Resistance Monitoring And Risk Assessment Of Pymetrozine In Small Brown Planthopper,Laodelphax Striatellus (Hemiptera:Delphacidae)

The small brown planthopper (SBPH), Laodelphax Striatellus (Fallen) is one of the major pests of rice in China. It causes great damage to many crops, including rice, wheat and maize, dueing to direct sap-sucking and as a plant virus vector. The population density and damage intensity increased dramatically in Jiangsu Province, Anhui Province, Shandong Province and Zhejiang Province from2000to2009, which posed a serious threat to the trait of high and stable yield of the crop. Currently, chemical control is still a major measure for suppressing L. Striatellus, while the development of insecticides resistance of Laodelphax Striatellus made the insecticides more and more inefficient. It is essential to monitor the resistance to commonly used insecticides in SBPH, the result will provide a scientific guideline for use of insecticides and resistance management in L. Striatellus. Pymtrozine are the major insecticide for controlling L. Striatellus since2008, and it is used more and more frequently. Therefore, it is very urgent to assess the resistance risk of pymetrozine, the aim is to delay the resistance development of this special insecticide.In this paper, rice seedling dipping method for resistance mornitoring and susceptible baselines in L. Striatellus was established, and resistance of L. Striatellus collected from Jiangsu Province, Anhui Province and Zhejiang Province wer mornitored and resistance risk to pymetrozine in SBPH was evaluated.The toxicity of chlorpyrifos and fipronil to L. striatellus were determined by stem-dipping method and seedling dipping method respectively, there are not obvious difference in the toxicities of two methods. The seedling dipping method was adopted as the resistance monitoring method in our study. The toxicity of five insecticides to the susceptible strain of L. striatellus was assayed by seedling dipping method, and the toxicity baselines of the four insecticides were consiedered as susceptible baselines.The resistance of L. striatellus collected from Jiangsu Province, Anhui Province and Zhejiang Province, were monitored by seedling dipping method during2009to2011. The results showed that substantial variations of the resistance level to different insecticides existed in L. striatellus. All the populations kept susceptible to minor resistance to thiamethoxam and nitenpyram in the three years (0.6to2.2-fold and0.8to3.8-fold, respectively). While the insects were in moderate to high level resistance to chlorpyrifos (11.6to83.6-fold) and the resistance increased obviously from2009to2011. First year, all three monitored populations were susceptible to pymetrozine (1.1to2.1-fold), however the resistance of60%populations increased to minor resistance (3.8to6.7-fold) in the second year. In the third year Wuxi, Nanjing and Yancheng populations deveploed low resistance to pymetrozine (5.3-to5.5-fold) and Huzhou, Jiaxing and Lujiang populations were in minor resistance (3.2to4.5-fold). Nine populations developed high to very high resistance to buprofezin and low to medium resistance to Alpha-cypermethrin in2011(136.4to271.1-fold and5.2to34.9-fold, respectively). Most of the field populations developed low to medium resistance (7.2to20.9-fold) to fipronil in2011, except for Wuxi population with2.3-fold. The eight populations of L. striatellus, collected from Jiangsu Province, Anhui Province and Zhejiang Province, were as susceptible to ethiprole and butylenes-fipronil as fipronil, and more susceptible to ethiprole than to butylenes-fipronil in2011. The toxicity of imidaclopyrid to the five populations of L. striatellus, collected from Jiangsu Province, Anhui Province and Zhejiang Province, did not increase obviously.After12-generation selection with pymetrozine in13generations, the resistance level of the small brown planthopper (a population of L. striatellus from the wheat field in Wuxi City, Jiangsu Province in2010) to pymetrozine increased by1.0-fold, with the LC50value increasing from29.724mg/L to58.886mg/L. The selected strain was7.4-fold resistance to pymetrozine compared with the susceptible strain. The overall mean estimate of h2of pymetrozine resistance in L.striatellus was0.0470, however, h2was0.2070in the stage (F4-F13) of continuous selection. When the h2=0.0470and the selection mortality is50%at each generation, the LC50increases tenfold in46generations, however, the same would happen in10generations if h2=0.2070. It suggested that L.striatellus had the definite risk of resistance to pymetrozine.