The Sodium Channel M918L Mutation Mediates The Resistance Of Rhopalosiphum Padi To Lambda-cyhalothrin

The bird cherry-oat aphid,Rhopalosiphum padi（L.）,is a major pest which damages a wheat worldwide.Control of R.padi is primarily dependented on spraying of chemical insecticides.Lambda-cyhalothrin is used in the control of R.padi in China for a long time.While,with the extensive use of pyrethroids,many insect species have developed resistance to these chemicals.Target insensitivity caused by point mutation of voltage-gated sodium channel（VGSC）gene is one of the main mechanisms for pyrethroid resistance.In this study,investigated the resistance level of the field populations of R.padi in Shaanxi to lambda-cyhalothrin and detected the mutation sites of VGSC in the R.padi.A highly resistant strain was established in laboratory,and bioassay was used to evaluate the resistance level of R.padi to type I and type II pyrethroids conferred by M918L.The fitness cost involved with the M918L target mutation was also investigated.The results are of importance for further analysis of the resistance mechanisms of R.padi to pyrothroids as well as for the resistance management of the pest.1.Resistance monitoring of R.padi to lambda-cyhalothrin in Shaanxi ProvinceWe analyzed the susceptibility of five R.padi field populations in Shaanxi Province to lambda-cyhalothrin.The results showed that the five populations varied in susceptibility to lambda-cyhalothrin.The maximum resistance level（24.14）was found in the field population from Jingyang（population code JY）.2.Mutation detection of VGSC gene in field population of R.padiThe full length of sodium channel VGSC gene of R.padi from field populations and laboratory susceptible strain was amplified.Only one super-kdr M918L mutation was detected in JY field population.No other mutation was found in Jingyang and the other four populations.The M918L mutation is caused by the nucleotide substitution from ATG mutation to TTG.The sequence chromatogram showed a mixed peak,indicating that the individual has a heterozygous genotype.No homozygous M918L mutation was detected in the Jingyang population.3.Establishment of lambda-cyhalothrin highly resistant strain of R.padi cross resistance of the strain to to several other insecticidesA highly resistant strain（LC-R）to lambda-cyhalothrin was established in laboratory,by selected the JY field population for 21 generations.The resistance level of F₂₁was 158.76.We assessed the cross resistance of LC-R strain to nine different insecticides including other pyrethroid,ganophosphatse,neonicotinoids and carbamates.The results showed that LC-R strain had the highest cross resistance to pyrethroid insecticides,with the resistant rato of 108.3 to deltamethrin,37.23 to bifenthrin and 36.99 to fenvalerate.LC-R strain showed different cross resistance levels to thiamethoxam,malathion,indoxacarb and methomyl,and all the levels were less than 30.No obvious cross resistance against imidacloprid and chlorpyrifos was found in the LC-R strain.4.Resistance level of R.padi to pyrethroids involved with M918LmutationSix mutant clones（s-kdr-RS）with only the M918L mutation and six wild type clones without any VGSC muation were established from samples collected from the same wheat field of Jingyang.Bioassay results showed that s-kdr-RS strains show high resistance to to bifenthrin and lambda-cyhalothrin,while the s-kdr-SS strains did not show obvious resistance to the two chemicals.The resistance of ss-kdr-RS strains to lambda-cyhalothrin were higher than those to bifenthrin.The mutant strains possibly had higher resistant levels to type I pyrethroids than to type II pyrethroids.5.Fitness cost involved with M918L mutation in R.padi resistant strainsLife table was constructed to explore the fitness cost of M918L mutation to R.padi.The results showed that the M918L mutation caused the development of the second-instar nymph to be significantly prolonged,and the total longevity was significantly shortened;the intrinsic growth rate（r）,the finite rate of increase（λ）,net reproduction rate（R₀）were significantly reduced,and the relative fitness cost was 0.757.No significant difference was found in the developmental of 1^st,3^rd,4^thinstar nymph and mean generation time（T）.The results showed that the M918L mutation was involved in the overall fitness of resistant strains.