MARSHALONIS*, D.; KNOWLTON, R.E.; MERCHANT, H.C.; George Washington University, Washington, DC; George Washington University, Washington, DC; George Washington University, Washington, DC: Acute Effects of Permethrin on Four Populations of Grass Shrimp, Palaemonetes pugio

Recent concern over the spread of the West Nile Virus has led to increased spraying of insecticides around Chesapeake Bay in hopes of reducing populations of the vector (various species of mosquitoes). Because permethrin is highly toxic to non-target organisms, especially crustaceans, and is capable of lingering in the environment for extended periods before degradation, there is some concern that it may be negatively affecting populations of grass shrimp and other Crustacea in Chesapeake Bay. In this study, the median lethal concentration (LC₅₀) of the insecticide permethrin was determined and compared for four distinct populations of grass shrimp, Palaemonetes pugio Holthuis. Organisms were collected from three field sites and acclimated to laboratory conditions for a minimum of four days, after which organisms were transferred to static test solutions for 48 hours. Shrimp collected from Pt. Lookout State Park, MD had significantly higher estimates (1.908 ppb) of LC₅₀ than those from Saxis, VA and Chincoteague, VA (0.548 ppb). There was no significant difference between LC₅₀ estimates for two populations of grass shrimp from Saxis, VA (0.731 and 0.604 ppb) collected at different times during the breeding season. Both populations of shrimp taken from Saxis had similar LC₅₀ estimates when compared to the shrimp population from Chincoteague, VA. The significant differences between LC₅₀ estimates for test populations may be a result of acquired resistance, in response to differences in exposure to permethrin in the Chesapeake Bay watershed. Differences in mean biomass and length between test populations seem to be an inherent quality of shrimp populations and are not likely to have contributed to differences in LC₅₀.