Studies On The Field Population Dynamics And Risk Of Evolution Of Resistance To Bt Maize In Spodoptera Frugiperda

Fall armyworm,Spodoptera frugiperda,is an important agricultural insect pest that has invaded in China in the past several years.S.frugiperda has wide ranges of host plants,strong reproductive and migratory abilities and serious detrimental effects on host plants,imposing serious threats to maize,sorghum and other crops in China.Hubei province is a suitable habitat and migration-transitional region for S.frugiperda through which the insect pest migrates from overwintering region in Southern China to the maize production region in Northern China.To date,there have been plenty of fundamental studies about this insect pest in China.However,studies concerning the field population dynamics in the migration-transitional region are rare.Planting Bt maize is an important method for controlling S.frugiperda,but the efficiency of Bt maize in population control and the risk of resistance to Bt maize have not been adequately assessed under the specific circumstances of China.In this dissertation,the field population dynamics of S.frugiperda in the maize planting areas of Hubei province were studied and analyzed,using both methods of field survey and modeling.In addition,the effects of population control and potential risk of resistance of S.frugiperda to Bt maize were assessed by models.The main results are as follows:(1)Field survey and analyse of larval population dynamics of S.frugiperda.Based on the data of field survey of larval population of S.frugiperda in the maize planting areas of Hubei province,the first time of larval observation,larval population dynamics and their spatial correlation were analyzed.The results showed that in 2020,the prevalence period of S.frugiperda was relatively longer and the population density was relatively higher in the sample sites of Southeastern and Central Hubei province,followed by those in the sample sites of Northeastern and Western Hubei province.In the Southeastern,Northeastern and Central Hubei province,which were the main prevalence regions in the study area,the main prevalence period was from the middle of August to the middle of October.The time series of the number of larvae per 100 plants among the sample sites of different regions were highly correlated with certain time lags.Based on the time lags,the yearly peaks of larval population density appeared relatively earlier in the sample sites of Central Hubei province,while relatively later in the sample sites of Southeastern and Northeastern Hubei province.The results demonstrated that there was cross-correlation among the field population dynamics of S.frugiperda in different regions of Hubei province.(2)Modeling analysis of the field populations of S.frugiperda Based on the effective-accumulative-temperature model and experimental parameters concerning development,survival and fecundity,a computer model of population dynamics of S.frugiperda was developed.The model was used to simulate the larval population dynamics of S.frugiperda in the sample sites of Dangyang,Xiantao,Wuxue,and Hongan,and the adult population dynamics S.frugiperda in a sample site of Dangyang.The model was also used to analyze the effects of the initial time and density of eggs,temperature rise,and oviposition age distribution on the population dynamics of S.frugiperda.Results showed that S.frugiperda had 4-5 generations in a year.The simulation results were largely consistent with observations in terms of the time and size of yearly peak.The sensitivity analyses of parameters demonstrated that the earlier the initial time of eggs,the earlier the time of yearly peak.It was also shown that the higher the initial population density of eggs,the higher the size of yearly peak.As far as the impact of temperature rise was concerned,the larger the temperature rise,the earlier the overall incidence and the higher the incidence level.Concerning the impact of oviposition age distribution,the shorter the oviposition period,the earlier the time of yearly peak and the larger the size of yearly peak.In summary,the initial time and population density of eggs,temperature rise,and oviposition age distribution all have important impacts on the field population dynamics of S.frugiperda.(3)Predictive analysis of population control of S.frugiperda by Bt maize in China.The effects of Bt maize on population control of S.frugiperda was predictively analyzed by a population dynamic model.In the cases of one-toxin Bt maize expressing Cry1 Ab and two-toxin Bt maize expressing Cry1Ab+Vip3Aa,both the population trend index and population density decrease as the proportion of Bt maize increases.However,the goal of population control,in which the population trend index is maintained below the value of 1,is easier to achieve in the case of two-toxin Bt maize than in the case of one-toxin Bt maize.When the population trend index on the conventional maize is less than or equal to 20,planting 95% of Bt maize could achieve the goal of population control within one year,while planting even 100% of Bt maize could not achieve the goal of population control.In conclusion,the two-toxin Bt maize is far more effective in population control of S.frugiperda than the one-toxin Bt maize.(4)Predictive analysis of resistance risk of S.frugiperda to Bt maize in China.A model of resistance risk analysis was developed to assess the risk of resistance of S.frugiperda to Bt maize in China,in which three varieties of Bt maize,i.e.Cry1Ab-maize,Vip3Aa-maize,Cry1Ab+Vip3Aa-maize,and two types of refuges,i.e.the structured refuge and seed mixed refuge,were considered.The model was also used to analyze the impact of cross-pollination and larval movement on the risk of resistance evolution.The results showed that in both cases of structured refuge and seed mixed refuge,the time to resistance,i.e.the time it takes for the frequencies of resistance genes to reach the threshold,is much longer for the two-toxin Cry1Ab+Vip3Aa-maize than for the one-toxin Cry1Ab-maize.In the case of cross-pollination and larval movement between Bt and non-Bt plants,use of seed mixed refuge results in much shorter time to resistance than use of structured refuge.When Cry1Ab+Vip3Aa-maize and 20%refuge are used,the Bt dominance of cross pollination is 0.5,and the probability of larval movement is 0.95,the time to resistance in the case of structured refuge is more than 200 generations,while that in the case of seed mixed refuge is only 59 generations.In conclusion,the risk of resistance to Cry1Ab-maize is much higher than that to Cry1Ab+Vip3Aa-maize,the risk of resistance in the case of seed mixed refuge is much higher than in the case of structured refuge,and in the case of seed mixed refuge,cross-pollination and larval movement could significantly increase the risk of resistance to Bt maize.In summary,predictive results concerning the field population dynamics of S.frugiperda in the maize planting areas of Hubei province and evolution of resistance of S.frugiperda to Bt maize were obtained by using the methods of modeling together with field survey and data analysis.The effects of relevant ecological and environmental factors on the field population dynamics and evolution of resistance of S.frugiperda were elucidated,which will be helpful for the decision making in preventive control of S.frugiperda and management of resistance risk of S.frugiperda to Bt maize.