| The chive gnat Bradysia odoriphaga Yang and Zhang(Diptera: Sciaridae) is the major insect pest damaging Chinese chive in China, and seriously affects production and quality of Chinese chive. Currently, the prevention of this pest mainly depended on organophosphorus insecticides. However, in recently, the application rates of pesticide were increasing and the control efficacy was declining in some area. Screening low toxic and high effective alternatives and their matched precise application technique against B. odoriphaga are need. To determine the potency of the second generation of new neonicotinoid insecticides to control B. odoriphaga and their scientific use, the toxic effects of thiamethoxam to B. odoriphaga and adsorption and leaching of in soil was studied. And their distribution in soil after the directional spray-washing, moving characteristic, distribution and degradation in the field, control effect against B. odoriphaga and impact to the growth index of Chinese chive were also investigated. The results were summarized as follow:1. The toxicities of thiamethoxam, and other 12 kinds of insecticides to B. odoriphaga by stomach-contact combination toxicity method were tested, meanwhile, acute toxicity of pesticides above to earthworms was determined using artificial soil method. Selective toxicity of pesticides above to B. odoriphaga and earthworms were confirmed by pot experiments. The results showed that the toxicity of imidacloprid, clothianidin, dinotefuran, thiacloprid and thiamethoxam to the 4th instar larvae of B. odoriphaga were significantly higher than other 8 pesticides and the LC50 of these five pesticides to tebufenozide were 101.6-, 55.0-, 32.9-, 27.2-, 13.6- folds respectively. For the 13 pesticides, the toxicity of imidacloprid, acetamiprid, clothianidin and dinotefuran to earthworms were medium while the other pesticides were low. In the pot experiments, the insecticidal effect and seedling protecting effect of imidacloprid, thiamethoxam, chlorpyrifos, fosthiazate and lambda-cyhalothrin were better than that of pesticides, and only thiamethoxam had inconspicuous lethality.2. The toxicities of thiamethoxam and its metabolic product clothianidin to different stages of B. odoriphaga were tested by stomach–contact combination toxicity method, the dripping quantitative liquid method; pesticide-membrane method. The results showed that the LC50 of thiamethoxam and clothianidin to B. odoriphaga larvae were 11.92 mg/L and 3.70 mg/L, their toxicities to eggs were low(thiamethoxam: LC50=113.45 mg/L; clothianidin: LC50=61.41mg/L), and their knockdown of thiamethoxam and clothianidin against B. odoriphaga adults were high(LC50 thiamethoxam: LD50=1.93 μg/cm2; clothianidin:LD50=1.44 μg/cm2).3. The standard contact and stomach bioassay method was used to assess the effects of sublethal(LC5=1.158 mg/L, LC20=3.615 mg/L) concentrations of thiamethoxam on the demographic parameters of B. odoriphaga, and data were interpreted based on the age-stage, two-sex life table theory. After thiamethoxam treatment, the intrinsic and finite rates of increase, net reproduction rate, survival rate, and reproductive value were all marked ly decreased, while the mean generation time, total preovipositional period, and larval and pupal duration were prolonged, compared with controls. The intrinsic rates of increase dropped from 0.1775/day to 0.1502-0.1136/day. Following LC5 and LC20 treatments, net reproduction rate dropped from 61.75 offspring/individual(control) to 43.36 and 20.75 offspring/individual, respectively. Sublethal concentrations of thiamethoxam decreased the developmental rate of laboratory populations of B. odoriphaga, suggesting that such doses may be useful in integrated pest management strategies.4. Analytical method for simultaneously detecting thiamethoxam and clothianidin residue in water and soil samples was established by HPLC and analytical method for simultaneously detecting thiamethoxam and clothianidin residue in leek samples was also established. The results showed that average recoveries of thiamethoxam in soil samples ranged from 89.67%-95.33%, the relative standard deviation was 1.67%-2.64%, its average recoveries in water samples ranged from 93.83%-97.17%, the relative standard deviation was 0.79%-1.66%, its average recoveries in leek samples ranged from 92.36%~106.42%, the relative standard deviation was 7.88%~8.86%; the average recoveries of clothianidin in soil samples ranged from 88.91%~93.34%, the relative standard deviation was 1.68%~2.15%, its average recoveries in leek samples ranged from 95.40%~101.16%, the relative standard deviation was 8.08%~9.46%. The lowest detectable concentration of thiamethoxam and clothianidin in leek and soil samples were 0.01 mg/kg, respectively.5. To study the influence of soil basic physical and chemical properties on the adsorption and leaching behavior of pesticide in soil, the adsorption and leaching of thiamethoxam onto three different soils including Sandy soil, Silt loam soil, Shajiang black soil, conducted by oscillation equilibrium experiment, soil thin layer chromatography and column leaching. And comparative concentration distributions of phoxim and thiamethoxam in soil after their application were also determined. The results showed that the adsorption of thiamethoxam in the soils could be well fitted by the empirical Freundich isotherm. The adsorption constants(Kd) were 1.25, 2.95 and 5.10 in Sandy soil, Silt loam soil, Shajiang black soil, respectively. Clay content was dominant factor affecting the adsorption of thiamethoxam in soils and the soil organic matter content was the next factor. The leaching of thiamethoxam was in order of Sandy soil>Silt loam soil>Shajiang black soil, and higher amount of leaching was the oil suspended agent and suspension concentrate, followed by water dispersible granule, the granulehad the lowest amount of leaching. Thiamethoxam had stronger mobility than phoxim, and was easier to move to the underground soil where biological targets lived in. These results demonstrated that thiamethoxam has potential risk of contaminating groundwater. Attention should be paid to its application, particularly in the soil of law clay content and organic matter content.6. Residual dynamic of thiamethoxam and clothianidin in soil was studied. Results for residual dynamic test showed that the degradation trends of thiamethoxam and clothianidin in soils were line with a kinetic equation. Equation of residues and degradation dynamics of thiamethoxam applied at 6.0 and12.0 kg a.i./ha were C=6.1015e-0.0258t(r2=0.9768)and C=12.01131e-0.0237t(r2=0.9579), their half-lives were 26.87 and 29.25 days; Equation of residues and degradation dynamics of thiamethoxam applied at 3.0 and 6.0 kg a.i./ha were C=2.5615e-0.0194t(r2=0.9804) and C=7.9136e-0.0192t(r2=0.9884), their half-lives were 26.87 and 29.25 days.7. The distributions of thiamethoxam and clothianidin in soil after the directional spray-washing and chemigation, moving characteristic in the field, control effect against B. odoriphaga and impact to the growth index of Chinese chive were studied. The results showed that application of insecticides using the directional spray-washing was easier to reach the target than those using the chemigation, and the concentration of insecticide in the soil decreased with the increase of soil depth; the concentration in the rhizosphere soil was higher than those in the inter-row soil of Chinese chive. Comparing to the distribution at the 7th day after application, insecticide moved much deeper and wider at the 120 th day after application. The control effect of thiamethoxam 12.0 kg a.i./ha and clothianidin 3.0 kg a.i./ha and 6.0 kg a.i./ha could last the whole root-rearing period and promote the growth of Chinese chive. Therefore, thiamethoxam and clothianidin could be used for the control B. odoriphaga in the root-rearing period of Chinese chive by the directional spray-washing application. |
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