Toxicity of a new molt-inducing insecticide (RH-5992) to aquatic macroinvertebrates.

The molt-inducing insecticide RH-5992, a potent ecdysone agonist, is being evaluated for potential use in forestry to control defoliating lepidopterans. The possible adverse effects of RH-5992 on nontarget aquatic organisms were studied in two test systems. Acute lethal effects were determined for one aquatic amphipod and 11 species of aquatic insects in laboratory flowthrough toxicity tests. Lethal and behavioral effects (drift response) on the amphipod and 8 species of stream insects were also evaluated under natural environmental conditions and more realistic exposure regimens in outdoor stream channels. There were no significant effects on drift or survival of the test species exposed to RH-5992 at the maximum test concentration of 3.5 mg/liter (100x the worst-case expected environmental concentration) in laboratory toxicity tests and stream channel treatments. Mortality of the amphipod Gammarus sp. in one toxicity test was considered an artifact, because there was no significant mortality in subsequent tests at concentrations up to 7.0 mg/liter, or in stream channels treated at 3.5 mg/liter. Yellow birch leaves were sprayed with RH-5992 at a rate of 50 g/ha and tested for residual toxic effects on two species of shredding invertebrates in the outdoor stream channels. There was no feeding inhibition or lethal effect on either test species resulting from consumption of the contaminated foliage. The candidate insecticide RH-5992 does not appear to pose undue risk of direct adverse effects to aquatic macroinvertebrates, particularly in water bodies where residues are likely to be short lived following aerial applications (e.g., lotic systems).