Cross-resistance and genetics of field-evolved resistance to chlorfenapyr in Plutella xylostella

Chlorfenapyr is a broad-spectrum halogenated pyrrole insecticide with a unique mode of action.Due to the misuse and overuse of this chemical,resistance has been reported in several arthropods,including Plutella xylostella,which is one of the most destructive insect pests afflicting crucifers worldwide.A better understanding of the cross-resistance and genetics of field-evolved chlorfenapyr resistance could effectively guide resistance management practices.Here,the chlorfenapyr resistance of a fieldderived population of P xylostella was introgressed into the susceptible IPP-S strain using a selection-assisted multigenerational backcrossing approach.The constructed nearisogenic strain,TH-BCsF2,shared 98.4%genetic background with the recurrent parent IPP-S strain.The TH-BCsF2 strain showed 275-fold resistance to chlorfenapyr,but no significant cross-resistance to spinosad,abamectin,chlorpyrifos,β-cypermethrin,indoxacarb,chlorantraniliprole,or broflanilide(no more than 4.2-fold).Genetic analysis revealed that resistance was autosomal,incompletely dominant,and conferred by 1 major gene or a few tightly linked loci.The synergism of metabolic inhibitors(PBO,DEM,and DEF)to chlorfenapyr was very weak(<1.7-fold),and the metabolic enzyme activities in the TH-BCsF2 strain were not significantly elevated compared with the IPP-S strain.The results enhances our understanding of the genetic traits of chlorfenapyr resistance,and provides essential information for improving resistance management strategies.

Baseline susceptibility of Helicoverpa armigera,Plutella xylostella,and Spodoptera frugiperda to the meta-diamide insecticide broflanilide

Broflanilide is a novel meta-diamide insecticide that acts as a y-aminobutyric acid-gated chloride channel allosteric modulator.With its unique mode of action,broflanilide has no known cross-resistance with existing insecticides and is expected to be an effective tool for the management of insecticide resistance.Establishing the base-line susceptibility to this insecticide is an essential step for developing and implementing effective resistance management strategies.Here we evaluated the baseline susceptibil-ity to broflanilide for 3 cosmopolitan lepidopteran pest species,Helicoverpa armigera,Plutella xylostella,and Spodoptera frugiperda.Broflanilide exhibited high activity against populations sampled in the major distribution range of these pests in China,with median lethal concentrations(LCso)ranging between 0.209 and 0.684,0.076 and 0.336,and 0.075 and 0.219 mg/L for H.armigera,P xylostella,and S.frugiperda,respectively.Among-population variability in susceptibility to broflanilide was moderate for H.armigera(3.3-fold),P xylostella(4.4-fold),and S.frugiperda(2.9-fold).The recommended diagnostic concentrations for H.armigera,P xylostella,and S.frugiperda were 8,4,and 2 mg/L,re-spectively.Little or no cross-resistance to broflanilide was detected in 3 diamide-resistant strains of P xylostella and 1 spinosyns-resistant strain of S.frugiperda.Our results provide critical information for the development of effective resistance management programs to sustain efficacy of broflanilide against these key lepidopteran pests.

Equivalent intensity but differential dominance of sodium channel blocker insecticide resistance conferred by F1845Y and V1848l mutations of the voltage-gated sodium channel in Plutella xylostella

Two point mutations(F1845Y and V1848I)in the voltage-gated sodium channel gene of Plutella xylostella are involved in the target-site resistance to sodium channel blocker insecticides(SCBIs).The contribution of the individual mutations to the SCBI resistance and the associated inheritance modes is as yet unclear.Through 2 rounds of single-pair crossing and marker-assisted selection,2 P xylostella strains(1845Y and 18481)bearing homozygous F1845Y or V18481 mutant alleles were successfully established from a field-collected population,and the contribution of each mutation to SCBI resistance,as well as associated inheritance patterns,was determined.When compared with the susceptible SZPS strain,each of the mutations individually conferred equally high-level resistance to indoxacarb(378 and 313 fold)and metaflumizone(734 and 674 fold),respectively.However,dominance levels of resistance to SCBIs were significantly different between the 2 resistant strains.Resistance of the 1845Y strain to indoxacarb and metaflumizone was inherited as an autosomal and incompletely dominant trait(D values ranged from 0.43 to 0.76).In contrast,that of the 1848I strain followed an autosomal but incompletely recessive to semidominant mode(D values:-0.24 to 0.09).Our findings enriched the current understanding of inheritance and mechanisms of SCBI resistance in P.xylostella,and will help develop resistance management programs for P:xylostella and othereconomic pests.

Whole-genome sequencing to detect mutations associated with resistance to insecticides and Bt proteins in Spodoptera frugiperda

The fall armyworm(FAW),Spodoptera frugiperda,is a major pest native to the Americas that has recently invaded the Old World.Point mutations in the target-site proteins acetylcholinesterase-1(ace-1),voltage-gated sodium channel(VGSC)and ryanodine receptor(RyR)have been identified in S.frugiperda as major resistance mechanisms to organophosphate,pyrethroid and diamide insecticides respectively.Mutations in the adenosine triphosphate-binding cassette transporter C2 gene(ABCC2)have also been identified to confer resistance to Cry IF protein.In this study,we applied a whole-genome sequencing(WGS)approach to identify point mutations in the target-site genes in 150 FAW individuals collected from China,Malawi,Uganda and Brazil.This approach revealed three amino acid substitutions(A201S,G227A and F290V)of S.frugiperda ace-1,which are known to be associated with organophosphate resistance.The Brazilian population had all three ace-1 point mutations and the 227A allele(mean frequency=0.54)was the most common.Populations from China,Malawi and Uganda harbored two of the three ace-1 point mutations(A201S and F290V)with the 290V allele(0.47-0.58)as the dominant allele.Point mutations in VGSC(T929I,L932F and L1014F)and RyR(I4790M and G4946E)were not detected in any of the 150 individuals.A novel 12-bp insertion mutation in exon 15 of the ABCC2 gene was identified in some of the Brazilian individuals but absent in the invasive populations.Our results not only demonstrate robustness of the WGS-based genomic approach for detection of resistance mutations,but also provide insights for improvement of resistance management tactics in S.frugiperda.