| As economic crops of the cruciferous family,cabbage and pakchoi occupy an important position in the output value and benefit of vegetables in our country.However,the frequent occurrence of pests and diseases on cabbage and pakchoi has a great impact on their quality and yield.The neonicotinoid insecticide imidacloprid and the broad-spectrum fungicide chlorothalonil have a wide range of applications and use.They are the main pesticide varieties to control aphids,pieris rapae and downy mildew.To ensure the efficient and safe use of imidacloprid and chlorothalonil,and further promote the process of pesticide reduction,this article uses chlorothalonil and imidacloprid as the target pesticides to(1)study the initial deposition concentration,residual digestion characteristics and temporal and spatial distribution of chlorothalonil and imidacloprid in cabbage and pakchoi under different application dosage,application period,planting area,application frequency,crop growth period and planting mode.(2)To clarify the pesticide deposition,digestion and metabolism law in the cabbage and pakchoi planting system in different regions.(3)To determine the application limit range of chlorothalonil and imidacloprid in the cabbage and pakchoi planting system.The main results are as follows:1.A quick and easy method for the analysis of chlorothalonil and imidacloprid residues on cabbage and pakchoi was established,which meets the requirements of pesticide residue analysis.Based on the recommended effective dose(X),four application doses were set(0.5X,0.75 X,1X and 1.5X).The initial deposition concentration of chlorothalonil and imidacloprid in the leaves,petioles,plant parts and planting soil showed an upward trend with the increase of the dosage.Chlorothalonil and imidacloprid are mainly retained on the leaves of cabbage and pakchoi plants.The sediment concentration in different parts of plants and planting soil was leaf> edible parts> petiole> root> planting soil.The initial deposition concentrations of chlorothalonil and imidacloprid under the same application dose in greenhouse was 1.1 to 2.5fold and 1.1 to 2.3 fold higher than that in open filed,respectively.The second application was higher than the first application.Under different application seasons,the deposition concentration of different parts of vegetable plants was December >November >October >May.The recommended doses of imidacloprid and chlorothalonil deposited on open filed in different regions were Beijing >Henan >Anhui >Hunan >Guangxi,Anhui >Hainan,respectively.The dissipation of chlorothalonil and imidacloprid in different parts of cabbage and pakchoi plants shows a trend of fast first and then slower.The dissipation rate in greenhouse was less than that in open field,and the dissipation half-life of the greenhouse was greater than that of the open field.The dissipation half-lives of chlorothalonil and imidacloprid in different parts and planting soil were planting soil> petiole> edible parts> leaves.The half-lives of imidacloprid in open-field cabbage in different growing areas were 5.5-9.9 d(Beijing),4.2-6.1 d(Henan),3.9-4.9 d(Anhui),1.2-1.7 d(Guangxi).The digestion half-life of chlorothalonil in cabbage different growth stages were in the order of maturity> rosette stage> seedling stage.The halflife of pesticides on cabbage and pakchoi in different seasons is November >October >May.2.Under the same dosage,the total retention rate of 70% imidacloprid water dispersible granules and 75% chlorothalonil wettable powder on cabbage and pakchoi plants first increased with the increase of the dosage and then tended to be relatively stable.Under different planting modes,the retention rate of chlorothalonil and imidacloprid in the greenhouse plants were 1.1to 6.7 fold higher than that of in the open field.The retention rate of chlorothalonil and imidacloprid in pakchoi was Beijing >Henan >Anhui >Hunan >Guangxi >Hainan.The use of chlorothalonil for downy mildew on pakchoi could be reduced by 19.9-40.0%,which were based on 80% minimum control requirements of fungicides.The amount of imidacloprid on pakchoi and cabbage could be reduced by 7.5-32.7% and 4.7-38.0%,respectively,which were based on 85% minimum field control requirements of pesticides.3.The greater the degree of insect damage to the cabbage ball,the greater the ability of imidacloprid to penetrate into the leaves of the ball.The residual concentration of imidacloprid on the cabbage balls gradually decreased from the outside to the inside.The residual concentration of the first layer of leaves was the largest.The residual concentration of different layers of leaves were shown as TR> FB> SR> FR> SB> TB.The wax content,growth rate,number of stomata and leaf structure of different layers could affect the penetration and distribution of pesticides on cabbage.4.Imidacloprid alkene(IMI-O),imidacloprid(IMI-U)and 6-chloronicotinic acid(6-CNA)were the main metabolites of imidacloprid in cabbage and pakchoi plants.The 4-hydroxy-2,5,6-trichloro-1,3-isophthalonitrile,2,4,5-trichloro-1,3-isophthalonitrile,2,5-dichloro-1,3-isophthalonitrile and 5-chloro-1,3-isophthalonitrile are the main metabolites of chlorothalonil on pakchoi and cabbage.The removal of chemical bonds and various functional groups during the degradation and metabolism of imidacloprid was explored at the molecular level by density functional theory.The van der Waals surface electrostatic potential analysis of imidacloprid and its metabolite molecular models was carried out.Five typical imidacloprid-water adsorption models were constructed and independent gradient model(IGM)analysis was carried out.The interaction between the molecules and the metabolic pathways of imidacloprid on cabbage and pakchoi were analyzed.5.The application limits of imidacloprid and chlorothalonil on cabbage and pakchoi were proposed through the minimum pesticide field control effect,dietary risk application threshold,environmental risk application threshold,and the highest dose of pesticide registration. |
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