrin, (S)-alphacyano-3-phenoxybenzyl (IR)-cis-3-(2, 2-dibromovinyl)-2,2,-dimethylcyclopropanecarboxylate. Hazard ratios were calculated for dimethoate and deltamethrin by dividing recommended field mass application rate in g a.i. ha-1 by LD50 in µg a.i. insect-1: this gave an indication of potential direct exposure hazard. The range of values for dimethoate (3,441 to 100,000) were well in excess of those for deltamethrin (28.6 to 500) because of its greater field application rate (340 g a.i. ha-1 compared with 6.25 g ai ha-1). LT50s were determined for adult C. septempunctata exposed to wheat leaves treated with either dimethoate or deltamethrin at full or half field rate within in-situ bioassays. At both rates, the dimethoate LT50s were shorter, ranging at full rate from 0.06 to 7.70 h, between 2 and 96 h after treatment, compared with 1.04 to 20.89 h after the same time intervals for deltamethrin. At half-field rate, the equivalent LT50 ranges were 0.52 to 13.65 h for dimethoate and 3.75 to 31.76 h for deltamethrin. The relative toxicities of the two insecticides, expressed as the ratio of log (x + 1) LT50 deltamethrin:dimethoate, narrowed greatly between 2 and 24 h, especially in the full rate treatment, indicating a more rapid loss of dimethoate than deltamethrin from foliage in the field shortly after spray application. The ecotoxicological consequences of exposure to dimethoate and deltamethrin are compared relative to their differing mass application rates, physicochemical properties and environmental fate. The role and value of laboratory-based toxicological testing within risk analysis procedures is discussed