Relative toxicity of decamethrin, chlorphoxim and temephos (abate) to simulium larvae

Tolerance levels Simulium larvae (S. ornatum and S. equinum) to chlorphoxim and decamethrin, relative to those with Abate, were determined in rapid through-flow laboratory test vessels; a 1-hour exposure followed by 24-hour mortality observations being the standard of comparison of the three chemicals. In addition, a further comparison between chlorphoxim and Abate was made on the basis of a 15-minute exposure in a miniature simulated stream, with the same mortality criteria. On the basis of LC95/1-hour data the results showed that decamethrin was approximately 4 times as lethal as chlorphoxim, and approximately 20 times more than Abate. On the basis of a 15-minute exposure, chlorphoxim was found to be even more toxic than Abate--approximately 10 times more--than with the 1-hour exposure--viz 5 times more. Concentration/mortality curves indicate that with both the organophosphorus larvicides there is a critical zone where mortality increases sharply with increase in concentration of toxinant. With decamethrin, there is a more uniform rate of increase in mortality with increasing concentration, these differences being illustrated by comparison at both LC50 and LC95 levels. These findings are discussed firstly, in relation to other laboratory test methods in use, and secondly with reference to the scarcity of comparable laboratory data for Simulium damnosum in the Onchosceriasis Control Programme (OCP) in the Volta River basin area of West Africa.