Demographic changes in Daphnia pulex (Leydig) after exposure to the insecticides spinosad and diazinon

The toxicity of the natural insecticide spinosad was assessed against Daphnia pulex (Leydig) using a demographic approach. Data were also generated for the commonly used organophosphorus insecticide diazinon as a comparison. Exposure to spinosad led to a concentration-dependent decline in survival, birth rate (b), net reproductive rate (R(0)), and intrinsic rate of increase (r(m)). Population extinction (-r(m)) occurred after exposure to spinosad concentrations >10 microg/L for 8 days. Exposure to increasing diazinon concentrations led to an initial increase in R(0) and r(m) followed by a sharp decline, with extinction occurring after exposure to >2 microg/L after 2 days. Based on concentrations of pesticide that caused population extinction, spinosad was five times less toxic than diazinon. The stable age distribution (after 65 days) of D. pulex changed after exposure to spinosad and diazinon. Increasing concentrations of spinosad resulted in a decrease in the percentages of individuals in the first juvenile and adult stages, increase in the third and fourth juvenile stages, and little or no change in the second juvenile and adolescent stages. Diazinon had a different effect on stable age distribution. Increasing concentrations of diazinon resulted in an increase in percentages of individuals in the first and second juvenile stages, little or no change in the third and fourth juvenile stages and adolescent stage, and a decrease in the adult stage. Although spinosad and diazinon are both neurotoxins, they have different modes of action and populations of D. pulex reacted differently to each pesticide. Results of this study indicate that spinosad is significantly less toxic than diazinon to D. pulex and because it is applied at lower concentrations than diazinon it should be less hazardous to this species.