Sublethal Effects And Resistance Mechanism Of Cantharidin On Plutella Xylostella

The diamondback moth,Plutella xylostella（L.）,is one of the most destructive pests of cruciferous vegetables worldwide.It has developed resistance to nearly all groups of insecticides.Currently,its control mainly relies on the chemical insecticides.Under such a background,the development and application of new insecticides will provide assurance for effective control.Cantharidin,a defensive toxin of blister beetles,has been considered to have good prospects of utilization and development with respect to its toxicity to P.xylostella and synergistic interactions with some traditional insecticides.To evaluate the effects of insecticides on pests comprehensively,the long-term effects on biological characteristics and population dynamics rather than mortality estimate have been suggested.In addition,the risk assessment of resistance and examination of resistance mechanism are crucial while choosing new insecticides.In this study,the sublethal effects and chronic sublethal effects of cantharidin on P.xylostella were investigated using life table method to assess the changes of development,reproduction and population dynamics when exposed to cantharidin for short or long time periods.With the laboratory-selected strain,the resistance mechanism of cantharidin on P.xylostella was preliminarily explored.In addition,the behavior and reproductive disadvantages of P.xylostella associated with evolution of resistance were also monitored in the selected strain.The main results are as follows:1.When the 3^rdinstar larvae of P.xylostella were fed with LC₁₀,LC₂₅5 and LC₅₀0 doses of cantharidin for 48 h,the pupal weight,pupation rate and adult emergence were remarkably decreased compared with the control in the parent generation.Besides such direct effects,indirect effects on the offspring such as disrupting growth and development,change in the trend of population development were also recorded.The sublethal effects of cantharidin on biological characteristics were more effective in higher concentrations,and these effects on the parent were more noticeable than on the offspring.It is worth noting that the lower sublethal concentration of cantharidin（LC₂）could stimulate reproduction in short time.2.The consistency of high-level susceptibility of P.xylostella to cantharidin was observed when exposed to sublethal concentration of cantharidin for a long-term.The sublethal concentration of cantharidin exhibited severe negative effects on biological characteristics and population growth such as lower survival rate of each developmental stage,clear extension of larval and pupal periods,decreasing fecundity and fertility,significant reduction in the net reproduction rate,intrinsic rate of increase and finite rate of increase;These all imply a serious adverse impact on population growth in P.xylostella.However,these effects gradually faded as the number of sublethal selection cycles increased.Additionally,malformation of wings was noticed in adults,and this phenomenon became more obvious along with the increase of exposure time.Although most biological characteristics like the duration of larval and pupal periods,adult longevity and oviposition period were recovered to normal when the selection pressure was removed,the relative fitness of population still maintained the disadvantage,and the population growth was suppressed as before.3.Selected by cantharidin(LC₅₀)for 12 generations,a laboratory-selected strain of P.xylostella with 5.68-fold resistance was obtained.The synergism bioassays were conducted in both selected and susceptible strains with PBO,DEM and TPP.Results revealed that PBO could increase the efficacy of cantharidin to both selected and susceptible strains,especially in the selected strain with the synergism ratio 1.60;TPP also increased the activity of cantharidin,but there was no significant difference between the two strains;DEM showed virtually no synergistic effect to cantharidin in both strains.4.The activities of the PNOD,GST and CarE were analyzed in both selected and susceptible strains.The results showed that the activity of PNOD from selected strain increased significantly,1.87 fold compared with that of the susceptible strain.No significant difference was observed in the activity of GST and CarE between selected and susceptible strains.5.Fourteen P450s genes belonging to CYP6 family were investigated in both selected and susceptible strains.Real-time quantitative PCR showed that most genes were overexpressed in the selected strain;the mRNA levels of six genes were increased significantly;one of the six genes was overexpressed by 3.95 fold.6.The changes of reproductive behaviors like calling,mating and oviposition,and the reproductive capacity in different mating type were monitored in the selected and susceptible strains.The lower calling rate,reduction of calling time and delaying onset time of calling were observed in the selected strain compared with the susceptible strain.In addition,the selected strain also exhibited disadvantages in mating ability,mating success and fecundity.These suggested that the selected strain had noticeable reproductive disadvantages compared to the susceptible strain.In summary,P.xylostella had low capability of developing resistance to cantharidin.Obvious reproductive disadvantages in the selected strain were observed.The sublethal concentration of cantharidin exhibited severe negative effects on population growth for longtime.Taken together,these results indicate that cantharidin has application potential for controlling P.xylostella.Additionally,the synergistic effect of PBO and the activity of PNOD were enhanced,and the mRNA levels of some CYP6 genes were overexpressed clearly in the selected strain.These indicated that P450s might be involved in the detoxification of cantharidin and the resistance emergence of P.xylostella to cantharidin.