Population-level effects of the neem insecticide, Neemix, on Daphnia pulex

Although natural insecticides from the neem tree are generally perceived as less harmful to the environment than synthetic insecticides, new evidence indicates that these products may pose a risk to certain nontarget organism s. In this paper, acute and chronic effects of commercial neem insecticides on the aquatic invertebrate, Daphnia pulex were examined. The acute toxici ty of two commercial neem insecticides, Neemix, Azatin and the experimental insecticide, RH-9999 to D. pulex was investigated using traditional 48 hr concentration-mortality estimates. Neemix and Azatin were equitoxic with LC 50s of 0.68 and 0.57 ppm; RH-9999 was significantly less toxic with an LC50 of 13 ppm. A 10 d population growth study was conducted for Neemix and a N eemix formulation blank (Neemix devoid of the active ingredients) to determ ine whether the active ingredients of Neemix and/or components of the formu lation were responsible for toxicity. D. pulex populations went to extincti on after exposure to a Neemix concentration of 0.45 ppm azadirachtin (equiv alent to the acute LC7). Neemix No Observable Effect Concentration (NOEC) a nd Lowest Observable Effect Concentration (LOEC) values for population grow th were 0.045 and 0.15 ppm azadirachtin, respectively. The mean number of o ffspring per surviving female (Ro) declined in a concentration-dependent ma nner after exposure to Neemix with no offspring being produced after exposu re to 0.45 ppm. Neemix NOEC and LOEC values for reproduction were 0.045 and 0.15 ppm, respectively. The formulation blank caused no mortality in the i ndividuals used to start the population growth study but reduced reproducti on and population growth accounting for 47% of the toxicity caused by Neemi x at a concentration of 0.15 ppm. Thus, the formulation contributes substan tially to the toxicity of Neemix but neem components are also toxic to D. p ulex. Because the NOEC for population growth and reproduction were higher t han the estimated environmental concentration of 0.035 ppm (a measure devel oped for forest pest mananagement), Neemix should pose little risk to popul ations of D. pulex.