Responses of Hyalella azteca to a pyrethroid mixture in a constructed wetland.

Constructed wetlands used to mitigate pesticide runoff from agricultural fields into receiving systems (e.g., lakes, rivers, streams) have been successful in reducing concentrations of non-point source pollutants such as agricultural insecticides (Moore et al., 2002; Leistra et al., 2003; Bouldin et al., 2005). Such wetlands also have important functions in enhancing the water quality and ecological values (Moore et al., 2002), and different phases (i.e., aqueous, sediment and detritus) have separate roles, as either sinks or sources, in determining effectiveness of wetlands in mitigating pesticide toxicity. For these reasons, the importance of elucidating potential effectiveness of wetlands in reducing pesticide toxicity to aquatic biota exposed to these different phases needs to be addressed. The pyrethroid insecticides, k-cyhalothrin and cyfluthrin, were used as model contaminants in a constructed wetland located in Leflore County, Mississippi, USA, designed to mitigate runoff from an agricultural field. Pyrethroids are used on a variety of agricultural crops in Mississippi, however most applications are primarily with cotton (Gossypium sp.) (USDA, 2004). Approximately 3,150 kg of k-cyhalothrin and 10,350 kg of cyfluthrin were applied in Mississippi in 2003 and, as a result, may contribute to non-point source contamination of aquatic environments (USDA, 2004). This study examined the use of a constructed wetland to mitigate ecological impacts of a simulated pyrethriod mixture (k-cyhalothrin and cyfluthrin) in runoff from agricultural fields to receiving aquatic systems by using 48 h aqueous, detrital and sediment bioassays with the freshwater test organism, Hyalella azteca.