Effects of triazine herbicides on organophosphate insecticide toxicity in Hyalella azteca

The frequent use of pesticides in agricultural and commercial settings has led some researchers to devote their attention to studying the effects of mixtures of these compounds as they co-occur in the environment. Recent studies have demonstrated the potentiating effects of triazine herbicides, such as atrazine and its analogs, to the toxicity of a variety of organophosphate (OP) insecticides. One such OP insecticide, chlorpyrifos, has been the topic of much concern because of its prevalence in the environment. This study focused on examining the effects of 10 select triazine herbicides at concentrations of 1 mumole/L (approximately 200 mug/L) to chlorpyrifos with Hyalella azteca. The compounds selected include atrazine, three of its degradation products, and six other herbicide active ingredients. Toxicity tests were performed using a two-way analysis of variance matrix design with effect levels determined by way of probit analysis. Atrazine was found to have the greatest acutely lethal effect to H. azteca, followed by its closest degradation product, deethylatrazine. Two of the six atrazine analogs, simazine and cyanazine, also showed significant effects to the insecticide's toxicity. Synergistic ratios (SRs) were calculated to compare the effect magnitudes for each of the herbicides. The highest ratio obtained was with atrazine (SR = 1.42). A majority of the past studies involving mixtures of triazines and OPs have examined the potentiation effects of active-use triazine herbicides on Chironomus species. However, compared with the acute effects previously obtained for Chironomus species, H. azteca show a higher tolerance to the presence of the triazine herbicides, even at levels often considered as being at the high end of environmentally relevant concentrations. When coupled with past studies from our laboratory, this research helps to provide a better understanding of the toxic effects of herbicide-insecticide interactions.