Analysis of the effects of rotational larviciding on aquatic fauna of two Guinean rivers: the case of permethrin

Within the Onchocerciasis Control Programme about 50,000 km of west African rivers have been regularly sprayed with larvicides to control the vector o f dermal filariasis caused by Onchocerca volvulus. Since the beginning of t he programme invertebrates and fish data were collected to monitor adverse effects on non-target organisms. The regular series of biological and hydro logical data collected in two Guinean rivers were analysed to evaluate the effects of rotational larviciding with particular attention to permethrin, as preliminary acute toxicology tests and semi-field experiments suggest it has stronger effects on non-target fauna in respect to other larvicides. I nvertebrates and fish variations in biomass and species richness are season al and flow-related and the results presented here do not support any evide nce of specific effects of permethrin application on the biological targets monitored. Larvicide applications influence community structures, putting pressure on some taxonomic groups, causing, for example, the rarefaction of some taxa. In spite of the above results, the scarcity of some invertebrat e systematic units does not result in a significant reduction of total inve rtebrate density because of the corresponding increase in other systematic units. In nature the studied aquatic communities would rarely be in equilib rium because of frequent natural stresses, such as drought and spate events , the biological variations discussed are to be considered ecologically acc eptable. (C) 2001 Elsevier Science Ltd. All rights reserved.