Tolerance levels of select stream macroinvertebrates to the Simulium larvicides, Chlorphoxim and decamethrin.

Tolerance levels of six non-target stream macroinvertebrates organophosphorus Simulium lavicide, chlorphoxim, and the synthetic pyrethroid decamethrin, were determined using rapid through-flow laboratory test vessels. A 1-hour exposure followed by 24-hour mortality recorded was adopted as a standard basis for comparison with the target. Simulium larvae. Additional comparisons between the reactions of Simulium larvae and non-targets to chlorphoxim were carried out on the basis of 15-minute exposures in a miniature stimulated stream or experimental channel. At the lowest concentration sufficient to produce 95% mortality in late instar Simulium larvae, decamethrin was highly lethal to all non-targets tested, with the exception of Ephemerella. Baetis and Gammarus proved to have a very high susceptibility to decamethrin, the former still recording 50% mortalities after exposure to concentrations as low as 0.005 parts per billion/1-hour. In the case of chlorphoxim, all the non-targets were found to be significantly more tolerant than Simulium larvae under identical conditions. The margin was narrowest in the case of Baetis (LC95 0.005 ppm as compared with 0.003 ppm for Simulium but wide enough in the case of the predaceous non-targets - Hydropsyche, Rhyacophila and Agrion - to suggest high survival at Simulium larvicide dosages. The results were compared with these previously obtained with other Simulium larvicides such as Abate, chlorpyrifos and permethrin under identical laboratory conditions. The applicability of these techniques to Simulium damnosum and its associated non-targets in West Africa is discussed, as also is the need for a comparable laboratory phase of evaluation in that programme.

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.

Relative susceptibility of stream macroinvertebrates to temephos and chlorpyrifos, determined in laboratory continuous-flow systems.

Laboratory techniques are described for evaluating the lethal and behavioral impact of pesticides on a range of stream macroinvertebrates under continuous through-flow and simulated stream conditions. The same basic test unit has been used, with slight modifications, to study the reactions of both Simulium larvae and non-target stream invertebrates. On the basis of a standard 1-hr exposure period to different concentrations followed by a 24-hr holding period in a continuous flow of clean water, different test organisms showed wide and consistent differences in tolerance to each of the two insecticides tested. The widest difference between two organisms occurred in the case of the Amphipod, Gammarus pulex (LC90-95, greater than 1 ppm) which was found to be about 5000 x more tolerant to temephos than are nymphs of the mayfly, Baetis rhodani. (LC 90-95, 0.001-0.002 ppm) The widest difference in the reactions of any one species is shown on the part of Gammarus which is about 100 times more susceptible to chlorpyrifos (LC 90-95, 0.05-0.1 ppm) than to temephos. The susceptibility levels of other indicator species such as Agrion, Hydropsyche, Brachycentrus, Ephemera, etc. are discussed in relation to susceptibility levels of Simulium larvae under the same test conditions, and also in relation to current field dosages of the two insecticides in practical and experimental Simulium control.