The toxicity of a neem insecticide to populations of culicidae and other aquatic invertebrates as assessed in in situ microcosms

Microcosm trials were conducted with the botanical insecticide Margosan-O(R ) to assess the potential hazards of the product to aquatic organisms. Labo ratory chronic bioassays with water from the treated microcosms were conduc ted to provide an estimate of the residual effect of Margosan-O. Results fr om chronic tests showed Margosan-O toxicity to be greater in the laboratory exposures than in situ with Culicidae larvae exposed to the same concentra tions. Residue analyses of the active ingredient, azadirachtin, determined that it had a half-life of 36 to 48 h in water exposed to natural sunlight. Two applications of Margosan-O at the recommended application rate for pes ts did not harm aquatic invertebrates that are categorized as planktonic an d filter feeding (Culex sp. and Daphnia sp.). However, the benthic inverteb rate (Chironomus riparius) was affected by multiple applications of neem. T hese results show that the use of Margosan-O and possibly other neem extrac ts in or near aquatic environments could lead to disturbances in benthic po pulations and may cause decreases in numbers of organisms that are importan t in food web and nutrient cycling processes.