View of Thrips tabaci (Lind.) (Thysanoptera, Thripidae), another vector for tomato spotted wilt virus in Finland

Research Note

Thrips tabaci (Lind.) (Thysanoptera, Thripidae), another vector for tomato spotted wilt virus in Finland

Anne^Lemmettyand IsaLindqvist

Lemmetty,A.^&Lindqvist,1.1993.Thripstabaci (Lind.) (Thysanoptera, Thripidae), another vector for tomato spotted wilt virusinFinland.Agric.Sci.Finl.2: 189-194.

(Agric.Res. Centre ofFinland,Inst.PI. Protect.,FIN-31600 Jokioinen,Finland.) Populationsof Thrips tabaci transmitted the isolate of tomato spotted wilt virus (TSWV)from infected china asters to healthy china astersinlaboratoryexperiments.

Greatvariationwasobserved among absorbance values betweenthripsinfested china asters. The highestabsorbance values wereobtained from stems ofthripsinoculated

plants.

According to our results, T. tabaci seems to bea noteworthy TSWV vectorin Finland,where it is theonly naturally occurringTSWV vectorspecies.It is alsooneof the main pests on greenhouse crops in addition to Frankliniella occidentalis, the primaryvector ofTSWV.

Key words: TSWV, china aster,thrips, transmission, detection,^ELISA

Introduction

Tomato spotted wilt vims (TSWV), a member of the tospovirus group, is vectored by seven thrips species; Frankliniella occidentalis (Perg.), F.

schultzei (Trybom),

F.fusca

The specificity between TSWV and these thrips species among 5000 known thrips species (Zur Strassen 1960)is not yetunderstood. According to Sakimura (1962), the thrips ^must acquire TSWV asnymphs in ordertotransmit the vims. F.

occidentalis is consideredtobe the primaryvector of TSWV in many countries(Allen and Broad-

bent 1986, Choetal. 1987).

Only one of the above mentioned species, T.

tabaci, is known to occur naturally in Finland, where it is one of the main pests of greenhouse vegetables and ornamentals. In^recentyears, also F.

occidentalis has become aserious and rathercom- mon pest in many Finnish greenhouses.

Although T. tabaci has been reported to be a vectorof TSWV (Best 1968),some studies indic- ate thatnotall insect populations orvims isolates areequally acquired (Mauetal. 1991)ortransmit- ted(Paliwal 1974, 1976)by this species.

InFinland,TSWV has been isolated fromtomato and chrysanthemum in 1989(Lemmetty 1991)and from cineraria in 1992. In bothcasestherewere sig- nificant populations ofF. occidentalisand T. tabaciin the greenhouses where the vims wasisolated.

The object of this studywastofindout whether TSWVcanbe vectored by T. tabaci alone.

Material and methods

The vims isolate used in this study was ob- tained from naturally infected cineraria⁽Senecio x

Agric.Sei.^{Fin l.}2⁽¹⁹⁹³⁾

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hybridus) by sap transmission. Identification of TSWV was based on symptoms on host plants, enzyme-linked immunosorbent assay (ELISA) and electron microscopy (Fig. 1).

To test thrips transmission, young and healthy china _asters (Callistephus chinensis Nees.) cv.

Remo original, planted in 10cmplastic pots, were sap inoculated(0.05 M phosphate buffer, pH 7.0) with TSWV isolate. Inoculated asters, which showed necrotic local lesions and a yellow-green systemic mosaic, weretested by ELISA twoweeks after inoculation. Asters of which sap gave absorb- ancevalues(A405^nm)between 1 and 2^werechosen for the experiments.

Three experiments were conducted during May toAugust 1992. Metal framed hard plastic cages, 60 x 60cm insize, wereused. Thetop of the cage was covered witha cloth,andon oneside therewas a narrowdoor. The cageswerestanding_{on a}metal plate with a 5 cm layer ofpeat oneach to allow

pupation of thrips. Approximately 50 thripswere transferred oneby one on each of the six TSWV inoculated asters, using a fine-tipped moistened brush. Mainly second instar larvae,but also some adults, were transferred on two or three leaves above the inoculated leaves. The plants were spaced apartfrom each other in the cage and the larvae were allowed free access to feed _on leaf surfaces for acquisition of the virus. The thrips completed their development and approximately three weekslater,after adult emergence, the TSWV inoculatedasterswerecutand lefton oneside of the cage for two to three days. Six young and healthy asters,cv. Carmen, 10-15cm high, with six to ten leaveson each, wereplacedonthe other side of the cage.

Two of the cages(1and2)wereplacedon atable in the laboratory and the third cage (3) intoagrow- ing chamber. The plants and thrips_weremaintained at

24±I°C

Fig. 1.Electron micrograph of spherical particles of TSWVon athin section of Senecio xhybridusleaf tissue.

Barrepresents250nm. (Photo Anne Lemmetty).

Agric.Sei.Finl. ^2(1993)

Agric. Sd.Fint.2⁽¹⁹⁹³⁾

the thrips used in the experiments were laboratory reared and originated from colonies maintainedon healthy chinaasters.

The development of feeding scars wasobserved and thrips populationsweremonitored. Two weeks after healthyastershad been placed into the cages, additional healthy plants were placed in the cages tominimize the feeding damage.

ELISA _{tests were} done _on leaf samples from healthy_astersapproximately three weeks after they had been exposedtoT. tabaci. Twoto threeleaves, depending _on the size and feeding damage, were removed from each plant for ELISA. Both old and young leaves were sampled. All plants were as- sayed for TSWV infection twice after the healthy astershad been placed in the cages.

Samples for the second ELISAweretaken from leaves of the sixastersin cage 1, fromstemsof the sixastersin cage2 and from leaves andstemsof the six astersin cage 3.

A commercial diagnostic kitwaspurchased from Loewe Biochemica GmbH, Germany. The antise- rum used in all ELISA tests was against thenu- cleocapsid protein of the strain CNPH (de Avilaet al. 1990). All sampleswere tested by direct double antibody ELISA accordingtothe instructions of the kit, using_a 1^to50 dilutionof pressed sap in sample buffer.

Absorbance values (A

405

afteronehour of substrate incubationatroomtem- perature. Each value ofoneplantwasthemeanof four wells. A samplewasconsidered TSWV posit- ive if the absorbance valuewas greaterthan three times the mean value of a healthy control. The healthy controlastersoriginated from the samelot asthose exposedtoTSWV.

Results

Populations of T. tabaci transmitted the TSWV isolate from infected china asters tohealthy china asters.The results of the three experiments repeated in successive weeks wereconsistent (Fig. 2).

After pupation, there were numerous adult T.

tabaci in each cage. Severe feeding damage, browning and scorching of foliage, was rapidly

obvious.Indeed,two china asters(cage 2) became so desiccated that only their stemtissue could be

tested.

TSWV was readily detected by ELISA after a three weeks’ exposure period of the healthy plants toT. tabaci. The first ELISAtestyielded generally low absorbance values.However, leaf samples of one plant gave an exceptionally high absorbance value comparedtothe others (Fig. 3).

In general, the longer the exposure time between transferring the healthy plants into the cage and Fig. 2. ^TSWVabsorbance values of china asters after exposure to TSWV transmission byT.tabaci for25 days (cage1) and24 days (cages 2and 3). Mean absorbance values of leaves of six china asters(cages 1and 3) and four china asters (cage 2)

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testing the plants, the higher the absorbance values (Figs. 3 and 4). The first maximum absorbance value (>2) was obtained from thestem of_achina asterin cage2 when the plants had been exposedto TSWV for 24 days. Maximum absorbance values were also measured from the stemsof five china astersin cage 3 after the plants had been exposedto TSWV transmission by T. tahaci for 32 days (Fig.

4). The absorbance values ofhealthy controls in six ELISAtestsranged from 0.003to0.008.

TSWV _wasdetected by ELISA inextractsfrom leavesorstemsof chinaasters. However, asshown in Fig. 4, thestem tissue gave higher absorbance values.

Discussion

Our laboratory experiments demonsrated that T.

tabaci is capable of obtaining and transmitting TSWVasalso reported by Sakimura^(1963),even though Paliwal (1976) and ^Reddy etal.(1983) could ^not confirm Sakimura’s report. Some as- sumptions have been presented to elucidate the nontransmissibility of TSWV by T. tabaci. Choet al.(1991)foundoutthat TSWV wasnottransstadi- ally passed from larvae toadults in T. tahaci al- though that happened in F. occidentalis, the prim- ary^vectorof TSWV. Zawirska (1976) explained that failuretospread TSWV by T. tahaci correlated with the absence ofmales in Polish T. tabaci popu- lations.

Two factors may have served aspreconditions for successful transmission of TSWV by T. tabaci in our experiment. Firstly, the TSWV isolate in- volved was detected with the antiserum raised against the CNPH strain. Itwas verylikely thatour TSWV isolate belonged to a common serogroup because some exceptional strains seem torequire special thrips species for transmission (Germanet al. 1992).Secondly, the TSWV isolate found in cinerariawasonlyoncetransferred by sap inocula- tion from cinerariato asters. It has been observed that TSWVcanlose itsvectortransmissibility after prolonged culture by sap transmission(Best 1968).

Ie (1982) also suggested that it is unlikely that the Fig. 3.TSWVabsorbance values of six china astersincage

1.Values wererecorded afterplants had been exposed to TSWVtransmission byT.tabaci for 19and25days.

Fig. 4.^TSWVabsorbance values of six china aster leaves and stems incage 3._Valueswererecorded afterplantshad been exposed to TSWV transmissionby T.tabaci for24 and 32 days.

Research Note Agric.Sei.Fin!. ^2(1993)

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defective form of TSWV would be transmitted by thrips.

Undoubtedly there are differences in transmis- sion efficiency of TSWV isolates by different spe- cies of thrips asdemonstrated by Paliwal (1976).

Inourtrial the density of T. tabaciwashigh; even if the transmission efficiency could have beenlow, the thrips transmittedTSWV. The number of larval thrips was unnecessarily high _{as our} assumption that the larvae would suffer from being transferred with _abrush turned_{out to}be incorrect.

We foundastersa very suitablesourceplant for T. tabaci-TSWV transmission assays. Firstly, 7.

tabaci fed readily on asters and, secondly, asters provided systemic infection of TSWV with relat- ively high virus titer for a prolonged period. In

somehost plants it is typical ofTSWV that thevims titer is maintained high for only one to two days beforeadramatic decrease(Best 1968).

The stageof larval thrips may affect the^trans-

mission efficiency of TSWV. In our trial, mostof the thripswere transferred_onthe infectedastersas second instars. Sakimura (1961) found that the feeding activity ofT. tabaci larvaeonEmilia leaves increased during the second instarstage.

Our results showedgreatvariation in absorbance values of the^astersinoculated by thrips. This might be duetothe fact that TSWV is unevenly distrib- uted in host plants. Itwasinterestingtofindoutthat the TSWV concentration was highest instems of asters. Also Allen_etal.(1990)detected TSWV in stemsof chrysanthemum by ELISA.

There are still open questions concerning the transmissibility of TSWV by T. tabaci, but this study provided information that T. tabaci popula- tions in greenhouses canbe_a link between TSWV infected and healthy plants. So far, the spread of TSWV infected plant material has been restricted in Finland andnoepidemic has yetbecome prevalent.

References

Allen, W. R. ^& Broadbent, A. B. 1986.Transmission of tomatospottedwilt virusinOntariogreenhouses bythe western flower thripsFrankliniella occidentalis (Per- gande).Can. J.PI._Path.8: 33-38.

—,Matteoni,J.A.^&Broadbent, A. B. 1990. Susceptibility of cultivars of florist’schrysanthemumto tomatospotted wilt virus. Can. J.PI.Path. 12: 417-423.

Avila, A.^C.de,Huguenot,C„ Resende, R. 0.,^Kitajima,E.

W., Goldbach,R. W. ^& Peters, D. 1990. Serological differentiation of20isolates of tomatospotted willvirus.

J. Gen. Virol.71:2801-2807.

Best, R.J. 1968.Tomatospottedwilt virus.In:Smith,K. M.

&Lauffer, M. A.(eds.). Adv. Virus Res. 13: 65-146.

Cho, J.J., Mitchell, W.^C.,Mau, R. F. L^&Sakimura,K.

1987. Epidemiologyof tomatospottedwilt virus disease oncrispheadlettuceinHawaii.PI.Dis.71: 505-508.

—, Mau, R. F. L., Ullman, D. E.^& Custer,D. M. 1991, Detection of the tomato spotted wilt virus (TSWV) within thrips. In: Hsu, H. T.^& Lawson, R. H. ^(eds.).

Virus-thrips-plant interactions of tomato spotted ^wilt virus ^.Proc. USDA workshop. ARS-87. Beltsville. p.

144-152.

German,T. L., Ullman, D. E. ^&Moyer, J.W. 1992.To- spoviruses: diagnosis, molecular biology, phylogeny, and vectorrelationships. Ann.Rev. Phytopath. 30: 315- 348.

le, T.S. 1982. A sap-transmissible defective form of tomato

spottedwilt virus. J. Gen. Virology.59:387-391, Lemmetty, A. 1991.First reported occurrence of tomato

spotted wilt virus in greenhouse crops in ^Finland, Växtskyddsnotiser55: 7-9.

Mau, R. F. L„ Bautista, R., Cho, J. J., Ullman, D. E., Gusukuma-Minuto, L.^&Custer, D. 1991.Factors af-

fectingthe epidemiologyof TSWVinfield crops: com- parativevirusacquisition efficiency of vectors and suit- ability of alternate hosts to Frankliniella occidentalis (Pergande). In: Hsu, H. T.^&Lawson, R. H. (eds.).Virus- thrips-plant interactions of tomato spotted wilt virus.

Proc,USDAworkshop. ^ARS-87.Beltsville. p.21-27.

Paliwal, Y.^C.1974.Someproperties andthriptransmission of tomatospotted^{wilt virus}inCanada. Can.J. Bot.52:

1177-1182.

1976.Somecharacteristics of thethripvectorrelationship of tomatospottedwilt virusinCanada. Can.J. Bot.54:

402-405.

Reddy,D. V. R„ Amin, P. W.,McDonald, D.^&Ghanekar, A. M. 1983.Epidemiology and control ofgroundnutbud necrosis and other diseases oflegume crops in India causedby TSWV. In: Plumb, R. T. ^& Thresh,J.M.

(eds.). Plant virus epidemiology.Oxford,p.93-102.

Sakimura,K. 1961.Techniques forhandling thrips intrans- missionexperiments with the tomatospottedwilt virus.

PI.Dis. Rep.45: 766-771.

1962.Thepresentstatusofthrips-borneviruses.In:Ma- Aghc. Sei.Fint. 2⁽¹⁹⁹³⁾

Research Note

ramorosch, K. ^(ed.). Biological transmission of disease agents. New York. p. 33-40.

South AfricanThysanoptera. ^J, Ent. Soc. S. Afr. 23:

321-367.

1963.Frankliniellafusca^{, an}additional vector for the tomatospottedwiltvirus, with noteson Thripslabaci,

another vector.Phytopath. 53: 412-415.

Manuscriptreceived February1993 Zawirska, I. 1976. Untersuchungen liber zweibiologische

typen^vonThripstabaci Lind. (Thysanoptera, Thripidae) inderVRPolen. Arch.Phytopath. Pfl.schutz. Berlin. 12:

411-422.

AnneLemmetty IsaLindqvist

AgriculturalResearch Centre of Finland Institute of Plant Protection

Zur Strassen,R. 1960. Cataloqueof the known speciesof FIN-31600 Jokioinen,Finland

SELOSTUS

Tupakkaripsiäinen, Thrips tabaci (Lind.),tomaatin pronssilaikkuviruksen ^(TSWV) toinen vektori Suomessa

Anne Lemmetit ja Isa^Lindqvist

Maatalouden tutkimuskeskus

Tomaatin pronssilaikkuvirusonaiheuttanut suuria vahinkoja eri puolilla maailmaa vihannes- ja koristekasviviljelmillä.

Suomessaviruslöydettiinensimmäisen kerran vuonna 1989 tomaatistajakrysanteemista sekä^vuonna 1992sineraariasta.

Virusta levittävät aikuisetripsiäiset,jotkaovattoukkavaihees^- sasyöneet viroottista kasvia.

Tehokkaimpanaviruksenlevittäjänä pidetäänkalifomian^- ripsiäistä (Frankliniella occidentalis^),muttakuuden muun ripsiäislajinonmyös todettu siirtävän virusta.Tupakkaripsiäi

■

sensiirrostuskyvystäonkuitenkin ristiriitaisia tietoja.

Laboratoriokokeen tarkoituksena oliselvittää,siirtääkö tu pakkaripsiäinen TSWV:tä. Kokeessa käytetty ripsiäispopu

■

laatio kasvatettiin terveissä astereissa laboratorio-olosuhteis- sa.Tutkittu virusisolaatti olieristettysineraariasta. Häkkiko- keessatupakkaripsiäistoukatsiirrettiin^TSWV;ninfektoimille kasveille, joitanesaivatvapaasti syödä.Kun koteloituneista toukista alkoikehittyä aikuisia,häkkeihin siirrettiin terveitä astereita syöttikasveiksi. Kasvien viroottisuus testattiin ELISA-menetelmää käyttäen.

Kokeessa todettiin tupakkaripsiäisen siirtäneen TSWV:n infektoituneista kasveista terveisiin kasveihin. Tupakkaripsi- äinenon ainoa maassamme luontaisestiesiintyvä TSWV:n vektorilaji. Sen yleisyyden huomioon ottaense on kokeen perusteellahuomionarvoinen TSWV:nsiirtäjäSuomessa.

Agric.Sei.Fin!. 2(1993)