Environmental impact of mosquito pesticides: Toxicity and anticholinesterase activity of chlorpyrifos to fish in a salt marsh habitat

Abstract

Chlorpyrifos was tested for its influence on thein vitro andin vivo brain acetylcholinesterase (AChE) activity ofFundulus heteroclitus under laboratory as well as field conditions. The concentration required for a 50% reduction in the mvitro enzyme activity (I₅₀) was 7.2 × 10⁻² mM for the parent compound and 4.1 × 10⁻⁶ mM for its oxygen analog. A 96-hr exposure of live fish to 1.0Μg/L chlorpyrifos resulted in a maximum AChE inhibition of 24%. At a concentration of 2.1Μg/L or higher, a 100% enzyme inhibition was observed after a 24-hr exposure period, followed by varying degrees of recovery during the next 24 hr. A second peak of AChE inhibition, proportional to the concentration of the insecticide, was observed 72 hr after the initial exposure and this was followed by a second phase of recovery during the next 24 hr. In spite of an initial 100% AChE inhibition at test concentrations of 2.1Μg/L and above, the fish mortality was less at lower concentration of the insecticide than at the higher levels. The 96-hr TL₅₀ and TL₅ (tolerance limit for 50% and 5% fish survival) of chlorpyrifos forF. heteroclitus were 4.7Μg/L and 12.2Μg/L, respectively. The LT₅₀ (lethal time in which 50% of the fish died) at a concentration of 5.6Μg/L was 49.5 hr.

Fish exposed to 4 successive field applications of chlorpyrifos granules showed AChE inhibition ranging from 56 to 100% and the insecticide effect was cumulative in nature. By 24 hr after the second application, 18.6% of the treatment fish had died; live fish collected at this time showed a 96% depression of AChE activity. AChE inhibition was still evident (62%) in fish 69 days after the final application of chlorpyrifos.