Toxicity of microencapsulated permethrin to selected nontarget aquatic invertebrates

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Abstract
Microencapsulated permethrin (penncapthrin) was evaluated under laboratory conditions for its toxicity toward several nontarget aquatic invertebrates. Average LC50 estimates for selected lotic invertebrates, based on a one hour dosing regime, were: 2.71 mg/L forSimulium vittatum, 4.59 mg/L forHydropsyche spp., and 13.41 mg/L forIsonychia bicolor. In acute static tests withDaphnia magna, there was no significant difference (p≤0.05) between the toxicity of penncapthrin at 96 h (LC50 range: 6.80–22.5 μg/L) and the EC formulation at 72 h (LC50 range: 0.6–21 μg/L). Comparatively, the toxicity of microencapsulated methyl parathion (penncap-m) was not significantly different from that of penncapthrin towardD. magna, the former having LC50 estimates ranging form 0.3–12.25 μg/L. LC50 estimates associated withDaphnia pulex ranged from 19 to 131 μg/L. The toxicity of penncapthrin and penncap-m towardD. pulex was difficult to determine because of frequent control mortality due to food deprivation resulting from the need to run tests for longer than 48 h. In successful tests, LC50 estimates ranged from 19 to 28 μg/L for penncapthrin and 0.08 to 25 μg/L for penncap-m after 72 h exposure. In long term toxicity tests, 95% of D. magna at 1 μg/L, 44% at 10 μg/L, and 20% at 15 μg/L survived after 39 days exposure. Less than 15% ofD. pulex survived over the same concentration range following 32 days exposure. Despite some drawbacks, long-term toxicity tests were more appropriate than short-term tests for evaluating microencapsulated sticides because of reduced variability in LC50 estimates and lower control mortality.