Plant viruses alter insect behavior to enhance their spread

Direct effects of barley yellow dwarf virus on the performance, parasitoid resistance, and feeding behavior of its vector Sitobion avenae (Hemiptera: Aphididae)

BACKGROUND

The complex interaction between plant viruses and their insect vectors is the basis for the epidemiology of plant viruses. The 'Vector Manipulation Hypothesis' (VMH) was proposed to demonstrate the evolution of strategies in plant viruses to enhance their transmission to new hosts through direct effects on insect vector behavior and/or physiology. However, the aphid vectors used in previous studies were mostly obtained by feeding on virus-infected plants and as a result, it was difficult to eliminate the confounding effects of infected host plants. Furthermore, the mechanisms of the direct effects of plant viruses on insect vectors have rarely been examined comprehensively.

RESULTS

We fed Sitobion avenae on an artificial diet infused with a purified suspension of Barley yellow dwarf virus (BYDV) PAV strain to obtain viruliferous aphids. We then examined their growth and reproduction performance, resistance to the parasitoid Aphidius gifuensis Ashmead, and feeding behavior. The results indicate that (1) viruliferous aphids had a shorter life span and a lower relative growth rate at the nymphal stage; (2) A. gifuensis had a lower parasitism rate, mummification rate, and emergence rate in viruliferous aphids; (3) Viruliferous aphids spent more time on non-probing and salivation behavior and had a shorter total duration of penetration and ingestion compared with healthy conspecifics.

CONCLUSION

These results suggest that plant virus infection may directly alter vector fitness and behavior that improves plant virus transmission, but not vector growth. These findings highlight the mechanisms of VMH and the ecological significance of vector manipulation by plant viruses, and have implications for plant virus disease and vector management. © 2024 Society of Chemical Industry.

Orthotospovirus iridimaculaflavi (iris yellow spot virus): An emerging threat to onion cultivation and its transmission by Thrips tabaci in India

The yellow spot disease caused by the virus species Orthotospovirus iridimaculaflavi (Iris yellow spot virus-IYSV), belonging to the genus Orthotospovirus, the family Tospoviridae, order Bunyavirales and transmitted by Thrips tabaci Lindeman. At present, emerging as a major threat in onion (Allium cepa) in Tamil Nadu, India. The yellow spot disease incidence was found to be 53-73 % in six districts out of eight major onion-growing districts surveyed in Tamil Nadu during 2021-2023. Among the onion cultivars surveyed, the cultivar CO 5 was the most susceptible to IYSV. The population of thrips was nearly 5-9/plant during vegetative and flowering stages. The thrips infestation was 34-60 %. The tospovirus involved was confirmed as IYSV through DAS-ELISA, followed by molecular confirmation through RT-PCR using the nucleocapsid (N) gene. The predominant thrips species present in onion crops throughout the growing seasons was confirmed as Thrips tabaci based on the nucleotide sequence of the MtCOI gene. The mechanical inoculation of IYSV in different hosts viz., Vigna unguiculata, Gomphrena globosa, Chenopodium amaranticolor, Chenopodium quinoa and Nicotiana benthamiana resulted in chlorotic and necrotic lesion symptoms. The electron microscopic studies with partially purified sap from onion lesions revealed the presence of spherical to pleomorphic particles measuring 100-230 nm diameter. The transmission of IYSV was successful with viruliferous adult Thrips tabaci in cowpea (Cv. CO7), which matured from 1st instar larva fed on infected cowpea leaves (24 h AAP). Small brown necrotic symptoms were produced on inoculated plants after an interval of four weeks. The settling preference of non-viruliferous and viruliferous T. tabaci towards healthy and infected onion leaves resulted in the increased preference of non-viruliferous thrips towards infected (onion-61.33 % and viruliferous thrips towards healthy onion leaves (75.33 %). The study isolates shared 99-100 % identity at a nucleotide and amino acid level with Indian isolates of IYSV in the N gene. The multiple alignment of the amino acid sequence of the N gene of IYSV isolates collected from different locations and IYSV isolates from the database revealed amino acid substitution in the isolate ITPR4. All the IYSV isolates from India exhibited characteristic amino acid substitution of serine at the 6th position in the place of threonine in the isolates from Australia, Japan and USA. The phylogenetic analysis revealed the monophyletic origin of the IYSV isolates in India.

Bacterial plant pathogens affect the locomotor behavior of the insect vector: a case study of Citrus volkameriana-Triozae erytreae-Candidatus Liberibacter asiaticus system

Plant pathogens can alter the behavior of their insect vectors as well as their survival and reproduction. The African psyllid, Trioza erytreae, is one of the vectors of Huanglongbing, a citrus disease caused mainly by "Candidatus Liberibacter asiaticus" (CLas). The purpose of this study was to characterize the effects of CLas on the psyllid, T. erytreae using Citrus volkamerina plants as the study system. The study focused more specifically on the CLas effects prior to and after its acquisition by the psyllid T. erytreae. Our results did not support the hypothesis that CLas effects psyllid probing behavior prior to acquisition; few differences were observed between uninfected T. erytrea feeding on CLas-infected versus control plants. On the other hand, compared to psyllids that had completed their development on control plants, the ones that had completed their development on a CLas-infected plant exhibited changes in their behavior (greater velocity), physiology (smaller mass) and biochemistry (lower water and lipid content). Altogether, our results confirm the existence of a marked postacquisition effect on the vector locomotor behavior and a minor preacquisition effect of CLas on the vector behavior, which can be partially explained by physiological and biochemical changes.

A viral movement protein targets host catalases for 26S proteasome-mediated degradation to facilitate viral infection and aphid transmission in wheat

The infection of host plants by many different viruses causes reactive oxygen species (ROS) accumulation and yellowing symptoms, but the mechanisms through which plant viruses counteract ROS-mediated immunity to facilitate infection and symptom development have not been fully elucidated. Most plant viruses are transmitted by insect vectors in the field, but the molecular mechanisms underlying virus‒host-insect interactions are unclear. In this study, we investigated the interactions among wheat, barley yellow dwarf virus (BYDV), and its aphid vector and found that the BYDV movement protein (MP) interacts with both wheat catalases (CATs) and the 26S proteasome ubiquitin receptor non-ATPase regulatory subunit 2 homolog (PSMD2) to facilitate the 26S proteasome-mediated degradation of CATs, promoting viral infection, disease symptom development, and aphid transmission. Overexpression of the BYDV MP gene in wheat enhanced the degradation of CATs, which leading to increased accumulation of ROS and thereby enhanced viral infection. Interestingly, transgenic wheat lines overexpressing BYDV MP showed significantly reduced proliferation of wingless aphids and an increased number of winged aphids. Consistent with this observation, silencing of CAT genes also enhanced viral accumulation and reduced the proliferation of wingless aphids but increased the occurrence of winged aphids. In contrast, transgenic wheat plants overexpressing TaCAT1 exhibited the opposite changes and showed increases in grain size and weight upon infection with BYDV. Biochemical assays demonstrated that BYDV MP interacts with PSMD2 and promotes 26S proteasome-mediated degradation of TaCAT1 likely in a ubiquitination-independent manner. Collectively, our study reveals a molecular mechanism by which a plant virus manipulates the ROS production system of host plants to facilitate viral infection and transmission, shedding new light on the sophisticated interactions among viruses, host plants, and insect vectors.

The Role of Plant Defense Signaling Pathways in Phytoplasma-Infected and Uninfected Aster Leafhoppers' Oviposition, Development, and Settling Behavior

In plant-microbe-insect systems, plant-mediated responses involve the regulation and interactions of plant defense signaling pathways of phytohormones jasmonic acid (JA), ethylene (ET), and salicylic acid (SA). Phytoplasma subgroup 16SrI is the causal agent of Aster Yellows (AY) disease and is primarily transmitted by populations of aster leafhoppers (Macrosteles quadrilineatus Forbes). Aster Yellows infection in plants is associated with the downregulation of the JA pathway and increased leafhopper oviposition. The extent to which the presence of intact phytohormone-mediated defensive pathways regulates aster leafhopper behavioral responses, such as oviposition or settling preferences, remains unknown. We conducted no-choice and two-choice bioassays using a selection of Arabidopsis thaliana lines that vary in their defense pathways and repeated the experiments using AY-infected aster leafhoppers to evaluate possible differences associated with phytoplasma infection. While nymphal development was similar among the different lines and groups of AY-uninfected and AY-infected insects, the number of offspring and individual female egg load of AY-uninfected and AY-infected insects differed in lines with mutated components of the JA and SA signaling pathways. In most cases, AY-uninfected insects preferred to settle on wild-type (WT) plants over mutant lines; no clear pattern was observed in the settling preference of AY-infected insects. These findings support previous observations in other plant pathosystems and suggest that plant signaling pathways and infection with a plant pathogen can affect insect behavioral responses in more than one manner. Potential differences with previous work on AY could be related to the specific subgroup of phytoplasma involved in each case.

New genetic resources for aphid resistance were identified from a newly developed wheat mutant library

Reports on development of resilient wheat mutants to aphid infestation-causing heavy losses to wheat production in many parts of the world, are scanty. The present study aimed to identify genetic diversity of wheat mutants in terms of varying degree of resistance to aphid infestation which can help protect wheat crop, improve yields and enhance food security. Resistance response to aphid infestation was studied on newly developed 33 wheat mutants, developed through irradiating seed of an elite wheat cultivar "Punjab-11" with gamma radiations, during three normal growing seasons at two sites. Data on various traits including aphid count per plant, biochemical traits, physiological traits and grain yield was recorded. Meteorological data was also collected to unravel the impact of environmental conditions on aphid infestation on wheat plants. Minimum average aphid infestation was found on Pb-M-2725, Pb-M-2550, and Pb-M-2719 as compared to the wild type. High yielding mutants Pb-M-1323, Pb-M-59, and Pb-M-1272 supported the moderate aphid infestation. The prevailing temperature up to 25 °C showed positive correlation (0.25) with aphid count. Among biochemical traits, POD (0.34), TSP (0.33), TFA (0.324) exhibited a high positive correlation with aphid count. In addition, CAT (0.31), TSS (0.294), and proline content (0.293) also showed a positive correlation with aphid count. However, all physiological traits depicted negative correlation with aphid count, while, a very weak correlation (0.12) was found between mean aphid count and grain yield. In PCA biplots, the biochemical variables clustered together with aphid count, while physiological variables grouped with grain yield. Biochemical parameters contributed most, towards first dimension of the PCA (48.6%) as compared to the physiological variables (13%). The FAMD revealed that mutant lines were major contributor towards total variation; Pb-M-1027, Pb-M-1323, Pb-M-59 were found to be the most diverse lines. The PCA revealed that biochemical parameters played a significant role in explaining variations in aphid resistance, emphasizing their importance in aphid defense mechanisms. The identified mutants can be utilized by the international wheat community for getting insight into the molecular circuits of resistant mechanism against aphids as well as for designing new KASP markers. This study also highlights the importance of considering both genetic and environmental factors in the development of resilient wheat varieties and pave the way for further investigations into the molecular mechanisms underpinning aphid resistance in wheat.

Aphid gene expression following polerovirus acquisition is host species dependent

Upon acquisition of persistent circulative viruses such as poleroviruses, the virus particles transcytose through membrane barriers of aphids at the midgut and salivary glands via hemolymph. Such intricate interactions can influence aphid behavior and fitness and induce associated gene expression in viruliferous aphids. Differential gene expression can be evaluated by omics approaches such as transcriptomics. Previously conducted aphid transcriptome studies used only one host species as the source of virus inoculum. Viruses typically have alternate hosts. Hence, it is not clear how alternate hosts infected with the same virus isolate alter gene expression in viruliferous vectors. To address the question, this study conducted a transcriptome analysis of viruliferous aphids that acquired the virus from different host species. A polerovirus, cotton leafroll dwarf virus (CLRDV), which induced gene expression in the cotton aphid, Aphis gossypii Glover, was assessed using four alternate hosts, viz., cotton, hibiscus, okra, and prickly sida. Among a total of 2,942 differentially expressed genes (DEGs), 750, 310, 1,193, and 689 genes were identified in A. gossypii that acquired CLRDV from infected cotton, hibiscus, okra, and prickly sida, respectively, compared with non-viruliferous aphids that developed on non-infected hosts. A higher proportion of aphid genes were overexpressed than underexpressed following CLRDV acquisition from cotton, hibiscus, and prickly sida. In contrast, more aphid genes were underexpressed than overexpressed following CLRDV acquisition from okra plants. Only four common DEGs (heat shock protein, juvenile hormone acid O-methyltransferase, and two unannotated genes) were identified among viruliferous aphids from four alternate hosts. Gene ontology (GO) enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) annotations indicated that the acquisition of CLRDV induced DEGs in aphids associated with virus infection, signal transduction, immune systems, and fitness. However, these induced changes were not consistent across four alternate hosts. These data indicate that alternate hosts could differentially influence gene expression in aphids and presumably aphid behavior and fitness despite being infected with the same virus isolate.

Preference of Pentalonia nigronervosa for infected banana plants tends to reverse after Banana bunchy top virus acquisition

Pentalonia nigronervosa Coquerel (Hemiptera: Aphididae) is the vector of the Banana Bunchy Top Virus (BBTV), the most serious viral disease of banana (Musa spp.) in the world. Before acquiring the virus, the vector is more attracted to infected banana plants in response to the increased emissions of volatile organic compounds (VOCs). Here, we test the hypothesis that BBTV acquisition directly modifies the preference of P. nigronervosa for infected banana plants, and that the change in preference results from the alteration of the organs linked to the VOC detection or to the behaviour of the vector. We found that the preference of P. nigronervosa for infected banana plants reverses after virus acquisition in dessert banana, while it remains similar between healthy and infected banana plants before and after the acquisition of BBTV. At the same time, aphids reared on infected bananas had smaller forewing areas and hind tibia length than aphids reared on healthy bananas, although the number of secondary rhinaria on the antennae was lower on dessert banana-reared aphids than plantain-reared aphids, this was not affected by the infection status of the aphid. These results support the "vector manipulation hypothesis-VMH" of pathogens to promote their spread. They have implications for the BBTV management.

The evolutionary trajectories of specialized metabolites towards antiviral defense system in plants

Viral infections in plants pose major challenges to agriculture and global food security in the twenty-first century. Plants have evolved a diverse range of specialized metabolites (PSMs) for defenses against pathogens. Although, PSMs-mediated plant-microorganism interactions have been widely discovered, these are mainly confined to plant-bacteria or plant-fungal interactions. PSM-mediated plant-virus interaction, however, is more complicated often due to the additional involvement of virus spreading vectors. Here, we review the major classes of PSMs and their emerging roles involved in antiviral resistances. In addition, evolutionary scenarios for PSM-mediated interactions between plant, virus and virus-transmitting vectors are presented. These advancements in comprehending the biochemical language of PSMs during plant-virus interactions not only lay the foundation for understanding potential co-evolution across life kingdoms, but also open a gateway to the fundamental principles of biological control strategies and beyond.

Barley Yellow Dwarf Virus Influences Its Vector's Endosymbionts but Not Its Thermotolerance

The barley yellow dwarf virus (BYDV) of cereals is thought to substantially increase the high-temperature tolerance of its aphid vector, Rhopalosiphum padi, which may enhance its transmission efficiency. This is based on experiments with North American strains of BYDV and R. padi. Here, we independently test these by measuring the temperature tolerance, via Critical Thermal Maximum (CTmax) and knockdown time, of Australian R. padi infected with a local BYDV isolate. We further consider the interaction between BYDV transmission, the primary endosymbiont of R. padi (Buchnera aphidicola), and a transinfected secondary endosymbiont (Rickettsiella viridis) which reduces the thermotolerance of other aphid species. We failed to find an increase in tolerance to high temperatures in BYDV-infected aphids or an impact of Rickettsiella on thermotolerance. However, BYDV interacted with R. padi endosymbionts in unexpected ways, suppressing the density of Buchnera and Rickettsiella. BYDV density was also fourfold higher in Rickettsiella-infected aphids. Our findings indicate that BYDV does not necessarily increase the temperature tolerance of the aphid transmission vector to increase its transmission potential, at least for the genotype combinations tested here. The interactions between BYDV and Rickettsiella suggest new ways in which aphid endosymbionts may influence how BYDV spreads, which needs further testing in a field context.

Endosymbionts modulate virus effects on aphid-plant interactions

Vector-borne pathogens frequently modify traits of their primary hosts and vectors in ways that influence disease transmission. Such effects can themselves be altered by the presence of other microbial symbionts, yet we currently have limited understanding of these interactions. Here we show that effects of pea enation mosaic virus (PEMV) on interactions between host plants and aphid vectors are modulated by the presence of different aphid endosymbionts. In a series of laboratory assays, we found strong interactive effects of virus infection and endosymbionts on aphid metabolomic profiles, population growth, behavior, and virus transmission during aphid feeding. Furthermore, the strongest effects-and those predicted to favor virus transmission-were most apparent in aphid lines harboring particular endosymbionts. These findings show that virus effects on host-vector interactions can be strongly influenced by other microbial symbionts and suggest a potentially important role for such interactions in disease ecology and evolution.

A Review of Interactions between Plants and Whitefly-Transmitted Begomoviruses

The transmission of plant viruses from infected to healthy host plants is a process in which insects play a major role, using various transmission strategies. Environmental factors have an impact on the transmission of viruses and the subsequent development of infections or diseases. When viruses are successful, plant virus diseases can reach epidemic proportions. Many plants across different regions are vulnerable to viral infections transmitted by the whitefly vector. Begomoviruses, which are transmitted by whiteflies, represent a significant threat to agriculture worldwide. The review highlights the mechanisms of virus acquisition and transmission by whiteflies and explores the factors influencing these interactions. Understanding the impacts of these changes is crucial for managing the spread of pests and mitigating damage to crops. It underscores the need for continued research to elucidate the mechanisms driving plant-insect-virus interactions and to identify new approaches for sustainable pest management.

Spatial spread of infectious diseases with conditional vector preferences

We explore the spatial spread of vector-borne infections with conditional vector preferences, meaning that vectors do not visit hosts at random. Vectors may be differentially attracted toward infected and uninfected hosts depending on whether they carry the pathogen or not. The model is expressed as a system of partial differential equations with vector diffusion. We first study the non-spatial model. We show that conditional vector preferences alone (in the absence of any epidemiological feedback on their population dynamics) may result in bistability between the disease-free equilibrium and an endemic equilibrium. A backward bifurcation may allow the disease to persist even though its basic reproductive number is less than one. Bistability can occur only if both infected and uninfected vectors prefer uninfected hosts. Back to the model with diffusion, we show that bistability in the local dynamics may generate travelling waves with either positive or negative spreading speeds, meaning that the disease either invades or retreats into space. In the monostable case, we show that the disease spreading speed depends on the preference of uninfected vectors for infected hosts, but also on the preference of infected vectors for uninfected hosts under some circumstances (when the spreading speed is not linearly determined). We discuss the implications of our results for vector-borne plant diseases, which are the main source of evidence for conditional vector preferences so far.

Integrating Community Ecology into Models of Vector-Borne Virus Transmission

Vector-borne plant viruses are a diverse and dynamic threat to agriculture with hundreds of economically damaging viruses and insect vector species. Mathematical models have greatly increased our understanding of how alterations of vector life history and host-vector-pathogen interactions can affect virus transmission. However, insect vectors also interact with species such as predators and competitors in food webs, and these interactions affect vector population size and behaviors in ways that mediate virus transmission. Studies assessing how species' interactions affect vector-borne pathogen transmission are limited in both number and scale, hampering the development of models that appropriately capture community-level effects on virus prevalence. Here, we review vector traits and community factors that affect virus transmission, explore the existing models of vector-borne virus transmission and areas where the principles of community ecology could improve the models and management, and finally evaluate virus transmission in agricultural systems. We conclude that models have expanded our understanding of disease dynamics through simulations of transmission but are limited in their ability to reflect the complexity of ecological interactions in real systems. We also document a need for experiments in agroecosystems, where the high availability of historical and remote-sensing data could serve to validate and improve vector-borne virus transmission models.

A plant virus enhances odorant-binding protein 5 (OBP5) in the vector whitefly for more actively olfactory orientation to the host plant

BACKGROUND

The whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) is a notorious agricultural pest and the effective vector of many plant viruses worldwide. Cucurbit chlorotic yellows virus (CCYV), exclusively transmitted by B. tabaci in a semipersistent manner, is a serious causal agent in cucurbit crops in many countries. Plant viruses can manipulate the behaviors of insect vectors to promote the spread of themselves, but underlying mechanisms are remaining unclear.

RESULTS

In this study, our observations indicated that B. tabaci, when carrying CCYV, oriented more actively to the host plant cucumber. Transcriptome analysis and quantitative polymerase chain reaction with reverse transcription analysis showed that the odorant-binding protein 5 (OBP5) was upregulated with viral acquisition. Sequence and phylogenetic analysis showed that BtabOBP5 was highly homologous with nine OBPs from other hemipteran insects. In addition, OBP5-silenced whiteflies significantly altered their orientation behavior towards cucumber plants and towards some typical volatile organic compounds released from cucumbers.

CONCLUSION

This study described a novel mechanism by which the olfactory system of vector insects could be regulated by a semipersistent plant virus, thereby affecting insect olfactory behavior and relationship with host plants. These results provided a basis for developing potential olfaction-based pest management strategies in the future. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Virus-Induced Plant Volatiles Promote Virus Acquisition and Transmission by Insect Vectors

Rice dwarf virus (RDV) is transmitted by insect vectors Nephotettix virescens and Nephotettix cincticeps (Hemiptera: Cicadellidae) that threatens rice yield and results in substantial economic losses. RDV induces two volatiles ((E)-β-caryophyllene (EBC) and 2-heptanol) to emit from RDV-infected rice plants. However, the effects of the two volatiles on the olfactory behavior of both non-viruliferous and viruliferous N. virescens are unknown, and whether the two volatiles could facilitate the spread and dispersal of RDV remains elusive. Combining the methods of insect behavior, chemical ecology, and molecular biology, we found that EBC and 2-heptanol influenced the olfactory behavior of non-viruliferous and viruliferous N. virescens, independently. EBC attracted non-viruliferous N. virescens towards RDV-infected rice plants, promoting virus acquisition by non-viruliferous vectors. The effect was confirmed by using oscas1 mutant rice plants (repressed EBC synthesis), but EBC had no effects on viruliferous N. virescens. 2-heptanol did not attract or repel non-viruliferous N. virescens. However, spraying experiments showed that 2-heptanol repelled viruliferous N. virescens to prefer RDV-free rice plants, which would be conducive to the transmission of the virus. These novel results reveal that rice plant volatiles modify the behavior of N. virescens vectors to promote RDV acquisition and transmission. They will provide new insights into virus-vector-plant interactions, and promote the development of new prevention and control strategies for disease management.

Onion Thrips (Thysanoptera: Thripidae) Host Plant Preference and Performance are Mediated by a Facultative Plant Pathogen of Onion

Insect vector and phytopathogen interactions are mediated by host plants. Insects interact with pathogens directly or indirectly and they may prefer host plants based on infection status. Performance on infected hosts varies depending on the type of pathogen involved. Species specific studies of economically important insects and phytopathogens are needed to understand how these interactions impact crop yields. Onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), is an economically devastating insect pest of onions (Allium cepa L., Asparagales: Amaryllidaceae) worldwide and it co-occurs simultaneously with many different pathogens. Colletotrichum coccodes (Wallr) (Glomerellales: Glomerellaceae) is a generalist fungal pathogen that attacks onion foliage, causing tan lesions and decreasing yield. Onion thrips and C. coccodes represent two important pests of onions, but the relationship between onion thrips and C. coccodes infected onions has not been studied, and it is unclear if onion thrips contribute to the spread of C. coccodes in onion fields. A four-choice test with control, artificially injured, artificially injured + symptomatic, and inoculated-symptomatic onion suggests that onion thrips distinguish between hosts based on health status. Furthermore, a two-choice test with control, inoculated-asymptomatic, and inoculated-symptomatic onion pairings revealed that onion thrips distinguish between hosts based on infection status and prefer inoculated-symptomatic hosts. In a no-choice test, onion thrips numbers increased on inoculated-symptomatic plants compared to control or inoculated-asymptomatic plants. Overall, we found that onion thrips preferred and performed best on C. coccodes infected plants.

Sugarcane multitrophic interactions: Integrating belowground and aboveground organisms

Sugarcane is a crop of major importance used mainly for sugar and biofuel production, and many additional applications of its byproducts are being developed. Sugarcane cultivation is plagued by many insect pests and pathogens that reduce sugarcane yields overall. Recently emerging studies have shown complex multitrophic interactions in cultivated areas, such as the induction of sugarcane defense-related proteins by insect herbivory that function against fungal pathogens that commonly appear after mechanical damage. Fungi and viruses infecting sugarcane also modulate insect behavior, for example, by causing changes in volatile compounds responsible for insect attraction or repelling natural vector enemies via a mechanism that increases pathogen dissemination from infected plants to healthy ones. Interestingly, the fungus Fusarium verticillioides is capable of being vertically transmitted to insect offspring, ensuring its persistence in the field. Understanding multitrophic complexes is important to develop better strategies for controlling pathosystems affecting sugarcane and other important crops and highlights the importance of not only studying binary interactions but also adding as many variables as possible to effectively translate laboratory research to real-life conditions.

Mechanisms of Microbial Plant Protection and Control of Plant Viruses

Plant viral diseases are major constraints causing significant yield losses worldwide in agricultural and horticultural crops. The commonly used methods cannot eliminate viral load in infected plants. Many unconventional methods are presently being employed to prevent viral infection; however, every time, these methods are not found promising. As a result, it is critical to identify the most promising and sustainable management strategies for economically important plant viral diseases. The genetic makeup of 90 percent of viral diseases constitutes a single-stranded RNA; the most promising way for management of any RNA viruses is through use ribonucleases. The scope of involving beneficial microbial organisms in the integrated management of viral diseases is of the utmost importance and is highly imperative. This review highlights the importance of prokaryotic plant growth-promoting rhizobacteria/endophytic bacteria, actinomycetes, and fungal organisms, as well as their possible mechanisms for suppressing viral infection in plants via cross-protection, ISR, and the accumulation of defensive enzymes, phenolic compounds, lipopeptides, protease, and RNase activity against plant virus infection.

Effects of PVY-Infected Tobacco Plants on the Adaptation of Myzus persicae (Hemiptera: Aphididae)

The indirect interaction between viruses and their insect vectors via the host plants can mediate viral transmission. Thus, elucidating these tripartite interactions is crucial for controlling the occurrence of viral diseases. This study examined the feeding behavior and life table parameters of the green peach aphid, Myzus persicae, using electropenetrography and an age-stage, two-sex life table on PVY-infected and uninfected tobacco plants. Furthermore, the amino acid and soluble sugar contents in tobacco tissue at different stages of PVY infection were determined. The results showed that PVY-infected plants exerted remarkable effects on the feeding activities of M. persicae. Aphids exhibited a reduced non-probing duration and increased phloem sap ingestion on infected plants. Although the nymph development time on the PVY-infected plants was significantly shorter than that of uninfected plants, M. persicae reared on infected plants had reduced fecundity and significantly shortened adult longevity. On day 12, the sugar: amino acid ratio of the PVY-infected plants was significantly higher than that of uninfected plants, whereas the opposite was observed on day 24. Our results demonstrated that PVY could alter the adaptability of M. persicae by modifying the nutritional quality of tobacco plants. In addition, divergent effects on aphids were observed at different infection stages, which are crucial to consider while exploring the interactions between viruses, insect vectors, and host plants. These results provided significant information for comprehending PVY spread and outbreaks.

Vector acquisition and co-inoculation of two plant viruses influences transmission, infection, and replication in new hosts

This study investigated the role of vector acquisition and transmission on the propagation of single and co-infections of tomato yellow leaf curl virus (TYLCV,) and tomato mottle virus (ToMoV) (Family: Geminiviridae, Genus: Begomovirus) by the whitefly vector Bemisia tabaci MEAM1 (Gennadius) in tomato. The aim of this research was to determine if the manner in which viruses are co-acquired and co-transmitted changes the probability of acquisition, transmission and new host infections. Whiteflies acquired virus by feeding on singly infected plants, co-infected plants, or by sequential feeding on singly infected plants. Viral titers were also quantified by qPCR in vector cohorts, in artificial diet, and plants after exposure to viruliferous vectors. Differences in transmission, infection status of plants, and titers of TYLCV and ToMoV were observed among treatments. All vector cohorts acquired both viruses, but co-acquisition/co-inoculation generally reduced transmission of both viruses as single and mixed infections. Co-inoculation of viruses by the vector also altered virus accumulation in plants regardless of whether one or both viruses were propagated in new hosts. These findings highlight the complex nature of vector-virus-plant interactions that influence the spread and replication of viruses as single and co-infections.

Maize Lethal Necrosis disease: review of molecular and genetic resistance mechanisms, socio-economic impacts, and mitigation strategies in sub-Saharan Africa

BACKGROUND

Maize lethal necrosis (MLN) disease is a significant constraint for maize producers in sub-Saharan Africa (SSA). The disease decimates the maize crop, in some cases, causing total crop failure with far-reaching impacts on regional food security.

RESULTS

In this review, we analyze the impacts of MLN in Africa, finding that resource-poor farmers and consumers are the most vulnerable populations. We examine the molecular mechanism of MLN virus transmission, role of vectors and host plant resistance identifying a range of potential opportunities for genetic and phytosanitary interventions to control MLN. We discuss the likely exacerbating effects of climate change on the MLN menace and describe a sobering example of negative genetic association between tolerance to heat/drought and susceptibility to viral infection. We also review role of microRNAs in host plant response to MLN causing viruses as well as heat/drought stress that can be carefully engineered to develop resistant varieties using novel molecular techniques.

CONCLUSIONS

With the dual drivers of increased crop loss due to MLN and increased demand of maize for food, the development and deployment of simple and safe technologies, like resistant cultivars developed through accelerated breeding or emerging gene editing technologies, will have substantial positive impact on livelihoods in the region. We have summarized the available genetic resources and identified a few large-effect QTLs that can be further exploited to accelerate conversion of existing farmer-preferred varieties into resistant cultivars.

Dynamic changes in virus-induced volatiles in cotton modulate the orientation and oviposition behavior of the whitefly Bemisia tabaci

Manipulation of insect vector behavior by virus-induced plant volatiles is well known. But how the viral disease progression alters the plant volatiles and its effect on vector behavior remains less explored. Our studies tracked changes in volatile profile in progressive infection stages of cotton leaf curl virus (CLCuV) infected plants and their effect on B. tabaci behavior. Significant differences in virus titers were noticed between progressive infection stages showing distinct symptoms. Whiteflies initially settled on CLCuV infected plants, but their preference was shifted to healthy plants over time. GC-MS analysis revealed subtle quantitative/qualitative changes in volatile organic compounds (VOCs) between the healthy and selected CLCuV infection stages. VOCs such as hexanal, (E)-2-hexen-1-ol, (+)-α-pinene, (−)-β-pinene, (Z)-3-hexen-1-ol, (+)-sylvestrene, and (1S,2E,6E, 10R)-3,7,11,11-tetramethylbicycloundeca-2,6-diene (Bicyclogermacrene) were associated with the infection stage showing upward curling of leaves; (E)-2-hexen-1-ol, β-myrcene, β-ocimene, and copaene were associated with the infection stage showing downward curling. Validation studies with eight synthetic VOCs indicated that γ-terpinene elicited attraction to B. tabaci (Olfactometric Preference Index (OPI) = 1.65), while β-ocimene exhibited strong repellence (OPI = 0.64) and oviposition reduction (66.01%–92.55%). Our studies have demonstrated that progression of CLCuV disease in cotton was associated with dynamic changes in volatile profile which influences the behavioural responses of whitefly, B.tabaci. Results have shown that VOCs such as (+)-α-pinene, (−)-β-pinene γ-Terpinene, α-guaiene; 4- hydroxy- 4 methyl-2- pentanone and β-ocimene emitted from Begomovirus infected plants could be the driving force for early attraction and later repellence/oviposition deterrence of B. tabaci on virus-infected plants. The findings of this study offer scope for the management of whitefly, B. tabaci through semiochemicals.

Rice Stripe Virus Modulates the Feeding Preference of Small Brown Planthopper from the Stems to Leaves of Rice Plants to Promote Virus Infection

Research on plant-virus-vector interactions has revealed that viruses can enhance their spread to new host plants by attracting nonviruliferous vectors to infected plants or driving viruliferous vectors to noninfected plants. However, whether viruses can also modulate the feeding preference of viruliferous vectors for different plant parts remains largely unknown. Here, by using rice stripe virus (RSV) and its vector, the small brown planthopper (SBPH), as a model, the effect of the virus on the feeding preference of its vector was studied by calculating the number of nonviruliferous and viruliferous SBPHs settling on different parts of rice plants. The results showed that the RSV-free SBPHs significantly preferred feeding on the stems of rice plants, whereas RSV-carrying SBPHs fed more on rice leaves. Moreover, the rice plants inoculated with RSV on the leaves showed more severe symptoms, with enhanced disease incidence and virus accumulation compared with rice plants inoculated at the top and bottom of stems, suggesting that the leaves are more susceptible to RSV than the stems of rice plants. These results demonstrate that RSV modulates the feeding preference of its transmitting vector SBPH from the stems to leaves of rice plants to promote virus infection. Interestingly, we also found that the leaves were more susceptible than the stems to rice black-streaked dwarf virus. This study proves that the feeding preference of insect vectors can be modulated by plant viruses to facilitate virus transmission.

Turnip Mosaic Virus Infection Differentially Modifies Cabbage Aphid Probing Behavior in Spring and Winter Oilseed Rape (Brassica napus)

Direct and indirect effects of plant virus infection on vector behavior have been discovered to improve virus transmission efficiency, but the impact of plant cultivars in virus-vector-plant interactions has received little attention. Electropenetrography (EPG) allows real-time tracking and quantification of stylet penetration behaviors, pathogen transmission, and plant resistance mechanisms. Quantitative probing behaviors on a spring oilseed rape cultivar, 'Xinyou17', and a winter oilseed rape cultivar, 'Zheping4', were investigated using EPG to compare turnip mosaic virus (TuMV) regulation of cabbage aphid probing behavior. Results for indirect effects showed that compared to mock-inoculated plants, cabbage aphids on infected plants increased brief probing frequency, cell penetration frequency, intracellular probing time, and decreased time to first probe and pathway time, potentially promoting viral acquisition. TuMV also directly influences aphid probing behavior. Viruliferous aphids had reduced pathway time, increased cell penetration frequency, increased intracellular probing time, increased salivation frequency, and ingested less sap than non-viruliferous aphids, primed for viral infection. Although oilseed rape cultivars can also influence aphid behavior, the main effect of cultivars was not significant on TuMV-infected plants.

Ecology of Yellow Dwarf Viruses in Crops and Grasslands: Interactions in the Context of Climate Change

Our understanding of the ecological interactions between plant viruses, their insect vectors, and their host plants has increased rapidly over the past decade. The suite of viruses known collectively as the yellow dwarf viruses infect an extensive range of cultivated and noncultivated grasses worldwide and is one of the best-studied plant virus systems. The yellow dwarf viruses are ubiquitous in cereal crops, where they can significantly limit yields, and there is growing recognition that they are also ubiquitous in grassland ecosystems, where they can influence community dynamics. Here, we discuss recent research that has explored (a) the extent and impact of yellow dwarf viruses in a diversity of plant communities, (b) the role of vector behavior in virus transmission, and (c) the prospects for impacts of climate change-including rising temperatures, drought, and elevated CO₂-on the epidemiology of yellow dwarf viruses.

Soybean Aphid (Hemiptera: Aphididae) Feeding Behavior is Largely Unchanged by Soybean Mosaic Virus but Significantly Altered by the Beetle-Transmitted Bean Pod Mottle Virus

The soybean aphid (Aphis glycines Matsumura) is an economically important invasive pest of soybean. In addition to damage caused by soybean aphid feeding on the phloem sap, this insect also transmits many plant viruses, including soybean mosaic virus (SMV). Previous work has shown that plant viruses can change plant host phenotypes to alter the behavior of their insect vectors to promote virus spread, known as the vector manipulation hypothesis. In this study, we used electropenetography (EPG) to examine the effects of two plant viruses on soybean aphid feeding behavior: SMV, which is transmitted by many aphid species including the soybean aphid, and bean pod mottle virus (BPMV), which is transmitted by chrysomelid and some coccinellid beetles but not aphids. These two viruses often co-occur in soybean production and can act synergistically. Surprisingly, our results showed little to no effect of SMV on soybean aphid feeding behaviors measured by EPG, but profound differences were observed in aphids feeding on BPMV-infected plants. Aphids took longer to find the vascular bundle of BPMV-infected plants, and once found, spent more time entering and conditioning the phloem than ingesting phloem sap. Interestingly, these observed alterations are similar to those of aphids feeding on insect-resistant soybean plants. The cause of these changes in feeding behavior is not known, and how they impact virus transmission and soybean aphid populations in the field will require further study.

The secret life of insect-associated microbes and how they shape insect-plant interactions

Insects are associated with a plethora of different microbes of which we are only starting to understand their role in shaping insect–plant interactions. Besides directly benefitting from symbiotic microbial metabolism, insects obtain and transmit microbes within their environment, making them ideal vectors and potential beneficiaries of plant diseases and microbes that alter plant defenses. To prevent damage, plants elicit stress-specific defenses to ward off insects and their microbiota. However, both insects and microbes harbor a wealth of adaptations that allow them to circumvent effective plant defense activation. In the past decades, it has become apparent that the enormous diversity and metabolic potential of insect-associated microbes may play a far more important role in shaping insect–plant interactions than previously anticipated. The latter may have implications for the development of sustainable pest control strategies. Therefore, this review sheds light on the current knowledge on multitrophic insect–microbe–plant interactions in a rapidly expanding field of research.

Can Mixed Intercropping Protect Cereals from Aphid-Borne Viruses? An Experimental Approach

Intercropping, i.e., association of two or more species, is promising to reduce insect populations in fields. The cereal aphid Rhopalosiphum padi, a vector of the Barley yellow dwarf virus PAV (BYDV-PAV), represents a major threat for cereal grain production. In this study, we tested the potential of winter barley intercropped with clover to reduce the size of R. padi populations and to lower the BYDV-PAV incidence in fields. We used arenas (i.e., sets of 36 barley plants) intercropped with or without clover plants (at different sown densities). In each arena, a single viruliferous founder, R. padi, (with an alate or a wingless morph) was deposited to introduce aphids and viruses in the experiment. Thirteen days later, the number of aphids in the arena, the percentage of plants hosting aphids and the infection rates were monitored. Data produced through this experimental design showed that clover alters the distribution of the aphid progeny (lower aphid spread) produced by an alate founder morph. Moreover, clover reduces the size of aphid populations produced by a wingless founder morph. However, despite the effects of clover on biological parameters of R. padi, the presence of clover in barley arena did not modify BYDV infections, suggesting complex mechanisms between partners of the BYDV pathosystem for plant-to-plant virus spread.

The Entomopathogenic Fungus Metarhizium anisopliae Affects Feeding Preference of Sogatella furcifera and Its Potential Targets’ Identification

The rice planthopper Sogatella furcifera is a unique vector of the southern rice black-streaked dwarf virus (SRBSDV). The feeding behavior of S. furcifera should directly affect the diffusion of this virus. In this study, we noted that the infection of Metarhizium anisopliae CQMa421 on S. furcifera disturbed the feeding behavior of this pest to SRBSDV-infected rice, from preference to non-preference. Then, we further investigated the potential targets of M. anisopliae CQMa421 on the feeding behavior of S. furcifera after 0 h, 24 h and 48 h of infection by transcriptomic analysis via Illumina deep sequencing. A total of 93.27 GB of data was collected after sequencing, from which 91,125 unigenes were annotated, including 75 newly annotated genes. There were 1380 vs. 2187 and 137 vs. 106 upregulated and downregulated differentially expressed genes (DEGs) detected at 24 h and 48 h, respectively. The biological functions and associated metabolic processes of these genes were determined with the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. The results suggested that major of DEGs are involved in energy metabolism, biosynthesis, immune response, the FoxO signaling pathway, the MAPK signaling pathway and apoptosis in response to the fungal infection. Noteworthily, several olfactory-related genes, including odorant receptors and odorant binding proteins, were screened from these differentially expressed genes, which played critical roles in regulating the olfactory behavior of insects. Taken together, these results provide new insights for understanding the molecular mechanisms underlying fungus and host insect interaction, especially for olfactory behavior regulated by fungus.

Differential gene expression in aphids following virus acquisition from plants or from an artificial medium

BACKGROUND

Poleroviruses, such as turnip yellows virus (TuYV), are plant viruses strictly transmitted by aphids in a persistent and circulative manner. Acquisition of either virus particles or plant material altered by virus infection is expected to induce gene expression deregulation in aphids which may ultimately alter their behavior.

RESULTS

By conducting an RNA-Seq analysis on viruliferous aphids fed either on TuYV-infected plants or on an artificial medium containing purified virus particles, we identified several hundreds of genes deregulated in Myzus persicae, despite non-replication of the virus in the vector. Only a few genes linked to receptor activities and/or vesicular transport were common between the two modes of acquisition with, however, a low level of deregulation. Behavioral studies on aphids after virus acquisition showed that M. persicae locomotion behavior was affected by feeding on TuYV-infected plants, but not by feeding on the artificial medium containing the purified virus particles. Consistent with this, genes potentially involved in aphid behavior were deregulated in aphids fed on infected plants, but not on the artificial medium.

CONCLUSIONS

These data show that TuYV particles acquisition alone is associated with a moderate deregulation of a few genes, while higher gene deregulation is associated with aphid ingestion of phloem from TuYV-infected plants. Our data are also in favor of a major role of infected plant components on aphid behavior.

Phytopathogens Increase the Preference of Insect Vectors to Volatiles Emitted by Healthy Host Plants

Phytopathogen infections not only affect the physiology of host plants but also the preference of insect vectors; these modifications may increase the spread of infection. For this, we determined the effects of "Candidatus Liberibacter asiaticus" (CLas) infection on the preference of an insect vector (Diaphorina citri) for its uninfected or CLas-infected host (Citrus sinensis) and found that the infected vector preferred uninfected citrus, while the uninfected vector preferred infected citrus. We identified two compounds, (Z)-3 hexenyl and methyl salicylate, that were differentially abundant in the volatiles emitted by infected and uninfected citrus and two odorant-binding protein (OBP) genes differentially expressed between infected and uninfected vectors. The results of receptor-ligand binding assays indicated that CLas upregulated OBP A10 expression in the infected vector to target (Z)-3 hexenyl acetate emitted by uninfected citrus and induced citrus to emit more methyl salicylate for binding to OBP2 in the uninfected vector. Our results might be useful for the effective control of CLas infections.

Manipulation of Insect Vectors’ Host Selection Behavior by Barley Yellow Dwarf Virus Is Dependent on the Host Plant Species and Viral Co-Infection

Previous studies have shown that vector-borne viruses can manipulate the host selection behavior of insect vectors, yet the tripartite interactions of pathogens, host plants and insect vectors have been documented only in a limited number of pathosystems. Here, we report that the host selection behavior of the insect vector of barley yellow dwarf virus-PAV (BYDV-PAV) and cereal yellow dwarf virus-RPS (CYDV-RPS) is dependent on the host plant species and viral co-infection. This study shows that a model cereal plant, Brachypodium distachyon, is a suitable host plant for examining tripartite interactions with BYDV-PAV and CYDV-RPS. We reveal that BYDV-PAV has a different effect on the host selection behavior of its insect vector depending on the host plant species. Viruliferous aphids significantly prefer non-infected plants to virus-infected wheat plants, whereas viral infection on a novel host plant, B. distachyon, is not implicated in the attraction of either viruliferous or nonviruliferous aphids. Furthermore, our findings show that multiple virus infections of wheat with BYDV-PAV and CYDV-RPS alter the preference of their vector aphid. This result indicates that BYDV-PAV acquisition alters the insect vector’s host selection, thereby varying the spread of multiple viruses.

Effector-mediated plant-virus-vector interactions

Hemipterans (such as aphids, whiteflies, and leafhoppers) are some of the most devastating insect pests due to the numerous plant pathogens they transmit as vectors, which are primarily viral. Over the past decade, tremendous progress has been made in broadening our understanding of plant-virus-vector interactions, yet on the molecular level, viruses and vectors have typically been studied in isolation of each other until recently. From that work, it is clear that both hemipteran vectors and viruses use effectors to manipulate host physiology and successfully colonize a plant and that co-evolutionary dynamics have resulted in effective host immune responses, as well as diverse mechanisms of counterattack by both challengers. In this review, we focus on advances in effector-mediated plant-virus-vector interactions and the underlying mechanisms. We propose that molecular synergisms in vector-virus interactions occur in cases where both the virus and vector benefit from the interaction (mutualism). To support this view, we show that mutualisms are common in virus-vector interactions and that virus and vector effectors target conserved mechanisms of plant immunity, including plant transcription factors, and plant protein degradation pathways. Finally, we outline ways to identify true effector synergisms in the future and propose future research directions concerning the roles effectors play in plant-virus-vector interactions.

Virus Infection and Host Plant Suitability Affect Feeding Behaviors of Cannabis Aphid (Hemiptera: Aphididae), a Newly Described Vector of Potato Virus Y

Aphids are the most prolific vectors of plant viruses resulting in significant yield losses to crops worldwide. Potato virus Y (PVY) is transmitted in a non-persistent manner by 65 species of aphids. With the increasing acreage of hemp (Cannabis sativa L.) (Rosales: Cannabaceae) in the United States, we were interested to know if the cannabis aphid (Phorodon cannabis Passerini) (Hemiptera: Aphididae) is a potential vector of PVY. Here, we conduct transmission assays and utilize the electrical penetration graph (EPG) technique to determine whether cannabis aphids can transmit PVY to hemp (host) and potato (non-host) (Solanum tuberosum L.) (Solanales: Solanaceace). We show for the first time that the cannabis aphid is an efficient vector of PVY to both hemp (96% transmission rate) and potato (91%) using cohorts of aphids. In contrast, individual aphids transmitted the virus more efficiently to hemp (63%) compared to potato (19%). During the initial 15 min of EPG recordings, aphids demonstrated lower number and time spent performing intracellular punctures on potato compared to hemp, which may in part explain low virus transmission to potato using individual aphids. During the entire 8-hour recording, viruliferous aphids spent less time ingesting phloem compared to non-viruliferous aphids on hemp. This reduced host acceptance could potentially cause viruliferous aphids to disperse thereby increasing virus transmission. Overall, our study shows that cannabis aphid is an efficient vector of PVY, and that virus infection and host plant suitability affect feeding behaviors of the cannabis aphid in ways which may increase virus transmission.

Do Options Matter? Settling Behavior, Stylet Sheath Counts, and Oviposition of Aster Leafhoppers (Hemiptera: Cicadellidae) in Two-Choice Bioassays

Polyphagous insects are characterized by a broad diet comprising plant species from different taxonomic groups. Within these insects, migratory species are of particular interest, given that they encounter unpredictable environments, with abrupt spatial and temporal changes in plant availability and density. Aster leafhoppers (Hemiptera: Cicadellidae: Macrosteles quadrilineatus Forbes) arrive in the Canadian Prairies in spring and early summer and are the main vector of a prokaryotic plant pathogen known as Aster Yellows Phytoplasma (AYp) (Candidatus Phytoplasma asteris). Host choice selection behavior of Aster leafhoppers was evaluated through two-choice bioassays, using domesticated and wild plants species commonly found in the Canadian Prairies. Leaf tissues from these plants were collected and stained to quantify the number of stylet sheaths and eggs. To assess possible effects due to insect infection, two-choice bioassays were repeated using leafhoppers infected with AYp and a subset of plant species. When two domesticated or wild plant species were presented together, similar numbers of uninfected Aster leafhoppers were observed on both plant species in most combinations. In domesticated-wild plant bioassays, uninfected Aster leafhoppers preferred to settle on the domesticated species. There was little to no association between settling preferences and stylet sheath and egg counts. These findings provide a better understanding of AY epidemiology and suggest that after domesticated species germination, leafhoppers could move from nearby wild plants into the preferred cereals (Poales: Poaceae) to settle on them, influencing the risk of AYp infection in some of these species.

Manipulation of Vector Host Preference by Pathogens: Implications for Virus Spread and Disease Management

Some plant pathogens manipulate the behavior and performance of their vectors, potentially enhancing pathogen spread. The implications are evolutionary and epidemiological but also economic for pathogens that cause disease in crops. Here we explore with models the effects of vector manipulation on crop yield loss to disease and on the economic returns for vector suppression. We use two frameworks, one that simulates the proportional occurrence of the pathogen in the vector population with the option to eliminate vectors by a single insecticidal treatment, and one that includes vector population dynamics and the potential for multiple insecticidal sprays in a season to suppress vectors. We parameterize the models with published data on vector manipulation, crop yields as affected by the age of the plant at infection, commodity prices and costs of vector control for three pathosystems. Using the first framework, maximum returns for treating vectors are greater with vector manipulation than without it by approximately US$10 per acre (US$24.7/ha) in peas infected by Pea enation mosaic virus and Bean leaf roll virus, and approximately US$50 per acre (US$124/ha) for potatoes infected by Potato leaf roll virus. Using the second framework, maximum returns for controlling the psyllid vectors of Candidatus Liberibacter solanacearum are 50% greater (approximately US$400/acre, US$988/ha) but additional returns for multiple weekly sprays diminish more with vector manipulation than without it. These results suggest that the economics of vector manipulation can be substantial and provide a framework that can inform management decisions.

Encounter With a Selfish Virus Sabotages Its Vector to Orient Toward Requisite Host Plant: A Case Study With Chili Leaf Curl Virus-Whitefly

Interactions of a virus with its vector and host plant have challenged entomologists, pathologists and biologists alike. Phytophagous insects depend on specific host volatile cues to locate suitable host plants for feeding and oviposition. Several studies have revealed that plant viruses modify their insect vector’s orientation toward specific host plants to facilitate their spread and survival. The ecological and molecular basis of this vector behavior modification remains largely unknown and was therefore explored in this study. Interestingly, host volatile preference for non-viruliferous female whiteflies [Bemisia tabaci (Genn.)] was found to be preferentially oriented toward infected chili plant [with chili leaf curl (ChLCV)] volatiles, while viruliferous whiteflies preferred healthy chili plant (Capsicum annum L.) volatiles in olfactometer. The electrophysiological studies involving electroantennogram (EAG) assays exhibited similar trend in EAG response amplitudes. Gas Chromatography linked electroantennodetection (GC EAD) revealed specific plant volatile cues responsible for altered host orientation behavior of the vector. Transcriptome profiling of the viruliferous and non-viruliferous whiteflies and Realtime qPCR validation showed differential expression of certain odorant binding proteins (OBPs) in viruliferous whiteflies. Our results suggest that there is a plant virus mediated altered chemoecological behavior in the vector with respect to orientation toward its host plant. Based on the findings we speculate that the virus mediates such change in the vector for a continued transmission success to the host.

Interactions of Liberibacter Species with Their Psyllid Vectors: Molecular, Biological and Behavioural Mechanisms

Liberibacter is a group of plant pathogenic bacteria, transmitted by insect vectors, psyllids (Hemiptera: Psylloidea), and has emerged as one of the most devastating pathogens which have penetrated into many parts of the world over the last 20 years. The pathogens are known to cause plant diseases, such as Huanglongbing (citrus greening disease), Zebra chip disease, and carrot yellowing, etc., threatening some very important agricultural sectors, including citrus, potato and others. Candidatus Liberibacter asiaticus (CLas), the causative agent of citrus greening disease, is one of the most important pathogens of this group. This pathogen has infected most of the citrus trees in the US, Brazil and China, causing tremendous decline in citrus productivity, and, consequently, a severely negative impact on economic and personnel associated with citrus and related industries in these countries. Like other members in this group, CLas is transmitted by the Asian citrus psyllid (ACP, Diaphorina citri) in a persistent circulative manner. An additional important member of this group is Ca. L. solanacearum (CLso), which possesses nine haplotypes and infects a variety of crops, depending on the specific haplotype and the insect vector species. Ongoing pathogen control strategies, that are mainly based on use of chemical pesticides, lack the necessary credentials of being technically feasible, and environmentally safe. For this reason, strategies based on interference with Liberibacter vector transmission have been adopted as alternative strategies for the prevention of infection by these pathogens. A significant amount of research has been conducted during the last 10-15 years to understand the aspects of transmission of these bacterial species by their psyllid vectors. These research efforts span biological, ecological, behavioural and molecular aspects of Liberibacter–psyllid interactions, and will be reviewed in this manuscript. These attempts directed towards devising new means of disease control, endeavoured to explore alternative strategies, instead of relying on using chemicals for reducing the vector populations, which is the sole strategy currently employed and which has profound negative effects on human health, beneficial organisms and the environment.

Groundnut Bud Necrosis Virus Modulates the Expression of Innate Immune, Endocytosis, and Cuticle Development-Associated Genes to Circulate and Propagate in Its Vector, Thrips palmi

Thrips palmi (Thysanoptera: Thripidae) is the predominant tospovirus vector in Asia-Pacific region. It transmits economically damaging groundnut bud necrosis virus (GBNV, family Tospoviridae) in a persistent propagative manner. Thrips serve as the alternate host, and virus reservoirs making tospovirus management very challenging. Insecticides and host plant resistance remain ineffective in managing thrips–tospoviruses. Recent genomic approaches have led to understanding the molecular interactions of thrips–tospoviruses and identifying novel genetic targets. However, most of the studies are limited to Frankliniella species and tomato spotted wilt virus (TSWV). Amidst the limited information available on T. palmi–tospovirus relationships, the present study is the first report of the transcriptome-wide response of T. palmi associated with GBNV infection. The differential expression analyses of the triplicate transcriptome of viruliferous vs. nonviruliferous adult T. palmi identified a total of 2,363 (1,383 upregulated and 980 downregulated) significant transcripts. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses showed the abundance of differentially expressed genes (DEGs) involved in innate immune response, endocytosis, cuticle development, and receptor binding and signaling that mediate the virus invasion and multiplication in the vector system. Also, the gene regulatory network (GRN) of most significant DEGs showed the genes like ABC transporter, cytochrome P450, endocuticle structural glycoprotein, gamma-aminobutyric acid (GABA) receptor, heat shock protein 70, larval and pupal cuticle proteins, nephrin, proline-rich protein, sperm-associated antigen, UHRF1-binding protein, serpin, tyrosine–protein kinase receptor, etc., were enriched with higher degrees of interactions. Further, the expression of the candidate genes in response to GBNV infection was validated in reverse transcriptase-quantitative real-time PCR (RT-qPCR). This study leads to an understanding of molecular interactions between T. palmi and GBNV and suggests potential genetic targets for generic pest control.

Resistance Management through Brassica Crop–TuMV–Aphid Interactions: Retrospect and Prospects

Turnip mosaic virus (TuMV) is an important threat to the yield and quality of brassica crops in China, and has brought serious losses to brassica crops in the Far East, including China and the north. Aphids (Hemiptera, Aphidoidea) are the main mediators of TuMV transmission in field production, and not only have strong virus transmission ability (small individuals, strong concealment, and strong fecundity), but are also influenced by the environment, making them difficult to control. Till now, there have been few studies on the resistance to aphids in brassica crops, which depended mainly on pesticide control in agriculture production. However, the control effect was temporarily effective, which also brought environmental pollution, pesticide residues in food products, and destroyed the ecological balance. This study reviews the relationship among brassica crop–TuMV, TuMV–aphid, and brassica crop–aphid interactions, and reveals the influence factors (light, temperature, and CO2 concentration) on brassica crop–TuMV–aphid interactions, summarizing the current research status and main scientific problems about brassica crop–TuMV–aphid interactions. It may provide theoretical guidance for opening up new ways of aphid and TuMV management in brassica crops.

Changes in Visual and Olfactory Cues in Virus-Infected Host Plants Alter the Behavior of Bemisia tabaci

The cucurbit chlorotic yellows virus (CCYV) has caused serious damage to melon crops in many countries in recent years. This plant virus is exclusively transmitted by the whitefly Bemisia tabaci (Gennadius) in a semi-persistent mode. Previous studies have shown that both persistent and non-persistent viruses can affect the orientation and performance of insect vectors, through changing host phenotype or interacting with insect vectors directly to facilitate the spread of viruses. However, how CCYV affects host-plant selection by B. tabaci has not been reported. In this study, we investigated the visual and olfactory preferences of B. tabaci between healthy and CCYV-infected host plants Cucumis sativus (Cucurbitaceae). Volatile profiles of healthy and CCYV-infected C. sativus plants were analyzed using gas chromatography-mass spectrometry (GC-MS). In the choice assay, whiteflies preferred to settle on CCYV-infected C. sativus seedlings. However, the concentrations of total volatiles and terpenes in C. sativus plants decreased after CCYV infection. Interestingly, in the Y-tube assay and vision preference test, whitefly B. tabaci adults showed significant visual preference to CCYV-infected host but showed olfactory preference to healthy plants. These results indicated that CCYV infection in plants differently affected the visual and olfactory-mediated orientation behaviors of vector whiteflies and implied that visual cues could play a more important role than olfactory cues in whiteflies in locating CCYV-infected host plants.

Effect of aphid biology and morphology on plant virus transmission

Aphids severely affect crop production by transmitting many plant viruses. Viruses are obligate intracellular pathogens that mostly depend on vectors for their transmission and survival. A majority of economically important plant viruses are transmitted by aphids. They transmit viruses either persistently (circulative or non-circulative) or non-persistently. Plant virus transmission by insects is a process that has evolved over time and is strongly influenced by insect morphological features and biology. Over the past century, a large body of research has provided detailed knowledge of the molecular processes underlying virus-vector interactions. In this review, we discuss how aphid biology and morphology can affect plant virus transmission. © 2021 Society of Chemical Industry.

Epidemiological and ecological consequences of virus manipulation of host and vector in plant virus transmission

Many plant viruses are transmitted by insect vectors. Transmission can be described as persistent or non-persistent depending on rates of acquisition, retention, and inoculation of virus. Much experimental evidence has accumulated indicating vectors can prefer to settle and/or feed on infected versus noninfected host plants. For persistent transmission, vector preference can also be conditional, depending on the vector’s own infection status. Since viruses can alter host plant quality as a resource for feeding, infection potentially also affects vector population dynamics. Here we use mathematical modelling to develop a theoretical framework addressing the effects of vector preferences for landing, settling and feeding–as well as potential effects of infection on vector population density–on plant virus epidemics. We explore the consequences of preferences that depend on the host (infected or healthy) and vector (viruliferous or nonviruliferous) phenotypes, and how this is affected by the form of transmission, persistent or non-persistent. We show how different components of vector preference have characteristic effects on both the basic reproduction number and the final incidence of disease. We also show how vector preference can induce bistability, in which the virus is able to persist even when it cannot invade from very low densities. Feedbacks between plant infection status, vector population dynamics and virus transmission potentially lead to very complex dynamics, including sustained oscillations. Our work is supported by an interactive interface https://plantdiseasevectorpreference.herokuapp.com/. Our model reiterates the importance of coupling virus infection to vector behaviour, life history and population dynamics to fully understand plant virus epidemics.

Plant virus diseases–which cause devastating epidemics in plant populations worldwide–are most often transmitted by insect vectors. Recent experimental evidence indicates how vectors do not choose between plants at random, but instead can be affected by whether plants are infected (or not). Virus infection can cause plants to “smell” different, because they produce different combinations of volatile chemicals, or “taste” different, due to chemical changes in infected tissues. Vector reproduction rates can also be affected when colonising infected versus uninfected plants. Potential effects on epidemic spread through a population of plants are not yet entirely understood. There are also interactions with the mode of virus transmission. Some viruses can be transmitted after only a brief probe by a vector, whereas others are only picked up after an extended feed on an infected plant. Furthermore there are differences in how long vectors remain able to transmit the virus. This ranges from a matter of minutes, right up to the entire lifetime of the insect, depending on the plant-virus-vector combination under consideration. Here we use mathematical modelling to synthesise all this complexity into a coherent theoretical framework. We illustrate our model via an online interface https://plantdiseasevectorpreference.herokuapp.com/.

Potato leafroll virus reduces Buchnera aphidocola titer and alters vector transcriptome responses

Viruses in the Luteoviridae family, such as Potato leafroll virus (PLRV), are transmitted by aphids in a circulative and nonpropagative mode. This means the virions enter the aphid body through the gut when they feed from infected plants and then the virions circulate through the hemolymph to enter the salivary glands before being released into the saliva. Although these viruses do not replicate in their insect vectors, previous studies have demonstrated viruliferous aphid behavior is altered and the obligate symbiont of aphids, Buchnera aphidocola, may be involved in transmission. Here we provide the transcriptome of green peach aphids (Myzus persicae) carrying PLRV and virus-free control aphids using Illumina sequencing. Over 150 million paired-end reads were obtained through Illumina sequencing, with an average of 19 million reads per library. The comparative analysis identified 134 differentially expressed genes (DEGs) between the M. persicae transcriptomes, including 64 and 70 genes that were up- and down-regulated in aphids carrying PLRV, respectively. Using functional classification in the GO databases, 80 of the DEGs were assigned to 391 functional subcategories at category level 2. The most highly up-regulated genes in aphids carrying PLRV were cytochrome p450s, genes related to cuticle production, and genes related to development, while genes related to heat shock proteins, histones, and histone modification were the most down-regulated. PLRV aphids had reduced Buchnera titer and lower abundance of several Buchnera transcripts related to stress responses and metabolism. These results suggest carrying PLRV may reduce both aphid and Buchnera genes in response to stress. This work provides valuable basis for further investigation into the complicated mechanisms of circulative and nonpropagative transmission.

Fungal phytopathogen modulates plant and insect responses to promote its dissemination

Vector-borne plant pathogens often change host traits to manipulate vector behavior in a way that favors their spread. By contrast, infection by opportunistic fungi does not depend on vectors, although damage caused by an herbivore may facilitate infection. Manipulation of hosts and vectors, such as insect herbivores, has not been demonstrated in interactions with fungal pathogens. Herein, we establish a new paradigm for the plant-insect-fungus association in sugarcane. It has long been assumed that Fusarium verticillioides is an opportunistic fungus, where it takes advantage of the openings left by Diatraea saccharalis caterpillar attack to infect the plant. In this work, we show that volatile emissions from F. verticillioides attract D. saccharalis caterpillars. Once they become adults, the fungus is transmitted vertically to their offspring, which continues the cycle by inoculating the fungus into healthy plants. Females not carrying the fungus prefer to lay their eggs on fungus-infected plants than mock plants, while females carrying the fungus prefer to lay their eggs on mock plants than fungus-infected plants. Even though the fungus impacts D. saccharalis sex behavior, larval weight and reproduction rate, most individuals complete their development. Our data demonstrate that the fungus manipulates both the host plant and insect herbivore across life cycle to promote its infection and dissemination.

Effect of Sugarcane Cultivars Infected with Sugarcane Yellow Leaf Virus (ScYLV) on Feeding Behavior and Biological Performance of Melanaphis sacchari (Hemiptera: Aphididae)

Sugarcane yellow leaf virus (ScYLV), Polerovirus, Luteoviridae, is one of the main viruses that infect sugarcane worldwide. The virus is transmitted by the aphid Melanaphis sacchari in a persistent, circulative manner. To better understand the interactions between ScYLV, sugarcane genotypes and M. sacchari, we explored the effect of sugarcane cultivars on the feeding behavior and biological performance of the vector. The number of nymphs, adults, winged, total number of aphids and dead aphids was assayed, and an electrical penetration graph (EPG) was used to monitor the stylet activities. Multivariate analysis showed changes in the vector’s behavior and biology on cultivars, identifying specific groups of resistance. In the cultivar 7569, only 5.5% of the insects were able to stay longer on sustained phloem ingestion, while in the other seven cultivars these values varied from 20% to 60%. M. sacchari showed low phloem activities in cultivars 7569 and Bio266. Overall, cultivar 7569 showed the worst biological performance of aphids, with the insects presenting mechanical difficulties for feeding and a shorter duration of the phloem period, and thus being considered the most resistant. We conclude that ScYLV virus infection in different sugarcane cultivars induced specific changes in the host plant, modifying the behavior of its main vector, which may favor or impair virus transmission.

Varroa destructor from the Laboratory to the Field: Control, Biocontrol and IPM Perspectives-A Review

Varroa destructor is a real challenger for beekeepers and scientists: fragile out of the hive, tenacious inside a bee colony. From all the research done on the topic, we have learned that a better understanding of this organism in its relationship with the bee but also for itself is necessary. Its biology relies mostly on semiochemicals for reproduction, nutrition, or orientation. Many treatments have been developed over the years based on hard or soft acaricides or even on biocontrol techniques. To date, no real sustainable solution exists to reduce the pressure of the mite without creating resistances or harming honeybees. Consequently, the development of alternative disruptive tools against the parasitic life cycle remains open. It requires the combination of both laboratory and field results through a holistic approach based on health biomarkers. Here, we advocate for a more integrative vision of V. destructor research, where in vitro and field studies are more systematically compared and compiled. Therefore, after a brief state-of-the-art about the mite's life cycle, we discuss what has been done and what can be done from the laboratory to the field against V. destructor through an integrative approach.

Virus-induced plant volatiles mediate the olfactory behaviour of its insect vectors

Plant viruses can manipulate their hosts to release odours that are attractive or repellent to their insect vectors. However, the volatile organic compounds (VOCs), either individually or as mixtures, which play a key role in the olfactory behaviour of insect vectors remains largely unknown. Our study focused on green rice leafhoppers (GRLHs) vectoring rice dwarf virus (RDV) revealed that RDV infection significantly induced the emission of (E)-β-caryophyllene and 2-heptanol by rice plants, which influenced the olfactory behaviour of both non-viruliferous and viruliferous GRLHs. (E)-β-caryophyllene attracted non-viruliferous GRLHs to settle on RDV-infected plants, but neither attracted nor repelled viruliferous GRLHs. In contrast, 2-heptanol repelled viruliferous GRLHs to settle on RDV-infected plants, but neither repelled nor attracted non-viruliferous GRLHs. Suppression of (E)-β-caryophyllene synthase OsCAS via CRISPR-Cas9 to generate oscas-1 plants enabled us to confirm the important role played by (E)-β-caryophyllene in modulating the virus-vector-host plant interaction. These novel results reveal the role of these virus-induced VOCs in modulating the behaviour of its GRLH insect vector and may facilitate the design of new strategies for disease control through manipulation of plant volatile emissions.