Bioefficacy and fate of fipronil and its metabolites in basmati rice under sub-tropical climatic conditions

Abstract

Fipronil is a systemic insecticide having persistence in the plant system and is used for the management of the yellow stem borer, Scirpophaga incertulas (Walker), and leaf folder, Cnaphalocrocis medinalis (Guenee). The nature and amounts of active metabolites of fipronil used in basmati rice are set to ensure the safety of the consumers and the environment. The bioefficay, persistence and metabolism of applications of fipronil (Regent 0.3G) at 45 (recommended dose) and 180 (higher dose) g a.i. ha−1 thirty days after transplanting of a basmati crop were studied. The fipronil gave long term insecticidal effectiveness for managing the stem borer and leaf folder at the tested doses on basmati rice throughout the cropping season. For residue analysis, samples of basmati rice plants were collected at 7, 15, 30, 45, 60, 90 and 120 days after application of the insecticide. Samples of basmati grains, bran, husk and straw were collected at harvest. After extraction and cleanup, fipronil and its metabolites were quantified by gas liquid chromatography (GLC) and confirmed by gas chromatography mass spectrometry (GCMS). The residues of fipronil and its metabolites were at a maximum at seven days after application, declining thereafter. Fipronil dominated as the parent compound and among the metabolites, the sulfone derivative was the most dominant followed by the sulfide, amide and desulfinyl, which were found below determination limit (0.01 mg kg−1) 45 and 90 days after the application of fipronil at recommended and higher dose, respectively. The samples of basmati straw, grains, bran and husk at harvest time did not contain any detectable residues of fipronil or its metabolites.