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.