R8 PM Pesticide Mixtures
Thursday, 17 November 2005: 1:50 PM - 5:30 PM in 331-332

(WAC-1117-728134) Temporal variation in mixture toxicity of chlorpyrifos and atrazine on Pimephales promelas and Xenopus laevis.

Wacksman, M¹, Lydy, M¹, ¹ Southern Illinois University Carbondale, Carbondale, IL, USA

ABSTRACT- Chlorpyrifos is often used in agricultural and residential settings to control worms, grubs, cockroaches, beetles and other insects. Atrazine is widely used in agriculture being the most heavily applied herbicide in the United States. These chemicals are often found in run-off water at elevated levels, causing exposure to non-target species. Past research has shown greater than additive toxicity when invertebrates were exposed to mixtures of atrazine and chlorpyrifos. This study expands this research to include vertebrate species, specifically fathead minnows and African clawed frogs. Groups of known age fathead minnows, Pimephales promelas, and African clawed frogs, Xenopus laevis were exposed to chlorpyrifos in binary mixtures with atrazine. These tests were performed using static renewal methods with 48 h pre-exposure to atrazine followed by 48 h of chlorpyrifos and atrazine exposure. Temporal variations were seen in the organisms' response to chlorpyrifos in a laboratory setting. The affect of atrazine on chlorpyrifos toxicity was shown in this study. These findings have been supported by acetylcholinesterase activity of whole fish and tadpole homogenates. A time-dependent acetylcholinesterase curve has also been examined for both species. These curves show low activity in young control organisms; with activity increasing during the organisms' maturation. These results may have ramifications concerning early life cycle testing with fathead minnows, since atrazine affects the cholinesterase system, therefore mixture toxicity may be underestimated until the organism's enzymatic system is fully developed.

Key words: Mixture, Pesticide, Fathead minnow, African clawed frog