Residue Characteristics And Degradation Mechanisms Of Common Pesticides In Wheat

Pesticide residue was one of the most important factors affecting wheat hygienic quality. Wheat plant was enriched in nutriment during flowering and grain-filling period, which might result in high incidence of diseases and pests. So many kinds of fungicides and insecticides were applied to control wheat diseases and pests. While these pesticides would inevitably deposit in wheat grains, plants and soil, causing potential harm to grain hygienic quality and field environment. So it was necessary to investigate the residue characteristics and degradation mechanisms of common pesticides in wheat so as to control residues in wheat. From 2003 to 2006, based on the results of systematic investigation and analysis of pesticide residue in seven counties in Jiangsu Province, the experiments on controlling measures of carbendazim, dimethoate, chlorpyrifos and imidacloprid against wheat pests were conducted on the Experimental Farm of Jiangsu Provincial Key Lab of Crop Genetics and Physiology of Yangzhou University to investigate residue characteristics, degradation mechanism and to find some cultivation measures to degrade residue, and integrated control approach of deseases, pests and small brown planthoppers in wheat. The main results were as follows.1 The residue detection rates of seven organophosphorus pesticides in grains and soil in wheat fields in seven counties of the middle region of Jiangsu Province were 95.2% and 100%, respectively, indicating that organophosphorus pesticide residues were very common in Jiangsu Province. Chlorpyrifos and dimethoate, which were middle-poisonous organophosphorus pesticides, were common in the middle region of Jiangsu. The average concentrations of seven kinds of organophosphorus pesticides in wheat grains were lower than National Residue Standard(GB16333-1996; GB5127-1998), indicating that there was a certain foundation for establishing a good quality, special-end-use and nuisance less wheat production system in Jiangsu Province.2 The residue characteristics and their differences of dimethoate, chlorpyrifos and imidacloprid in different parts of wheat were systematicly found. Dimethoate, chlorpyrifos and imidacloprid were sprayed on wheat plants after anthesis. The detection results indicated that the residues of these three pesticides decreased in ears, leaf blades, stems and leaf sheaths of wheat. The degradation rate of dimethoate and chlorpyrifos was synchronous in different up-ground parts of wheat plants during the early period of degradation, but the degradation of imidacloprid was not synchronous, which indicated that the dominant factors of imidacloprid degradation were different from dimethoate and chlorpyrifos. Degradation of dimethoate, chlorpyrifos and imidacloprid in ears, leaf blades, stems and leaf sheaths of wheat could be expressed by first-order kinetic equation. The half-life of dimethoate, chlorpyrifos and imidacloprid in wheat were within the ranges of 1.23~4.75d, 1.17~3.36d, and 1.33~5.78d, respectively and the effect of the pesticide dosages on the half-lives of the pesticides was insignificant. The half-life had significantly positive correlation with the degradation percentage of pesticides on the 3rd day after spraying. The residues of dimethoate and chlorpyrifos were mainly in bran, glumes and soil at maturity in wheat. According to National Standard (GB 5127-1998; GB 16333-1996), grains after harvest were safe in quality if dimethoate and chlorpyrifos were spayed with a dosage less than double recommend dosage before 10th day prior to harvest. But the earlier the spray was conducted, the more the dimethoate could be detected in the flour. The residue of imidacloprid at maturity was not detected, which indicated that imidacloprid was safer than dimethoate and chlorpyrifos. Degradation of dimethoate, chlorpyrifos and imidacloprid in the soil of wheat could be described by the first-order kinetic equation, and it decreased after pesticides spay. But the residue of dimethoate and chlorpyrifos elevated first and then dropped in the rhizosphere soil. The half-lives of dimethoate and chlorpyrifos in different parts of wheat followed the order of soil>ear>stem and leaf sheath≥leaf blade, and the half-lives of imidacloprid were in the order of soil>stem and leaf sheath >leaf blade≈ear.3 Studies on the degradation and residue characteristics of carbendazim in wheat plants above ground and the effects of insecticide imidacloprid and chlorpyrifos on the residue of carbendazim showed that when more carbendazim was used, greater residue could be detected in wheat grains, ears, leaf blades, stems and leaf sheaths. The residue of carbendazim was mainly in leaf blades and ears; while in grains, stems and leaf sheaths, the residue was too low to be detected. The degradation of carbendazim in wheat ears, leaf blades, stems and leaf sheaths followed one-level dynamic equation Ct=C0e-KT. Different organs had different deposits in the early time after carbendazim was used, indicating that the deposit in ears and leaf blades was higher than in stems and leaf sheaths. The degradation dynamic of each part showed that the residue decreased along with the growing process. The lowest level was detected at the ripen stage. At the earlier stage, the degradation rate was faster, but in the latter, it became slower. The half-life of carbendazim in ears was shorter than in leaf blades, stems & sheaths. Differences in degradation equation and the half-life existed because different growth and environmental conditions between two growing seasons. When imidacloprid or chlorpyrifos was blended with carbendazim separately, both produced great effects on the residue of carbendazim. Imidacloprid lengthened the half life of carbendazim, while the chlorpyrifos made it shorter. The result in different parts of wheat plants was similar. The effects on ears were more significant. According to the treatment of this experiment, the residue of carbendazim in grains was lower than National Standard, 0.5mg.kg-1 (GB 14870-1994 in China), so grains were hygienically edible.4 Possible pathway of pesticide degradation in wheat was found. During the grain filling period after pesticides were spayed, air temperature and ear fresh weight were the two main factors influencing the initial deposition content of dimethoate and chlorpyrifos in ears. Ear fresh weight was the main factor affecting the initial deposition content of imidacloprid in ears; Air temperature, blade fresh weight and wind speed were the three main factors influencing the initial deposition content of dimethoate in leaf blades. Air temperature and leaf blade fresh weight were the two main factors influencing the initial deposition content of chlorpyrifos in leaf blades. Leaf blade fresh weight was the main factor affectingthe initial deposition content of imidacloprid in leaf blades. For these three pesticides, stem and leaf sheath fresh weight was the main factor influencing the initial deposition content in stems and sheaths. During the early degradation period, air temperature was the main factor influencing the degradation speed of dimethoate and chlorpyrifos, and air temperature mainly impacted the volatilization rate of pesticides. Probably the decreased concentration of dimethoate and chlorpyrifos at the earlier stage was mainly caused by volatilization rather than degradation. Sunshine duration was the main factor influencing imidacloprid degradation, but it was not a significant impact on the imidacloprid degradation in stems and sheathes. Perhaps it was related to light distribution. Photodegradation might be a pathway of imidacloprid degradation at the earlier stage after spraying.5 Spraying NaOH and Na2CO3 (alkaline) in wheat field could significantly degrade the residue of dimethoate, chlorpyrifos and imidacloprid, but spraying KH2PO4 (acid) and NaCl (neutral) solution did not have significant impacts on the degradation of the three pesticides. Acetone and NaOH could significantly degrade the residue of carbendazim in wheat bran. Riboflavin and FeCl3 could significantly degradethe residue of carbendazim in wheat glumes. Compared with urea and ammonium, organic fertilizer was better in degrading pesticide. In contrast to KCl and K2CO3, plant ash was more efficient in the degradation.6 Carbendazim, chlorpyrifos and dimethoate were observed to have boosted the population of samll brown planthopper (Laodelphax striatellus Fallén) on the 20th day after sparying. The population density of small brown planthoppers in plots treated with chlorpyrifos or chlorpyrifos and carbendazim were 255.2% or 425.6% higher than the control, respectively. Free amino acid, deoxidized sugar, total phenolic content, invertase (Inv.), polyphenol oxidase (PPO) activity in leaf blades and ears of wheat were investigated on the 27th day after anthesis. The results showed that compared with the control, all pesticide treatments except imidacloprid and prochloraz caused increment of free amino acid, deoxidized sugar content and Inv activity, and decrement of total phenolic content and PPO activity in leaf blades and ears in wheat. Of all the treatments, chlorpyrifos plus carbendazim treatment was the most serious. Results from correlation analysis showed that the population density of small brown planthoppers were positively correlated to free amino acid, deoxidized sugar content and Inv activity in leaf blades and ears in wheat, and negatively correlated to total phenolic content and PPO activity in leaf blades in wheat, suggesting that the influence of pesticides on the small brown planthoppers might be the results of physiological and biochemical changes in wheat plants. Given the population density of small brown planthoppers, it was suggested that carbendazim and prochloraz were applied alternately to control wheat scab, and that imidacloprid were applied to control wheat aphids.