Monitoring Of Insecticide Resistance And Studies On Resistance Risk For Tebufenozide In Spodoptera Exigua(Hübner)

The beet armyworm,Spodoptera exigua(H(?)bner),is a worldwide distributed and polyphagous pest,which has developed resistance to many insecticides,including chlorinated hydrocarbons,organophosphates,carbarnates,pyrethroids,benzoylphenylureas, and Bt formulations.In recent years the damage by the beet armyworm to crops increased dramatically in China.Tebufenozide is considered as one of a few highly effective insecticides for controlling beet armyworm.In order to make good use of this insecticide and implement effective resistance management strategy,we surveyed resistance of field populations of beet armyworm from some areas of Jiangsu and Henan provinces in China to several classes of insecticides.Furthermore,the resistant strain was selected with tebufenozide,the resistance realized heritability(h²) was estimated and resistance development rate was projected.In the same time,cross-resistance,inhetitance of resitance, fitness cost and synergistic effects of three synergists to tebufenozide were studied.The results were summarized as follows:1 Resistance monitoring in beet armywormSusceptibility to several classes of insecticides was determined using leaf-dipping method in field populations of Spodoptera exigua from some areas of Jiangsu and Henan Provinces,in China.The results showed that the majority of populations were susceptible or decreased susceptible to a few novel insecticides,such as emamectin benzoate, methoxyfenozede,JS118,tebufenozede,chlorfenapy and indoxacarb,as well as thoidicarb, but two populations from Jiangning and Sheyang counties of Jiangsu province first developed a moderate levels of resistance to abamectin,and one population from Fengxian county of Jiangsu province had a low level of resistance to indoxacarb.Two populations from Jiangning and Funing counties of Jiangsu province and one population from Xinye county of Henan province developed very high levels resitance with 195.7-,407.2-and 365.5-fold and one population from Sheyang county of Jiangsu province had a moderate level resistance(38.3) to chlorfuazuron,and only one populaton collected from Fengxian county of Jiangsu province in 2002 showed susceptible to chlorfuazuron.All tested populations developed very high level resistance to cyhalothrin(181.1～1865.6), low-moderate levels resistance to phoxim(6.9～11.7) and decreased susceptible to low or moderate levels resistance to chlorpyrifos(3.6～11.4),trichlorphon(3.4～10.0) and beta-cypermethrin(3.5～9.9).2 Resistanc selection and risk assessment to tebufenozide in beet armywormA field population of beet armyworm,Spodoptera exigua(H(?)bner)(Lepidoptera: Noctuidae),from Shenzhen,Guangdong province was used by dietary exposure to select for resistance to tebufenozide in the laboratory.After continuous selection with tebufenozide 62 times during 75generations,a resistant strain was achieved with resistance ratio of 39.2-fold to tebufenozide when compared with the original reference strain (141.3-fold compared with a susceptible laboratory strain).Estimation of Realized heritability(h²) of resistance in different selection stage was based on the method described by Tabashnik & McGaughey.The results suggested that h² to tebufenozide in 5 successive selection stages was 0.0471(F₀～F₈),0.2606(F₂₂～F₂₈),0.3613(F₃₁～F₃₈),0.1228(F₄₅～F₅₅) and 0.1849(F₅₇～F₇₅),respectively,h² for the entire selection experiment was 0.1556.The time course of resistant development appeared to slow at first then speed up,afterwards become slow then speed up.The resistance development rate increases as h² and selection intensity increase.Thus,assuming h²=0.1556 and selection kills 80%～90%of the populations,then a 10-fold increase in LC₅₀ is expected in 10.2～12.9 generations.The susceptibility to tebufenozide increased 2 times when the culture was kept for 21 generation without exposure to the chemical pressure after 43 times selection.3 Cross-resistance to other insecticides of tebufenozide-selected beet armyworm and synergistic effects of synergists to tebufenozideSusceptibity to other insecticides was determined in selected population after 47 and 64 selection in the laboratory,respectively.Bioassay revealed that this resistant strain showed high levels resistance to tebufenozide(F₄₇:50.7-fold,F₆₄:85-fold),and showed high levels cross-resistance to chlorfuazuron(increasing resistance fold RR′＞312.5-fold),high levels cross-resistance to methoxyfenozide,RR′71.4(leaf-dipping method) and RR′46(dietary exposure) in F₄₇ and RR′123.1(leaf-dipping method) and RR′133(dietary exposure) in F₆₄, low(RR′4.7 and 7.4-fold with leaf-dipping method) or moderate(RR′14.9 and 22.3-fold with dietary exposure) levels cross-resistance to JS118,moderate levels cross-resistance to abamectin(F₄₇:RR′13.1)and emamectin benzoate(F₆₄:RR′20.4),low levels crossresistance to indoxacarb(RR′8.4 and 8.7),a little cross-resistance to chlorfenapy(RR′1.9 and 1.3),and no cross-resistance to thiodicarb(RR′0.3). Synergism experiment in vitro with DEM,PBO and DEF was performed to see the roles of detoxication enzymes in tebufenozide resistance.The results revealed that PBO had significant synergistic effects on tebufenozide in resistant(SR 3.6) and susceptible strain (SR1.9).Synergistic ratio between resistant and susceptible strains was 1.9.DEM and DEF had apparent synergistic effects in resistant strain(SR 1.6 and 1.4),but no effect in susceptible strain.This implied that mixed-function oxidase played an important role in resistance to tebufenozide,but esterase and glutathione-S-tranferase might be involved in the tebufenozide metabolism.4 Resistance inheritance to tebufenozide in beet armywormLogit regression analysis of F-1 reciprocal crosses between resistant and un-selected strain of beet armyworm indicated that LC₅₀ of the progenies from F₁ and F₁′were 0.523(0.279～0.772) and 0.453(0.402～0.507) mg/L,respectively,and there were no significant difference between the two LC₅₀s.This results showed the gene(or genes) that confers resistance to tebufenozide was on autosome.According to the formula of Stone(1968),the dominance degree(D) of F₁ and F₁′were -0.4217 and -0.4982,respectively.This indicated that resistance was inherited as an incompeletely recessive trait(-1＞D＞0).Similarly,Logit regression analysis of F₁ reciprocal crosses between resistant and laboratory susceptible strain of beet armyworm indicated that LC₅₀ of the progenies from F₁ and F₁′were 0.514 (0.463～0.570) and 0.500(0.317～0.717) mg/L,respectively,and there were no significant difference between the two LC₅₀s.This results also showed the gene(or genes) that confers resistance to tebufenozide was on autosome.According to the formula of Stone(1968),the dominace degree(D) of F₁ and F₁′were -0.2651 and -0.2773,respectively.This indicated that resistance was inhetited as an incompeletely recessive trait(-1＞D＞0).LD-P curves for progenies of two backcrosses and F₂,F₂′were apparently different from correspongding expected curves assuming monogenic inheritance.Theχ² analysis confirmed that observed mortalities failed to fit the monogenetic model,which supports the conclusion that resistance to tebufenozide conferred by more than one factor.5 Fitness cost of resistance to tebufenozide in beet armywormThe results of the relative fitness indicated that the resistant strain possessed significantly biological disadvantages,including lower survivor rate of fourth to fifth instar,prolonged larval and pupal duration,lighter pupa weight,lower pupation and egg number laid per female when compared with other strains.Intrinsic rate of natural increase of resistant, susceptible strains and RS,SR were 341.3,241.6,613.0 and 358.4,respectively.The resistant strain and RS,SR were calculated to have 0.71,1.80 and 1.05 of fitness value relative to susceptible strain.