Subcellular effects and localization of binding sites of phytohemagglutinin in the potato leafhopper, Empoasca fabae (Insecta: Homoptera: Cicadellidae)

To identify the means by which phytohemagglutinin (PHA) exerts its toxicity on the potato leafhopper, four different methods (thick and semi-thin sectioning combined with immunofluorescent staining, in vitro receptor autoradiography, and immunoelectron microscopy) were used to elucidate the PHA target tissue, binding site, and its effects on this tissue. Sixteen 1- or 2-day-old female potato leafhoppers were fed for 36 h on each of three treatments: a control, diet or a diet containing either the PHA-E subunit or the PHA-L subunit. The PHA-E subunit, but not PHA-L, had previously been shown to be lethal. The insects were then prepared for both light and confocal microscopy. Analysis of images showed that PHA bound only to the surface of midgut epithelial cells of the potato leafhopper. PHA-E caused severe disruption, disorganization, and elongation of the brush border microvilli, and swelling of the epithelial cells into the lumen of the gut, leading to complete closure of the lumen. Furthermore, PHA-E stimulated the division of midgut epithelial cell nuclei, leading to two nuclei in each cell. Nuclei later elongated and degraded. In contrast, PHA-L had little effect on the epithelial cells of the midgut. It did not strongly bind to the surface of epithelial cells and caused much less disruption of brush-border microvilli, less disorganization of the cells and less elongation of nuclei. Strong binding of PHA occurred solely on the cell membrane of the brush border microvilli of epithelial cells. In contrast, the controls (i.e., midgut tissue, blocking agent, PHA, and antibodies) showed that midgut tissue was not autofluorescent and showed no fluorescent binding signal. Analysis of both bright- and dark-field images obtained by autoradiography and immunoelectron microscopy confirmed these findings.     

Variation in tolerance and resistance to the leafhopper Empoasca fabae (Hemiptera: Cicadellidae) among potato cultivars: implications for action thresholds

The potato leafhopper, Empoasca fabae (Harris) (Hemiptera: Cicadellidae), is an emerging pest of potato and insecticide applications to control this insect have increased in recent years. Based on field observations of leafhopper-crop dynamics, however, currently recommended action thresholds seem to be overly conservative. As a result, we initiated two experiments designed to quantify the impact of leafhoppers on potato yield, and determine how the magnitude of this effect changes among cultivars. In experiment 1, leafhoppers were manipulated (control versus insecticide-treated plots) on 17 potato varieties. In experiment 2, three cultivars (Superior, Atlantic, and Snowden) were planted representing early-, mid-, and late-season maturing lines, and six insecticide spray regimes were imposed (early-, late-, and full-season applications at high and low rates). In both experiments, leafhopper abundance, plant damage, and potato yield were measured. Overall, leafhoppers reduced yield in control plots by 15.7% relative to insecticide-treated plots. Leafhopper impact, however, varied among cultivars; a significant effect of leafhoppers on yield was detected in 6, 12, and 59% of cultivars tested in each of three trials. Of the 44 cases in which leafhoppers exceeded action thresholds, yield loss was only documented in 13 cases. Data from these experiments provide evidence that such variable effects ofleafhoppers on yield are explained by cultivar-specific resistance and tolerance traits. Our results suggest that potato growers can accept higher leafhopper densities than current thresholds recommend, particularly when cultivating resistant and/or tolerant varieties.     

Glandular-haired alfalfa resistance to potato leafhopper (Homoptera: Cicadellidae) and hopperburn: development of resistance indices

Eight proprietary genotypes of glandular-haired alfalfa, Medicago sativa L., supplied by two different companies, were compared for the degree and types of resistance to the potato leafhopper, Ernpoasca fabae (Harris), and hopperburn. A tube cage no-choice bioassay was developed to test leafhopper mortality, feeding, settling preferences, severity of hopperburn symptoms (in this case, defined as both yellowing and stem growth reduction), and trichome density and type on feeding sites. Leafhopper mortality was both strongly and significantly associated with feeding and leaf trichome density; decreased hopperburn symptom severity was weakly, although significantly, associated with increased mortality. To quantify hopperburn in terms of both yellowing and stem growth reduction, we developed a ranking system that reduces overall hopperburn expression to a single number that considers the varying responses to both types of symptoms. Great variability in leafhopper settling, leafhopper mortality, and stem glandular trichome density was detected among alfalfa genotypes, suggesting that genotypic differences may be based on the concentration and/or chemical constituency of the trichome exudates. We postulate that, among variably resistant genotypes of glandular-haired alfalfa, differences among leafhopper responses and hopperburn severity are linked to forced movement from the stems to the leaves as refuge feeding sites. Principal component analysis was performed to reduce the 10 variables down to five biologically significant factors. Scores for these factors were then used to develop resistance indices for potato leafhopper resistance, hopperburn resistance, and an overall glandular-haired alfalfa resistance index.     

Aspects of potato leafhopper (Homoptera: Cicadellidae) biology on Solanum berthaultii and other potato genotypes

Survival of nymphs and adult males and females of potato leafhopper, Empoasca fabae (Harris), on six potato (Solanum spp.) genotypes with varied levels of resistance was evaluated and related to type, density, and droplet/head size of foliar glandular trichomes. Survival was greater on Solanum tuberosum cultivars ('Allegany' and 'Elba') compared with accessions of the wild S. berthaultii (PI 473331 and PI 473334). The barrier provided by glandular trichomes may affect survival of E.fabae by restricting nymph and adult feeding. PI 473331 was the most pubescent host, bearing type A and type B trichomes and the most unsuitable host for E. fabae. Allegany possesses leaflets with low trichome densities and small gland diameter and was the most suitable host for adult E. fabae survival. Females generally had higher survival rates than males. Nymphal and adult leafhoppers differed greatly in their survival rates, particularly on S. tuberosum x S. berthaultii hybrids (Q174-2 and NY123). Interestingly, nymphs had longer (190 d) mean lethal time (LT50) than adults (31 d) when held on Q174-2, and a lower LT50 (19 d) when held on NY123 compared with adults (104 d). Starvation, dehydration, or both were likely associated with mortality of nymphs confined on PI 473331 and PI 473334. Nymphs on these accessions died at a comparable rate to those confined on a starvation treatment and more rapidly than those supplied with water only, sucrose solution or other potato genotypes. Darkened trichome exudates were observed on insects held on PI 473331 and PI 473334, consistent with previous reports that glandular trichomes of S. berthaultii contain phenolic oxidation chemistry. This study aids in the identification of the probable mechanisms of resistance to E. fabae used by S. berthaulti and its hybrids with S. tuberosum.     

Within-field manipulation of potato leafhopper (Homoptera: Cicadellidae) and insect predator populations using an uncut alfalfa strip

The effects of a 3-m wide uncut alfalfa strip on the within field distribution of Empoasca fabae (Harris) (Homoptera: Cicadellidae) and several species of insect predators were examined for 3 yr (1998-2000). The objectives were to determine whether this uncut strip would serve as a trap crop for E. fabae and a refuge for insect predators at first harvest. Empoasca fabae and insect predators in the families Coccinellidae, Nabidae, Anthocoridae, Chrysopidae, and Hemerobiidae were collected weekly using sweep samples and sticky traps from the uncut strips and up to 40 m into the alfalfa regrowth. For 2 yr, both small- (0.34 ha) and large-scale (approximately 11.3 ha) field trials showed higher numbers of E. fabae in 73% of the uncut strips for 2-3 wk after harvest. Similarly, the number of insect predators found within <50% of the uncut strips was also higher during the first or second week after harvest. In 1999, however, we did not observe higher numbers of E. fabae in the uncut strips. Differences may be because of higher E. fabae population numbers in 1999 compared with 1998 and 2000. This research provides alfalfa growers a potential cultural management technique for E. fabae while conserving predatory insects.     

Chromosomal polymorphism in an Australian leafhopper (Homoptera: Cicadellidae)

Variation in chromosome number from 2n=9 to 2n=7 was found in a species of leafhopper, Alodeltocephalus draba. It is argued that this variation is due to chromosome fusion and that A. draba is presently evolving to a lower number. The process of reduction is more advanced in some populations than in others.Seconded from: Department of Biology, University of Oregon, Eugene, Oregon, U.S.A.     

Role of the glandular trichomes in resistance of perennial alfalfa to the potato leafhopper (Homoptera: Cicadellidae)

Experiments were performed to elucidate resistance of glandular-haired alfalfa, Medicago sativa L., to the potato leafhopper, Empoasca fabae (Harris). The primary objective was to examine the potential role of the glandular trichomes and stem lignification for imparting resistance to this pest. During free-choice foliar discoloration experiments, the resistant alfalfa clone FGplh13 expressed lower levels of injury than the susceptible P5373 after 13 d. No-choice tests performed using nymphal potato leafhoppers showed higher levels of mortality associated with the resistant glandular-haired clone FGplh13, and no nymphs survived after 48 h. Additional no-choice experiments using FGplh13 and P5373 alfalfa with the glandular and nonglandular trichomes intact or removed show that mortality of nymphs and adults decreased after removal of the glandular trichomes from FGplh13. Nymphal and adult mortality and nymphal development time did not differ on FGplh13 with the trichomes removed and P5373 with the trichomes intact or removed. Firstinstar, and possibly second-instar, potato leafhoppers were entrapped by an exudate produced by the glandular trichomes on the FGplh13 alfalfa. Stem lignification and the number of vascular bundles did not differ between FGplh13 and P5373 alfalfas. The glandular trichomes on FGplh13 alfalfa appeared to provide the major host resistance factor, with resistance to adults being chemically based and resistance to nymphs being chemically and mechanically based. To maintain levels of potato leafhopper resistance, breeders would appear to benefit by continuing to select for the expression of the glandular trichome phenotype.     

Transmission of Spiroplasma citri by the aster leafhopper Macrosteles fascifrons (Homoptera: Cicadellidae)

The aster leafhopper (Macrosteles fascifrons), injected with an isolate of Spiroplasma citri obtained from brittle root-diseased horseradish (Armoracia rusticana), transmitted the spiroplasma to horseradish and China aster (Callistephus chinensis.) After feeding on plants infected with S. citri, M. fascifrons transmitted the spiroplasma from aster to aster and horseradish, from yellow rocket (Barbarea vulgaris) to aster, and from turnip (Brassica rapa) to turnip. Symptoms in infected horseradish were chlorosis and stunting of newly formed leaves, discoloration of root phloem, and reduced plant growth typical of brittle root disease. Chlorosis, stunting, and asymmetry of young leaves occurred in affected aster and turnip. Flowers of infected aster were small and pale in colour and occasionally showed other symptoms including asymmetry, petal distortion, or light green petals. Spiroplasmas were isolated from all plants showing symptoms. Transmission rates by M. fascifrons which acquired S. citri by feeding on infected plants were very low, but injected leafhoppers transmitted more frequently. This is the first report of the transmission of S. citri from diseased to healthy plants by M. fascifrons.     

Glandular trichomes of Solanum berthaultii and its hybrids with Solanum tuberosum affect nymphal emergence, development, and survival of Empoasca fabae (Homoptera: Cicadellidae)

The influence of foliar glandular trichomes on developmental biology of the potato leafhopper, Empoasca fabae (Harris) (Homoptera: Cicadellidae), was examined by selective removal of trichomes on resistant and susceptible potato germplasm comprising two glandular trichome-bearing genotypes (PI 473331 and PI 473334) of wild potato, Solanum berthaultii (Hawkes), two interspecific hybrids of S. berthaultii with Solanum tuberosum L. (Q174-2 and NY123), and two commercial S. tuberosum cultivars ('Elba' and 'Allegany'). Adult mortality was affected, whereas nymphal emergence was unaffected by removal of glandular trichome exudates. No nymphs were observed emerging from leaves of PI 473331 or PI 473334. Regardless of the trichome removal treatment, no nymphs completed development on PI 473331, and only a small percentage of nymphs survived to adulthood on NY123 and PI 473334. PI 473331 had the greatest frequency of adults and nymphs with trichome exudate deposition on their bodies, followed by PI 473334. Nymphs, but not adults, had exudate accumulation when caged on foliage of Q174-2. Removal of type A trichome heads and type B droplets eliminated exudate deposition. The results of these studies provide evidence that phenolic oxidation chemistry and the physical barrier imposed by trichomes of S. berthaultii are responsible for much of the observed resistance but that other factors possibly linked to the presence of trichomes aid in the expression on resistance.     

Effect of temperature on distribution and success of introduction of an Empoasca fabae (Homoptera: Cicadellidae) isolate of Erynia radicans (Zygomycetes: Entomophthoraceae)

Empoasca fabae is a serious leafhopper pest of agriculturally important crops in the midwestern and northeastern United States. The geographic distribution of Erynia radicans, a fungal pathogen of E. fabae, led us to examine the effects of temperature on tn vitro and in vivo growth and conidial germination of E. radicans. In addition, we examined the effects of temperature on the establishment of E. radicans in the field. E. radicans grew at constant temperatures of 28°C and below, both in vivo and in vitro, but did not grow at 30°C or above. Conidial germination followed a similar pattern; however, E. radicans survived exposure periods of up to 24 hr at 32°C in vivo. E. radicans did not cycle through a caged population of E. fabae in a growth chamber at 22°C despite the occurrence of infections immediately following fungal release. E. radicans did not become established after introduction into field populations of E. fabae at three sites in Illinois. We observed initial infections of leafhoppers at two of these sites and evidence suggests that horizontal transmission occurred at one of them. Epizootics, however, did not occur despite high populations of leafhoppers. Temperatures exceeded the laboratory thresholds for growth of E. radicans at each of the three sites during the first week after release, but for different durations.     

Determination of traits associated with leafhopper (Empoasca fabae and Empoasca kraemeri) resistance and dissection of leafhopper damage symptoms in the common bean (Phaseolus vulgaris)

The typical presentation of potato leafhopper injury in beans includes necrosis at the leaf margins (leaf burn or hopperburn), and downward curling or “cupping” of the leaves. To evaluate potato leafhopper damage a visual score that combines the overall severity of leaf burn, leaf curling and stunting symptoms is usually used. Nonetheless, it may be useful to evaluate these symptoms separately since they may be the result of separate mechanisms of damage, controlled by separate genes. A population of 108 recombinant inbred lines (RILs), derived from a cross between a leafhopper‐susceptible Ontario cultivar (Berna) and a resistant line (EMP 419) were scored for injury after natural infestation with Empoasca fabae in Canada and Empoasca kraemeri in Colombia. Leaf burn and leaf curl were significantly rank‐correlated (0.37–0.74, P<0.001) in all environments. However, several RILs consistently exhibited high scores for leaf curl but low values for leaf burn, which suggests that genetic dissection of these characters may be possible. Indeterminate growth habit was associated with slightly lower damage scores in Colombia and Ontario, Canada (P<0.05) while white‐seeded lines had lower damage scores in Colombia (P<0.05). The resistant parental line had significantly lower nymph counts than did the susceptible parent. A positive relationship between damage scores and nymph counts was also observed in the F₃ families and the F_5:6 RILs.     

Climate Change and Phenology: Empoasca fabae (Hemiptera: Cicadellidae) Migration and Severity of Impact

Climate change can benefit individual species, but when pest species are enhanced by warmer temperatures agricultural productivity may be placed at greater risk. We analyzed the effects of temperature anomaly on arrival date and infestation severity of potato leafhopper, Empoasca fabae Harris, a classic new world long distance migrant, and a significant pest in several agricultural crops. We compiled E. fabae arrival dates and infestation severity data at different states in USA from existing literature reviews and agricultural extension records from 1951–2012, and examined the influence of temperature anomalies at each target state or overwintering range on the date of arrival and severity of infestation. Average E. fabae arrival date at different states reveal a clear trend along the south-north axis, with earliest arrival closest to the overwintering range. E. fabae arrival has advanced by 10 days over the last 62 years. E. fabae arrived earlier in warmer years in relation to each target state level temperature anomaly (3.0 days / °C increase in temperature anomaly). Increased temperature had a significant and positive effect on the severity of infestation, and arrival date had a marginal negative effect on severity. These relationships suggest that continued warming could advance the time of E. fabae colonization and increase their impact on affected crops.     

Host effects of glandular-haired alfalfa on alfalfa weevil (Coleoptera: Curculionidae) and potato leafhopper (Homoptera: Cicadellidae) populations in Virginia

Cultivars of glandular-haired alfalfa, Medicago sativa L., such as '54H69', are currently available and marketed as being resistant to potato leafhopper, Empoasca fabae (Harris). Between 2000 and 2002, studies were conducted to compare the effects of '54H69' and a standard, nonglandular-haired alfalfa cultivar, 'Choice', on alfalfa weevil, Hypera postica (Gyllenhal), and potato leafhopper populations at Campbell and Montgomery counties, Virginia. '54H69' had no effect on alfalfa weevil populations. At each location, densities of alfalfa weevil in '54H69' and 'Choice' were similar, but pest pressure was higher at Campbell Co. than at Montgomery Co. and always exceeded the economic threshold before insecticide was applied. Densities of potato leafhopper also did not differ between '54H69' and 'Choice' in any year at the two locations. Insecticide treatment effectively reduced potato leafhopper densities in the two cultivars, although populations were below the economic threshold at both locations when the insecticides were applied. Overall, postinsecticide treatment comparisons showed that the densities of alfalfa weevil and potato leafhoppers were similar or higher in untreated '54H69' compared with insecticide-treated 'Choice'. In addition, there were no differences in seasonal dry yields between '54H69' and 'Choice' in any year at either location. Our results indicate that the glandular-haired alfalfa '54H69' does not provide acceptable resistance to potato leafhopper and also does not offer a yield advantage to growers in Virginia.     

Larval development of Empoasca vitis and Edwardsiana rosae (Homoptera: Cicadellidae) at different temperatures on grapevine leaves

The grape leafhopper Empoasca vitis (Homoptera: Cicadellidae) is regarded as a major insect pest in many European grapevine growing areas, with an increasing importance realized in recent years maybe as a result of climatic change. Both larvae and adults feed on the phloem vessels of the leaves, causing characteristic symptoms also referred to as hopperburn. Phenology of adult leafhoppers was monitored in one vineyard in three successive years and indicated that immigration of a few hibernated E. vitis individuals into vineyards might take place already quite early in the year depending on winter temperatures and starts to progress in substantial numbers right at grapevine bud burst. In addition, these monitoring studies have shown that there are several other leafhopper species occurring on grapevine plants besides E. vitis, such as the rose leafhopper Edwardsiana rosae (Homoptera: Cicadellidae). Here, we report on the development of larval instars of both leafhopper species, E. vitis and E. rosae on grapevine leaves under different temperature regimes in the laboratory. Shortest larval developmental time was observed at night temperatures of 13–15°C and day temperatures of 23–25°C, which was in agreement with predicted optimal temperatures for both species. At the temperature regime of 20°C night and 30°C day temperature, either no egg hatch was observed or early development of first‐instar larvae was not successful for both species. These results suggest that warm (18°C) nights and moderately warm (28°C) days are representing the upper thermal threshold for development of both E. vitis and E. rosae embryonic stages on grapevine leaves, questioning current assumptions of an increasing importance of E. vitis as a grapevine pest under future climate change.     

Diversity and abundance of leafhopper species (Homoptera: Cicadellidae) among red maple clones

A survey was conducted in the 2001 growing season to examine the leafhopper diversity and abundance among trees of 17 red maple (Acer rubrum) clones. Yellow sticky traps were used to qualify and quantify the number of aerial leafhoppers from 1 May 2001 until 4 September 2001. A total of 45 species from eight different leafhopper subfamilies, for a total of 6055 individuals, were considered in this study. The mean number of leafhoppers collected, mean species richness, diversity and evenness were significantly lower on traps of trees for October Glory than for the other clones. Yet, none of the leafhopper species dominated the weekly samples. Species similarity among clones ranged from 56 to 90%. No two clones had complete similarity. Franksred and trees of a controlled cross between October Glory × Autumn Flame shared the highest degree of species similarity, while clones from PA, USA and RI, USA were the least similar. The development of new clones did not create new pest problems, but suppressed populations of damaging pests, and maintained the diversity of low abundance species.     

假眼小绿叶蝉微卫星位点的生物信息学分析

【目的】为开发假眼小绿叶蝉Empoasca vitis分子标记,采用高通量测序技术对假眼小绿叶蝉DNA进行了测序与分析。【方法】本研究基于Illumina Hi Seq测序技术,构建了PE文库(~400bp),对获得的测序数据利用生物信息学分析手段完成全基因组扫描,并进一步使用MISA分析鉴定基因组序列中出现的微卫星序列(SSR)。针对微卫星序列共设计10对引物,并使用3步法进行引物多态性筛选。【结果】共计检测Scaffold数量为183 194条,其中包含SSR的Scaffold共计1 545条,共计筛选出1 569个SSR位点。在假眼小绿叶蝉的微卫星中,共包括87种重复基元类型,二核苷酸与三核苷酸重复序列为主要重复类型,分别占SSRs总数的70.26%和27.84%;二核苷酸重复基元CA/TG和三核苷酸重复基元AAT/ATT是优势重复基元,分别占SSRs总数的33.96%和5.86%。在设计的10对引物中,5对具有多态性,在8个假眼小绿叶蝉个体中共发现16个等位基因。【结论】结果说明假眼小绿叶蝉SSR位点在多态性方面具有极大的可开发性,具有多态性的SSR位点可对假眼小绿叶蝉种群间的分化,种群间的扩散机理和途径及影响因素等问题提供分子视角。     

Management of corn leafhopper (Homoptera: Cicadellidae) and corn stunt disease in sweet corn using reflective mulch

Plastic reflective mulches significantly reduced populations of corn leafhopper, Dalbulus maidis (DeLong & Wolcott), adults and the incidence of corn stunt disease caused by Spiroplasma kunkelii (CSS) in late planted sweet corn (Zea mays L.). The reflective mulches were more effective than were either foliar or soil applied insecticides in managing both the leafhopper and the pathogen it transmits. Yields of marketable ears were 1.5 to 2 times greater in reflective mulch plots than from fallow plots. This was due to larger ears (individual ear weight and length) rather than an increase in the number of ears. The use of reflective mulches provides an alternative strategy to insecticides in the management of both D. maidis and corn stunt disease. Such a strategy may prove useful to growers in Latin America and to limited resource growers and organic growers in the United States who wish to grow corn without the use of insecticides.