Seasonal flight activity of two Homalodisca species (Homoptera: Cicadellidae) that spread Xylella fastidiosa in southern California.

Homalodisca coagulata (Say) and Homalodisca lacerta (Fowler) are vectors of a new bacterial disease of oleander in California known as oleander leaf scorch, induced by the bacterium Xylella fastidiosa. H. coagulata also has been implicated in the spread of the strain of X. fastidiosa that induces Pierce's disease of grapevines in California. We monitored the flight activity of H. coagulata and H. lacerta in oleander and citrus by using yellow sticky cards at three southern California locations where outbreaks of oleander leaf scorch have been documented, and where vector compliments are different. Areas sampled included a mesic coastal area (Irvine, CA) that supports predominantly H. coagulata and few H. lacerta, a dry inland location (Palm Desert, CA) that supports predominantly H. lacerta and few H. coagulata, and an intermediate area (Riverside, CA) supporting both Homalodisca species. From November 1996 to October 1999 peak catches of both Homalodisca species occurred during the midsummer at all locations. H. coagulata was trapped in greater numbers in citrus than in oleander at both the Riverside and the Irvine sites. Likewise, H. lacerta in Riverside was more associated with citrus than oleander, yet H. lacerta in Palm Desert was trapped in greater numbers in oleander than citrus.     

Impact of a screen barrier on dispersion behavior of Homalodisca coagulata (Hemiptera: Cicadellidae)

The recent incursion of the sharpshooter Homalodisca coagulata (Say) (Hemiptera: Cicadellidae), a vector of the plant pathogenic bacterium Xylellafastidiosa, into southern California has caused new epidemics of plant diseases. The potential of H. coagulata to spread throughout the state and disseminate disease has encouraged the development of techniques to limit further spread of the vector and to manage disease epidemics where the insect already exists. We evaluated a unique tactic to curtail the immigration of H. coagulata into high-value crops, including nursery stock, where they can be spread via commercial transportation throughout the state and into disease-susceptible vineyards. This tactic consists of a 5-m-high screen barrier surrounding the crop that is impenetrable to H. coagulata. We examined H. coagulata orientation and flight direction when placed near or on the screen barrier, and determined the proportion of insects that flew over it. When released midway between a barrier and adjacent vegetation 71.5% of H. coagulata flew away from the barrier and in the direction of the vegetation; whereas 29.5% flew in the direction of the barrier. Of the total number of H. coagulata released, 7.5% flew over the barrier. When placed on the barrier, H. coagulata generally climbed up an average of 1.16 m before flying away from the structure. Of the total number of H. coagulata placed on the barrier, 6% flew over it. These results suggest that a screen barrier can be a part of a management strategy to reduce number of H. coagulata in high-value crops.     

Transmission of Xylella fastidiosa to grapevines by Homalodisca coagulata (Hemiptera: Cicadellidae)

Pierce's disease (PD) of grapevines is caused by a xylem-limited bacterium Xylella fastidiosa (Wells, Raju, Hung, Weisburg, Mandelco-Paul, and Brenner) that is transmitted to plants by xylem sap-feeding insects. The introduction of the sharpshooter leafhopper Homalodisca coagulata (Say) into California has initiated new PD epidemics in southern California. In laboratory experiments, the major characteristics of H. coagulata's transmission of X. fastidiosa to grapevines were the same as reported for other vectors: short or absent latent period; nymphs transmitted but lost infectivity after molting and regained infectivity after feeding on infected plants; and infectivity persisted in adults. Adult H. coagulata acquired and inoculated X. fastidiosa in <1 h of access time on a plant. Inoculation rates increased with access time, but acquisition efficiency (20% per individual) did not increase significantly beyond 6-h access. Estimated inoculation efficiency per individual per day was 19.6, 17.9, and 10.3% for experiments where plant access was 1, 2, and 4 d, respectively. Freshly molted adults and nymphs acquired and transmitted X. fastidiosa more efficiently than did older, field-collected insects. H. coagulata transmitted X. fastidiosa to 2-yr-old woody tissues of grapevines as efficiently as to green shoots. H. coagulata transmitted X. fastidiosa 3.5 mo after acquisition, demonstrating persistence of infectivity in adults. About half (14/29) of the H. coagulata from which we failed to culture X. fostidiosa from homogenized heads (with a detection threshold of 265 CFU/head) transmitted the pathogen to grape, and 17 of 24 from which we cultured X. fastidiosa transmitted.     

Feasibility of tracking within-field movements of Homalodisca coagulata (Hemiptera: Cicadellidae) and estimating its densities using fluorescent dusts in mark-release-recapture experiments

A mark-release-recapture technique was developed and tested for use in tracking the field movements of adult glassy-winged sharpshooters, Homalodisca coagulata (Say) (Hemiptera: Cicadellidae), in various agricultural and urban plantings. Greenhouse experiments in which adult H. coagulata were marked with one of five colored fluorescent dusts (Aurora Pink-All, Horizon Blue-A19, Blaze Orange-A15N, Saturn Yellow-A17, and Corona Magenta-A21) and released into cages with citrus seedlings showed that their mortality rates during a 30-d period were statistically similar to that of the undusted controls. Adults marked with a sixth dust color (Signal Green-A18N) suffered higher rates of mortality than did the undusted controls and thus were eliminated from further consideration. Adult H. coagulata marked with one of the five accepted colors of fluorescent dust were able to fly beyond 100 m in a field devoid of vegetation within minutes of their release, and the marking did not affect overall flight behavior significantly compared with that of the undusted controls. However, at wind speeds above 5 m/s, percentage recapture was significantly reduced, which indicates that both dusted and undusted adults were unable to orient their flight. In total, 41,124 marked and unmarked adults were released in the three field experiments in southern California (Riverside and Kern counties) during 2000 and 2001 to evaluate flight dispersal and estimate densities of H. coagulata. The mark-release-recapture and feral data obtained during the June, July, and August 2001 studies, when coupled with the Lincoln index, yielded estimates of adult H. coagulata of 1.2 and 2.2 million per ha, respectively, at a San Joaquin Valley (Kern Co.) and a southern California (Riverside Co.) citrus grove. The use of colored dusts to mark H. coagulata proved to be reliable, cost-effective, and time-efficient for mark-release-recapture studies with this insect within a citrus grove, but they are less likely to be useful for studies of adult H. coagulata movements among plantings.     

Spatial and temporal dynamics of overwintering Homalodisca coagulata (Hemiptera: Cicadellidae)

A 4-yr landscape-scale study was conducted to investigate spatial and temporal dynamics of overwintering Homalodisca coagulata (Say) (Hemiptera: Cicadellidae) in the lower San Joaquin Valley, California. Spatial structures of H. coagulata distributions were characterized with Moran's I index, and spatial associations between H. coagulata and the surrounding environment were investigated with a geographic information system. H. coagulata was caught consistently with sticky traps throughout the winter, and trap catches formed a distinctive peak in December or January, indicating active flight of H. coagulata during the winter. In 2000-2001, the mean +/-SE trap count was 4.8 +/- 1.21 per trap per wk, and H. coagulata trap catches were spatially autocorrelated within approximately 1.3 km. Approximately 49% of H. coagulata were caught in citrus, 23% in stone fruit, and 11% in grape. After a control program began in spring 2001, the mean trap count was considerably lower (0.041 +/- 0.0004 per trap per wk), and no spatial autocorrelations were detected in 2001-2004. H. coagulata trap catch-crop associations also changed after initiation of the control program. Between 25 and 38% of H. coagulata trap catches were from citrus, between 8 and 20% were from stone fruit, and between 11 and 25% were from grape. Potential for winter-season spread and management of Xylella fastidiosa Wells et al., a pathogen causing Pierce's disease, are discussed.     

Distribution and management of citrus in California: implications for management of glassy-winged sharpshooter

The epidemiology of Pierce's disease of grape (Vitis spp.) in California has changed over the past 10 yr due to the introduction of an exotic vector, Homalodisca vitripennis (Germar), the glassy-winged sharpshooter. Although this insect is highly polyphagous, citrus (Citrus spp.) is considered a preferred host and proximity to citrus has been implicated as a significant risk factor in recent epidemics of Pierce's disease in southern California. Consequently, a detailed knowledge of the distribution and management of citrus in relation to grape is needed to improve insect and disease management. Analysis of data on the area planted to these two commodities indicates that only five counties in California concomitantly grow >1,000 ha of grape and >1,000 ha of citrus: Riverside, Kern, Tulare, Fresno, and Madera counties. Comparison of the distribution of grape and citrus within each of these counties indicates that the percentage of grape that is in proximity to citrus is greatest for Riverside County, but the total area of grape that is in proximity to citrus is greater for Fresno, Kern, and Tulare counties. The use of carbamates, neonicotinoids, organophosphates, and pyrethroids as part of the citrus pest management program for control of key insect pests was compared among the same five counties plus Ventura County from 1995 to 2006. Ventura County was included in this analysis as this county grows >10,000 ha of citrus and has established glassy-winged sharpshooter populations. The use of these broad-spectrum insecticides was lowest in Riverside and Ventura counties compared with the other four counties. Analysis of historical trapping data at the county scale indicates a negative association of broad-spectrum insecticide use with glassy-winged sharpshooter abundance. These results are used to retrospectively analyze the Pierce's disease outbreaks in Kern and Riverside counties.     

Biochemical monitoring of acetylcholinesterase sensitivity to organophosphorus insecticides in glassy-winged sharpshooter Homalodisca coagulata Say (Homoptera: Cicadellidae) and smoke-tree sharpshooter H. lacerta Fowler

The glassy-winged sharpshooter Homalodisca coagulata Say (Homoptera: Cicadellidae) is a new pest to California agriculture. It is the principal vector of several plant pathogenic diseases, particularly Pierce's Disease in grapevines, and oleander leaf scorch. A microplate-based assay is described that measures the sensitivity of acetylcholinesterase (AChE) activity to inhibition by organophosphorus (OP) insecticides in this important pest. The technique provides users with an accurate measure of the efficacy of OP binding to this target site, and is a valuable tool for monitoring field populations of the insect to determine whether the use of OP insecticides has selected for resistant individuals. The technique will also measure AChE sensitivity in the smoke-tree sharpshooter, H. lacerta Fowler. This species is native to California and is regarded only as a minor pest. Both inhibition and kinetic measurements for the AChE enzymes in these sharpshooters demonstrate the close phylogenetic relationships between the two species.     

Phenology and demography of Homalodisca coagulata (Hemiptera: Cicadellidae) in southern California citrus and implications for management

Populations of Homalodisca coagulata (Say) were sampled from citrus orchards in southern California, USA to characterize and quantify seasonal occurrences of nymphs and adults with the goal of identifying management opportunities through well-timed treatments and/or natural enemy releases. Higher densities of H. coagulata in 2001 contributed to a complete seasonal profile that began in early spring with the emergence of first instar nymphs and their progression through five nymphal instars lasting until mid-August. Adult emergence began in mid-June with peak adult densities attained from mid to late August followed by a gradual decline through autumn. A persistent and significant male bias was observed in the adult sex ratio from the time of first emergence through mid-October in oranges; the same trend was present in lemons, but with more variability. Adult densities gradually declined through the winter months into the following spring before rapidly increasing again in June as the 2002 spring generation of nymphs began emerging as adults. The seasonal timing of nymphs and adults in 2002 was nearly identical to that observed the previous year. Phenology data from both years were incorporated into a stochastic, temperature-dependent model that predicts the occurrences of H. coagulata stages through time. Applications of imidacloprid early in the spring generation of nymphs proved very effective at reducing nymphs and sustaining lower densities of adults through summer.     

Evaluation of methods for extracting Xylella fastidiosa DNA from the glassy-winged sharpshooter

The recent spread of the plant pathogenic bacterium Xylclla fastidiosa Wells et al. by an invasive vector species, Homalodisca coagulata Say, in southern California has resulted in new epidemics of Pierce's disease of grapevine. Our goal is to develop an efficient method to detect low titers of X. fastidiosa in H. coagulata that is amenable to large sample sizes for epidemiological studies. Detection of the plant pathogenic bacterium X. fastidiosa in its insect vector is complicated by low titers of bacteria, difficulty in releasing it from the insect mouthparts and foregut, and the presence of substances in the insect that inhibit polymerase chain reaction (PCr). To select the optimal protocol for DNA extraction to be used with PCR, we compared three standard methods and 11 commercially available kits for relative efficiency of X. fastidiosa DNA extraction in the presence of insect tissue. All of the protocols tested were proficient at extracting DNA from pure bacterial culture (1 x 10(5) cells), and all but one protocol successfully extracted sufficient bacterial DNA in the presence of insect tissue. Three DNA extraction techniques, immunomagnetic separation, the DNeasy Tissue kit (Qiagen, Hercules, CA), and Genomic DNA Purification kit (Fermentus, Hanover, MD), were compared more closely using a dilution series of X. fastidiosa (5000-0 cells) with and without insect tissue present. The DNeasy Tissue kit was the best kit tested, allowing detection of 5 x 10(3) X. fastidiosa cells with an insect head background.     

Effect of selected insecticides on Homalodisca coagulata (Homoptera: Cicadellidae) and transmission of oleander leaf scorch in a greenhouse study.

Homalodisca coagulata (Say) is a recent introduction to California. It is known to spread a strain of the bacterium Xylella fastidiosa Wells, Raju, Hung, Weisberg, Mandelco-Paul & Brenner that induces oleander leaf scorch disease in oleander, Nerium oleander L. Oleander leaf scorch is lethal to oleander and threatens to decimate one of the most important landscape shrubs in California. Towards developing a management strategy for H. coagulata-spread oleander leaf scorch, we documented the affects of selected insecticides on H. coagulata mortality, feeding behavior, and disease transmission in a greenhouse study. Oleanders treated with fenpropathrin, fenpropathrin + acephate, and imidacloprid caused significant mortality to caged H. coagulata within 4 h of exposure. Within 24 h, these pesticides caused nearly 100% mortality 3 wk after treatment. In other experiments, acetamiprid and fenpropathrin treatments reduced time spent feeding and total time on plants. H. coagulata on fenpropathrin-, acetamiprid-, and imidacloprid-treated oleander died in less than 13 min on average. Oleander leaf scorch transmission by H. coagulata was blocked by applications of foliar-applied acetamiprid, and soil-applied imidacloprid and thiamethoxam.     

Effects of temperature on the flight activity of Graphocephala atropunctata (Hemiptera: Cicadellidae)

Graphocephala atropunctata (Signoret) is the principal vector of Xylella fastidiosa (Wells, Raju, Hung, Weisberg, Mandelco-Paul and Brenner), the bacterium that causes Pierce's disease of grapevine in coastal California. Monitoring the activity of C. atropunctata in the early spring is important for timing insecticide sprays and assessing the potential for disease spread to adjacent vineyards. Trapping studies with yellow sticky traps over 3 yr in Napa Valley, CA, established a significant correlation between early spring trap catch and temperature. Sticky trap catches of G. atropunctata occurred in the springs of 1996-1998 only when temperature was greater than or equal to 14.5 degrees C. In 1997 and 1998, the degree-hours (> 14.5 degrees C) per day from sunrise to sunset during March and April, but not in May, correlated significantly with trap catches. The temperature threshold of 14.5 degrees C in the early spring can be used to improve the timing of insecticidal applications aimed at reducing C. atropunctata populations in vineyards affected by Pierce's disease.     

Detection and differentiation of Xylella fastidiosa strains acquired and retained by glassy-winged sharpshooters (Hemiptera: Cicadellidae) using a mixture of strain-specific primer sets

Xylella fastidiosa Wells is a bacterial pathogen that causes a variety of plant diseases, including Pierce's disease (PD) of grapevine, almond leaf scorch, alfalfa dwarf, citrus variegated chlorosis, and oleander leaf scorch (OLS). Numerous strains of this pathogen have been genetically characterized, and several different strains occur in the United States. The dominant vector in southern California is the glassy-winged sharpshooter, Homalodisca coagulata (Say) (Hemiptera: Cicadellidae). The high mobility of this insect, and its use of large numbers of host plant species, provides this vector with ample exposure to multiple strains of X. fastidiosa during its lifetime. To learn more about the ability of this vector to acquire, retain, and transmit multiple strains of the pathogen, we developed a polymerase chain reaction (PCR)-based method to detect and differentiate strains of X. fastidiosa present in individual glassy-winged sharpshooter adults. Insects were sequentially exposed to plants infected with a PD strain in grapevine and an OLS strain in oleander. After sequential exposure, a few insects tested positive for both strains (7%); however, in most cases individuals tested positive for only one strain (29% PD, 41% OLS). In transmission studies, individual adults transmitted either the PD or OLS strain of the pathogen at a rate (39%) similar to that previously reported after exposure to a single strain, but no single individual transmitted both strains of the pathogen. PD and OLS strains of X. fastidiosa remained detectable in glassy-winged sharpshooter, even when insects were fed on a plant species that was not a host of the strain for 1 wk.     

Estimation of feeding threshold for Homalodisca vitripennis (Hemiptera: Cicadellidae) and its application to prediction of overwintering mortality

The glassy-winged sharpshooter, Homalodisca vitripennis (Germar), vectors the bacterium Xylella fastidiosa that induces Pierce's disease of grape. This study determined the effect of temperature on the feeding activity of H. vitripennis adults and the resulting production of excreta. The Logan type I model described a nonlinear pattern that showed excreta production increased up to an optimal temperature (33.1°C), followed by an abrupt decline near an estimated upper threshold (36.4°C). A temperature threshold for feeding, at or below which adults cease feeding, was estimated to be 10°C using a linear regression model based on the percentage of adults producing excreta over a range of constant temperatures. A simulated winter-temperature experiment using fluctuating thermal cycles confirmed that a time period above the temperature threshold for feeding was a critical factor in determining adult survival. Using data from the simulated temperature study, a predictive model was constructed by quantifying the relationship between cumulative mortality and cooling degree-hours. In field validation experiments, the model accurately predicted the temporal pattern of overwintering mortality of H. vitripennis adults held under winter temperatures simulating conditions in Bakersfield and Riverside, California, in 2006-2007. Model prediction using winter temperature data from a Riverside weather station indicated that H. vitripennis adults would experience an average of 92% overwintering mortality before reproduction in the spring, but levels of mortality varied depending on winter temperatures. The potential for temperature-based indices to predict temporal and spatial dynamics of H. vitripennis overwintering is discussed.     

Establishment of baseline susceptibility data to various insecticides for Homalodisca coagulata (Homoptera: Cicadellidae) by comparative bioassay techniques

Homalodisca coagulata Say, adults from three locations in California were subjected to insecticide bioassays to establish baseline toxicity. Initially, two bioassay techniques, petri dish and leaf dip, were compared to determine the most useful method to establish baseline susceptibility data under laboratory and greenhouse conditions. Comparative dose-response data were determined by both techniques to endosulfan, dimethoate, cyfluthrin, and acetamiprid. Toxic values were similar to some insecticides with both techniques but not for all insecticides, revealing susceptibility differences among the three populations of H. coagulata. In subsequent tests, the petri dish technique was selected to establish baseline susceptibility data to various contact insecticides. A systemic uptake bioassay was adapted to estimate dose-mortality responses to a systemic insecticide, imidacloprid. A 2-yr comparison of toxicological responses showed all three populations of H. coagulata to be highly susceptible to 10 insecticides, including chlorpyrifos, dimethoate, endosulfan, bifenthrin, cyfluthrin, esfenvalerate, fenpropathrin, acetamiprid, imidacloprid, and thiamethoxam. In general, two pyrethroids, bifenthrin and esfenvalerate, were the most toxic compounds, followed by two neonicotinoids, acetamiprid and imidacloprid. The LC50 values for all insecticides tested were lower than concentrations used as recommended field rates. Baseline data varied for the three geographically distinct H. coagulata populations with the petri dish technique. Adult H. coagulata collected from San Bernardino County were significantly more susceptible to select pyrethroids compared with adults from Riverside or Kern counties. Adults from San Bernardino County also were more sensitive to two neonicotinoids, acetamiprid and imidacloprid. The highest LC50 values were to endosulfan, which nonetheless proved highly toxic to H. coagulata from all three regions. In the majority of the tests, mortality increased over time resulting in increased susceptibility at 48 h compared with 24 h. These results indicate a wide selection of highly effective insecticides that could aid in managing H. coagulata populations in California.     

A SYBR green-based real-time polymerase chain reaction protocol and novel DNA extraction technique to detect Xylella fastidiosa in Homalodisca coagulata

Homalodisca coagulata Say (Hemiptera: Cicadellidae) is a major agronomic pest because it transmits Xylella fastidiosa (Wells), the bacterium that causes Pierce's disease of grapevine. The ability to easily detect X. fastidiosa in populations of H. coagulata facilitates epidemiological studies and development of a monitoring program supporting disease management. Such a program depends on a detection protocol that is rapid, reproducible, and amenable to large sample sizes, while remaining sensitive enough to detect low amounts of pathogen DNA. In this study, we developed an improved method to speed DNA extraction by implementing a simple vacuum step that replaces labor- and time-intensive maceration of tissue samples and that is compatible with manufactured DNA extraction kits. Additionally, we have developed a SYBR Green-based real-time (RT)-polymerase chain reaction (PCR) system, which uses traditional PCR primers that are relatively inexpensive and effective. Using this extraction/RT-PCR system, we found no statistically significant differences in the detection of X. fastidiosa among samples that were either immediately extracted or stored dry or in mineral oil for 10 d at -4 degrees C. In further testing, we found no significant reduction in detection capabilities for X. fastidiosa-fed H. coagulata left in the sun on yellow sticky cards for up to 6 d. Therefore, we recommend a field-based detection system that includes recovery of H. coagulata from sticky traps for up to 6 d after trapping, subsequent freezing of samples for as long as 10 d before vacuum extraction is performed, and detection of the bacterium by SYBR Green-based RT-PCR.     

Estimating Xylella fastidiosa transmission parameters: decoupling sharpshooter number and feeding period

Compared to human- and wildlife-transmitted pathogens, less emphasis has been placed on developing models of plant pathogen transmission by insects. Here, we describe the transmission ecology of the bacterium Xylella fastidiosa Wells et al., the causal agent of Pierce's disease in grapevines, by its leafhopper vectors. First, we performed a meta-analysis of transmission studies of X. fastidiosa by its two most important vectors in the Western USA, the invasive glassy-winged sharpshooter, Homalodisca vitripennis Germar, and the native blue-green sharpshooter, Graphocephala atropunctata Signoret (both Hemiptera: Cicadellidae). The importance of vector number, pathogen acquisition period, and inoculation access period (IAP) for transmission differed between the two species. We fit these transmission datasets to two biologically derived transmission models, i.e., a binomial and a Poisson probability model. The Poisson model provided substantially better fit and produced estimates of H. vitripennis transmission efficiency that were dramatically lower than for G. atropunctata . We also conducted a separate pair of experiments that decoupled vector number from IAP. These experiments supported the results of the meta-analysis. Interestingly, high vector loads not only increased transmission rate, but also shortened X. fastidiosa incubation period in grapevines. This work provides quantitative estimates of transmission of an economically important pathogen that is analogous to risk models for arthropod-vectored human and wildlife diseases. In addition, this work suggests that heterogeneous vector loads may accelerate the disease cycle, increasing the potential for secondary spread in vineyards.     

Correlation of stylet activities by the glassy-winged sharpshooter, Homalodisca coagulata (Say), with electrical penetration graph (EPG) waveforms

Glassy-winged sharpshooter, Homalodisca coagulata (Say), is an efficient vector of Xylella fastidiosa (Xf), the causal bacterium of Pierce's disease, and leaf scorch in almond and oleander. Acquisition and inoculation of Xf occur sometime during the process of stylet penetration into the plant. That process is most rigorously studied via electrical penetration graph (EPG) monitoring of insect feeding. This study provides part of the crucial biological meanings that define the waveforms of each new insect species recorded by EPG. By synchronizing AC EPG waveforms with high-magnification video of H. coagulata stylet penetration in artifical diet, we correlated stylet activities with three previously described EPG pathway waveforms, A1, B1 and B2, as well as one ingestion waveform, C. Waveform A1 occured at the beginning of stylet penetration. This waveform was correlated with salivary sheath trunk formation, repetitive stylet movements involving retraction of both maxillary stylets and one mandibular stylet, extension of the stylet fascicle, and the fluttering-like movements of the maxillary stylet tips. Waveform B1 was ubitquious, interspersed throughout the other waveforms. B1 sub-type B1w was correlated with salivation followed by maxillary tip fluttering. This tip fluttering also occurred before and during B1 sub-type B1s, but was not directly correlated with either the occurrence or frequency of this waveform. Waveform B2 was correlated with sawing-like maxillary stylet movements, which usually occurred during salivary sheath branching. Waveform C was correlated with ingestion. Fluid outflow was also observed as a mechanism to clear the maxillary tips from debris during waveform C. This detailed understanding of stylet penetration behaviors of H. coagulata is an important step toward identifying the instant of bacterial inoculation which, in turn, will be applied to studies of disease epidemiology and development of host plant resistance.     

Spatial genetic structure of a vector-borne generalist pathogen

Vector-borne generalist pathogens colonize several reservoir species and are usually dependent on polyphagous arthropods for dispersal; however, their spatial genetic structure is generally poorly understood. Using fast-evolving genetic markers (20 simple sequence repeat loci, resulting in a total of 119 alleles), we studied the genetic structure of the vector-borne plant-pathogenic bacterium Xylella fastidiosa in Napa Valley, CA, where it causes Pierce's disease when it is transmitted to grapevines from reservoir plants in adjacent riparian vegetation. Eighty-three different X. fastidiosa multilocus microsatellite genotypes were found in 93 isolates obtained from five vineyards, resulting in an index of clonal fraction closer to 0 and a Simpson's genotypic diversity index (D) closer to a maximum value of 1. Moderate values of Nei's gene diversity (H(Nei); average H(Nei) = 0.41) were observed for most of the X. fastidiosa populations. The low Wright's index of genetic diversity among populations calculated by the FSTAT software (Wright's F(ST) index) among population pairs (0.0096 to 0.1080) indicated a weak or absent genetic structure among the five populations; a panmictic population was inferred by Bayesian analyses (with the STRUCTURE and BAPS programs). Furthermore, a Mantel test showed no significant genetic isolation by distance when both Nei (r = -0.3459, P = 0.268) and linearized (r = -0.3106, P = 0.269) indices were used. These results suggest that the riparian vegetation from which vectors acquire the pathogen prior to inoculation of grapevines supports a diverse population of X. fastidiosa.     

Seasonal flight activity by the Asian citrus psyllid in east central Florida

The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae), is an important invasive citrus pest in the USA because it vectors a bacterium responsible for huanglongbing, a devastating disease of citrus. Information was lacking on seasonal aspects of flight activity by D. citri, which could have ramifications on psyllid management as well as our understanding of epidemiology of the disease. Of interest from a pest management standpoint would be whether D. citri regularly disperses to or away from citrus on a predictable schedule. In research presented here, seasonal flight activity by D. citri was investigated using yellow sticky traps deployed in citrus trees and in fallow areas adjacent to citrus. Results indicated that flight activity by both male and female D. citri away from citrus can occur at any time of the year with consistent dispersal activity during the spring. The research further indicated citrus is continually subject to infestation by immigrating adults and that there is no time during the year that a citrus grower could be assured immigration would not occur. Growers should be aware that adult dispersal occurs regularly during spring and they should time management tactics accordingly. Adult flight activity 2 m from a citrus tree was more pronounced at 1 m above ground than at 2 or 3 m high. At distances of 8–60 m from trees, numbers of adults on traps were similar among the three heights. Males and females were similar with respect to seasonal flight activity. Numbers of adults captured on traps distant from citrus were not correlated with wind speed, sunlight, or air temperature, but there was some evidence that relative humidity influenced flight activity. Although the D. citri life cycle is dependent on flush, data from these studies did not confirm that psyllid dispersal from citrus consistently increases as citrus flush abundance decreases.     

Seasonal abundance and spatio-temporal distribution of dominant xylem fluid-feeding hemiptera in vineyards of central Texas and surrounding habitats

A survey of xylem fluid-feeding insects (Hemiptera) exhibiting potential for transmission of Xylella fastidiosa, the bacterium causing Pierce's disease of grapevine, was conducted from 2004 to 2006 in the Hill Country grape growing region of central Texas. Nineteen insect species were collected from yellow sticky traps. Among these, two leafhoppers and one spittlebug comprised 94.57% of the xylem specialists caught in this region. Homalodisca vitripennis (Germar), Graphocephala versuta (Say), and Clastoptera xanthocephala Germar trap catches varied significantly over time, with greatest counts usually recorded between May or June and August and among localities. A comparison of insect counts from traps placed inside and outside vineyards indicated that G. versuta is always more likely captured on the vegetation adjacent to the vineyard. C. xanthocephala was caught inside the vineyard during the summer. Between October and December, the natural habitat offers more suitable host plants, and insects were absent from the vineyards after the first freezes. H. vitripennis was caught in higher numbers inside the vineyards throughout the grape vegetative season. However, insects were also caught in the habitat near the affected crop throughout the year, and residual populations overwintering near vineyards were also recorded. This study shed new light on the fauna of xylem fluid-feeding insects of Texas. These results also provide critical information to vineyard managers for timely applications of insecticides before insect feeding and vectoring to susceptible grapevines.