Imidacloprid residues in Willapa Bay (Washington State) water and sediment following application for control of burrowing shrimp

Pesticides have been used in the Willapa Bay estuary in western Washington State to control the exotic invasive plant species Spartina alterniflora (cordgrass) and the native species of burrowing shrimp (Callianassa sp.; Upogebia sp.) that affect oyster production. Carbaryl, the only registered insecticide for control of burrowing shrimp, has not been extensively studied in the Willapa Bay. However, carbaryl use has been severely restricted, and alternatives likely to have less severe environmental impacts are being sought. Imidacloprid applied directly to exposed sediments when the tide is out is efficacious for burrowing shrimp control but lacks studies of its behavior in the estuary. For this study, imidacloprid dissipation was monitored as the tide was rising in Willapa Bay, Over 99% of applied material dissipated from small plots within 24 h, but residues near the analytical detection limit were found in sediments 28 days later. At a distance of 152 m along a transect from the plot in the direction of tidal flow, imidacloprid residues in water peaked within 10 min after initiation of tidal flow. Within 30 min, imidacloprid residues were not detected, nor were residues detected in the water any time over the next month after application. Carbaryl residues in water were also monitored, and they exhibited the same rise and fall at the 152 m distance from the experimental plot as did the imidacloprid residues. However, carbaryl levels significantly above the detection limit were still present in water over the next month after application. The rapid dissipation of imidacloprid from water was hypothesized to be due to extensive dilution by the tide. The hypothesis was tested in batch equilibration sorption studies with radiolabeled imidacloprid and Willapa Bay sediment. Sorption distribution coefficients were <1 mL/g, and hysteresis was not observed during two desorption cycles, suggesting that imidacloprid was widely dispersed to extremely low levels soon after application.