Resistance Monitoring And Resistance Mechanism To Beta-Cypermethrin In Wheat Aphids

Aphids are the main pests of wheat?Triticum aestivum L.?.Rhopalosiphum padi?Linnaeus?and Sitobion avenae?Fabricius?are predominant species in the wheat field,distributing in all wheat-growing regions in China.The application of insecticides remains the most effective method for aphids control.Extensive use of insecticides has contributed to the development of insecticide resistance in the target insects.In order to evaluate the insecticide resistance levels of R.padi and S.avenae and prolong the application life in the field,we collected aphids from wheat fields and analyzed the regional susceptibilities of R.padi and S.avenae to imidacloprid,thiamethoxam,sulfoxaflor,beta-cypermethrin,bifenthrin,avermectins,omethoate and chlorpyrifos by leaf dip method.Additionally,the cross-resistance spectrum,the metabolic resistance mechanisms and fitness costs of beta-cypermethrin in R.padi were investigated by a resistant strain with resistance ratio of4588.48-fold.The results are as follows:1. Insecticide resistance monitoring of R.padi and S.avenae in the field populationsThe susceptibilities of twenty-four field populations of R.padi and twenty-seven field populations of S.avenae to eight frequently used insecticides were measured in fifteen provinces of China from 2018 to 2019 by leaf dip method.The results showed a large number of wheat aphids populations have developed low to moderate levels of resistance to beta-cypermethrin and bifenthrin,and a few populations including the population SXY-2019?Yangling,Shaanxi?of R.padi has developed high level of resistance to beta-cypermethrin?Resistance Ratio,RR=454.16?and bifenthrin?RR=301.92?.The population NXS-2018?Shizuishan,Ningxia?has developed high level of resistance to beta-cypermethrin?RR=130.58?and JSY-2019?Yangzhou,Jiangsu?has developed high level of resistance to bifenthrin?RR=329.35?,respectively.Furthermore,the population HNX-2018?Xinxiang,Henan?of S.avenae has developed high level of resistance to beta-cypermethrin?RR=139.64?,and YNK-2019?Kunming,Yunnan?has developed high level of resistance to bifenthrin?RR=301.29?.Most wheat aphids populations have developed low to moderate level of resistance to imidacloprid,thiamethoxam and avermectin except the population YNK-2019,which has developed high level of resistance to sulfoxaflor?RR=136.57?.In addition,most wheat aphids populations showed susceptibility or low level of resistance to omethoate and chlorpyrifos.The correlation between the LC₅₀ value of each insecticide was analyzed,indicating that the susceptibility of R.padi to imidacloprid was significantly positive correlated with the responses to thiamethoxam,chlorpyrifos,omethoate and sulfoxaflor;the susceptibility to sulfoxaflor was significantly positive correlated with thiamethoxam,omethoate and chlorpyrifos;and there is a significantly positive correlation between beta-cypermethrin and bifenthrin.Correlation analysis indicated positive and significant correlation to S.avenae resistance between bifenthrin and omethoate,and a very significantly positive correlation between bifenthrin and sulfoxaflor.The toxicity difference between R.padi and S.avenae was analyzed by LC₅₀ values,suggesting that imidacloprid,thiamethazine,beta-cypermethrin and omethoate have higher toxicity to R.padi than S.avenae.2. Fitness costs of beta-cypermethrin resistance in R.padiTo determine the fitness costs of beta-cypermethrin resistant strain in R.padi,life table was performed between the beta-cypermethrin resistant and susceptible strains?SS?.The results showed that the development of first-instar nymphs,total pre-oviposition period?TPOP?and mean generation time?T?were significantly prolonged,but the oviposition period,number of nymphs per female,intrinsic rate of increase?r?,finite rate of increase??and net reproductive rate?R₀?were significantly reduced.No significant difference was found in the development of 2^nd,3^rd,4^th instar nymphs,adult longevity,mean longevity and pre-adult survival rate.The resistant strain had a relative fitness of 0.85.These results indicated that development of the resistance may have a significant fitness cost in the resistant strain.3. Biochemical mechanism of beta-cypermethrin resistance in R.padiBeta-cypermethrin resistant strain showed significant levels of cross-resistance to bifenthrin,sulfoxaflor,thiamethoxam,imidacloprid and low cross-resistance to avermectins.However,they exhibited lower negative cross-resistance to omethoate and chlorpyrifos.The activity of beta-cypermethrin was synergized by the synergists piperonyl butoxide?PBO?and diethyl maleate?DEM?in the resistant strain with 2.33-fold and 1.22-fold relative synergistic ratios compared with the susceptible strain,respectively.The cytochrome P450 monooxygenase?P450s?and glutathione S-transferases?GSTs?activities in the resistant strain were significantly higher than in the susceptible strain.The results indicated that the enhanced detoxification by P450s and GSTs enzymes are associated with beta-cypermethrin resistance.4. Detection of mutations in ?S4–?S6 region of the R.padi voltage-gated sodium channel geneIn this study,corresponding fragments in ?S4–?S6 region of sodium channel were amplified from resistant and susceptible strains to illuminate the molecular basis to beta-cypermethrin resistance in R.padi.The mutation super-kdr M918L,which has been found to associate with pyrethroid resistance,was detected in the resistant strain.We concluded that the enchanced activities of P450s and GSTs in combination with mutation of M918L mainly contributed to the high-level resistance to beta-cyremethtin in R.padi.