Comparison of the acute toxicity for gamma-cyhalothrin and lambda-cyhalothrin to zebra fish and shrimp

Gamma-cyhalothrin 15CS (GCH) contains only the active stereoisomer of the two isomers found in lambda-cyhalothrin 25EW (LCH). GCH (0.5 × rate) provides equivalent overall insect control as LCH (1 × rate). Both formulations showed high acute toxicity to zebra fish (Brachydanio rerio H.B.) and shrimp (Macrobrachium nippoensis de Haan). The 96-h LC50(zebra fish,GCH) is 1.93 μg a.i/L and LC50(zebra fish,LCH) is 1.94 μg a.i/L. LC50(shrimp,GCH) is 0.28 μg a.i./L and LC50(shrimp,LCH) 0.04 μg a.i./L. This indicates that the toxicity to shrimp is likely stereochemistry-dependent. The fates of GCH and LCH are similar in laboratory simulated rice paddy water and their concentrations decrease rapidly, with no GCH or LCH detected after 3 or 4 days. Both are toxic to shrimp in a simulated paddy irrigation reservoir even though treated return water is diluted 5 times. No shrimp fatality is shown in the GCH-treated paddy water after a 4-day holding period, and longer than 5 days is necessary to reach a zero fatality rate for LCH. This is compatible with the 7-day water holding period considered reasonable in agricultural practice.