FIELD EFFICACY, PERSISTENCE AND METABOLISM OF THIAMETHOXAM IN MUSTARD AGAINST APHID
Abstract
Field assessment was done to evaluate the bioefficacy of thiamethoxam 25% WG at different doses at 12.5 (T1), 16.67 (T2), 25 (T3), 37.5 (T4), 50 (T5) g a.i. per ha, standard check of dimethoate 30% EC at 200 (T6) g a.i. per ha and untreated control (water spray) (T7) over Lipaphis erysimi sprayed twice with an interval of 10 days. In 2020-21 and 2021-22, T5, T4 and T3 gave lowest aphid population and were statistically at par in terms of their effectiveness followed by T6, T2, T1 and T7. The plots treated with thiamethoxam at 50 g a.i. per ha gave highest seed yield and benefits over the cost incurred and it was in line with the results of thiamethoxam 37.5 and 25 g a.i. per ha. The persistence and metabolism of thiamethoxam in mustard leaves, flowers and soil was studied following thiamethoxam application at 12.5, 16.67, 25, 37.5, 50 g a.i. per ha. Mustard leaves, flowers and soil samples were analyzed by Reverse phase-Ultra High-Performance Liquid Chromatography. During 2020-21, the initial deposits of thiamethoxam and its metabolites were found to be 2.20, 2.63, 4.43, 6.86 and 9.40 mg kg-1 in leaves collected at 0 (1 hr after application), however, during 2021-22, the residues were 2.17, 2.76, 4.32, 6.71 and 8.42 mg kg-1, respectively. During 2020-21, the initial deposits of thiamethoxam and its metabolites were found to be 1.43, 1.85, 2.63, 4.37 and 5.73 mg kg-1 in flowers collected at 0 (1 hr after application), however, during 2021-22, the residues were 1.36, 1.73, 2.54, 4.24 and 5.46 mg kg-1, respectively. The residue in mustard leaves and flowers reached below LOQ (0.05 mg kg-1) after 30 days of application of thiamethoxam during both the years. But in soils, residues of thiamethoxam reached LOQ after 30 days of treatment except when applied at 12.5 and 16.67 g a.i. per ha where it took 15 days to reach below LOQ. The samples of oil and seed cake did not reveal the presence of thiamethoxam residues at harvest during both the years. Samples of clay loam, silt loam and sandy loam soil were collected from different geographical locations of Bihar. Each soil samples were fortified at three levels of thiamethoxam i.e. 100, 200 and 400 mg kg-1. The whole experiment was conducted at 25 ± 2°C under laboratory conditions. The limit of quantification of thiamethoxam and its metabolites was worked out to be 0.05 mg kg-1. The total residues of thiamethoxam and its metabolites in clay loam, silt loam and sandy loam soil after 7 days of its application @ 400 mg kg-1 were found to be 212.94, 191.01, 184.55 mg kg-1, respectively. The persistence of thiamethoxam was found to be more in clay loam followed by silt loam and sandy loam soil. Thiamethoxam after degradation resulted in the formation of clothianidin (main metabolite) followed by 1- methyl-3 nitroguanidine. Out of the various microbial isolates that were isolated from the soil and tested for thiamethoxam degradation, Bacillus cereus and Acinetobacter rudis were found to be prominent in degrading thiamethoxam as compared to control. Soil amended with B. cereus, A. rudis and consortium, the residues of thiamethoxam and its metabolites reached LOQ after 42 days of treatment when fortified at 50 mg kg-1 of soil whereas it took 56 days to reach LOQ when unamended.