Mechanism And Measures Of Agricultural And Chemical Control To Phyllotreta Humilis Weise In Seedling Stage Of Spring-rape

Spring rape is one of the main oil crops in northern China and the planting acreage of the spring rape in the central and western regions of Inner Mongolia accounts for about one quarter of the planting acreage in northern region. Phyllotreta humilis is one of the major pests in rape seedling period and can bring huge losses to production. Currently, chemical pestiides are mainly used for prevention and treatment. Therefore, this paper mainly studied the prevention and treatment of the P. humilis in the agricultural cultivation practices and chemical control. The results were as follows:Effects of different sowing densities on the P. humilis harm and rape yield. Three sowing densities at 250 g/mu,300 g/mu and 400 g/mu were chosen and the effects of different sowing amount on the P. humilis harm and rape yield were measured. The results showed that:the insect population density of the P. humilis increased with the increase of the sowing amount and the insect population density of the P. humilis at the three sowing amount were in the order of 400 g/mu> 300 g/mu> 250 g/mu. Young seedling leaf harming index also increased with the sowing amount, which was in the same trend with the insect density changes. The effect order of the sowing yield was 300 g/mu> 400 g/mu > 250 g/mu, suggesting that the 300 g/mu was the best sowing amount.Effects of different spring rape varieties on the P. humilis harm and rape yield. Five rape varieties of big yellow rape, Jin rape No.6, space Meng IV, green rape No.14 and green Za No.3 were selected and the P. humilis harm and rape yield were measured. Survey results of two years showed that:the densities of the P. humilis in the five varieties of rapes were as follows in order:space Meng Ⅳ< big yellow rape< green Za No.3< Jin rape No.6< green rape No.14. The harming index survey also found that the harm indexes of the space Meng Ⅳ in various stages of seedling were the lowest with all the indexes no more than 0.35 and the harm indexes of the green rape were the highest with all the indexes no more than 0.39. In the aspect of yield, the yield of the green rape No.14 was the lowest with a highest yield of 102.26kg/mu, the highest yield of the space Meng IV was 132.93 kg/mu and the highest yield of the green Za No.3 was 128.22 kg/mu. There were no significant differences in the yield between two years in the investigation. The rape yields of the five rapes were in the following order:green rape No.14< big yellow rape< Jin rape No.6< green Za No.3< space Meng Ⅳ.Effects of seed dressing on spring rape young seedling growth and major pest density were studied by differet pesticides. Imidacloprid, thiamethoxam, chlorpyrifos, carbosulfan and fipronil were selected to conduct seed dressing treatment. The results showed that there was no significant effect in on the time and rate of seedling emergence through seedling dressing and the rate of emergence through seedling dressing. Root length of rape was promoted significantly by chorpyrifos and carbosulfan. The other three insecticides had a significant influence on seedling height besides except fipronil. In the aspects of prevention and treatment, the imidacloprid and carbosulfan could significantly reduce the insect population densities of P. humilis, plutella xylostella and lipaphis erysimi. All the pesticides had the highest amount at the 7th day of the rape seedling emergence with a highest concentration all above 13.69 mg/kg and then the pesticides amount gradually reduced with a content lower than 1.2 mg/kg at the 35 days. The highest content of the chlorpyrifos at the 7th day was 23.2 mg/kg and the metabolisms of the thiamethoxam and imidacloprid were slower than that of the chlorpyrifos.Toxicities and function mechanisms of different pesticides on P. humilis. The results showed that the toxicity of chlorpyrifos to the P. humilis was 5.543 mg/L, land it was the lowest among the four pesticides tested. The LC50 of the carbosulfun was 6.016 mg/L, the LC50 of the thiamethoxam and imidacloprid was 12.236 mg/kg and s23.998 mg/kg The results suggestied that the P. humilis was mostly sensitive to the chlorpyrifos. The inhibition of insecticides to acetylcholinesterase showed that the I50 value of the carbosulfan was 0.3476 mmol/L and the I50 value of the chlorpyrifos was 0.03195 mmol/L; and the median inhibition time (IT50) of the carbosulfan was 11.85 min and the IT50 of the chlorpyrifos was 6.02min. In the highest concentration(1x10-3mol/L), the inhibition of the imidacloprid to the acetylcholinesterase was only about 20%.