Specific Insect-Virus Interactions Are Responsible for Variation in Competency of Different Thrips tabaci Isolines to Transmit Different Tomato Spotted Wilt Virus Isolates

Local adaptation between sympatric host and parasite populations driven by vector genetics appears to be a factor that influences dynamics of disease epidemics and evolution of insect-vectored viruses. Although T. tabaci is the primary vector of Tomato spotted wilt virus (TSWV) in some areas of the world, it is not an important vector of this economically important plant virus in many areas where it occurs. Previous studies suggest that genetic variation of thrips populations, virus isolates, or both are important factors underlying the localized importance of this species as a vector of TSWV. This study was undertaken to quantify variation in transmissibility of TSWV isolates by T. tabaci, in the ability of T. tabaci to transmit isolates of TSWV, and to examine the possibility that genetic interactions and local adaptation contribute to the localized nature of this species as a vector of TSWV. Isofemale lines of Thrips tabaci from multiple locations were tested for their ability to transmit multiple TSWV isolates collected at the same and different locations as the thrips. Results revealed that the probability of an isofemale line transmitting TSWV varied among virus isolates, and the probability of an isolate being transmitted varied among isofemale lines. These results indicate that the interaction of T. tabaci and TSWV isolate genetic determinants underlie successful transmission of TSWV by T. tabaci. Further analysis revealed sympatric vector-virus pairing resulted in higher transmission than allopatric pairing, which suggests that local adaptation is occurring between T. tabaci and TSWV isolates.     

Effects of tomato spotted wilt virus isolates, host plants, and temperature on survival, size, and development time of Frankliniella occidentalis

Tomato spotted wilt virus (TSWV) replicates in both its plant hosts and its thrips vectors. Replication of TSWV within thrips suggests the potential for pathological effects that could affect the fitness of its vectors directly, whereas infection of the plant may alter its suitability as a host for thrips development. This study was undertaken to examine the influence of TSWV isolate, host plant, and temperature on potential direct and host-mediated effects of virus infection of the thrips and the plant on Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), an important vector of TSWV. Neonate F. occidentalis were reared to adult eclosion on excised foliage of Datura stramonium (L.) (Solanaceae) or Emilia sonchifolia (L.) (Compositae) infected with either the CFL or RG2 isolate of TSWV, or not infected. Effects of the TSWV isolates and host plants on thrips were measured at 18.3, 23.9, and 29.4 °C. Results demonstrate significantly improved survival and a small but significant decrease in development time of F. occidentalis on TSWV-infected plants. These effects resulted from the combined influence of the direct effects of the virus on infected thrips and plant-mediated effects resulting from virus infection of the thrips’ host plant. Our results extend previous findings and help to explain inconsistencies among previously published reports by demonstrating that the manifestation and magnitude of effects of TSWV on F. occidentalis are dependent on host plant, virus isolate, and temperature.     

Vector and virus induce plant responses that benefit a non-vector herbivore

The negative cross-talk between induced plant defences against pathogens and arthropod herbivores is exploited by vectors of plant pathogens: a plant challenged by pathogens reduces investment in defences that would otherwise be elicited by herbivores. This negative cross-talk may also be exploited by non-vector herbivores which elicit similar anti-herbivore defences in the plant. We studied how damage by the thrips Frankliniella occidentalis and/or infection with Tomato spotted wilt virus (TSWV) affect the performance of a non-vector arthropod: the two-spotted spider mite Tetranychus urticae, a parenchym feeder just like F. occidentalis. Juvenile survival of spider mites on plants inoculated with TSWV by thrips was higher than on control and on thrips-damaged plants. However, thrips damage did not reduce spider-mite survival as compared to the control, suggesting that the positive effect of TSWV on spider-mite survival is independent of anti-thrips defence. Developmental and oviposition rates were enhanced on plants inoculated with TSWV by thrips and on plants with thrips damage. Therefore, spider mites benefit from TSWV-infection of pepper plants, but also from the response of plants to thrips damage. We suggest that the positive effects of TSWV on this non-vector species cannot be explained exclusively by cross-talk between anti-herbivore and anti-pathogen plant defences.     

Role of insecticides in reducing thrips injury to plants and incidence of tomato spotted wilt virus in Virginia market-type peanut

Tomato spotted wilt virus (family Bunyaviridae, genus Tospovirus, TSWV), transmitted by many thrips species, is a devastating pathogen of peanut, Arachis hypogaea L. TSWV has become a serious problem in the Virginia/Carolina peanut-growing region of the United States. During 2002, TSWV was present in 47% of the North Carolina hectarage and caused a 5% yield reduction in Virginia. Factors influencing levels of TSWV in runner market-type peanut cultivars, which are primarily grown in Alabama, Flordia, Georgia, and Texas, have been integrated into an advisory to help those peanut growers reduce losses. An advisory based on the southeast runner market-type version is currently under development for virginia market-type peanut cultivars that are grown primarily in the Virginia/ Carolina region. A version based on preliminary field experiments was released in 2003. One factor used in both advisories relates to insecticide use to reduce the vector populations and disease incidence. This research elucidated the influence of insecticides on thrips populations, thrips plant injury, incidence of TSWV, and pod yield in virginia market-type peanut. Eight field trials from 2003 to 2005 were conducted at two locations. In-furrow application of aldicarb and phorate resulted in significant levels of thrips control, significant reductions in thrips injury to seedlings, reduced incidence of TSWV, and significant increases in pod yield. Foliar application of acephate after aldicarb or phorate applied in the seed furrow further reduced thrips plant injury and incidence of TSWV and improved yield. These findings will be used to improve the current virginia market-type TSWV advisory.     

Tactics for management of thrips (Thysanoptera: Thripidae) and tomato spotted wilt virus in tomato

Four studies were conducted in Georgia during spring 1999, 2000, 2001, and 2002 to evaluate various management tactics for reducing thrips and thrips-vectored tomato spotted wilt virus (TSWV) in tomato and their interactions relative to fruit yield. Populations of thrips vectors of TSWV, Frankliniella occidentalis (Pergande) and Frankliniella fusca (Hinds), were determined using flower and sticky trap samples. The management practices evaluated were host plant resistance, insecticide treatments, and silver or metallic reflective mulch. Averaged over all tests, the TSWV-resistant tomato 'BHN444' on silver mulch treatment had the largest effect in terms of reducing thrips and spotted wilt and increasing marketable yield. Of the insecticide treatments tested, the imidacloprid soil treatment followed by early applications of a thrips-effective foliar insecticide treatment provided significant increase in yield over other treatments. Tomato yield was negatively correlated with the number of F. fusca and percentage of TSWV incidence. F. occidentalis per blossom was positively correlated with percentage of TSWV incidence, but not with yield. No significant interactions were observed between cultivar reflective mulch main plot treatments and insecticide subplot treatments; thus, treatment seemed to be additive in reducing the economic impact of thrips-vectored TSWV. Control tactics that manage thrips early in the growing season significantly increased tomato yield in years when the incidence of TSWV was high (>17%).     

Impact of early-season thrips management on reducing the risks of spotted wilt virus and suppressing aphid populations in Flue-cured tobacco

The influence of tray drench (TD) treatments, with and without foliar applications of the plant activator acibenzolar-S-methyl (Actigard), was examined in replicated field plots in 2000--2002. TD treatments of Actigard, imidacloprid (Admire), and these two products combined had little effect on seasonal mean thrips populations; however, thrips densities were lower in the Admire-treated plots at 4 and 5 wk after transplanting. Actigard and Admire TD treatments significantly reduced the seasonal incidence of tomato spotted wilt virus (TSWV) symptomatic plants in 2 yr in the study. The combination of both products was better in reducing TSWV than Actigard alone. Three early-season foliar sprays of Actigard had no effect on thrips population densities, but they did reduce TSWV incidence. The tobacco thrips, Frankliniella fusca (Hinds), comprised 92-95% of the thrips complex each year. Other thrips collected on tobacco foliage at very low densities included Haplothrips spp., Chirothrips spp., Limothrips cerealium (Haliday), other Frankliniella spp. and other unidentified species. Using nonstructural TSWV protein enzyme-linked immunosorbent assay, 1.5-2.3% of the F. fusca tested positive for nonstructural TSWV protein. Cured yields were higher in the TD treatments and the Actigard foliar treatments in the years with high TSWV in the untreated plots. The TD treatments and foliar Actigard had little impact on plant height or grade index; however, TD treatments with Admire had low tobacco aphid, Myzus nicotianae Blackman, populations through 10 wk after transplanting. The early-season Actigard and Admire treatment options are management decisions that can effectively reduce the risks of TSWV incidence in flue-cured tobacco.     

Occurrence and localization of spherical viruslike particles in tissues of apparently healthy tobacco thrips,Frankliniella fusca,a vector of tomato spotted wilt virus

Spherical viruslike particles (VLP) were found in the tissues of apparently healthy tobacco thrips, Frankliniella fusca. The particles occurred in abundance in thrips from Ontario but were absent in thrips from Oklahoma reared under identical conditions. The VLP were not transmissible to any of the seven plant hosts (in four families) of F. fusca suggesting that they may be an insect virus. Transmission of tomato spotted wilt virus (TSWV) by F. fusca, a known vector, was not affected by the presence or absence of the VLP. No TSWV particles were detected in tissues of F. fusca that transmitted TSWV to test plants. The VLP occurred in several internal organs and hemocoele of the thrips and were isolated in vitro by preparing homogenates of gut tissues. Infection of oocytes and presence of VLP in young nymphs suggested transovarial transmission of the particles. The VLP measured 62 ± 4 nm in diameter and usually occurred in dense viroplasms in the cell cytoplasm. Development of the particles within the viroplasms is discussed.     

Effect of Watermelon Silver Mottle Virus on the Life History and Feeding Preference of Thrips palmi

Thrips-borne tospoviruses cause numerous plant diseases that produce severe economic losses worldwide. In the disease system, thrips not only damage plants through feeding but also transmit causative agents of epidemics. In addition, thrips are infected with tospoviruses in the course of virus transmission. Most studies on the effect of tospoviruses on vector thrips have focused on the Tomato spotted wilt virus–Frankliniella occidentalis system. Thus, we focused on another thrips-borne tospovirus, Watermelon silver mottle virus (WSMoV), to examine the effect of virus infection on its vector, Thrips palmi. In this study, the direct and indirect effects of WSMoV on the life history traits and feeding preference of T. palmi were examined. The survival rate and developmental time of the WSMoV-infected larval thrips did not differ significantly from those of the virus-free thrips. Comparing the developmental time of larval thrips fed on the healthy plants, thrips-damaged plants, and thrips-inoculated plants (the WSMoV-infected plants caused by thrips feeding), feeding on the thrips-damaged plants reduced the developmental time, and the WSMoV infection in host plants partially canceled the effect of thrips damage on the developmental time. In addition, no significant variations between the virus-free and WSMoV-infected adult thrips regarding longevity and fecundity were observed. These results implied that WSMoV did not directly affect the life history traits of T. palmi, but the WSMoV infection indirectly affected the development of T. palmi through the virus-infected plants. Furthermore, feeding preference tests indicated that T. palmi preferred feeding on either the thrips-damaged plants or the thrips-inoculated plants to the healthy plants. The effect of tospoviruses on the life history and feeding preference of vector thrips might vary among host plants, virus species, vector species, and environmental factors.     

Differential Susceptibilities between Leaf Disks and Plants in the Transmission of Tomato Spotted Wilt Virus by Frankliniella occidentalis to TSWV Hosts and Transgenic Plants

The efficiency by which tomato spotted wilt virus (TSWV) was transmitted to plants and leaf disks by the vector Frankliniella occidentalis , was analysed. The virus was efficiently transmitted to Datura stramonium, Impatiens sp. and tobacco plants, i.e. 60–100% of the plants became infected when 1–3 viruliferous thrips were confined per plant for a period of 3 days. However, lettuceexhibited a lower susceptibility since only 25% of the test plants were infected when challenged by 10 viruliferous thrips per plant for 3 days. In contrast, complete resistance was found when transgenic tobacco plants, expressing the nucleocapsid protein of TSWV, were challenged with up to 10 viruliferous thrips per plant, whereas all non-transgenic control plants were infected when 5 viruliferous thrips per plant were used. To improve and accelerate the tramission studies, the applicability of leaf disks in these studies was tested. Leaf disks of 16 different plant species appeared to be highly susceptible. Infection ratings ranging from 51.6 to 95.0% were obtained when one viruliferous adult was placed singly on these leaf disks for a period for 24 h. The leaf disk assay was also employed to screen resistance of transgenic plants expressing the nucleocapsid protein of TSWV. One transgenic tomato line displayed complete immunity whereas a second line appeared to be susceptible. For the transgenic tobacco line, positive ELISA reactions were found for a few leaf disks (7.5%) suggesting that some virus replication did occur. However, the ELISA readings for these disks were significantly lower than those for leaf disks of non-transgenic controls. Finally, the significance of the use of the leaf disks and test plants in virus-vector studies is discussed.     

Effects of tomato spotted wilt virus (TSWV) isolates, host plants, and temperature on survival, size, and development time of Frankliniella fusca

The effects of different isolates of the tomato spotted wilt tospovirus (TSWV), host plants, and temperatures on Frankliniella fusca (Hinds) (Thysanoptera: Thripidae), the most important vector of TSWV in North Carolina, were measured in the laboratory. Thrips were reared at either 18.3, 23.9, or 29.4 °C until adult eclosion on excised leaves of Datura stramonium L. or Emilia sonchifolia (L.). Plants were either infected with the TSWV isolates CFL or RG2, or left uninfected (control). The results revealed a positive relationship between larval survival and temperature, regardless of host plant or TSWV isolate. Both survival to adult and percentage transmission of TSWV by F. fusca were significantly affected by the interaction between host plant and TSWV isolate. The consequence of this interaction was that the cohort‐based percentage transmission from infected E. sonchifolia plants for CFL was 1.3‐fold greater than that of RG2, whereas the percentage transmission from infected D. stramonium plants for RG2 was twice that of CFL. Both host plant and TSWV isolates showed significant effects on thrips development time to adult and head capsule width of adult thrips, as well as on the incidence of thrips infection with TSWV. The infection status of these thrips was determined by ELISA for the NSs viral protein. Infected thrips reared on infected host foliage took longer to develop to adult and were smaller than non‐infected thrips which had also been reared on infected host foliage, demonstrating a direct effect of the TSWV on thrips. However, non‐infected thrips reared on non‐infected leaves took longer to develop than non‐infected thrips reared on infected leaves, suggesting an effect of the plant tissue on thrips. In addition, adult thrips reared on TSWV‐infected D. stramonium at 29.4 °C developed smaller head capsules than thrips developing on infected foliage at lower temperatures and on non‐infected leaves of D. stramonium or E. sonchifolia. Both TSWV isolates and host plants differentially affected females more than males. In conclusion, both the infection of thrips by TSWV and TSWV‐mediated changes in host plant quality were found to have significant biological effects on F. fusca.     

Antagonistic plant defense system regulated by phytohormones assists interactions among vector insect, thrips and a tospovirus

The western flower thrips (Frankliniella occidentalis) is a polyphagous herbivore that causes serious damage to many agricultural plants. In addition to causing feeding damage, it is also a vector insect that transmits tospoviruses such as Tomato spotted wilt virus (TSWV). We previously reported that thrips feeding on plants induces a jasmonate (JA)-regulated plant defense, which negatively affects both the performance and preference (i.e. host plant attractiveness) of the thrips. The antagonistic interaction between a JA-regulated plant defense and a salicylic acid (SA)-regulated plant defense is well known. Here we report that TSWV infection allows thrips to feed heavily and multiply on Arabidopsis plants. TSWV infection elevated SA contents and induced SA-regulated gene expression in the plants. On the other hand, TSWV infection decreased the level of JA-regulated gene expression induced by thrips feeding. Importantly, we also demonstrated that thrips significantly preferred TSWV-infected plants to uninfected plants. In JA-insensitive coi1-1 mutants, however, thrips did not show a preference for TSWV-infected plants. In addition, SA application to wild-type plants increased their attractiveness to thrips. Our results suggest the following mechanism: TSWV infection suppresses the anti-herbivore response in plants and attracts its vector, thrips, to virus-infected plants by exploiting the antagonistic SA-JA plant defense systems.     

Variety,mulch and stage of inoculation effects on incidence of tomato spotted wilt virus disease in cucumber (Cucumis sativus L.)

Abstract

Tomato spotted wilt virus (TSWV) vectored by thrips is one of the major diseases affecting cucumber yield. Control of thrips is an underlying factor in its management. A study was conducted to determine the effect of time of inoculation, variety and mulch on disease incidence. Four varieties were inoculated with TSWV at cotyledon, 3 – 4 leaf and flower bud stages in a RCBD experiment replicated four times in a greenhouse. In the field, a 2×8 factorial design where two cucumber varieties were raised on seven types of mulches (red, yellow, silver, clear, black, white, and straw) with unmulched plots as controls was used. Variety Marketer was more tolerant to the disease compared to other varieties. Most varieties were generally tolerant to TSWV at cotyledonous but susceptible at 3 – 4 leaf and flower bud stages. Silver and clear mulches significantly suppressed thrip populations, yield and quality under field conditions.     

Expression and characterization of a soluble form of tomato spotted wilt virus glycoprotein GN

Tomato spotted wilt virus (TSWV), a member of the Tospovirus genus within the Bunyaviridae, is an economically important plant pathogen with a worldwide distribution. TSWV is transmitted to plants via thrips (Thysanoptera: Thripidae), which transmit the virus in a persistent propagative manner. The envelope glycoproteins, G(N) and G(C), are critical for the infection of thrips, but they are not required for the initial infection of plants. Thus, it is assumed that the envelope glycoproteins play important roles in the entry of TSWV into the insect midgut, the first site of infection. To directly test the hypothesis that G(N) plays a role in TSWV acquisition by thrips, we expressed and purified a soluble, recombinant form of the G(N) protein (G(N)-S). The expression of G(N)-S allowed us to examine the function of G(N) in the absence of other viral proteins. We detected specific binding to thrips midguts when purified G(N)-S was fed to thrips in an in vivo binding assay. The TSWV nucleocapsid protein and human cytomegalovirus glycoprotein B did not bind to thrips midguts, indicating that the G(N)-S-thrips midgut interaction is specific. TSWV acquisition inhibition assays revealed that thrips that were concomitantly fed purified TSWV and G(N)-S had reduced amounts of virus in their midguts compared to thrips that were fed TSWV only. Our findings that G(N)-S binds to larval thrips guts and decreases TSWV acquisition provide evidence that G(N) may serve as a viral ligand that mediates the attachment of TSWV to receptors displayed on the epithelial cells of the thrips midgut.     

Herbivore benefits from vectoring plant virus through reduction of period of vulnerability to predation

Herbivores can profit from vectoring plant pathogens because the induced defence of plants against pathogens sometimes interferes with the induced defence of plants against herbivores. Plants can also defend themselves indirectly by the action of the natural enemies of the herbivores. It is unknown whether the defence against pathogens induced in the plant also interferes with the indirect defence against herbivores mediated via the third trophic level. We previously showed that infection of plants with Tomato spotted wilt virus (TSWV) increased the developmental rate of and juvenile survival of its vector, the thrips Frankliniella occidentalis. Here, we present the results of a study on the effects of TSWV infections of plants on the effectiveness of three species of natural enemies of F. occidentalis: the predatory mites Neoseiulus cucumeris and Iphiseius degenerans, and the predatory bug Orius laevigatus. The growth rate of thrips larvae was positively affected by the presence of virus in the host plant. Because large larvae are invulnerable to predation by the two species of predatory mites, this resulted in a shorter period of vulnerability to predation for thrips that developed on plants with virus than thrips developing on uninfected plants (4.4 vs. 7.9 days, respectively). Because large thrips larvae are not invulnerable to predation by the predatory bug Orius laevigatus, infection of the plant did not affect the predation risk of thrips larvae from this predator. This is the first demonstration of a negative effect of a plant pathogen on the predation risk of its vector.     

Weed species in tomato production and their role as alternate hosts of Tomato spotted wilt virus and its vector Frankliniella occidentalis

Tomato spotted wilt virus (TSWV) is an important plant virus that infects a wide range of hosts including weeds making its management difficult. A survey was undertaken to establish the occurrence of weed species in tomato production systems in Kenya and their role as hosts of TSWV and its vectors. Selected weed species were further evaluated for their reaction to TSWV, transmission efficiency by Frankliniella occidentalis and ability to support thrips reproduction. Of the 43 weed species identified in the field, 29 species had been reported as hosts of TSWV, two were non‐hosts and 11 had no record of their status. Among the more common species, Amaranthus hybridus, Solanum nigrum, Tagetes minuta and Datura stramonium were susceptible to the virus and supported high levels of thrips reproduction. The TSWV could not be transmitted to Galinsoga parviflora and Sonchus oleraceus by F. occidentalis despite them being highly susceptible in mechanical transmission tests. There was a significant correlation between feeding damage and number of larvae of F. occidentalis on different weeds. Occurrence of weeds that support thrips reproduction and are good hosts of TSWV is a clear indicator of their role in epidemiology and the importance of their management for disease control.     

Suppressing spread of Tomato spotted wilt virus by drenching infected source or healthy recipient plants with neonicotinoid insecticides to control thrips vectors

Field experiments were done to determine whether drenching plants with two systemically active neonicotinoid insecticides, thiamethoxam and imidacloprid, suppresses spread of Tomato spotted wilt virus (TSWV) by thrips vectors. Separate treatments to TSWV ‘infector’ tomato (source) and healthy lettuce (recipient) plants provided information on the relative importance of targeting control at virus acquisition by nymphs versus virus transmission to healthy plants by adults. Drenches were applied either to seedlings just before transplanting or to soil around plants. The thrips vectors recorded were Frankliniella occidentalis, F. schultzei and Thrips tabaci, but F. schultzei and T. tabaci predominated. Overall ratios of external to internal TSWV spread into and within plots without insecticide ranged from 1 : 2.3 to 1 : 2.8 between field experiments. Applying thiamethoxam as a soil drench to both young source plants and recipient seedling transplants suppressed TSWV incidence by 86%, while such application to either young source or recipient seedlings diminished incidence by 67–70%. When thiamethoxam was applied either as a soil drench to old source plants and concurrently as a seedling drench to recipient plants or as a seedling drench to recipient plants alone, incidence was suppressed by 65–73% and 54–73%, respectively. Thiamethoxam applied as a soil drench to old source plants diminished incidence by only 33% or not significantly. Imidacloprid applied either as a soil drench to old source plants and concurently as a seedling drench or as a seedling drench alone, suppressed TSWV incidence by 90–92% and 80% respectively. Although adult vector thrips and nymph numbers were low, fewer adults and/or nymphs were sometimes recorded due to insecticide application. Drenching healthy seedlings with neonicotinoid insecticides just before transplanting can be an effective chemical control measure to include in integrated disease management strategies to suppress TSWV epidemics in short‐duration crops like lettuce.     

Management of spotted wilt vectored by Frankliniella fusca (Thysanoptera: Thripidae) in Virginia market-type peanut

Field tests were conducted during 2001 and 2002 in northeastern North Carolina to evaluate the impact of cultural practices and in-furrow insecticides on the incidence of Tomato spotted wilt virus (genus Tospovirus, family Bunyaviridae, TSWV), which is transmitted to peanut, Arachis hypogaea L., primarily by tobacco thrips, Frankliniella fusca Hinds (Thysanoptera: Thripidae). Treatments included in row plant populations of 7, 13, and 17 plants per meter; the virginia market-type 'NC V-11' and 'Perry'; planting dates of early and late May; and phorate and aldicarb insecticide applied in-furrow. The incidence of plants expressing visual symptoms of spotted wilt was recorded from mid-June through mid-September. Treatment factors that reduced the incidence of symptoms of plants expressing spotted wilt symptoms included establishing higher plant densities, delaying planting from early May until late May, and applying the in-furrow insecticide phorate. Peanut cultivar did not have a consistent, significant effect on the incidence of symptomatic plants in this experiment.     

Summer weeds as hosts for Frankliniella occidentalis and Frankliniella fusca (Thysanoptera: Thripidae) and as reservoirs for tomato spotted wilt Tospovirus in North Carolina

In North Carolina, Tomato spotted wilt tospovirus (family Bunyaviridae, genus Tospovirus, TSWV) is vectored primarily by the tobacco thrips, Frankliniella fusca (Hinds), and the western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae). TSWV overwinters in winter annual weeds from which it is spread to susceptible crops in spring. Because most susceptible crops are destroyed after harvest before winter weeds emerge in the fall, infected summer weeds are thought to be the principal source for spread of TSWV to winter annual weeds in fall. A survey of summer weeds associated with TSWV-susceptible crops in the coastal plain of North Carolina conducted between May and October revealed that relatively few species were commonly infected with TSWV and supported populations of F. fusca or F. occidentalis. F. occidentalis made up > 75% of vector species collected from 15 summer weed species during 2002. The number of F. occidentalis and F. fusca immatures collected from plant samples varied significantly among plant species. Ipomoea purpurea (L.) Roth, Mollugo verticillata L., Cassia obtusifolia L., and Amaranthus palmeri S. Wats supported the largest numbers of immature F. occidentalis. Richardia scabra L., M. verticillata, and Ipomoea hederacea (L.) supported the largest numbers of F. fusca immatures. TSWV was present at 16 of 17 locations, and naturally occurring infections were found in 14 of 29 weed species tested. Five of the TSWV-infected species have not previously been reported as hosts of TSWV (A. palmeri, Solidago altissima L., Ipomoea lacunosa L., I. purpurea, and Phytolacca americana L.). Estimated rates of infection were highest in I. purpurea (6.8%), M. verticillata (5.3%), and I. hederacea (1.9%). When both the incidence of infection by TSWV and the populations of F. occidentalis and F. fusca associated with each weed species are considered, the following summer weed species have the potential to act as significant sources for spread of TSWV to winter annual weeds in fall: I. purpurea, I. hederacea, M. verticillata, A. palmeri, C. obtusifolia, R. scabra, Ambrosia artemisiifolia L., Polygonum pensylvanicum L., and Chenopodium album L.