Xylella fastidiosa inoculation behaviors (EPG X wave) are performed differently by blue-green sharpshooters based on infection status of prior probing host

Does Xylella fastidiosa, a bacterial plant pathogen with noncirculative foregut-borne transmission, manipulate behavior of its sharpshooter vector to facilitate its own inoculation? To answer this question, blue-green sharpshooters, Graphocephala atropunctata (Signoret), were reared on basil to clean their foreguts, then removed from the colony and given one of four pre-electropenetrography (EPG) treatments: i) old colony adults on basil, ii) young colony adults on basil, iii) young colony adults held on healthy grapevine for 4 days, and iv) young colony adults held on Xf-infected (symptomatic) grapevine for 4 days. After treatments, stylet probing behaviors were recorded on healthy grapevine via AC-DC electropenetrography. Waveforms representing putative Xf inoculation (XB1 [salivation and rinsing egestion] and XC1 [discharging egestion]) and other behaviors were statistically compared among treatments. Mean number of events per insect and 'total' duration per insect of XB1 and XC1 were highest for insects from healthy grape, lowest for basil (regardless of insect age), and intermediate for Xf-infected grape. The surprising results showed that prior exposure to healthy grapevines had a stronger effect on subsequent performance of inoculation behaviors on healthy grapevine than did prior exposure to Xf-infected grapevine. It is hypothesized that non-Xf microbes were acquired from healthy grapevine, causing greater clogging of the precibarium, leading to more performance of inoculation behaviors. This study shows for the first time that presence of noncirculative, foregut-borne microbes can directly manipulate a vector's behavior to increase inoculation. Also, EPG can uniquely visualize the dynamic interactions between vectors and the microbes they carry.

A new mouse of the Peromyscus maniculatus species complex (Cricetidae) from the highlands of central Mexico

The Peromyscus maniculatus species complex is one of the most widespread group of small mammals in North America. However, the taxonomy and phylogenetic relationships among its constituent taxa remain unclear. As part of a revision of Peromyscus specimens from the highlands of the Trans-Mexican Volcanic Belt in central Mexico, we identified five individuals collected in 1968 that differed externally from other Peromyscus specimens, although morphologically similar to P. labecula and P. melanotis, both latter in the P. maniculatus species complex. Based on cranial measurements and mitochondrial DNA sequences, we aimed to more accurately determine the phylogenetic relationships and the taxonomic status of these individuals. Molecular phylogenetic analyses showed that the specimens formed a monophyletic clade sister to the P. maniculatus species complex. Pairwise genetic distances between those specimens and other species within the P. maniculatus species complex were greater than 7.91%. In addition, morphological analyses clearly distinguished the test specimens from P. melanotis and P. labecula. Based on the results of our molecular and morphological analyses, we conclude that these specimens represent an undescribed species of the P. maniculatus species complex, which we describe herein.

Quantitative Differences in Feeding Behavior of Lygus lineolaris (Hemiptera: Miridae) on Transgenic and Nontransgenic Cotton

Lygus lineolaris (Palisot de Beauvois) is one of the most important pests on cotton in the United States. Previous research showed that transgenic cotton plants expressing the Bacillus thurigiensis (Bt) crystalline protein Cry51Aa2.834_16 (designated MON 88702) have insecticidal effects on nymphal L. lineolaris. The present study is the first to examine effects of a Bt-expressing cotton on feeding by a heteropteran like L. lineolaris. We compared stylet probing behaviors of third-instar nymphs on pin-head squares (i.e., buds <3 mm wide) of MON 88702 cotton versus nontransgenic (control) DP393 plants using AC-DC electropenetrography. Waveforms were quantified based on appearances previously characterized and correlated with adult L. lineolaris feeding behaviors; nymphal and adult waveforms had the same appearance. Generalized third-instar feeding included maceration of tissues during cell rupturing (waveform CR), tasting/testing during a waveform called transition (T), and ingestion (I); all were similar between MON 88702 and DP393 plants. However, the number of events and duration of each waveform were different between treatments. Relative to nymphs on DP393, those on MON 88702 spent more time overall in stylet probing, due to increased number of maceration events per probe and longer durations of tasting/testing, per waveform event, per probe, and per insect; yet, ingestion events were shorter and more frequent. These findings support that MON 88702 cotton plants were less palatable and/or preorally digestible to L. lineolaris nymphs than DP393, suggesting antixenosis for MON 88702. Transgenic cotton antixenosis could positively affect cotton pest management by reducing feeding of L. lineolaris nymphs and protecting crop yield.

Assessing the Temporal Effects of Squash vein yellowing virus Infection on Settling and Feeding Behavior of Bemisia tabaci (MEAM1) (Hemiptera: Aleyrodidae)

Insect vector behavior and biology can be affected by pathogen-induced changes in the physiology and morphology of the host plant. Herein, we examined the temporal effects of Squash vein yellowing virus (family Potyviridae, genus Ipomovirus) infection on the settling, oviposition preference, and feeding behavior of its whitefly vector, Bemisia tabaci (Gennadius) Middle East-Asia Minor 1 (MEAM1), formerly known as B. tabaci biotype B. Settling and oviposition behavioral choice assays were conducted on pairs of infected and mock-inoculated watermelon (Citrullus lanatus (Thunb) Matsum and Nakai) (Cucurbitales: Cucurbitaceae) at 5-6 days post inoculation (DPI) and 10-12 DPI. Electropenetrography, or electrical penetration graph (both abbreviated EPG), was used to assess differences in feeding behaviors of whitefly on mock-inoculated, 5-6 and 10-12 DPI infected watermelon plants. Whiteflies showed no preference in settling or oviposition on the infected and mock-inoculated plants at 5-6 DPI. However, at 10-12 DPI, whiteflies initially settled on infected plants but then preference of settling shifted to mock-inoculated plants after 8 h. Only at 10-12 DPI, females laid significantly more eggs on mock-inoculated plants than infected plants. EPG revealed no differences in whitefly feeding behaviors among mock-inoculated, 5-6 DPI infected and 10-12 DPI infected plants. The results highlighted the need to examine plant disease progression and its effect on vector behavior and performance, which could play a crucial role in Squash vein yellowing virus spread.

EPG waveform library for Graphocephala atropunctata (Hemiptera: Cicadellidae): Effect of adhesive, input resistor, and voltage levels on waveform appearance and stylet probing behaviors

Blue-green sharpshooter, Graphocephala atropunctata, is a native California vector of Xylella fastidiosa (Xf), a foregut-borne bacterium that is the causal agent of Pierce's disease in grapevines. A 3rd-generation, AC-DC electropenetrograph (EPG monitor) was used to record stylet probing and ingestion behaviors of adult G. atropunctata on healthy grapevines. This study presents for the first time a complete, updated waveform library for this species, as well as effects of different electropenetrograph settings and adhesives on waveform appearances. Both AC and DC applied signals were used with input resistor (Ri) levels (amplifier sensitivities) of 10⁶, 10⁷, 10⁸ and 10⁹ Ohms, as well as two type of adhesives, conducting silver paint and handmade silver glue. Waveform description, characterization of electrical origins (R versus emf components), and proposed biological meanings of waveforms are reported, as well as qualitative differences in waveform appearances observed with different electropenetrograph settings and adhesives. In addition, a quantitative study with AC signal, using two applied voltage levels (50 and 200 mV) and two Ri levels (10⁷ and 10⁹ Ohms) was performed. Intermediate Ri levels 10⁷ and 10⁸ Ohms provided EPG waveforms with the greatest amount of information, because both levels captured similar proportions of R and emf components, as supported by appearance, clarity, and definition of waveforms. Similarly, use of a gold wire loop plus handmade silver glue provided more definition of waveforms than a gold wire loop plus commercial conducting silver paint. Qualitative/observational evidence suggested that AC applied signal caused fewer aberrant behaviors/waveforms than DC applied signal. In the quantitative study, behavioral components of the sharpshooter X wave were the most affected by changes in Ri and voltage level. Because the X wave probably represents X. fastidiosa inoculation behavior, future studies of X. fastidiosa inoculation via EPG will require carefully determined instrument settings. An intermediate Ri level such as 10⁸ Ohms with low voltage, AC applied signal, and gold wire loop plus silver glue is recommended as the best electropenetrograph methods to conduct future EPG studies of sharpshooter inoculation behaviors on Xf-resistant and -susceptible grapevine.

Certain applied electrical signals during EPG cause negative effects on stylet probing behaviors by adult Lygus lineolaris (Hemiptera: Miridae)

This study is the first to fully evaluate whether electrical signals applied to large insects during electropenetrography (EPG; also called electrical penetration graph) negatively affect insect behavior. During EPG, electrical signals are applied to plants, and thus to the gold-wire-tethered insects feeding on them. The insect completes an electrical circuit whose changes in voltage reflect the insect's stylet probing/penetration behaviors, recorded as waveform output. For nearly 50 years of EPG science, evidence has supported that there are no or negligible effects on tiny insects from applied electricity during EPG. Recently however, EPG studies of large-bodied hemipterans such as heteropterans and sharpshooter leafhoppers have been published. The wider stylet diameters of such large insects cause them to have lower inherent resistances to applied signals compared with smaller insects, conveying more electrical current. The present study asked whether such increased currents would affect insect stylet probing, by comparing Lygus lineolaris behaviors on pin-head cotton squares using an AC-DC electropenetrograph. Effects of AC or DC applied signals were separately examined in two factorial studies, each comparing four input resistor (Ri) levels (10⁶, 10⁷, 10⁸ and 10⁹ Ω) and four applied voltage levels (2, 60, 150 and 250 mV). Results showed that changes in both probing and non-probing behaviors were indeed caused by changing signal type, Ri level, or applied voltage. Negative effects on feeding were numerically greater overall for DC than AC applied signals, perhaps due to muscular tetany from DC; however, AC versus DC could not be statistically tested. Results strongly support the need for flexible Ri and applied voltage levels and types, to tailor instrument settings to the size and special needs of each insect subject. Our findings will facilitate further EPG studies of Lygus spp., such as host plant resistance or insecticidal assays/bioassays to assess mode of action and appropriate dosage. It is hoped that this study will also inform EPG studies of similar, large heteropterans in the future.

Correlation of Electropenetrography Waveforms From Lygus lineolaris (Hemiptera: Miridae) Feeding on Cotton Squares With Chemical Evidence of Inducible Tannins

Probing behavior of Lygus lineolaris (Palisot de Beauvois) has previously been characterized with electropenetrography (EPG). Cell rupturing (CR) and ingestion (I) EPG waveforms were identified as the two main stylet-probing behaviors by adult L. lineolaris. However, characterization and identification of EPG waveforms are not complete until specific events of a particular waveform are correlated to insect probing. With the use of EPG, histology, microscopy, and chemical analysis, probing behavior of L. lineolaris on pin-head cotton squares was studied. Occurrences of waveforms CR and I were artificially terminated during the EPG recording. Histological samples of probed cotton squares were prepared and analyzed to correlate specific types and occurrences of feeding damage location and plant responses to insect feeding. Both CR and I occurred in the staminal column of the cotton square. Cell rupturing events elicited the production of dark-red deposits seen in histological staining that were demonstrated via chemical analysis to contain condensed tannins. We hypothesize that wounding and saliva secreted during CR triggered release of tannins, because tannin production was positively correlated with the number of probes with single CR events performed by L. lineolaris. Degraded plant tissue and tannins were removed from the staminal column during occurrence of waveform I. These results conclude the process of defining CR and I as probing waveforms performed by L. lineolaris on pin-head cotton squares. These biological definitions will now allow EPG to be used to quantitatively compare L. lineolaris feeding among different plant treatments, with the goal of improving pest management tactics against this pest.

Strain specificity and simultaneous transmission of closely related strains of a Potyvirus by Myzus persicae

Potato virus Y (PVY), a Potyvirus, is transmitted by aphids in a nonpersistent manner. PVY severely affects potato production worldwide. Single and mixed infections of PVY strains, namely PVY(O), PVY(NTN), and PVY(N:O) are a common occurrence in potato systems. However, information available on the ability of aphids to simultaneously transmit multiple PVY strains, specificity associated with simultaneous transmission, and factors affecting specificity are limited. Aphid-mediated transmission experiments were conducted to test the ability of individual aphids to transmit multiple strains using a PVY indicator host. Preliminary results revealed that aphids can transmit at least two viral strains simultaneously. Subsequently, aphid-mediated transmission of three dual-strain combinations was tested using potato plants. Individual aphids transmitted two viral strains simultaneously for all three dual-strain combinations. In all aphid-mediated dual-strain infections involving PVY(NTN), the rate of PVY(NTN) infection was greater than the infection rates of the second strain and dual-strain combinations, indicating specificity associated with transmission of PVY strains. Results of aphid-mediated transmission experiments were compared with results obtained through mechanical transmission. In general, PVY infection rates from aphid-mediated transmission were lower than the rates obtained through mechanical transmission. Unlike aphid-mediated transmission, component strains in dual-strain inoculations were not eliminated during mechanical transmission. These results suggest that there may also be interference associated with aphid-mediated transmission of closely related PVY strains. Perhaps, the observed specificity and/or interference may explain the increase in the incidence of PVY(NTN) and other necrotic strains in recent years.

Within plant distribution of Potato Virus Y in hairy nightshade (Solanum sarrachoides): an inoculum source affecting PVY aphid transmission

Potato virus Y (PVY) is vectored by several potato-colonizing and non-colonizing aphid species in a non-persistent manner and has a wide host range. It occurs naturally in several plant families. Myzus persicae and Macrosiphum euphorbiae are the most efficient potato-colonizing aphid vectors of PVY. Rhopalosiphum padi, a cereal aphid that migrates in large numbers through potato fields during the middle of the growing season, does not colonize potato plants but can transmit PVY. Hairy nightshade, Solanum sarrachoides, a prevalent annual solanaceous weed in the Pacific Northwest (PNW) of the United States, is an alternative host for PVY and a preferred host for M. persicae and M. euphorbiae. Hence, hairy nightshade plants might play an important role as an inoculum source in the epidemiology of PVY. We looked at titre accumulation and distribution of PVY(O), PVY(N:O) and PVY(NTN) in S. sarrachoides and potato after aphid inoculation with M. persicae and studied the transmission of PVY(O) and PVY(NTN), by M. persicae, M. euphorbiae and R. padi from hairy nightshade to potato plants. Virus titre at different positions on the plant was similar in S. sarrachoides and potato plants with strains PVY(O) and PVY(N:O). Titres of PVY(NTN) were similar in S. sarrachoides and potato but differences in titre were observed at different positions within the plant depending on the plant phenology. Percentage transmission of PVY(NTN) by M. persicae and M. euphorbiae was twice as high (46 and 34%, respectively) from hairy nightshade to potato than from potato to potato (20 and 14%). Percentage transmission of PVY(O) by M. persicae and M. euphorbiae was not affected by the inoculum source. No effect of the inoculum source was observed in the transmission of either PVY strain by R. padi. These results show that hairy nightshade may be an equal or better virus reservoir than potato and thus, important in the epidemiology of PVY.