Browsing by Author "Sakthiselvi, T."

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    Residue dynamics and mobility of flonicamid and dinotefuran insecticides in soil and assimilation by rice (Oryza sativa (L.))
    (Department of Soil Science and Agricultural Chemistry, College of Agriculture, Vellayani , 2024-12-19) Sakthiselvi, T.; Thomas George
    A study entitled “Residue dynamics and mobility of flonicamid and dinotefuran insecticides in soil and assimilation by rice (Oryza sativa (L.))” was carried out at the Department of Soil Science and Agricultural Chemistry and Pesticide Residue Research and Analytical Laboratory, College of Agriculture, Vellayani, Thiruvananthapuram during 20202024, with the objective to investigate the persistence, transformation and mobility of flonicamid and dinotefuran formulations in soils, loss through leachate and the uptake and dissipation of residues in rice. The experiment comprised of the following lead topics viz. determination of initial physicochemical properties of sandy clay loam soils, method validation, persistence study, mobility experiment, leachate experiment, field experiment, effect of processing operations on residues and effect of residues on soil microbial activity. Initial properties of the experimental soil were determined to relate their possible influence on insecticide residue behaviour. The soil had sandy clay loam texture (56.28 % sand, 14.30 % silt and 29.42 % clay) and was strongly acidic (pH: 5.05), non-saline (EC: 0.2 dS m-1) with high organic matter (1.59 %) content. An efficient method for extraction and estimation of residues was validated by conventional liquid-liquid extraction and QuEChERS extraction method in rice crop, soil and water matrices for flonicamid, TFNA, TFNA-AM, TFNG, dinotefuran, DN-HCl and DNurea. Conventional method showed lower recovery, however, QuEChERS method was highly efficient with recovery rates within acceptable limits of 70-120 % for both parent compounds and their metabolites. QuEChERS method further validated in terms of linearity had correlation coefficient above 0.99. Precision evaluated in terms of relative standard deviation (RSD %) and Horwitz ratio had deviations within the tolerable limits of 20 % and 2, respectively. Low to medium matrix effect was obtained for all the analytes, therefore, matrixmatched standards were used for calibration to compensate it. Sensitivity determined in terms of limit of detection (LOD) and limit of quantification (LOQ) was 0.003 and 0.01 µg g-1. Thus, QuEChERS method which was proved best in terms of validation parameters of recovery, precision, linearity and matrix effect was adopted for all the experiments for sample processing. Persistence experiment was executed under two soil moisture conditions viz. field capacity and flooded condition with four insecticide concentrations viz. 0 (control), 1, 2 and 4 mg kg-1 in the laboratory. The initial residues were higher for flonicamid in the range of 0.5531.315 and 0.514-1.153 mg kg-1, respectively, under field capacity and flooded condition, than dinotefuran in the range of 0.532-1.044 and 0.238-1.041 mg kg-1. TFNA was found to be the major flonicamid metabolite under both moisture conditions with persistence till 15th day under field capacity and 80th day under flooded condition. TFNG and TFNA-AM appeared during the first week at higher concentrations under both moisture conditions. DN-HCl and DN-urea were formed only under flooded condition whereas degradation did not happen under field capacity. Dissipation data showed that flonicamid and total flonicamid had faster dissipation under field capacity with half-lives (DT50) of 1.41-3.33 days than flooded condition with DT50 of 16.90-49.50 days, whereas dinotefuran and total dinotefuran degraded faster under flooded condition with DT50 of 11.55-106.61 days than field capacity with 138.60-346.50 days. The mobility experiment involved assessing the residues of soil cores and leachates after loading 150 µg of standards of flonicamid and dinotefuran in separate columns of 50 cm length and subsequent elution with 20, 40, 80 and 160 ml of water. Significant differences in residue retention of 61.85, 58.79, 44.15 and 25.80 % for flonicamid and 72.19, 55.65, 38.62 and 21.96 % for dinotefuran were observed in the top 20 cm layer respectively after 20, 40, 80 and 160 ml of water application. Mobility of metabolites (TFNA and DN-HCl) increased significantly with increased water application. In leachates, flonicamid and dinotefuran in the range of 0.022-1.115 and 0.032-1.393 µg, respectively, were detected after 40, 80 and 160 ml of water application. TFNA was the only metabolite found in leachates at a concentration of 0.069-0.199 µg. Leachate experiment was carried out in metallic trays of 60 x 30 x 20 cm3 volume. The trays were three-fourths filled with soil, planted with rice seedlings and were irrigated every day to maintain the flooded condition. Plants were sprayed with flonicamid (Ulala 50 % WG) and dinotefuran (Token 20 % SG) after 50 days of planting at the recommended dosage of 75 and 30 g a.i. ha-1, respectively and samples were analysed for residues. Dissipation data showed that flonicamid degraded with DT50 of 6.36, 1.96 and 9.49 days, whereas, dinotefuran had DT50 of 10.19, 2.81 and 6.93 days in plant, soil and leachate water, respectively. TFNA, TFNA-AM and TFNG appeared as major flonicamid metabolites in plants with DT50 of 9.49, 8.25 and 12.38 days, respectively. In soil and leachate samples, TFNA was the only major metabolite with DT50 of 22.35 and 7.62 days, respectively. However, DN-urea was the only dinotefuran metabolite formed in plants. Persistence and dissipation kinetics of insecticides under open field rice ecosystem were ascertained by conduction of supervised field trials with treatments including two formulations viz. flonicamid and dinotefuran and three application frequencies (25, 25 & 50 and 25, 50 & 75 days after transplanting). In rice leaves, higher initial residues were recorded for flonicamid of 14.80-17.38 µg g-1 than dinotefuran of 2.56-3.48 µg g-1. The residues persisted up to 20th and 10th day with DT50 in the range of 2.75-3.13 and 1.62-2.21 days for flonicamid and dinotefuran, respectively. The safe waiting period for fodder use was calculated to be around 25 days for flonicamid, however, it was found to be safe immediately after application for dinotefuran. Significant differences in the DT50 values were obtained for chemical type, however, no significant effect was observed among application frequencies. In soils, residues were not detected in any of the treatments irrespective of chemical type and application frequencies. In rice grains, initial residues of 3.02 and 1.13 µg g-1 with DT50 of 9.01 and 7.05 days for flonicamid and dinotefuran, respectively, were registered. The residues persisted till 20th day for dinotefuran, while flonicamid residues persisted during the entire study period of 25th day. The harvested field samples including straw, soil and grain samples were processed and residue occurrence was studied. Only the grain samples obtained from triple application frequencies contained flonicamid residues above LOQ which were subjected to processing operations like parboiling, hulling, milling and cooking to determine the effect on residues. Cooking was proved to be most effective followed by parboiling in the removal of residues. The human dietary risk assessment was assessed in terms of estimated daily intake (EDI) and risk quotient (RQ). The calculated EDI was less than the acceptable daily intake (ADI) value of flonicamid and dinotefuran. RQ values were worked out to be < 1, suggesting that consumption of rice grains was under acceptable level of risks. The effect of flonicamid and dinotefuran residues on soil microbes was studied through dehydrogenase activity and microbial population by analysing the treated samples of field soil from 0th to 45th day. Soil dehydrogenase activity significantly reduced after each treatment with increased application frequency showing greater reduction. Among the chemical types, dinotefuran had much impact with activity range of 67.85-77.31 µg TPF g-1 hr-1 than flonicamid with 73.37-83.25 µg TPF g-1 hr-1 on the day of application. The activity started recovering after a month of application in all the treatments. The impact of residues on the population showed that, bacteria were highly affected followed by actinomycetes and fungi were least affected. The effect was higher in plots treated with dinotefuran three times. In a nutshell, the persistence of dinotefuran was significantly higher than flonicamid which could be influenced by soil properties. The mobility and leachate study confirmed that, the chemicals have higher leaching potential, hence careful management under flooded conditions is necessary to prevent contamination of water bodies. The field study showed that, both the insecticides degraded at a faster rate with very short DT50 of within 10 days (nonpersistent) in rice and soil samples. Hence, foliar spray of flonicamid and dinotefuran at the recommended dose in repeated sprays were safe for consumption which is supported by the associated dietary RQ of < 1. Processing operations of harvested samples showed that the extent of residue reduction were observed to be cooking (82.79 %) > Parboiling (61.06 %) > Milling (42.25 %) > Hulling (26.80 %). Increased number of sprayings and dinotefuran chemical type were found to have a significant impact on soil dehydrogenase activity and microbial population for one month period which recovered to the original level presumably due to dissipation of residues. The research underscored the relatively safe nature of flonicamid and dinotefuran in rice field ecosystem under Kerala’s climatic conditions, implying the adoption of these chemicals have the potential for sustainable management of sucking insects.