Rajagopalbabu Srinivasan

Thrips Vectors of Tospoviruses

Tospoviruses belong to the sole phytovirus genus, Tospovirus , in the family Bunyaviridae . Tospoviruses are known to be exclusively transmitted by thrips belonging to the family Thripidae and subfamily Thripinae. Of the known 1,710 species of Thripidae only 14 thrips species are currently reported to transmit tospoviruses. Thrips-transmitted tospoviruses cause severe yield losses to several economically important crops in the United States and worldwide. For instance, a single Tospovirus ( Tomato spotted wilt virus ) alone caused an estimated

Effect of Mixed Viral Infections (Potato Virus Y–Potato Leafroll Virus) on Biology and Preference of Vectors Myzus persicae and Macrosiphum euphorbiae (Hemiptera: Aphididae)

1.4 billion in losses in the U.S. over 10 years. Global trade and associated movement of plant materials across borders have introduced tospoviruses and their vectors into newer areas. Advances in serological and molecular techniques have also led to identification of new tospoviruses. This scenario has also initiated new vector-pathogen interactions between introduced and native thrips species and tospoviruses. The goal of this manuscript is to provide a comprehensive and updated list of thrips species that serve as vectors of tospoviruses along with information pertaining to common names, key diagnostic characters, distribution, important crops economically affected, and thrips and Tospovirus -induced symptoms. The manuscript is prepared with special emphasis to the U.S., but information pertaining to other countries is also included.

Laboratory And Field Evaluations Of Silwet L-77 And Kinetic Alone and in Combination with Imidacloprid and Abamectin for the Management of the Asian Citrus Psyllid, Diaphorina Citri (Hemiptera: Psyllidae)

Abstract Mixed viral infections of heterologous viruses such as Potato virus Y (familyPotyviridae, genusPotyvirus, PVY) and Potato leafroll virus (familyLuteoviridae, genusPolerovirus, PLRV) are a regular occurrence in Idaho’s potato,Solanum tuberosum (L.), cropping systems. An increased number of plant samples from Idaho’s potato fields over the past 2 yr has serologically tested positive for both PVY and PLRV via double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) and exhibited more severe symptoms than singly-infected plants (PVY or PLRV). Several studies have extensively examined the mixed infection phenomenon but to the best of our knowledge, none have examined the effect of such infections on vector biology and preference. Laboratory studies were conducted to examine the effect of mixed viral (PVY–PLRV) infection on the fecundity and preference of two of the most efficient PVY and PLRV vectors, the green peach aphid,Myzus persicae (Sulzer), and the potato aphid,Macrosiphum euphorbiae (Thomas) (Hemiptera: Aphididae).M.persicae andM.euphorbiae adults were clip-caged (one adult per cage) to leaflets of PVY, PLRV, PVY–PLRV-infected, and noninfected potato plants. The number of nymphs produced in all four treatments was recorded after 96 h.M.persicae andM.euphorbiae fecundity was significantly higher on mixed infected plants than on singly infected plants or noninfected plants. Preference of alatae and apterae ofM.persicae andM.euphorbiae was determined with the use of settling bioassays. Both alatae and apterae ofM.persicae andM.euphorbiae preferentially settled on PVY–PLRV-infected plants than on singly infected plants (PVY or PLRV) or noninfected plants.

Evaluation of Hairy Nightshade as an Inoculum Source for Aphid-Mediated Transmission of Potato Leafroll Virus

Abstract Silwet L-77, an organosilicone adjuvant, was used to enhance coverage of an entomopathogenic fungus in field trials conducted in a central Florida citrus research grove. The results indicated that Silwet L-77, by itself, was toxic to nymphs of the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae). Laboratory bioassays were conducted to confirm the toxicity of the adjuvant to eggs, nymphs, and adults of D. citri. Silwet L-77 at 0.05% (500 ppm) killed all nymphs, but was not as effective against eggs and adults. However, Silwet L-77, when combined with a reduced rate (one-tenth the lowest label rate = LLR) of imidacloprid, killed >90% of eggs and adults in laboratory tests. In a subsequent field trial, the combination of Silwet L-77 and one-tenth the LLR of imidacloprid gave good control of nymphs, but exhibited weak residual toxicity to adults when compared to imidacloprid at the LLR. Additional trials were then conducted with Silwet L-77 and Kinetic, another organosilicone adjuvant, alone and in combination with different rates of imidacloprid or abamectin using potted citrus trees in the greenhouse. Combining Silwet L-77 or Kinetic with one-fourth or one-half the LLR of imidacloprid killed as many eggs, nymphs, or adults as the LLR of imidacloprid. The combination of Silwet L-77 or Kinetic with one-fourth or one-half the LLR of abamectin killed as many eggs and nymphs as the LLR, but did not control adults as well. The results are discussed in terms of managing the Asian citrus psyllid in Florida now that citrus greening disease is endemic.

Transmission of Iris Yellow Spot Virus by Frankliniella fusca and Thrips tabaci (Thysanoptera: Thripidae)

Abstract Potato leafroll virus (PLRV) causes one of the most serious aphid-transmitted diseases affecting yield and quality of potatoes, Solanum tuberosum (L.), grown in the United States. The green peach aphid, Myzus persicae (Sulzer), is considered to be by far the most efficient vector of this virus. Even the most strict aphid control strategy may not prevent the spread of PLRV unless measures also are taken to keep virus source plants within and outside the crop at a minimum. Hairy nightshade, Solanum sarrachoides (Sendtner), is one of the preferred weed hosts for green peach aphid. The potential of this weed as an aphid reservoir and virus source and its spread or perpetuation were investigated. With the use of double antibody sandwich enzyme-linked immunosorbent assay, it was confirmed that green peach aphid can transmit PLRV to hairy nightshade and that aphids can become viruliferous after feeding on infected hairy nightshade plants. Transmission from hairy nightshade to potato is 4 times the rate of potato to potato or potato to hairy nightshade. The green peach aphid preferred hairy nightshade over potato plants and reproduced at a higher rate on hairy nightshade than on potato. Therefore, a low level of PLRV–hairy nightshade infection could enhance the disease spread in the field.

Effect of an Alternate Weed Host, Hairy Nightshade, Solanum sarrachoides, on the Biology of the Two Most Important Potato Leafroll Virus (Luteoviridae: Polerovirus) Vectors, Myzus persicae and Macrosiphum euphorbiae (Aphididae: Homoptera)

ABSTRACT Thrips-transmitted Iris yellow spot virus (IYSV) (Family Bunyaviridae, Genus Tospovirus) affects onion production in the United States and worldwide. The presence of IYSV in Georgia was confirmed in 2003. Two important thrips species that transmit tospoviruses, the onion thrips (Thrips tabaci (Lindeman)) and the tobacco thrips (Frankliniella fusca (Hinds)) are known to infest onion in Georgia. However, T. tabaci is the only confirmed vector of IYSV. Experiments were conducted to test the vector status of F. fusca in comparison with T. tabaci. F. fusca and T. tabaci larvae and adults reared on IYSV-infected hosts were tested with antiserum specific to the nonstructural protein of IYSV through an antigen coated plate ELISA. The detection rates for F. fusca larvae and adults were 4.5 and 5.1%, respectively, and for T. tabaci larvae and adults they were 20.0 and 24.0%, respectively, indicating that both F. fusca and T. tabaci can transmit IYSV. Further, transmission efficiencies of F. fusca and T. tabaci were evaluated by using an indicator host, lisianthus (Eustoma russellianum (Salisbury)). Both F. fusca and T. tabaci transmitted IYSV at 18.3 and 76.6%, respectively. Results confirmed that F. fusca also can transmit IYSV but at a lower efficiency than T. tabaci. To attest if low vector competency of our laboratory-reared F. fusca population affected its IYSV transmission capability, a Tomato spotted wilt virus (Family Bunyaviridae, Genus Tospovirus) transmission experiment was conducted. F. fusca transmitted Tomato spotted wilt virus at a competent rate (90%) suggesting that the transmission efficiency of a competent thrips vector can widely vary between two closely related viruses.

Direct and indirect effects of a thrips-transmitted Tospovirus on the preference and fitness of its vector, Frankliniella fusca

Abstract Hairy nightshade, Solanum sarrachoides (Sendtner), is a ubiquitous weed in potato agro-ecosystems and nonagricultural lands of southeastern Idaho and the Pacific Northwest. This weed increases the complexity of the Potato leafroll virus (PLRV) (Luteoviridae: Polervirus)-potato pathosystem by serving as aphid and virus reservoir. Previous field studies showed higher densities of green peach aphid, Myzus persicae (Sulzer), and potato aphid, Macrosiphum euphorbiae (Thomas), the two most important vectors of PLRV, on S. sarrachoides compared with potato plants in the same fields. Some of the S. sarrachoides plants sampled in these surveys tested positive for PLRV. Viral infections can alter the physiology of plant hosts and aphid performance on such plants. To understand better the potential effects of S. sarrachoides on the PLRV-potato-aphid pathosystem, the life histories of M. persicae and M. euphorbiae were compared on virus-free and PLRV-infected S. sarrachoides and potato. Individual nymphs of each aphid species were held in clip cages on plants from each treatment to monitor their development, survival, and reproductive output. Nymphal survival for both aphids across plant species was higher on S. sarrachoides than on potato, and, within plant species, it was higher on PLRV-infected plants than on noninfected plants. With a few exceptions, similar patterns occurred for fecundity, reproductive periods, adult longevity, and intrinsic rate of increase. The enhanced performance of aphids on S. sarrachoides and on PLRV-infected plants could alter the vector population dynamics and thus the PLRV-disease epidemiology in fields infested with this weed.

Host Plant Resistance Against Tomato spotted wilt virus in Peanut (Arachis hypogaea) and Its Impact on Susceptibility to the Virus, Virus Population Genetics, and Vector Feeding Behavior and Survival

Phytoviruses including tospoviruses are known to affect the behavior and fitness of their vectors both positively and negatively. In this study, we investigated the effects of Tomato spotted wilt virus (TSWV) (family Bunyaviridae, genus Tospovirus) infection on the fitness and feeding ability of tobacco thrips, Frankliniella fusca (Hinds) (Thysanoptera: Thripidae) using peanut, Arachis hypogaea L. (Fabaceae), as a host. Potentially viruliferous F. fusca laid more eggs than non‐viruliferous F. fusca. In contrast, fewer potentially viruliferous F. fusca developed into adults and required a longer developmental time than non‐viruliferous F. fusca, indicating a direct negative effect of the virus on thrips fitness. In addition, no‐choice feeding tests indicated that non‐viruliferous F. fusca fed more rapidly than potentially viruliferous F. fusca. Typically, phytovirus infections are known to enhance the availability of vital nutrients such as free amino acids in infected host plants and to affect other important physiological processes negatively. Free amino acids are known to play a vital role in egg production and development. Further investigations in this study revealed that leaflets of infected plants had ca. 15 times more free amino acids than non‐infected leaflets. TSWV‐infected leaflets were used to rear potentially viruliferous thrips. Higher amino acid levels in TSWV‐infected leaflets than in non‐infected leaflets could have contributed to increased oviposition by potentially viruliferous F. fusca compared to non‐viruliferous F. fusca. Taken together, these results suggest that increased concentrations of free amino acids in TSWV‐infected plants might serve as an incentive for thrips feeding on otherwise unsuitable hosts, thereby facilitating TSWV acquisition and transmission.

Transmission of Pantoea ananatis and P. agglomerans, causal agents of center rot of onion (Allium cepa), by onion thrips (Thrips tabaci) through feces.

Tomato spotted wilt virus (TSWV) severely affects peanut production in the southeastern United States. Breeding efforts over the last three decades resulted in the release of numerous peanut genotypes with field resistance to TSWV. The degree of field resistance in these genotypes has steadily increased over time, with recently released genotypes exhibiting a higher degree of field resistance than older genotypes. However, most new genotypes have never been evaluated in the greenhouse or laboratory against TSWV or thrips, and the mechanism of resistance is unknown. In this study, TSWV-resistant and -susceptible genotypes were subjected to TSWV mechanical inoculation. The incidence of TSWV infection was 71.7 to 87.2%. Estimation of TSWV nucleocapsid (N) gene copies did not reveal significant differences between resistant and susceptible genotypes. Parsimony and principal component analyses of N gene nucleotide sequences revealed inconsistent differences between virus isolates collected from resistant and susceptible genotypes and between old (collected in 1998) and new (2010) isolates. Amino acid sequence analyses indicated consistent differences between old and new isolates. In addition, we found evidence for overabundance of nonsynonymous substitutions. However, there was no evidence for positive selection. Purifying selection, population expansion, and differentiation seem to have influenced the TSWV populations temporally rather than positive selection induced by host resistance. Choice and no-choice tests indicated that resistant and susceptible genotypes differentially affected thrips feeding and survival. Thrips feeding and survival were suppressed on some resistant genotypes compared with susceptible genotypes. These findings reveal how TSWV resistance in peanut could influence evolution, epidemiology, and management of TSWV.

Whitefly Population Dynamics and Evaluation of Whitefly-Transmitted Tomato Yellow Leaf Curl Virus (TYLCV)-Resistant Tomato Genotypes as Whitefly and TYLCV Reservoirs

Frankliniella fusca, the tobacco thrips, has been shown to acquire and transmit Pantoea ananatis, one of the causal agents of the center rot of onion. Although Thrips tabaci, the onion thrips, is a common pest of onions, its role as a vector of P. ananatis has been unknown. The bacterium, P. agglomerans, is also associated with the center rot of onion, but its transmission by thrips has not been previously investigated. In this study, we investigated the relationship of T. tabaci with P. ananatis and P. agglomerans. Surface-sterilized T. tabaci were provided with various acquisition access periods (AAP) on onion leaves inoculated with either P. ananatis or P. agglomerans. A positive exponential relationship was observed between thrips AAP duration and P. ananatis (R² = 0.967; P = 0.023) or P. agglomerans acquisition (R² = 0.958; P = 0.017). Transmission experiments conducted with T. tabaci adults indicated that 70% of the seedlings developed center rot symptoms 15 days after inoculation. Immunofluorescence microscopy with antibodies specific to P. ananatis revealed that the bacterium was localized only in the gut of T. tabaci adults. Mechanical inoculation of onion seedlings with fecal rinsates alone produced center rot but not with salivary secretions. Together these results suggested that T. tabaci could efficiently transmit P. ananatis and P. agglomerans.