Icon® rice seed treatment toxicity to crayfish ( Procambarus clarkii ) in experimental rice paddies

Outdoor pools (2.3 × 2.3 m) were used to simulate typical rice agricultural practices in Louisiana, USA, to evaluate the toxicity of ICON® (active ingredient [a.i.] fipronil) and its degradates to crayfish ( Procambarus clarkii ). Six paddies were planted with seed treated with ICON 6.2 FS at an exaggerated application rate of 0.05 kg a.i./ha (recommended rate, 0.042 kg a.i./ha), simulating three rice‐planting scenarios. Two reference paddies were planted with untreated seed. Crayfish were exposed to tail water within 24 to 48 h after seeding, simulating standard Louisiana agricultural and water management practices. At 50 d after planting, a separate group of crayfish was caged in situ for 14 d to evaluate toxicity. An additional 50 crayfish were added to two paddies approximately 100 d after rice planting and held for 29 weeks to evaluate bioaccumulation. Residues of fipronil and its degradates in water and soil were similar to residue concentrations measured from rice fields in Louisiana. Tail water from the treated paddies was not toxic to crayfish. The fipronil 96‐h median lethal concentration (LC50) for adult crayfish was 180 μg/L, which would provide at least a sixfold safety factor between the maximum fipronil concentration in tail water and the crayfish LC50. In situ exposures of crayfish also were not toxic. Concentrations of fipronil and its degradates after 29 weeks of exposure were less than 5 μg/kg in crayfish tail muscle tissue. These results demonstrate that label instructions adequately protect crayfish in a rice‐crayfish cropping scenario when ICON is applied at maximum application rates as a seed treatment.