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

Résumé 0

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 x L-1 for 10 min in river, with a cany of 20 km at 100m3 xsec-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% al 100 µg x L-1 for 10 min against 17% for temephos at the same dose and 59% for chlorphoxim at 50 µg x 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 LC50 for Chrysichthys nigrodigitatus is 150 µg x L-1 and that for Pollimyrus isidori 170 µg x L-1, values which are not very different from the operational dose of the larvicide (100 µg xL-1 for 10 min.). Nevertheless, in a river, no fish mortality was recorded. Based on fish LC50 and drift of benthic insects, pyraclofos at 100 µg x L-1 was judged to be less toxic to aquatic fauna in the short term than permethrin and carbosulfan.