Discrepancy of Transmission Rate of the Rice Stripe Virus by Nymphs and Adults of the Small Brown Planthopper Laodelphax striatellus Fallen(Homoptera: Delphacidae)? Corresponding authors:E-mail:wanghd61@126.com;zrzhu@zju.edu.cn

Discrepancy of Transmission Rate of the Rice Stripe Virus by Nymphs and Adults of the Small Brown Planthopper Laodelphax striatellus Fallen(Homoptera: Delphacidae)? Corresponding authors:E-mail:wanghd61@126.com;zrzhu@zju.edu.cn

1 Introduction

The small brown planthopper,Laodelphax striatellus Fallen(Homoptera:Delphacidae)is a widely geographical distributed planthopper in the Euro-Asian continents(Zhu et al.2005).It is the vector of the rice stripe disease caused by the rice stripe virus(RSV).RSV has been a destructive rice disease in the Eastern Asian countries(Hibino,1996;Cheng et al.2002;Zhu et al.2005;Zhu et al.2007).The transmission process and efficiency of a plant virus by their vectors was influenced by many factors(Gray&Banejee,1999),for instance,environmental temperature,plant species(Jin et al.1985),plant age(Wang et al.2007),but no information was documented on the role of the planthopper vector's age on the rice stripe virus.In the year 2006 and 2007,we collected nymphs and adults of the vector planthoppers from overwintering habits and biologically tested their transmission rates.

2 Materials and Methods

2.1 Insects

In 2006,the small planthoppers in growth stages of large nymphs and macropterous and brachypterous adults were collected from overwintering habits such wheat,barley,rye fields and grasses in bank fields of 5 sites,e.g.Haiyan,Haining,Jiashan,Huzhou and Changxin,Northern Zhejiang Province.In 2007,the vectors were collected from 7 sites,e.g.Haiyan,Haining,Xiuzhou,Jiashan,Tongxiang and Changxin.More than 100 planthoppers were collected from each site each year.

2.2 Rice seedling

The most widely grown rice cultivar Xiushui 110,a Japonica type bred by Jiaxing Academy of Agricultural Sciences,was sown and maintained in greenhouse plots and the seedlings at age of 15 days were used for the bioassay.

2.3 Bioassay

After they were collected and transported in healthy rice seedlings to the laboratory,the planthoppers were separated into groups of larger nymphs,macropterous and brachypterous adults as described in Sun et al.(2007).One rice seedling was set in a glass tube in size of 15cm×2cm and enclosed in one end with nylon mesh.After one planthopper of either nymph or adult was put into each glass tube,the second end was enclosed as for other the end and the glass tubes were marked for the vector's site etc.All the tubes and plants with planthoppers were sitting vertically on trays with still water to maintain the plants alive.The trays were kept in a greenhouse of RH75%,25～28℃ with 14L:10D.After 24 hours after infection,the seedlings were transplanted into plots in a screen house and covered by net cages in size of 30 cm in height×10cm in diameter individually.The seedlings were monitored and recorded daily for the incidence of the rice stripe disease till 30 days.

2.4 Data analysis

Since all the incidence data of percentages of infected rice plants were lying within the range of 0 to 30%,the square-root transformation was used before analysis of variance.The Generic linear model(GLM)was applied to separate the factors and their interactions on the variance.Thereafter,the least significant difference(LSD)was used to compare the difference among the transmission rates of different status of vectors.The data analysis was performed in the DPS program(Tang and Feng 2006).

3 Results and Analysis

The age and site of the vector had significant effect on the transmission rate(ANOVA,F(2,32)=82.85,P=0.0001;and F(6,32)=8.54,P=0.0098,Table 1),but the factor of year and the two-paired interactions among all the three factors were not significant(P>0.05,Table 1).This indicates that transmission competencies of the vectors collected from different sites were consistent over these two years.In both the years of 2006 and 2007 for each site,the transmission rate of large nymph was higher than those of brachypter and macropter,respectively,and the mean of the former was significantly higher than those of later(Table 2).The ratio of such transmission rates was nymph:macropter:brachypter=1:0.69:0.35 in 2006 and 1:0.68:0.45 in 2007.

SourceofvarianceSSDegreeoffreedomdfMeansquaresMSFProbabilitylevel,PAge223.02572111.512982.84770.0001Site68.9879611.49808.54230.0098Year7.041717.04175.23160.0622Year×Age2.320621.16030.86200.4687Year×Site17.451335.81714.32180.0604Site×Age44.6304123.71922.76320.1107Error8.076061.3460Total335.229532

Table 1 Analysis of variance of factors affecting the transmission rate of overwintering Laodelphax striatellus Fallen

SBPHoriginationsAgeofsmallbrownplanthopper(SBPH)20062007MacropterBrachypterLargenymphMacropterBrachypterLargenymphHaiyan3.36.258.233.525.026.12Haining3.855.136.992.143.035.81Jiashang2.783.836.222.594.216.25Tongxiang3.547.6913.64Xiuzhou3.595.267.13Changxin2.604.228.505.636.487.69Huzhou1.829.0911.29Mean±SE2.87±0.345.7±0.948.25±0.873.5±0.495.28±0.677.77±0.215%cbababa1%BAABABA

Table 2 Comparison of transmission rate (%) of the rice stripe virus by different age of the small brown planthopper (SBPH), Laodelphax striatellus Fallen

4 Discussion

4.1 Consistence of the decrease of virus transmission ability of insect vectors with age

The experiment presented in the paper showed that the large nymphs of the planthopper vector had higher transmission efficiency than their adults.Such discrepancy between adults and younger stages are found in many plant virus vectors.For both sexes of Frankliniella occidentalis(Pergande)(Thysanoptera:Thripidae),the vector of the tomato spotted wilt virus,the transmission efficiency dropped with age,simultaneously with the consumption rate(van de Wetering et al.1999).The transmission of Tomato spotted wilt virus by Thrips tabaci adults decreased with the age too at which the virus was acquired by larvae(Chatzivassiliou et al.,2002).In aphid vector of the persistent transmission of plant viruses,transmission efficiency decline with age though they remain infectivefor a long time,possibly over their whole life.Infectivity is not affected by molting,important because of existence of the virus as aphids mature.Foregut and hind gut is lined with cuticle that is shed with the molt;viruses transmitted in a persistent fashion must be either in midgut or within the body.But Ling(1975)noted that adult green leafhopper(Nephotettix virescens)is three times more efficient vector than nymphs in rice tungro transmission.

The acquisition access period(AAP)and inoculation access feeding periods(IAP)are two of the important factors determining the transmission efficiency.The proportion of aster leafhoppers,Macrosteles quadrilineatus Forbes,that became vectors was significantly higher for bolt strain of aster yellows phytoplasma when leafhoppers acquired aster yellows phytoplasma as nymphs than as adults.Once leafhoppers became inoculative,the rate of transmission remained constant over their life spans(Murral et al.1996).Acquisition only occurs in the first and early second nymphal stages of the life-cycle and adult thrips cannot acquire the virus(Moritz et al.,2004).

Due to the lack of strong evidence to elucidate the mechanism undergoing the different transmission ability between the ages and wing-morphs of the small brown planthopper,more experiments are necessary to carried out to draw the clear pictures.

4.2 Implication of higher RSV transmission competency in the large nymphs of L. striatellus

When winter crops such wheat,ryegrass are spatially connected with or temporally followed by rice seedling nursery or transplanted rice,viruliferous planthopper nymphs remained in the nearby winter crops or grasses will move by jumping to the newly-planted rice plants and transmit the virus to rice resulting in severer RSV incidence than viruliferous adults.Such event should be prevent through spatially and temporally isolation.Postpone of sowing date of rice has been tested experimentally as one of the effective approach to avoid RSV disease outbreak in rice(Zhu,et al.2007,in review).

Additionally,the data can be used in a rice stripe disease epidemiological model to evaluate strategies for the disease management.

Acknowledgement

The research presented in the paper is part of the Zhejiang Provincial Key Projects(2005C32033,2004E60055),China National High-Tech(863)Program(2007AA10Z220).