The Mechanism Of Diverse Tolerance To Insecticides Induced By Phytotoxins In Beet Armyworm Spodoptera Exigua

In plant-insect interactions,phytotoxins such as gossypol exert a defensive role on behalf of the plant by interfering with the essential metabolic,biochemical and physiological pathways of herbivorous insects.The beet armyworm,Spodoptera exigua（Hübner）,is a key pest for many important crops including a range of vegetables,ornamentals,and cotton.For this study,we investigated how resistance to deltamethrin relates to enzyme activity in gossypol-pretreated larvae of S.exigua.Through selection with deltamethrin insecticides on gossypol-pretreated larvae for 10 generations,the Gos-SEL population developed a 113.29-fold resistance.Under the same conditions,the Delta-SEL selected population showed a69.76-fold increase resistance along with corresponding levels of xenobiotic defense enzymes activity.Similarly,the fecundity of the Delta-SEL population along with male and female longevity were found to be significantly lower when compared to the Gos-SEL population and the lab susceptible-strain group（SS-strain）.In addition,the activities of cytochrome P450s in S.exigua were significantly enhanced when the insects were fed on a deltamethrin and gossypol-pretreated diet compared with being fed on deltamethrin alone.In conclusion,the reproductive capacity of S.exigua is significantly reduced in the Delta-SEL and Gos-SEL populations compared to the control group（SS-strain）.Elevation of the major detoxification enzyme cytochrome P450 monooxygenase and esterase might play an important role to induce tolerance to the deltamethrin in gossypol-fed S.exigua populations.Tannins are among the most important phytotoxins in cotton field.We showed that tannic acid enhance the toxic effect of chlorantraniliprole on S.exigua when presented in combination.Bioassays using third-instar S.exigua larvae on an artificial diet showed that consumption of tannic acid with chlorantraniliprole at the concentration of(2 mg/g and LC₅₀0.018 mg/L)had higher toxicity when compared to either chlorantraniliprole or tannic acid alone(LC₅₀ 0.027 mg/L).The diet containing tannic acid with chlorantraniliprole significantly prolonged larval and pupal developmental time and extended mean generation time and total pre-oviposition period compared to either chemical alone.Moreover,fecundity,survival rate,reproductive value,intrinsic rate of increase,finite rate of increase and net reproduction rate declined significantly when exposed to the combined treatment.No difference was observed between tannic acid and the control.Meanwhile,tannic acid with chlorantraniliprole had markedly antifeedant effects;causing significant decline in the relative growth rate（RGR）,the relative consumption rate（RCR）,the efficiency of conversion of ingested food（ECI）,the efficiency of conversion of digested food and an increase in the approximate digestibility（AD）compared to either chemical alone.Tannic acid with chlorantraniliprole also decreased the insect’s carbohydrate,lipid and protein contents significantly.The results showed that the interaction between tannic acid and chlorantraniliprole on the growth inhibition of larvae was additive and tannic acid increased the toxicity of chlorantraniliprole to insects.The results of this study provide information useful in integrated pest management programs for S.exigua and show that tannic acid combined with chlorantraniliprole may be a route to reducing the use of synthetic pesticides.Plants employ an intricate and dynamic defense system that includes physiological,biochemical,and molecular mechanisms to counteract the effects of herbivorous attacks.In addition to their tolerance to phytotoxins,beet armyworm has quickly developed resistance to deltamethrin;a widely used pyrethroid insecticide in cotton fields.The lethal concentration（LC50）required to kill 50%of the population of deltamethrin to gossypol-fed Spodoptera exigua larvae was 2.34-fold higher than the control group,suggesting a reduced sensitivity as a consequence of the gossypol diet.Piperonyl butoxide（PBO）treatment was found to synergize with deltamethrin in gossypol-fed S.exigua larvae.To counteract these defensive plant secondary metabolites,beet armyworm elevates their production of detoxification enzymes,including cytochrome P450 monooxygenases（P450s）.Gossypol-fed beet armyworm larvae showed higher 7-ethoxycoumarin-O-deethylase（ECOD）activities and exhibited enhanced tolerance to deltamethrin after 48 and 72 h when compared to the control.Moreover,gossypol pretreated S.exigua larvae showed faster weight gain than the control group after transferring to a deltamethrin-supplemented diet.Meanwhile,gossypol-induced P450s exhibited high divergence in the expression level of two P450 genes:CYP6AB14 and CYP9A98 in the midguts and fat bodies contributed to beet armyworm tolerance to deltamethrin.Knocking down of CYP6AB14 and CYP9A98,via double-stranded RNAs（dsRNA）in a controlled diet,rendered the larvae more sensitive to the insecticide.These data demonstrate that generalist insects can exploit secondary metabolites from host plants to enhance their defense systems against other toxic chemicals.Impairing this defense pathway by RNA interference（RNAi）holds a potential to eliminate the pest’s tolerance to insecticides and,therefore,reduce the required dosages of agrochemicals in pest control.