Toxicity of stormwater runoff after dormant spray application of Diazinon and Esfenvalerate (ASANA®) in a French Prune Orchard, Glenn County, California, USA

Organophosphate pesticides (OPs), in particular diazinon and chlorpyrifos, have frequently been detected in toxic concentrations in waterways draining agricultural and urban areas in California's Sacramento and San Joaquin River watersheds (US Geological Survey 1997). Toxicity has in part been linked to stormwater runoff of OP pesticides applied during the dormant season on stonefruit and almond orchards. State Water Quality Plans have now been implemented by regulatory agencies to prevent movement of OPs into surface water, and growers have reduced the application of OPs. Simultaneously, the use of so-called reduced-risk alternatives, such as pyrethroid insecticides and Bacillus thuringiensis bloom sprays, has increased dramatically. Best management practices (BMPs) are aimed at reducing off-site movement of pesticides into surface waters. Pyrethroid pesticides, among them the widely used esfenvalerate (Asana registered ) are considerably more hydrophobic (solubility in water: 0.4 mu g/L) than the relatively soluble OP pesticide diazinon (solubility in water: 40,000 mu g/L). Although runoff of pyrethroids is believed to be minimal thus reducing pesticide impact on surface waters, esfenvalerate has been shown to be toxic to fish at extremely low concentrations ( less than or equal to 1 ug/L), and potentially poses a significantly higher risk to these organisms than OP pesticides. In addition, its potential to bioaccumulate and bioconcentrate is high. A second recommended method for reducing toxic runoff from orchards is the use of different orchard floor cover crops. Cover crops are believed to enhance water infiltration. This study was performed to measure the effectiveness of these two BMPs in reducing the toxicity of stormwater runoff. Experiments were carried out in a French prune orchard at the Talbot - Vereschagin Ranch, Glenn County, California.