UC Berkeley

In this dissertation, I examined the occurrence of insecticides in streams in intensive soy production regions of South America, and their effects on invertebrate communities. Recently soy has become a major export crop in South America, and the insecticides used are highly toxic to aquatic invertebrates. I adapted the Species at Risk pesticide index (SPEARpesticides), which was developed in Europe to assess effects of pesticide contamination in agricultural streams. I then explored the relative importance of insecticides in comparison to other agricultural stressors, and the potential for riparian buffers to mitigate pesticide transport and impacts. My study sites were on small streams adjacent to agricultural fields in four soy production regions: two regions in the Argentine Pampas (La Plata-Magdalena and Arrecifes), and one region each in the Atlantic forest habitat of Brazil and Paraguay.

Commonly used insecticides were detected at high frequencies in all three countries, and pyrethroids insecticides were the most likely to occur at acutely toxic concentrations. Samples with highest toxicity were collected from streams with riparian buffer width less than 20 m, and buffer width was the most important predictor variable in explaining insecticide levels. I evaluated the toxicity of the four most commonly detected insecticides to Hyalella curvispina, a freshwater amphipod that is widespread in South America. The lowest LC50 values were found for the pyrethroid insecticides lambda-cyhalothrin and cypermethrin, followed by chlorpyrifos and alpha-endosulfan.

After adapting the SPEARpesticides index for local invertebrate communities in the Argentina streams, I found that SPEARpesticides was the only response metric that was significantly correlated with insecticide levels. Multiple regression showed that insecticide toxicity was the most important stressorin explaining variability in the SPEARpesticide index.

I then evaluated the relative importance of insecticides and other agricultural stressors on invertebrate communities in Atlantic Forest streams. Although buffer widths in Brazil streams were negatively correlated with insecticide concentrations, and had a moderate importance in mitigating effects on some sensitive taxa, insecticides had little importance in explaining variability in invertebrate communities. The forested riparian buffer zones are likely to have mitigated the effects of pesticides on stream invertebrate communities in these regions.