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

Harnessing the Power of Electrical Penetration Graph Technology to Understand Psyllid-Transmitted Fastidious Bacterial Diseases

Psyllids, also called plant lice, are hemipteran insects that feed on phloem sap. In addition to the direct damage they cause to plants, they are vectors of many phloem-restricted bacterial pathogens belonging to the 'Candidatus Liberibacter' spp. and 'Candidatus Phytoplasma' spp. from the apple proliferation group (16SrX). Although 'Candidatus Liberibacter' spp. cells possess cell walls unlike those of the phytoplasmas, they both share a reduced genome and unavailability in culture. In addition, psyllids transmit both species of bacteria in a persistent, circulative, and propagative manner. Because of the similarity of these pathosystems, the electrical penetration graph (EPG) was employed to study the probing behavior of psyllids. Such studies may assist in understanding the specific interactions between the fastidious bacteria, plant hosts, and insect vectors and lead to innovative control strategies. Herein, we discuss the potential of the EPG to study and understand the tritrophic interactions that secure a successful transmission from plant to plant. In addition, the use of the EPG in evaluating psyllid control strategies including pesticides and tolerant varieties is reviewed.

AC-DC Electropenetrography as a Tool to Quantify Probing and Ingestion Behaviors of the Yellow Fever Mosquito (Aedes aegypti) on Mice in Biocontainment

A detailed understanding of mosquito probing and ingestion behaviors is crucial in developing novel interventions to interrupt the transmission of important human and veterinary pathogens, but these behaviors are difficult to observe as the mouthparts are inserted into the skin of the host. Electropenetrography (EPG) allows indirect observation, recording, and quantification of probing and ingestion behaviors of arthropods by visualizing the electrical waveform associated with these behaviors. The study of mosquito probing and ingestion behaviors has been limited to the use of human hands as host, which is not suitable for pathogen transmission studies. Mouse models of mosquito-borne diseases are a widely used and indispensable tool in this research, but previous attempts to use direct current EPG to study mosquito probing behaviors on mice have been unsuccessful. Accordingly, the present study used alternating or direct current (AC-DC) EPG to observe the ingestion behaviors of adult Aedes aegypti mosquitoes on a mouse host in real time under BSL-2 containment conditions with enhanced BSL-3 practices. Our results show that waveform families previously identified during Ae. aegypti probing and ingestion on human hands were observed using 100 mV of AC at an input resistance (Ri) of 107 Ohms (Ω) on CD1 mice. This work is a proof of concept for using mouse models for studying mosquito probing and ingestion behaviors with AC-DC EPG. In addition, these data show that the experimental setup used in these experiments is sufficient for conducting studies on mosquito probing and ingestion behaviors under BSL-2 containment conditions enhanced with BSL-3 practices. This work will serve as a foundation for using EPG to investigate the effects of pathogen infection on mosquito probing behaviors and to understand the real-time dynamics of pathogen transmission.

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.

Determination of Mosquito Probing and Feeding Time to Evaluate Mosquito Blood Feeding

Hematophagous arthropods are animals that feed on vertebrate blood for egg production. Mosquitoes must pierce the host skin, locate blood vessels, and extract blood without being noticed. Mosquito stylets lacerate host tissues, triggering the activation of the three branches of hemostasis, or stopping of blood flow: vasoconstriction, platelet aggregation, and coagulation. Mosquitoes inject saliva into the host skin during their intradermal search for blood (also called probing), and salivary proteins counteract hemostasis. Blood feeding dynamics have been traditionally described by observational studies, in which researchers using magnifying glasses watched mosquitoes in the act of blood feeding. These studies provided the foundation for protocols to evaluate mosquito blood feeding in a more quantitative manner. Here, we introduce mosquito blood feeding biology with a focus on the feeding steps, which include penetration, probing, and feeding. Understanding blood feeding dynamics is crucial for evaluating probing time and other relevant parameters derived from blood feeding, such as blood meal size, fecundity, and fertility. Other considerations, including the relationship between probing and pathogen transmission and novel technologies to address blood feeding, are also discussed.

Common resistance mechanisms are deployed by plants against sap-feeding herbivorous insects: insights from a meta-analysis and systematic review

Despite their abundance and economic importance, the mechanism of plant resistance to sap-feeding insects remains poorly understood. Here we deploy meta-analysis and data synthesis methods to evaluate the results from electrophysiological studies describing feeding behaviour experiments where resistance mechanisms were identified, focussing on studies describing host-plant resistance and non-host resistance mechanisms. Data were extracted from 108 studies, comprising 41 insect species across eight insect taxa and 12 host-plant families representing over 30 species. Results demonstrate that mechanisms deployed by resistant plants have common consequences on the feeding behaviour of diverse insect groups. We show that insects feeding on resistant plants take longer to establish a feeding site and have their feeding duration suppressed two-fold compared with insects feeding on susceptible plants. Our results reveal that traits contributing towards resistant phenotypes are conserved across plant families, deployed against taxonomically diverse insect groups, and that the underlying resistance mechanisms are conserved. These findings provide a new insight into plant-insect interaction and highlight the need for further mechanistic studies across diverse taxa.

Field-Collected Glassy-Winged Sharpshooters (Hemiptera: Cicadellidae) Perform More Xylella fastidiosa-Inoculating Behaviors on Susceptible Vitis vinifera cv. 'Chardonnay' Than on Resistant Vitis champinii Grapevines

The glassy-winged sharpshooter, Homalodisca vitripennis (Germar) (Hemiptera: Cicadellidae: Cicadellinae), is an introduced vector of the xylem-dwelling bacterium Xylella fastidiosa Wells et al. (Xanthomonadales: Xanthomonadaceae) in California. Once acquired, X. fastidiosa colonizes the functional foregut of the vector. Bacteria can be inoculated directly into grapevine xylem during the xylem cell acceptance process in sharpshooter stylet probing, represented by the X wave using electropenetrography (EPG). Since 2001, an effort has been underway to develop PD-resistant grapevines, Vitis vinifera L., through classical breeding of various species of resistant wild grapevines with more susceptible V. vinifera. The present study used EPG to compare H. vitripennis stylet probing behaviors in a factorial experiment between V. champinii (a V. candicans/V. rupestris natural hybrid with moderate trichomes) and V. vinifera cv. 'Chardonnay' (which lacks trichomes) that had been gently scraped to remove trichomes or was not scraped. Results showed that sharpshooters performed significantly more X waves/X. fastidiosa inoculation behaviors of overall longer duration on Chardonnay than on V. champinii, regardless of shaving or not-shaving to remove trichomes. In addition, trichomes caused more frequent standing/walking/test-probing behaviors on V. champinii, whose xylem was rapidly accepted for sharpshooter ingestion once probing began. Thus, EPG can detect a novel type of grapevine resistance to X. fastidiosa-to the vector's probing process and inoculation of bacteria-in addition to the bacterial infection and symptom development processes that are the basis for most resistance breeding today. Future research could use EPG to screen grapevines for this novel type of resistance.

Different Host Plants Distinctly Influence the Feeding Ability of the Brown Citrus Aphid Toxoptera citricida

Simple Summary

The brown citrus aphid, Toxoptera citricida, is an important pest of citrus and causes serious damage in the main production areas. Host plant resistance is an environmentally friendly method to manage aphid infestations and is becoming increasingly important as aphids develop greater resistance to insecticides. The aim of this study was to assess T. citricida resistance on seven widespread and common hosts using electropenetrography combined with a population development test. We showed that the feeding parameters of the brown citrus aphid differed significantly depending on the host plants. Toxoptera citricida spent more time in the pathway stage and less time in the phloem stage on Eureka, while the opposite was observed on Yuzu and Rough Lemon. Measurements of population development of aphids showed that on the Eureka, aphids developed more slowly. Our data suggest that different host plants distinctly influence the ability of T. citricida to feed. The brown citrus aphid did not prefer to feed on Eureka Lemon compared to the other six host plants.

Abstract

Piercing–sucking insects are important crop pests, and an understanding of their feeding behavior and population development plays a crucial role in studying insect population dynamics and crop resistance. In our study, we examined the probing behavior of the brown citrus aphid, Toxoptera citricida, using electropenetrography and assessed its population development after 8 days on seven host plants: Yuzu, Citrus junos Sieb. ex Tanaka; Rough Lemon, C. jambhiri Lush.; ‘Luofu’ kumquat, Fortunella margarita Swingle; ‘Olinda’ valencia orange, C. sinensis (L.) Osbeck; ‘Yanxiwanlu’ Ponkan, C. reticulata Blanco; ‘Rohde Red’ valencia orange, C. sinensis; and ‘Eureka’ lemon, C. limon (L.) Osbeck. The results demonstrated that probing by the brown citrus aphid differed significantly according to the target hosts. Toxoptera citricida produced significantly more pathway activities on Eureka than on Rough Lemon and Yuzu. Toxoptera citricida spent more time from the first probe to first salivation into phloem sieve elements on Eureka compared to Yuzu. In addition, the total duration of ingestion from sieve cells of each aphid in the phloem-feeding phase was shortest on Eureka, and this was significantly shorter than that on Yuzu, Rough Lemon, Luofu, and Olinda. The population number of T. citricida on Eureka after 8 days was significantly lower than that on the other hosts. Overall, Eureka was found to have obvious resistance to T. citricida, whereas Yuzu and Rough Lemon were susceptible host plants. These results provide a theoretical basis for exploring aphid-resistant fruit tree resources using resistant varieties.

Influence of ontogenetic and migration stage on feeding behavior of Cacopsylla picta on 'Candidatus Phytoplasma mali' infected and non-infected apple plants

The summer apple psyllid Cacopsylla picta (Foerster) is the vector of 'Candidatus Phytoplasma mali', the causal agent of apple proliferation disease (AP). During its phloem-feeding activities it transmits this biotrophic bacterium from infected to healthy apple trees (Malus domestica Borkh.) causing high economic losses. During its life cycle, C.picta performs two host switches: In summer, the new adult generation (emigrants) hatch on apples before they emigrate to their overwintering host conifers. The following spring, the overwintered adult generation (remigrants) remigrate into apple orchards for mating and oviposition. The preimaginal stages (nymphs) develop on apple. It is known that phytopathogen-induced changes in plant physiology can affect insect-plant-interactions. In 12 h recordings of electrical penetration graphs (EPG) it was assessed whether 'Ca. P. mali' infection of the plant affected probing and feeding behavior of the vector C.picta. Its life stage and the infection status of the host plant (and the interaction between these factors) significantly affected the first occurrence, duration and frequency of probing and feeding phases. On 'Ca. P. mali' infected plants, the phloem salivation phase occurred later than on non-infected plants. Even though all life stages fed both on phloem and xylem, significant differences were found in the frequency and duration of phloem and xylem ingestion phases. Nymphs spent the shortest time non-probing, earlier started the first leaf penetration and longer ingested xylem compared with adults. Further, phloem phases differed between migratory stages; remigrants had higher numbers of phloem ingestion events and spent longer ingesting phloem than emigrants. For emigrants, however, phloem contact was very rarely observed during our recordings. The impact of our findings for understanding the multitrophic interactions between host plant, pathogen and behavior of vector insects are discussed with regard to the epidemiology of AP and pest control strategies of the vector.

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.

Probing Behavior of Diaphorina citri (Hemiptera: Liviidae) on Valencia Orange Influenced by Sex, Color, and Size

Candidatus Liberibacter asiaticus Jagoueix, Bové, and Garnier (Rhizobiales: Rhizobiaceae) is transmitted by the psyllid Diaphorina citri Kuwayama and putatively causes Huanglongbing disease in citrus. Huanglongbing has reduced yields by 68% relative to pre-disease yields in Florida. Disease management is partly through vector control. Understanding vector biology is essential in this endeavor. Our goal was to document differences in probing behavior linked to sex. Based on both a literature review and our results, we conclude that there is either no effect of sex or that identifying such an effect requires a sample size at least four times larger than standard methodologies. Including both color and sex in statistical models did not improve model performance. Both sex and color are correlated with body size, and body size has not been considered in previous studies on sex in D. citri in terms of probing behavior. An effect of body size was found wherein larger psyllids took longer to reach ingestion behaviors and larger individuals spent more time-ingesting phloem, but these relationships explained little of the variability in these data. We suggest that the effects of sex can be ignored when running EPG experiments on healthy psyllids.