Resistance Monitoring And Resistance Mechanism To λ-cyhalothrin Of Melon Aphids In Central China

Aphis gossypii Glover is disastrous pests on cucurbitaceous crops significantly,which causes damage by feeding plant juice and transmitting plant pathogenic viruses.At present,the control of A.gossypii mainly relies on chemical insecticides.However,the long-term use of insecticides reduced the sensitivity of A.gossypii to insecticides.In order to clarify the levels of resistance to A.gossypii in central China.In this study,the sensitivity of A.gossypii to 7 insecticides（imidacloprid,thiamethoxam,sulfoxaflor,flonicamid,chlorpyrifos,pirimicarb andλ-cyhalothrin）was systematically monitored from 2020 to 2021 in central China,including Henan,Hubei,and Hunan provinces.Furthermore,the resistance mechanism of A.gossypii toλ-cyhalothrin was explored by using the susceptible andλ-cyhalothrin resistant strains of A.gossypii.The main results are as follows:1.The resistance levels of 15 A.gossypii field populations collected from Hubei,Henan,and Hunan provinces to 7 insecticides were systematically monitored by leaf-dipping method from 2020 to 2021.The results showed that A.gossypii field populations developed low to high levels of resistance（RR=9.48-801.67-fold）to sulfoxaflor except Yichang population（HBYC-2020）.Most of the A.gossypii field populations were sensitive toλ-cyhalothrin,while the populations collected from Jinzhou（HBZJ-2020,HBZJ-2021）,Xinyang（HNXY-2020,HNXY-2021）,and Xinxiang（HNXX-2021）evolved moderate levels of resistance toλ-cyhalothrin（RR=16.96-47.38-fold）.The populations of Xiangyang（HBXY-2020）,Yichang（HBYC-2020）,Changsha（HNCS-2020）,and Xinxiang（HNXX-2020）were highly sensitive to imidacloprid,while other populations developed low to high levels of resistance to imidacloprid（RR=8.29-2352.96-fold）.Most of A.gossypii field populations developed low to high levels of resistance to thiamethoxam（RR=5.98-2194.96-fold）.Most of A.gossypii field populations were sensitive to chlorpyrifos.The tested A.gossypii populations have developed low to high levels of resistance to pirimicarb（RR=6.90-283-fold）.Most of field A.gossypii populations exhibited low to high resistance levels to flonicamid（RR=7.18-306.07-fold）.The results of correlation analysis showed that there was prominent positive correlation between the enzyme activity of cytochrome P450 and the logarithm of LC₅₀to sulfoxaflor,thiamethoxam,and pirimicarb.In addition,carboxylesterase（Car E）was positively correlated withλ-cyhalothrin,while glutathione S-transferase（GST）was negatively correlated withλ-cyhalothrin.The toxicity of sulfoxaflor to A.gossypii were positively correlated with that of the toxicity of imidacloprid and thiamethoxam.2.Aλ-cyhalothrin resistant strain of A.gossypii was obtained by continuous selection for 10 generations（λ-cyh-R,RR=35.45）.Compared to the susceptible strain,theλ-cyhalothrin resistant strain of A.gossypii developed cross-resistance to flonicamid andβ-cyhalothrin,the resistance ratios were 11.70 and 5.22,respectively.However,the resistant strain showed no cross-resistance to sulfoxaflor,imidacloprid,thiamethoxam,and chlorpyrifos.The results of synergism bioassays demonstrated that PBO and TPP had significant synergistic effects on two resistant strains.Furthermore,the results of enzyme activities determination showed that the activities of P450 and Car E were significantly increased in two resistant strains,which suggests that P450 and Car E might play the key roles in the resistance development of A.gossypii toλ-cyhalothrin.3.The transcriptional levels of P450 genes were detected by real-time quantitative PCR,and the results showed that the transcript levels of CYP6CY18,CYP6CY19,CYP6CY22,and CYP4G51 were significantly up-regulated inλ-cyh-R.Furthermore,the results of gene network map analysis indicated that the expression levels of CYP6CY18,CYP6CY22,and CYP4G51 were closely related to the development trend ofλ-cyhalothrin resistance.The expression of P450 genes can be induced by different sublethal concentrations ofλ-cyhalothrin in a time pattern.When treated with LC₂₅ and LC₅₀ concentrations ofλ-cyhalothrin for 6,12,24,and 48 h,the expression levels of CYP6CY18,CYP6CY22,and CYP4G51 were significantly increased compared to the control.The expression level of CYP6CY22 was increased even higher more than 200-folds by treatment with LC₂₅ ofλ-cyhalothrin for 12 h.Moreover,we found that silencing of the expression of CYP6CY18 and CYP6CY22 by RNAi can significantly restore the sensitivity of resistant A.gossypii toλ-cyhalothrin.