Toxicity of Fipronil to the Midge, Cricotopus lebetis Sublette

Fipronil, a relatively new insecticide more recently developed than organophosphates and pyrethroids, has been detected in surface water draining from agricultural and urban-developed areas. This insecticide is primarily lost through subsurface and surface drainage from terrestrial areas where it has been applied. Invasive aquatic plants often need to be managed in these receiving water bodies to prevent loss of recreational and functional values (e.g., drainage), especially in subtropical and tropical areas. One insect of particular interest is the chironomid midge Cricotopus lebetis Sublette, which may be a useful augmentative biocontrol agent for the invasive aquatic weed Hydrilla verticillata L.f. Royale. Exposure of aquatic organisms, especially insects, to fipronil may significantly impact nontarget populations. These studies investigated the sensitivity of C. lebetis to fipronil exposures ranging from 24 to 96 h. The LC50 observed for each exposure interval was 7.26 g/L (24 h), 2.61 g/L (48 h), 1.78 g/L (72 h), and 1.06 g/L (96 h). The LC90 values observed were 47.18 g/L (24 h), 9.55 g/L (48 h), 6.45 g/L (72 h), and 4.81 g/L (96 h). Behavioral changes were seen at all fipronil concentration levels, where larvae exited the plant and exhibited abnormal behavior, such as restricted movement and lack of feeding. Results indicate that acute lethality occurred at environmentally relevant concentrations of fipronil.