Monitoring And Management Of Insecticide Resistance In Chilo Suppressalis (Walker) And Screening Of Alternative Insecticides For Replacing Highly Toxic Insecticides

The rice stem borer,Chilo suppressalis(Walker)(Lepidoptera:Pyralidae),is one of the economically important rice insects in China.In the last 10 years,population density and its damage intensity increased dramatically in China,which posed a serious threat to the trait of high and stable yield of the crop.Currently,control of C.suppressalis relies mainly on chemical insecticides.Five highly toxic organophosphates including methamidophos, parathion,methyl-parathion,monocrotophos,and phosphamidon will be banned in 2007 by the Ministry of Agriculture in China.In addition,populations of C.suppressalis in many rice regions of China have developed high levels of resistance to monosultap(bisultap) and triazophos,which are two additional conventional insecticides for the control of C. suppressalis.Resistance to a high efficacy novel insecticide fipronil has also been observed in some field populations in the last several years.Because of toxicity risk and increasing resistance development in C.suppressalis to conventional insecticides,it is urgent to screen available insecticides to find effective alternatives for replacing highly toxic OPs and other conventional insecticides.The aims of present study include:to provide a foundation for resistance management of C.suppressalis,the dynamics of resistance of C.suppressalis to long-history-use insecticides such as monosultap and triazophos and the newly introduced insecticides fipronil and abamectin are monitored,and the possible underlying mechanisms of resistance to the conventional insecticides in field populations of C.suppressalis are studied; to provide scientific evidence for assess the feasibility of pyrethroids for rice insect control, the susceptibilities to pyrethroids in field populations of C.suppressalis are examined and the risk to develop resistance to pyrethroids in C.suppressalis is estimated;under the ban policy for the highly toxic Ops,a mass of compounds are chosen from insecticides in being, and are evaluated for their activities against C.suppressalis,in order to searching for candidate alternatives for field tests and for selecting additiveness and synergism of insecticide mixtures.To monitor the dynamics of resistance of C.suppressalis,a total of eight populations were collected from 2003 to 2006 in five growing provinces in China and were used to examine their susceptibility levels to monosultap,triazophos,fipronil and abamectin using the topical application method.Results indicated that All 9 field populations collected in 2003～2006 were susceptible to abamectin(RR＜5).Most field populations were susceptible to fipronil(RR＜3),but the populations from Ruian and Cangnan,Zhejiang in 2006 showed moderate levels of resistance to fipronil(RR=19.1～23.4).Variable susceptibilities to triazophos and monosultap were detected in different populations.Those populations from the southeast Zhejiang province,had extremely high levels of resistance to triazophos and of high levels to monosultap;the population from Gaochun,Jiangsu had high levels of resistance to triazophos and of extremely high levels to monosultap;the two populations from Nanchang,Jiangxi and Guilin,Guangxi,evolved resistance of moderate or high levels to monosultap and triazophos;the population from Qingpu,Shanghai was low-level resistant to triazophos and moderate-level resistant to monosultap;the population from Changsu,Jiangsu had low levels of resistance to the two insecticides,and the population from Liangyungang,Jiangsu was low-level resistant to monosultap and susceptible to triazophos.The resistance levels in the RA population of rice stem borer exhibited an increasing trend(or with fluctuation) over a five-year period(2002～2006) for fipronil, monosultap and triazophos,and reached a maximal level in 2006 for all four insecticides.To assess the feasibility of pyrethroids for rice insect control,we examined susceptibilities of four field populations(LYG,CS,RA and GL) of C.suppressalis to ten pyrethroids using the topical application method in laboratory in 2004 and 2005.Our results showed that the seven pyrethroids with high fish-toxicity(i.e.beta-cyfluthrin, lambda-eyhalothrin,beta-cypermethrin,deltamethrin,S-fenvalerate,alpha-cypermethrin, and fenpropathrin) were more effective against C.suppressalis than the three compounds with low fish-toxicity(i.e.cycloprothrin,etofenprox,and silafluofen).The results also showed that all ten of the pyrethroids were much more effective than methamidophos and monosultap for C.suppressalis control.In addition,we found that susceptibilities of some field populations of C.suppressalis to some high fish-toxicity pyrethroids were significantly reduced,and our results indicated that a Ruian(RA) field population showed a year-to-year variation in susceptibility to most tested pyrethroids between 2004 and 2005. Our data indicated that the tolerance levels increased dramatically in RA population, especially to beta-cyfluthrin and deltamethrin(with the resistance ratios from 16.2-and 10.6-fold,respectively,in 2004,to 131.4-and 60.4-fold in 2005,respectively).In addition, a close correlation between resistance ratios to the 10 compounds and differences of the structures of these compounds seems being established in the RA05 population,which was resistant to cyclopropane carboxylate pyrethroids with ethenyl and a-cyano while it was still very susceptible to the isovalerate pyrethroid fenvalerate with no cross resistance.This correlation is probably due to the detoxification mediated by the cytochrome P450-dependent monooxygenases(P450s),according to the study on biochemical resistance mechanism to insecticides in C.suppressalis described in Chapter 6.Furthermore,the risk to develop resistance to pyrethroids in C.suppressalis is estimated.QP04 population was used to start cycloprithrin selection.It had resistance of a low level to triazophos before selection(RR=5.6).After 8 incontinuous selection cycles (generations),selected strain was 15.2-fold resistant to cycloprithrin,2.72 times higher than that pre-selection.Realized heritability(h²)estimated from selection was 0.1474(0.2303 for F₈-F₁₁,continuous selection).A similar deltamethrin selection experiment was initiated with LYG05 population,which was very susceptible to deltamethrin(RR=2.2).After 5 incontinuous selection cycles,deltamethrin resistance ratio of screened strain increased to 5.8-fold,2.67 times higher than that pre-selection,and the corresponding h² estimate was equal to 0.6618(1.4314 for F₀-F₂,continuous selection).It was preliminary indicated that this insect had a lower risk to evolve resistance to cycloprithrin than to develop resistance to deltamethrin.To investigate the biochemical resistance mechanism to conventional insecticides in field population of C.suppressalis,synergism studies and detoxicating enzyme assays were conducted in 2007 on a field population of RA07,which was detected to be multi-resistant to triazophos(RR=204.8),monosultap(RR=164.8),pyrethroids(with 42.9-fold RR to deltamethrin),fipronil(RR=6.5) and etc.Synergism studies showed that piperonyl butoxide (PBO) had significant synergistic effect(4.93-fold) to deltamethrin,while S,S,S-tributyl phosphorotrithioate(DEF) and maleic acid diethyl ester(DEM) had no synergistic effect to deltamethrin;DEF had weak synergistic effect(1.38-fold) to triazophos,while DEM and PBO had no synergistic effect to triazophos;DEM,PBO and DEF had no synergistic effect to monosultap and fipronil.In vitro activity of esterase detected usingα-NA as a substrate and microsomal-O-demethylase using p-NA as a substrate showed significant differences between resistant(RA07) and susceptible(LYG05) strains with the increase ratios of 1.32-. and 1.74-fold,respectively.These results suggested that in this multi-resistance population, elevation in microsomal-O-demethylase metabolic activity was an importance mechanism of resistance to deltamethrin,esterase mediated detoxification might be a factor for resistance to triazophos,resistance to monosultap and fipronil had no close relations with three main metabolic enzymes(P450s,esterases and GSTs).To provide alternative insecticides for the control of C.suppressalis,the topical application method was used to evaluate 22 insecticides which represented 6 different insecticide classes for their activities against four field populations of C.suppressalis with different resistance levels to conventional insecticides in 2005.Insecticides of different classes had the similar toxicity sequences in four populations.The rice stem borer was most sensitive to avermectins and fipronil,while C.suppressalis exhibited the least sensitivity to endosulfan and monosultap.Insect growth regulators(tebufenozide,JS118,hexaflumuron and chlorfluazuron) showed great efficacy against C.suppressalis,especially against the populations which have developed resistance to conventional insecticides.Efficacy data showed that seven novel insecticides(abamectin,emamectin bezoate,fipronil,tebufenozide, JS118,hexaflumuron and chlorfluazuron) and a few organophosphates(such as phoxim, chlorpyrifos,quinaphos and pyridaphenthion) were potential alternatives for replacing highly toxic pesticides for field trials.