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 co mpound dimethoate, O,O-dimethyl S-methylcarbamoylmethyl phosphorodithi oate, 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 atric apillus, Trechus quadristriatus, Bembidion obtusum and Nebria brevicol lis. LD(50) 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 mu g a.i. g(-1) body weight. These values were similar to those obtained in anot her study by Wiles and Jepson (1992) for the pyrethroid insecticide de ltamethrin, (S)-alpha-cyano-3-phenoxybenzyl ibromovinyl)-2,2,-dimethyl cyclopropanecarboxylate. Hazard ratios were calculated for dimethoate and deltamethrin by dividing recommended field mass application rate i n g a.i. ha(-1) by LD(50) in mu g a.i. insect(-1): this gave an indica tion of potential direct exposure hazard. The range of values for dime thoate (3,441 to 100,000) were well in excess of those for deltamethri n (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)). LT(50)s were determined fo r adult C. septempunctata exposed to wheat leaves treated with either dimethoate or deltamethrin at full or half field rate within in-situ b ioassays. At both rates, the dimethoate LT(50)s 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 deltam ethrin. At half-field rate, the equivalent LT(50) ranges were 0.52 to 13.65 h for dimethoate and 3.75 to 31.76 h for deltamethrin. The relat ive toxicities of the two insecticides, expressed as the ratio of log (x + 1) LT(50) 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 exposur e to dimethoate and deltamethrin are compared relative to their differ ing mass application rates, physicochemical properties and environment al fate. The role and value of laboratory-based toxicological testing within risk analysis procedures is discussed.