Biological Traits Of Near Isogenic Line Strains Resistant To Bacillus Thuringiensis Cry1Ac Toxin In Thuringiensis Cry1Ac Toxin In Diamondback Moth,Plutella Xylostella(Lepidoptera:Plutellidae)

Toxins from Bacillus thuringiensis (Bt) is the most widely used as biological pesticide for pest management with remarkable characteristics of harmless to environment. Diamondback moth (DBM), Plutella xylostella (Lepidoptera:Plutellidae), is one of the most notorious pests of cruciferous vegetables worldwide, which was the first insect to evolve resistance to Bt spray in the open-field. The high ability of reproduction and frequent occurrence of this lepidopteran pest led to great reduction of the production and quality of cruciferous vegetables, and the cost of DBM populations control worldwide has been estimated as USS4-5billion annually.In this paper, a colony of NIL-R, near isogenic to the susceptible strain (DBM1Ac-S) with about5,000-fold resistance to Bt CrylAc was investigated, and the results will provide new insight into CrylAc resistance management strategies in the future.(1) the life table results showed that there were not significantly difference in the mean egg width and length between the DNM1Ac-S and the NIL-R. No difference was found in pupae weight regardless of gender in both DBMlAc-S and NIL-R. The egg stage and the survival rate of DBM1Ac-S and NIL-R consistent with the egg size, there was not significantly difference. Which were the same as the development of larve, the longevity of adult, APOP, TPOP and fecundity of every female adult compared with DBM1Ac-S, indicating that the resistance to CrylAc did not lead to developmental asynchrony.(2) the parameter of life table:the intrinsic rate of increase (r), the finite rate of increase (λ), the net reproductive rate (R0), gross reproductive (GRR) were no difference in both two strains, which indicated the resistance to CrylAc in the NIL-R strain was very stable. The relative fitness of P.xylostella, were ratios of1.016,0.940and0.932respectively in repeat of life table study, which reflected the lack-of-fitness costs of CrylAc resistance in NIL-R strain. The results showed that there had no effect on the developmental rate and fecundity of DBM.(3) Compared with the susceptible DBM1Ac-S strain, Leaf disc dip bioassays were conducted to determine susceptibility to CrylAb、Cry1Ac、Cry1Ah、Cry1Ca and CrylIe in DBM1Ac-S and NIL-R. The results showed that NIL-R developed4850-fold resistance to CrylAc, exceeding2000-fold resistance to CrylAb, exceeding63-fold resistance to CrylAh,1.82-fold and0.74-fold resistance to CrylCa and CrylIe, indicating that the CrylAc-selected NIL-R strain had high cross-resistance to CrylAb, had a moderately cross-resistance to CrylAh and had a slight cross-resistance to CrylCa, but had no cross-resistance to CrylIe. (4) The heritability of Cry1Ac resistance was analysed through reciprocal genetic crosses between DBMlAc-S and NIL-R. The LC50values of F1and F1’ progeny were close to the LC50values, and degree of dominance of F1and F1’ progeny were-0.74and-0.71, respectively. The result indicated that the genetic basis of resistance was incomplete recessiveness. The LC50values of F1and F1’ progeny were not significantly difference, log dose-probit lines of F1and F1’ progeny were mainly superposed, which showed that the resistance to Cry1Ac was autosomal rather than sex-linked. The log dose-probit line of backcross BC progeny had obvious plateaus at about50%mortality, indicating a single gene is responsible for resistance. According to a chi-square goodness-of-fit test, sum χ2i (10.425)<0.05(12.592) and there was no significantly difference between observed mortality and excepted mortality, suggesting that the resistance was responsible for a single gene.(5) Proteinase activities were compared in midgut obtained from larvae of DBM1Ac-S and NIL-R. The results showed that there were no significant differences for chymotrypsin-like proteinase activities from the midgut of two P. xylostella strains. However, the activities of total protease and the trypsin-like proteinase from NIL-R strain were significantly higher than the susceptible strain. Therefore trypsin-like proteinase may be associated with resistance to Cry1Ac toxin in this resistant strain of P. xylostella.Conclude:The fitness costs and mode of inheritance of resistance have serious implications on resistance management strategies. Our study showed that there were lack-of-fitness costs for resistance to CrylAc, the resistance to Cry1Ac in NIL-R was autosomal and incompletely recessive, which was controlled by monogenic gene. The results indicated the resistance to Cry1Ac in the NIL-R strains was very stable and might not reduce in the absence of CrylAc. However, if this population is found in the field, there would ineffective control P. xylostella with Cry1Ac toxin.