Background: Fipronil is an insecticidal pyrazole that is commonly used as an insecticide in field crops, urban pesticide and veterinary medicine, but there are no reports of its evaluation against stored-product insects. Three series of laboratory bioassays were conducted to assess fipronil as a potential grain protectant against adults of Sitophilus oryzae (L.), Tribolium confusum Jacquelin du Val, Rhyzopertha dominica (F.) and Prostephanus truncatus (Horn). Factors such as dose (0.01, 0.1, 1 and 10 mg fipronil kg(-1) grain), exposure interval (24 h, 48 h, 7 days and 14 days), temperature (20, 25 and 30 degrees C), relative humidity (RH; 55 and 65%) and commodity (wheat, maize, barley and paddy rice) were evaluated with regard to their impact on the insecticidal activity of fipronil. Progeny production was assessed after 64 days of exposure.
Results: At doses of < 1 mg fipronil kg(-1) grain, efficacy of fipronil was enhanced by an increase in temperature from 20 to 25 degrees C against S. oryzae or R. dominica for exposures of > 48 h and against T. confusum or P. truncatus for exposures of > 24 h. A further increase in temperature from 25 to 30 degrees C, although it enhanced efficacy at doses of < 1 mg fipronil kg(-1) grain against S. oryzae at all exposure intervals and against T. confusum after 7 days of exposure, it was negatively associated with efficacy against R. dominica or P. truncatus for exposures of > 24 h. By contrast, increase in RH did not have a significant impact on efficacy of fipronil. Although the performance of fipronil among the tested commodities was species dependent, this substance appeared to be less effective in paddy rice than in barley, maize and wheat. Regardless of the treated commodity, progeny production of all the tested species was almost suppressed with doses higher than 0.1 mg fipronil kg(-1) grain.
Conclusions: Fipronil appeared to be a very effective alternative to the existing substances in stored-grain protection at doses equal to or higher than 1 mg fipronil kg(-1) grain, and thus these doses require further evaluation in terms of safety for the consumer and residues on stored products.