Ecological effects of experimental insecticide applications on invertebrates in Sahelian temporary ponds

The ecological impact of four insecticides used to control desert locusts w as investigated during experimental field trials in natural temporary ponds in a cultivated savannah area of Senegal, West Africa. The insecticide for mulations, dose rates, and application techniques were similar to those use d in operational desert locust control. Average initial concentrations of f enitrothion, diflubenzuron, deltamethrin, and bendiocarb in pond water were 80, 10.4, 0.45, and 24.0 mu g/L, respectively. Diflubenzuron and deltameth rin virtually disappeared in 24 h. Pseudo-first-order half-lives were 34 h for fenitrothion and 17 d fur bendiocarb. Fenitrothion and deltamethrin sig nificantly reduced population densities of backswimmers of the genus Anisop s (Hemiptera, Notonectidae) and in addition caused an extensive kill of oth er species of insects. Both insecticide applications were also followed by reductions of zooplankton densities, especially Cladocera, and deltamethrin eradicated populations of fairy shrimp (Streptocephalus spp,: Branchiopoda , Anostraca). Diflubenzuron only affected crustaceans, i.e., cladocerans an d fairy shrimp. The least harmful insecticide in the study was bendiocarb, which only reduced the density of cladocerans. Recovery proceeded at fixed rates, which were different for each taxon. Anisops spp. recovered from the treatments in 1 to 4 weeks, most likely through aerial migration. Cladocer ans returned to normal densities in 3 to 6 weeks. Streptocephalus spp., how ever, whose resting eggs presumably need desiccation during the dry season to be able to hatch, did not reappear until the next rainy season, one year later. It is concluded that contamination of temporary ponds in the Sahel by insecticide drift from desert locust control operations with any of the four insecticides should he avoided.