| Rice is the second most important staple food crop consumed by more than half of the world population.However,the competition between rice and its insect pest has long been remained a major constraint in the production and quality of rice around the globe.Insects cost millions of dollars due to damage and consumption of insecticides in worldwide.Application of insecticide is still the main and important tool for controlling insects.Therefore the formulation and application of novel,effective and environment friendly insecticide is necessary to overcome yield loss caused by insect pest?s outbreak.However,the effectiveness is endangered by the development of resistance in many species of insect pests like Sogatella furcifera.S.furcifera(Horvath)(Hemiptera: Delphacidae)is a notorious pest of rice in many developing countries of Asia,and it damages rice crop by sucking phloem sap and ovipositing in rice stems.Thus chemicals are the only means as necessary inputs to gain high yield of rice controlling this rice insect pest.Due to extensive and frequently uses of chemicals in the rice field,S.furcifera has exhibited resistance to 12 different active ingredients,including buprofezin.Buprofezin is one of the insect growth regulator insecticide,belongs to a thiadiazine group which prevents chitin synthesis of insect pests,the most important part of the insect cuticle.At present,buprofezin insecticide is mostly applied against homopteran pests in rice fields.Due to high application frequency,this insecticide has developed different level of resistance in rice field populations of white-backed planthopper in China.Therefore,an investigation of resistance monitoring and the sublethal effects of buprofezin in the field population,the fitness cost as well as inheritance,cross resistance risk and metabolic mechanisms in laboratory selected buprofezin resistance strain were made to observe the resistance concerns in S.furcifera and the main results are as follow 1.Resistance monitoring in the field populations of S.furciferaIn this study,the resistance level of buprofezin in forty-three(43)field populations of S.furcifera from sixteen(16)locations of five provinces were monitored during 2014 to 2018,to know precisely the current resistance status of WBPH against buprofezin insecticide.The mean LC50 value of buprofezin for 43 field population ranged from 0.61 to 7.36 mg ai/L,indicating that S.furcifera resistance is distributed from low to high level resistance(RR=13.8–167.3-fold)against buprofezin.Notably,the level of resistance of S.furcifera to buprofezin indicated an increasing trend in the period 2014-2018.In addition synergism effects were assessed in the three field populations of high resistance level to buprofezin and found that PBO and DEM produced slight synergism of buprofezin effects in the WX-18,XG-2018 and AH-18 field populations(1.9,1.7,1.9-fold and 1.6,1.7,and 1.6-fold,respectively)where TPP had 1.5,1.3 and 1.5-fold synergistic effect.Furthermore,the susceptibility of field populations of S.furcifera to buprofezin was significantly correlated with the responses of other tested insecticides except for isoprocarb and etofenprox.These findings showed that buprofezin could be an effective alternative chemical for the management of S.furcifera,which is immediately needed to avoid or reduce further increases of resistance in S.furcifera.To prevent further development of the resistance,use of buprofezin should be limited and rotated with alternative insecticides with different modes of action.2.Sublethal effect of buprofezin on the S.furciferaThe effects of sublethal concentrations of buprofezin on life-table traits of S.furcifera had evaluated for two consecutive generations(F0 and F1).Our results exhibited that the fecundity,life span(longevity)and hatchability of the F0 and F1 generations were significantly decreased at LC30 compared to the control(untreated).However,copulation was not significantly affected for the F0 or F1 generations at sublethal concentrations.The female life span was affected negatively at both treatments in F0 and at LC30 in F1,compared to the control.Furthermore,significant effects of the sublethal concentrations were found on the developmental rate of all instars of nymphs except the 3rd instar of F1 individuals.However,the pre-adult period,total pre-oviposition period(TPOP)and adult pre-oviposition period(APOP)significantly increased in F1 individuals at LC30 and LC10 compared to the control.Our findings revealed that demographic characters(survival rate,intrinsic rate of increase(ri),finite rate of increase(λ),net reproductive rate(R0),and gross reproductive rate(GRR))of the F1 generation(from F0 parents)significantly decreased compared to the untreated group;However,the generation time(T)increased at LC10.The results suggested that buprofezin could adversely affect individuals in the successive generation.3.Fitness cost and inheritance of resistant and susceptible strains of S.furciferaA buprofezin-resistant strain of S.furcifera was developed by continuous selection with buprofezin from a field strain after several generations in the laboratory to observe the fitness cost and inheritance of buprofezin resistance.A moderate level of resistance to buprofezin in the original strain of S.furcifera was developed.The fitness cost of resistant and susceptible strain from which moderate level of buprofezin-resistant strain obtained was compared.Life fitness traits of both strains(buprofezin resistant and unselected)of S.furcifera were investigated by using the age-stage,two sex life table method.The relative fitness of buprofezin resistant and the susceptible strain was 0.53,with a decreased intrinsic rate of increase(ri)and net reproductive rate(R0).Variations were also observed in some life-table traits of the buprofezin-resistant strain compared to the susceptible strain.The development time of 1st,3rd,4th and 5th instar nymphs,the adult pre-oviposition period(APOP)and total pre-oviposition period(TPOP)of the buprofezin-resistant strain were significantly increased.Whereas the fecundity(eggs/female),longevity,total male and female longevity were significantly decreased in the buprofezin resistant strain of S.furcifera.The lower net reproductive rate(R0)and intrinsic rate of increase(ri)seemed to be largely due to development time and number of eggs laid.Moreover,the genetic inheritance of buprofezin resistance was also conducted by crossing the susceptible and buprofezin-resistant strains.The results of inheritance of buprofezin resistance have shown an autosomal and incompletely recessive inheritance for buprofezin resistance in the resistant strain of S.furcifera.This investigation provided valuable information for facilitating the potential management tactics to reduce development of buprofezin resistance in S.furcifera.4.Metabolic mechanisms in buprofezin resistant strainTo study metabolic mechanisms,the resistant strain of S.furcifera was evolved in the laboratory from a field population with buprofezin exposure.After continuous selection for 47 generations,the strain had developed moderate resistance to buprofezin.The mean LC50 level of this population has increased to 36.71 mg/L from F0 LC50(1.68 mg/L)after 47 generations.Furthermore,cross-resistance risk and mechanisms of buprofezin resistance were investigated in this planthopper species.Buprofezin-resistant strain showed no obvious crossresistance to other tested insecticides.The result of metabolic enzymes showed that gluthathoine-s-transferase activity of the buprofezin-resistant strain was a 1.99-fold higher than that of the unselected strain and the cytochrome P450 monooxygenase activitiy of the resistant strain was 5.7-fold higher than the unselected strain.Esterase activity has no significant effect of the resistant strain compared to the unselected strain.The synergistic tests of PBO,DEM and TPP to buprofezin-resistant and unselected strains were conducted for further investigation to assess the detoxification enzymes involved with buprofezin resistance in S.furcifera.PBO(3.9-fold)and DEM(1.8-fold)had significant synergistic effect in the resistant strain compared to the unselected strain.No synergistic effect was detected for TPP in both strains.Synergism data and enzyme activities data suggested potential involvement of cytochrome P450 s and GST in buprofezin resistant strain of S.furcifera.In addition,the m RNA expression levels of cytochrome P450(CYP)and GST genes in the buprofezin resistant and unselected strains were analysed by quantitative real-time PCR.The results indicated that sixteen P450 and one GST genes were significantly expressed higher in the buprofezin-resistant strain,including four P450 genes in CYP4 Clade(CYP4DD1,CYP425B1,CYP4C65,and CYP439A3),four in CYP3 Clade(CYP6BD5,CYP6FU1,CYP418A2 and CYP6FJ3),five in CYP2 Clade(CYP304H1,CYP306A2,CYP303A1,CYP15G1 and CYP305A13)and three in Mito Clade(CYP302A1,CYP301B1,CYP314A1),and one GSTTt.Among these CYP450 upregulated genes,CYP302A1,CYP304H1,CYP306A2 and CYP4DD1 had high expression levels in the buprofezin-resistant strain compared to that in unselected strain,with 11.4-fold,6.3-fold,4.2-fold and 4.1-fold variation,respectively.It is concluded that multiple overexpressed CYP450 genes were potentially involved with buprofezin resistance,suggested that these genes may be associated with buprofezin resistance in S.furcifera.This study also increases our knowledge of the buprofezin resistance mechanism in S.furcifera and provides a useful reference for integrated pest management(IPM)strategies. |
