Substrate-mediated toxicity of deltamethrin residues to beneficial invertebrates: estimation of toxicity factors to aid risk assessment.

Abstract

Laboratory bioassays were carried out to determine the toxicity of residues of the synthetic pyrethroid insecticide, deltamethrin ((S)-α-cyano-3-phenoxybenzyl (1R)-cis-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropanecarboxylate) to a range of beneficial invertebrates exposed to treated wheat foliage, sandy loam soil, and glass surfaces. The test invertebrates represented a range of predators and a parasitoid that inhabit the foliage and ground layers of temperate cereal crops, i.e., Pterostichus melanarius (Illiger), Nebria brevicollis (F.), Demetrias atricapillus (L.) and Bembidion obtusum (Serville) (Coleoptera: Carabidae), Tachyporus hypnorum (F.) (Coleoptera: Staphylinidae), Coccinella septempunctata (L.) (Coleoptera: Coccinellidae) adults (A) and larvae (L), Episyrphus balteatus (Degeer) (Diptera: Syrphidae), and Aphidius rhopalosiphi (De Stefani-Perez) (Hymenoptera: Braconidae). In 2 h flag leaf exposure bioassays, the LD₅₀ values varied from 0.4 g ai/ha to >50 g ai/ha. The susceptibility ranking, from most to least susceptible, was C. septempunctata (L) >T. hypnorum >C. septempunctata (A) >E. balteatus >A. rhopalosiphi >D. atricapillus. On soil, the LD₅₀ values were 52.8 g ai/ha for T. hypnorum and 97.8 g ai/ha for C. septempunctata (A) after 2 h,exposure. This period of exposure was, however, found to be of insufficient duration to separate the susceptibilities of the other test species adequately, and therefore 72 h exposure bioassays were also carried out. The 72 h LD₅₀ values varied between 4.2 g ai/ha and 267 g ai/ha. The susceptibility ranking, from most to least susceptible, was T. hypnorum >B. obtusum >C. septempunctata (A) >P. melanarius >N. brevicollis >D. atricapillus. In 2 h glass bioassays the LD₅₀ range was 1.2 g ai/ha to > 37 g ai/ha and the susceptibility ranking was T. hypnorum >C. septempunctata (A) >N. brevicollis, and D. atricapillus. Residual toxicities in the different 2 h exposure bioassays were compared for T. hypnorum and C. septempunctata (A) by iterating sequences of lethal dose ratios between LD₁₀ and LD₉₀ from the dose-response statistics for each respective pair of substrates. The mean ratios were termed 'toxicity factors' (Tf). Tf values comparing glass and flag leaf assays, were 0.98 and 1.23, respectively for T. hypnorum and C. septempunctata (A), indicating that the toxicity of fresh deltamethrin residues to both species was similar on these surfaces. Tf values comparing either glass or flag leaf and soil were however in the range of 50-60 for T. hypnorum and C. septempunctata (A), indicating much lower effects on soil. Estimates of the bio-available half-life on leaf and soil, obtained from in situ bioassays, indicated that effects in the field may vary two- to three-fold as a result of differences in the duration of toxicity on these substrates. Overall, T. hypnorum could be at more than 150 times greater risk on leaf compared to soil surfaces. The potential for extrapolating laboratory toxicity data to the field is discussed.