Effects of the herbicides hexazinone and triclopyr ester on aquatic insects

Experiments were conducted to measure acute lethal response of aquatic insects to hexazinone (Velpar L) and triclopyr ester (Garlon 4) in flow-through laboratory bioassays, and to determine lethal and behavioral effects of these herbicides on insects in outdoor stream channels. No significant mortality (chi 2 P greater than 0.05) occurred in 13 test species exposed to hexazinone in laboratory flow-through bioassays (1-hr exposure, 48-hr observation) at the maximum test concentration of 80 mg/liter. The survival of insects exposed to 80 mg/liter hexazinone in outdoor stream channels was likewise unaffected. Significant drift (chi 2 P less than 0.001) of Isonychia sp. occurred during a hexazinone treatment of the stream channels, but only at the maximum concentration of 80 mg/liter, and survival of the displaced Isonychia sp. was not affected. In flow-through bioassays with triclopyr ester, 10 of 12 test species showed no significant mortality at concentrations greater than 80 mg/liter. Survival of Isogenoides sp. and Dolophilodes distinctus was significantly affected at less than 80 mg/liter. Lethal concentrations were estimated by probit analysis of concentration-response data (1-hr exposure, 48-hr observation) for Simulium sp. (LC50 = 303 mg/liter), Isogenoides sp. (LC50 = 61.7 mg/liter), and D. distinctus (LC50 = 0.6 mg/liter). Triclopyr ester applications to the stream channels resulted in significant drift and mortality of D. distinctus at 3.2 mg/liter (no effects at 0.32 mg/liter), Isogenoides sp. at 32 mg/liter, and Hydropsyche sp. and Epeorus vitrea at 320 mg/liter. The risk to aquatic insects of these herbicides used in forest vegetation management is discussed.