Sublethal Effects Of Serratia Marcescens On Brown Planthopper

The brown planthopper,Nilaparvata lugens Stal is one of the serious rice pests in China and other Asian countries.At present,the control of N.lugens mainly depend on chemical pesticides,however,the unreasonable use of chemicals has resulted in reduced susceptibility,even high resistance to many insecticides.Therefore,it is urgent to explore safe and efficient techniques to control N.lugens.Insect pathogens are important resources for biological control,they not only have direct lethal effect,but also have sublethal effect on target pests.Seratia marcescens is a kind of bacteria widely existed in nature.Based on the isolation of S.marcescens from N.lugens,our study focus on the sublethal effect of the bacteria on the insect pest.We investigate the impact of S.marcescens infection on heat tolerance,chemical insecticide sensitivity and facultative symbionts infection of N.lugens.The main results are as follows:(1)Comparison on the biological activity of different concentrations of S,marcescens indicated that,the infectivity increased with the concentration of S.marcescens.The mortality caused by 109cfu/ml S.marescens was significantly higher than that caused by 108 cfu/ml S.marcescens.Whereas there was no difference in killing speed among the three concentrations of S.marcescens.The observation on the symptom of S.marcescens infected insect pest showed that,the body color of the dead brown planthoppers were red,accompanied by other symptoms such as deformity,inability of emergence.(2)The study on sublethal effect of S.marcescens on heat tolerance and chemical insecticide sensitivity of N.lugens showed that,no effect on tolerance to 40? was found when newly emerged N.lugens infected with 3×108 cfu/ml S.marcescens for 72h,the cumulative mortality of the bacteria-treated pests and the control at 40? for 144h were 85.5%and 80%respectively,and there was no significant difference.However,S.marcescens infection improved sensitivity to chemical insecticides,following exposure to bacteria,the nymphs were more sensitive to thiamethoxam and pymetrozine.Whereas there was no effect on the kill speed of the chemical insecticides.(3)The effect of S,marcescens on the infection frequencies of symbionts in N.lugens were investigated.The results showed that,N.lugens collected from Nantong,Jiangsu province,China,harbored four symbionts including Wolbachia,Serratia,Arsenophonus and Acinetobacter,and 109cfu/ml S.marcescens significantly improved the infection frequencies of Wolbachia,Serratia,and the co-infection frequencies of Wolbachia and Serratia,while 108 cfu/ml S.marcescens only increased the infection frequencies of Serratia,and had no effect on the infection frequencies of Wolbachia.Both of the two concentrations of S.marcescens did not affect the infection frequencies of Arsenophonus and Acinetobacter.