Dissipation of Spirotetramat and Imidacloprid in Four Different Tropical Soils-Confirmation of Residues by Electrospray Tandem Mass Spectrometry

A field study was conducted to evaluate the fate of imidacloprid and spirotetramat in sandy loam, loamy sand, sandy clay and clay soils following single application of combinational insecticide formulation Spirotetramat 12% + Imidacloprid 36% supplied by Bayer Crop science Limited, Mumbai, India. Two different dosages were applied on the moist soil plots at spirotetramat 90 g.a.i and imidacloprid 270 g.a.i/ha (750 ml formulation/ha – recommended dose) and spirotetramat 180 g.a.i and imidacloprid 540 g.a.i/ha (1500 ml formulation/ha – double the recommended dose). Field experiment plots were laid in Randomized Block Design (RBD) with three replications for each treatment. Untreated control soil plots were also maintained for comparison. The composite soil sample was collected from 0–15 cm depth 2 h after the application (0^th d) and continued the sampling on different predetermined intervals 1^st, 3^rd, 5^th, 7^th and 10^th d after spraying. The collected soil samples were extracted and analysed for the residues of spirotetramat and imidacloprid by a validated High Performance Liquid Chromatography with Ultra Violet detector (HPLC-UV). The soil extraction method showed recovery of spirotetramat 87 + 5% and imidacloprid 90 + 3%. This method has lowest limit of quantification (LOQ) 0.03 mg kg⁻¹ for spirotetramat and imidacloprid. Apart from the residues of imidacloprid and spirotetramat, ‘enol’ metabolite of spirotetramat was also analysed. The LOQ established for the metabolite enol was 0.03 mg kg⁻¹ and the recovery 86 + 4%.