Nontarget effects of neem-based insecticides on aquatic invertebrates.

Neem-based insecticides are being investigated as alternatives to synthetic insecticides for the control of forest insect pests in Canada. Because the proposed use pattern against forest defoliators often necessitates spraying near shoreline areas, an assessment of adverse effects of neem on nontarget aquatic invertebrates was warranted. Acute lethal effects of two neem-based formulations on eight species of macroinvertebrates were determined in flow-through screening tests at 10 times the expected environmental concentration (0.35 mg/liter). Significant mortality occurred only in the mayfly Isonychia bicolor/rufa exposed to one formulation, and further tests were conducted to determine a concentration-dependent response. The LC50 for I. bicolor/rufa was estimated at 1.12 mg/liter; the LC10 was 0.63 mg/liter. Three detritivorous species were tested at expected environmental concentrations (0.035 mg/liter) at longer exposures in aquatic microcosms to determine effects of the two formulations on feeding rates and survival. Neither formulation caused significant mortality or antifeedant effects after a 28-day exposure. The implications of these results for forest pest management operations are discussed.