Short-term toxicity of pyraclofos used as a blackfly larvicide on non-target aquatic fauna in a tropical environment

Abstract

Among the many larvicides tested for the control of Simulium damnosum s.l. larvae, the vector of human onchocerciasis in West Africa, pyraclofos proved to be 100% effective at 100 μg × L −1 for 10 min in river, with a carry of 20 km at 100 m 3 × sec −1. Tests were then performed both in laboratory and field conditions to evaluate its toxicity on the non-target aquatic fauna. In experimental short-term gutter tests, the detachment of the total benthic insects was 35% at 100 μg × L −1 for 10 min against 17% for temephos at the same dose and 59% for chlorphoxim at 50 μg × L −1 for 10 min. Centroptilum , Baetis and Pseudopannota bertrandi were the most affected organisms. The treatment of a river resulted in a considerable detachment of the same taxonomic groups, plus Orthocladiinae. On the other hand, investigations conducted in tanks showed that the 24-hr LC 50 for Chrysichthys nigrodigitatus is 150 μg × L −1 and that for Pollimyrus isidori 170 μg × L −1, values which are not very different from the operational dose of the larvicide (100 μg × L −1 for 10 min.). Nevertheless, in a river, no fish mortality was recorded. Based on fish LC 50 and drift of benthic insects, pyraclofos at 100 μg × L −1 was judged to be less toxic to aquatic fauna in the short term than permethrin and carbosulfan.