The acute toxicity to third-instar Aedes aegypti (L.) of two 1-h exposures to each of five insecticides (technical permethrin, microencapsulated permethrin, fenitrothion, carbaryl, and carbofuran) was determined with static toxicity tests. The toxicity of two 1-h exposures to these test compounds, separated by a 6-h insecticide-free period, was compared to that of continuous 2-h exposures. Acute toxicity was expressed as LC50 values based on survival to the adult stage. The LC50 for two 1-h exposures to microencapsulated permethrin (180 μg/L) was significantly lower than that for a continuous 2-h exposure (250 μg/L). There was no significant difference between LC50 values for two 1-h and continuous 2-h exposures for each of technical permethrin (2.03–2.32 μg/L), fenitrothion (49.4–48.8 μg/L), carbaryl (3,040–3,470 μg/L, and carbofuran (1,590–2,130 μg/L).
Increasing time between two 1-h exposures to carbaryl from 6 to 24 h did not significantly affect acute toxicity (LC50 values were 3.0 mg/L and 3.8 mg/L, respectively). The LC50 for four 1-h exposures to carbaryl each separated by a 12-h period in clean water (LC50 = 1.7 mg/L) was not significantly different from that of a continuous 4-h exposure (LC50 = 1.4 mg/L). These results suggest that there was no recovery from the effect of insecticide poisoning during the insecticide-free period between exposures.