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Title: DEMOGRAPHIC CHANGES IN DAPHNIA PULEX (LEYDIG) AFTER EXPOSURE TO THE INSECTICIDES SPINOSAD AND DIAZINON

Author
item STARK, JOHN - WASHINGTON STATE UNIV
item Vargas, Roger

Submitted to: Ecotoxicology and Environmental Safety
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2002
Publication Date: 3/4/2003
Citation: Stark, J.D., Vargas, R.I. 2003. Demographic changes in daphnia pulex (leydig) after exposure to the insecticides spinosad and diazinon. Ecotoxicology and Environmental Safety. 56(3):334-338.

Interpretive Summary: Spinosad is a new environmentally benign pesticide developed by DowElanco in response to criticism to the negative effects of pesticides. Little is known about the effect spinosad might have on aquatic ecosystems. In this study the toxicity of spinosad was assessed against Daphnia pulex a common aquatic invertebrate. Toxicology data were also generated for the commonly used organophosphorus insecticide diazinon as a comparison. Exposure to spinosad led to a decline in survival, birth rate, reproduction and population increase. Population extinction occurred after exposure to spinosad concentrations greater than 10 parts per million for 8 days. Exposure to increasing diazinon concentrations led to an initial increase in reproduction and rate of population growth, followed by a sharp decline, with extinction occurring after exposure to concentrations greater than 2 parts per million after 2 days. Spinosad was five times less toxic than diazinon. 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.

Technical Abstract: 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 organophosphorous insecticide diazinon as a comparison. Exposure to spinosad led to a concentration-dependent decline in survival, birth rate (b), net reproduction (Ro) and intrinsic rate of increase (rm). Population extinction (-rm) occurred after exposure to spinosad concentrations > 10 ug/L for 8 days. Exposure to increasing diazinon concentrations led to an initial increase in Ro and rm followed by a sharp decline, with extinction occurring after exposure to > 2 ug/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 3rd and 4th juvenile stages and little or no change in the 2nd juveniles 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 1st and 2nd juvenile stages, little or no change in the 3rd and 4th 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.