Comparison of AC electronic monitoring and field data for estimating tolerance to Empoasca kraemeri (Homoptera: Cicadellidae) in common bean genotypes

Evaluating resistance of the soybean block technology cultivars to the Neotropical brown stink bug, Euschistus heros (F.)

Laboratory studies were conducted with the Neotropical brown stink bug, Euschistus heros (F.), to evaluate nymphal and adult biology on immature pods of soybean, Glycine max (L.) Merrill (Fabaceae), bearing the block technology (resistant to stink bug damage - cvs. BRS 391, BRS 543 RR and BRS 1003 IPRO) compared to a susceptible cultivar (BRS 5601 RR). Results indicated that nymphs' developmental time and survivorship were similar on all cultivars tested. The same was observed for adult survivorship and reproductive performance. However, data from electropenetrography (EPG) demonstrated that adults of E. heros spent significantly less time in feeding activities on resistant plants compared to the susceptible one. Large differences were observed in feeding activities on seeds; on resistant plants, the insects dedicated a shorter period of time to feed on seed endosperm than on BRS 5601. In addition, when bugs fed on seeds of block cultivars, the majority of probes were composed of only laceration/maceration activities (Eh3a waveform) without ingestion events of the cell contents (Eh3b waveform). In contrast, on the susceptible cultivar, Eh3a waveform events were repeated much more frequently (3-5X) with more probes also containing ingestion of seed contents. These results suggest that the soybean cultivars bearing the block technology presented a lower preference (antixenosis) by the bugs with fewer feeding activities, primarily in the seed endosperm, compared to the susceptible one tested.

AC-DC electropenetrography: fundamentals, controversies, and perspectives for arthropod pest management

Studying the intimate association of arthropods with their physical substrate is both important and challenging. It is important because substrate is a key determinant for organism fitness; challenging because the intricacies of this association are dynamic, and difficult to record and resolve. The advent of electropenetrography (EPG) and subsequent developments allowed researchers to overcome this challenge. Nonetheless, EPG research has been historically restricted to piercing-sucking hemipteran plant pests. Recently, its potential use has been greatly broadened for additional pests with instrument advances. Thus, blood-feeding arthropods and chewing feeders, as well as non-feeding behaviors like oviposition by both pests and parasitoids, are novel new targets for EPG research, with critical consequences for integrated pest management. EPG can explain mechanisms of crop damage, plant or animal pathogen transmission, and the effects of insecticides, antifeedants, repellents, or transgenic plants and animals, on specific behaviors of damage or transmission. This review broadly covers the principles and development of EPG technology, emphasizing controversies and challenges remaining with suggested research to overcome them. In addition, it summarizes 60+ years of basic and applied EPG research, and previews future directions for pest management. The goal is to stimulate new applications for this unique enabling technology. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.

Electropenetrographic Comparison of Feeding Behavior of Dichelops furcatus (Hemiptera: Heteroptera: Pentatomidae) on Soybean and Spring Cereals

We used electropenetrography to quantify and compare counts and durations of selected waveforms, produced by adult females of the stink bug Dichelops furcatus (F.). Insects fed on immature soybean pods and immature seed heads of four spring cereals: wheat, black oat, barley, and rye. On all foods, bugs spent over 60% of their plant access time in non-probing activities. This total waveform duration was significantly longer on barley and rye compared to those on soybean and oat; wheat was intermediate. Considering only probing activities, bugs spent longer durations (ca. 2×), on soybean and oat compared to barley, rye, and wheat plants. Bugs produced significantly more pathway events on soybean and rye than on wheat and barley; with a significantly shorter duration per event on rye. The counts and durations of xylem ingestion did not differ among foods. Cell rupturing activities on seeds were longer on soybean (ca. 23%) and oat (ca. 21%), than on barley and rye (ca. 6%). The durations of ingestion events on seeds were significantly shorter on soybean (over 3×) compared to those on barley and wheat; oat and rye were intermediate. However, the ingestion duration per insect did not show significant difference among foods. Results demonstrated that D. furcatus spent more time overall in probing activities on soybean and oat; whereas, rye and barley presented the worst feeding behavior. This study provides important background information for further quantitative studies of stink bugs on different plants, such as development of resistant host plants.

The Power of Electropenetrography in Enhancing Our Understanding of Host Plant-Vector Interactions

The invasive Asian citrus psyllid, Diaphorina citri (Hemiptera: Liviidae), is the primary vector of the phloem-infecting bacterium, Candidatus Liberibacter asiaticus. Candidatus L. asiaticus is the putative causal agent of Huanglongbing (HLB) disease, a destructive disease of Citrus. While many Citrus species are susceptible to D. citri probing and HLB disease, there are marked behavioral differences in D. citri probing responses and Ca. Liberibacter asiaticus infection severity among Citrus species. Using four mandarin hybrid selections and pummelo plants variably resistant to D. citri probing, oviposition, and survival, we explored probing differences using electropenetrography (EPG), conducted an oviposition and survival study, and determined host plant metabolites using gas-chromatography mass-spectroscopy (GC-MS). We found thirty-seven D. citri probing variables to be significantly different among tested mandarin selections and pummelo, in addition to differential oviposition and survivorship abilities on tested plants. We found sixty-three leaf metabolites with eight being significantly different among tested mandarin selections and pummelo. Detailed analysis of probing behavior, oviposition, survivorship, and host plant metabolite concentrations reveals the complex, layered resistance mechanisms utilized by resistant Citrus against D. citri probing. EPG is a powerful technology for screening Asian citrus psyllid resistant Citrus to elucidate host plant-vector interactions, with an aim to minimize vector probing and eliminate the spread of the bacterial pathogen, Ca. L. asiaticus.

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.

Probing behavior of Empoasca vitis (Homoptera: Cicadellidae) on resistant and susceptible cultivars of tea plants

Feeding activities of the tea green leafhopper, Empoasca vitis (Gothe) (Homoptera: Cicadellidae), on resistant and susceptible cultivars of tea plants (Camellia sinensis L.) were recorded and analyzed using the direct current electrical penetration graph (EPG) system. Six distinct EPG waveforms characterizing the feeding behavior of the tea green leafhopper, categorized as waveforms A, C, E, S, F, and R, were obtained during the investigation. Duration of passive ingestion, possibly of phloem (E), was the longest among all the probing waveforms on susceptible cultivars, whereas durations of the salivation (S) waveform and stylet work waveform (F) became longer on resistant cultivars. The durations of waveforms S and F on the resistant cultivar Jiandecha were slightly longer than those on the less resistant cultivar Yunguidaye, and both were significantly longer than those on the susceptible cultivars Hangzhoudaye and Zhushan-1. Waveform E was shorter on the resistant cultivar Jiandecha than on the less resistant cultivars Yunguidaye and was significantly shorter than on the susceptible cultivars (Hangzhoudaye and Zhushan-1). It is suggested that E, S, and F are the important waveforms related to leafhopper feeding behavior and tea plant resistance. Based on the results, the resistance levels of tea cultivars against the tea leafhopper can be evaluated quickly by direct current EPG.

Host plant determines the phytoplasma transmission competence of Empoasca decipiens (Hemiptera: Cicadellidae)

Phytoplasmas are phloem-restricted plant pathogens transmitted by leafhoppers, planthoppers, and psyllids (Hemiptera). Most known phytoplasma vectors belong to the Cicadellidae, but many are still unknown. Within this family, Empoasca spp. (Typhlocybinae) have tested positive for the presence of some phytoplasmas, and phytoplasma transmission has been proven for one species. The aim of this work was to investigate the ability of Empoasca decipiens Paoli in transmitting chrysanthemum yellows phytoplasma (CYP, "Candidatus Phytoplasma asteris", 16SrI-B) and Flavescence dorée phytoplasma (FDP, 16SrV-C) to Chrysanthemum carinatum Schousboe (tricolor daisy) and Viciafaba (L.) (broad bean). Euscelidius variegatus Kirschbaum, a known vector of CYP and FDP, was caged together with Em. decipiens on the same source plants as a positive control of acquisition. Em. decipiens acquired CYP from daisies, but not from broad beans, and inoculated the pathogen to daisies with alow efficiency, but not to broad beans. Em. decipiens did not acquire FDP from the broad bean source. Consistent with the low transmission rate, CYP was found in the salivary glands of very few phytoplasma-infected Em. decipiens, indicating these organs represent a barrier to phytoplasma colonization. In the same experiments, the vector Eu. variegatus efficiently acquired both phytoplasmas, and consistently CYP was detected in the salivary glands of most samples of this species. The identity of the CYP strain in leafhoppers and plants was confirmed by polymerase chain reaction (PCR)-restriction fragment length polymorphism. The CYP titer in Em. decipiens was monitored over time by real-time PCR. The damage caused by Em. decipiens feeding punctures was depicted. Differences in feeding behavior on different plant species may explain the different phytoplasma transmission capability. Em. decipiens proved to be an experimental vector of CYP.

The AC-DC correlation monitor: New EPG design with flexible input resistors to detect both R and emf components for any piercing-sucking hemipteran

Much of what is known today about hemipteran feeding biology, as well as mechanisms of their host plant interactions and transmission of phytopathogens, has been learned via use of electrical penetration graph (EPG) technology, originally called electronic monitoring of insect feeding. Key to all of this information has been the electronic designs of EPG monitors. It has been 45 years since the publication of the original EPG, the AC monitor, and 30 years since introduction of the DC monitor, an important improvement for EPG science. Herein we describe our new AC-DC Correlation Monitor, the first major improvement in design since the DC monitor. We provide the monitor's block diagram and circuit description, and discuss (as a first example) its application to aphid feeding waveforms. Our instrument combines design features from the existing AC Missouri monitor and the DC Tjallingii monitor, plus several new innovations. It can produce three simultaneous, time-synchronized, output signals from a single insect, via AC and DC signal processing circuitry, as well as using either AC, DC, AC-plus-DC, or 0V substrate voltage. Our research conclusively demonstrates that AC signal processing can be designed to duplicate the level of detail and fidelity of aphid waveforms previously provided solely by the DC monitor, including all R- and emf-component waveforms. Availability of either AC or DC applied voltages will allow similar high-resolution recording of insects that appear to be sensitive to DC applied voltages. We also begin to determine the subtle reasons why published waveforms from older AC and DC monitors appear to differ so greatly. Our instrument is a single, flexible, universal monitor that can provide maximum, R-plus-emf waveform information from any piercing-sucking species, especially non-aphid species with sensitivity to DC applied voltage.

Physiological response of glandular-haired alfalfa to potato leafhopper (Hemiptera: Cicadellidae) injury

Plant tolerance to herbivory is a key approach for managing pests. In alfalfa, Medicago sativa, the potato leafhopper, Empoasca fabae, is a major pest as a result of the cascade of plant responses to piercing-sucking injury. To identify tolerance to its injury based on alfalfa physiology, experiments were conducted in the field and greenhouse. In our comparison of the response of field-grown alfalfa cultivars to standardized leafhopper densities, net photosynthesis and transpiration rates of 'Geneva' leaves were reduced by 18 and 21%, respectively, by leafhopper presence compared with a rate change of <1% of resistant 'EverGreen' leaves. Under greenhouse conditions, alfalfa clones varied in their level of gas exchange (net photosynthesis and transpiration) and stem elongation responses to leafhopper injury. For example, in the comparison of seven clones, net photosynthesis declined an average of 40.7% with leafhopper injury, although individual clones varied from 26.6 to 74.3% reduction. Internode elongation after 2 d was 60.3% less on injured stems compared with healthy stems, but again, the individual clones varied from 17.3 to 91.9%. In a time-course study of selected clones, clones varied in their level of injury just after and 3 d after insect removal. Gas exchange responses of all clones recovered by 7 d after cessation of injury. In a choice test, leafhoppers spent similar amounts of time on the susceptible clone and the most tolerant clone; however, their precise feeding behaviors were not measured. Thus, the variable response of clones to injury may be either true physiological tolerance or antixenosis from a change in feeding behavior. This study showed putative tolerance to leafhopper injury among alfalfa genotypes, suggesting that tolerance could be the basis for crop protection in alfalfa from potato leafhopper injury.

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.

Mechanisms of hopperburn: an overview of insect taxonomy, behavior, and physiology

Hopperburn is a non-contagious disease of plants caused by the direct feeding damage of certain leafhoppers and planthoppers. Although long studied, especially with Empoasca spp. leafhoppers (Cicadellidae: Typhlocybinae), the mechanisms underlying hopperburn have only recently been elucidated. Hopperburn is caused by a dynamic interaction between complex insect feeding stimuli (termed hopperburn initiation) and complex plant responses (termed the hopperburn cascade). Herein we review the nature of the feeding stimuli in hopperburn initiation, especially for Empoasca spp., which we also compare with the planthopper Nilaparvata lugens. Contrary to previous reports, Empoasca hopperburn is not caused solely by toxic saliva. Instead, it is caused by a plant wound response triggered by a unique type of stylet movement, which is then exacerbated by saliva. Electrical penetration graph monitoring has revealed that all Empoasca spp. are cell rupture feeders, not sheath feeders, and that certain tactics of that feeding strategy are more damaging than others. Measuring the proportions of the most damaging feeding led to development of a resistance index, the Stylet Penetration Index, which can predict hopperburn severity in different plants or under different environmental conditions and can supplement or replace traditional, field-based resistance indices.