EFSA 4th European Conference on Xylella – Report part 3

EFSA 4th European Conference on Xylella fastidiosa - Report part 3

In August 2023, the European Food Safety Authority (EFSA) held its 4th European Conference on Xylella fastidiosa in Lyon, France as a satellite event to the ICPP 2023. Experts from the BIOVEXO Project were key participants, alongside experts from the BeXyl projectERC MultiX project, and the Euphresco network, as well as experts from a range of research institutions, and national and international bodies. This blog is the third in a series intended to provide an in-depth summary of the conference.
In August 2023, the European Food Safety Authority (EFSA) held its 4th European Conference on Xylella fastidiosa in Lyon, France as a satellite event to the ICPP 2023. Experts from the BIOVEXO Project were key participants, alongside experts from the BeXyl projectERC MultiX project, and the Euphresco network, as well as experts from a range of research institutions, and national and international bodies. This blog is the third in a series intended to provide an in-depth summary of the conference.

After session 1, on the Current status and research updates on Xylella fastidiosa in the EU, and session 2, on the latest findings on Xylella fastidiosa resistance and control, session 3 looked at Xylella fastidiosa vectors biology.

Session 3 consisted of 6 presentations (click on each below to skip to the summary), followed by an open Q&A discussion:

Nicola Bodino (IPSP-CNR) introduced his talk on the reproductive biology and egg parasitoids of Philaenus spumarius (a.k.a. the spittlebug) by outlining the potential such research holds for helping in spittlebug population control. Spittlebugs have a year-long lifecycle characterised by a long adult life (which includes an extended period of sexual dormancy which is ended by environmental triggers, meaning all female spittlebugs lay eggs at the same time) and a period of over-winter dormancy (a.k.a. diapause) in the egg stage. There is some evidence of an association between Philaenus spumarius and the bacterial parasite Wolbachia, especially in colder climates, though the nature of this relationship requires further research. To date only one egg parasitoid of Philaenus spumarius has been identified in Europe and this is another potential line for further research.

Nicola Bodino explained work they have done to:

i) Identify molecular markers that may predict when eggs-laying will occur;

ii) Understand the relationship between Philaenus spumarius and Wolbachia; and,

iii) Investigate communities of egg parasitoids in northern Italy.

In the first investigation, correlation was observed between levels of Vitellogenin and the number of eggs in spittlebugs’ ovaries. Furthermore, the expression of Vitellogenin was found to occur earlier at higher altitudes or latitudes (i.e. colder climates). The second investigation confirmed higher levels of Wolbachia in spittlebugs in these colder climates, suggesting that the extended sexual dormancy in warmer climates inhibits the ability of Wolbachia to be transmitted from parent to offspring. In the third investigation, spittlebug eggs were exposed at different sites at varying times of winter. The subsequent emergence of parasitoids was observed. Only a single species was observed: Ooctonus vulgatus.

Nicola Bodino concluded that the knowledge gained could assist in the design of Integrated Pest Management (IPM) strategies and the disruption of oviposition/egg development.

Júlia López-Mercadal (UIB) explained how her doctoral research built upon existing understandings of how various factors can influence the populations of known vectors of Xylella fastidiosa. In the Balearic islands the known vectors are spittlebugs (specifically, two species: Philaenus spumaris, and Neophilaenus campestris). They extracted data from existing research to try and uncover the key ecological drivers behind density of vector populations. They were able to see that both species of spittlebugs were equally likely to be infected with Xylella bacteria, and equally efficient at transmitting it; however, there was a degree of variance in between the two species of spittlebugs in terms of their presence and abundance at different times of year and in preference for different crops.  

João Lopes (ESALQ) presented research into Cercopoidea Clastopteridade, a different family of spittlebug, which is suspected of being a vector for Xylella in olives in Brazil. Xylella is a causing significant problems in olive cultivation in Brazil, due at least partially to the large number of vectors (which, in addition to spittlebugs, also include sharpshooters). In previous studies, including different methods of insect capturing, almost 100 subspecies of xylem-feeding insect were detected in the olive orchards. However, only one was found to colonize the olive trees (i.e. to live in the olive tree throughout all its developmental stages, from first instar to adult): a newly discovered subspecies of Cercopoidea Clastopteridade, which they have, for the moment, named Clastoptera sp.1.

João Lopes explained that this new species feeds upon numerous host plants. It has not yet been confirmed whether or not these other plants also are Xylella hosts, but it is likely. As Clastoptera sp.1 is prevalent in Brazilian olive orchards, is known to colonize the olive trees, and is a natural host of Xylella, it is likely to be one of the key factors in the Xylella outbreaks. As such, there is work to be done, both in terms of taxonomy and assessing Clastoptera sp.1‘s ability to transmit Xylella.

Lindsey Burbank (USDA) discussed the work of their team on Xylella in grape (most commonly subspecies fastidiosa) and in almond (also subspecies multiplex) in California. They noted that subspecies pauca is not currently found in North America, though the olive industry in California is concerned about what may happen should pauca or another olive-pathogenic strain of Xylella spread there due to the large number of vectors known to exist in the region, for example, various species of sharpshooter.

Their work has focussed on analysing the risk of introduction of an olive-pathogenic strain of Xylella into California. Their first objective was evaluating whether olive cultivars commonly grown in California are susceptible to European Xylella fastidiosa strain DeDonno. Their second objective was to investigate the potential of the Glassy-winged sharpshooter to act as a vector for Xylella fastidiosa in olive.

Production in California is mainly for table olives, though olive oil production has risen in the last decade. Over 99% of olives grown for table olives in California are one of two cultivars, Manzanillo and Sevillano, with the former by far the most common. This lack of diversity makes it particularly important to understand the susceptibility to Xylella. Lindsey Burbank explained that she and her colleagues tested three different cultivars, Mission, Manzanillo, and Sevillano, and found them all to be susceptible to Xylella fastidiosa DeDonno.

The Glassy-winged sharpshooter, itself an invasive species in California, is already established in significant parts of at least one of the major olive producing counties in California and others are at risk. Thus, it is again important to understand its suitability as a vector. Insects were caged on inoculated specimens of each of the cultivars, Mission, Manzanillo, and Sevillano and tested for Xylella acquisition after 3 days. These tests were repeated at various intervals post-inoculation of the plant. Whilst acquisition rates were low (the median rates were between 1 and 6 percent, depending on the cultivar), it was confirmed that all cultivars could infect the sharpshooters with Xylella.

Alberto Fereres (ICA-CSIC), a member of the BeXyl project consortium, reminded the audience of the situation with Xylella in Spain, where it mainly affects almonds, and reviewed the lifecycle of Philaenus spumarius. He noted that certain questions related to the lifecycle of the spittlebug remain unanswered, in particular:

i) When can spittlebugs’ eggs hatch?

ii) What environmental factors trigger, or are required for, the hatching of eggs?

iii) What factors influence the development of the spittlebug from egg stage to first nymph stage?

Answering these questions is important to allow scientists and farmers to predict when eggs will hatch in different locations and enable them to take actions that may help control spittlebug populations as part of an integrated pest management plan.

Spittlebug nymphs (3rd instar) ©CNR IPSP & V. Cavalieri

Alberto Fereres explained that the goal of their research was the development of a decision support tool to predict the optimal times for interventions to manage the spittlebugs’ population as the nymphal stage is when the vector is most vulnerable. They collected data from various sites across Spain on oviposition (egg-laying) dates and egg hatching dates, factoring in altitude, and used this to construct a model, which they calibrated depending on temperature. Their model was subsequently validated in systematic field surveys over a 5-year time period (2016-2021). It was found that they could predict the hatching dates in Spain and that with two treatments (with timings based on temperatures), the majority of the spittlebug population could be controlled. The hatching prediction and guide for treatment tool is available publicly (though it is restricted to Spain).

He concluded that, for effective nymphal control, it is essential to understand the spittlebug’s diapause. Temperature is a key factor in determining egg hatching times. Their decision support tool is able to predict the best times to apply treatments at a given site in Spain in order to maximise nymph mortality. The finished by pointing to a recently published paper in Environmental Entomology which provides further details of the work.

The final presentation of the session was made by the session’s co-chair, Astrid Cruaud (INRAE), who began by reminding the audience of the fact that monitoring for Xylella is usually done by testing of symptomatic plants, and the drawbacks of this approach (specifically, that the bacteria is not evenly distributed throughout the plant, leading to false negatives; and, plants are often asymptomatic, leading to them not being tested). Thus, their research focusses on using insect vectors as sentinels to complement plant surveillance. The wide distribution of Philnaenus spumarius in Europe makes it an ideal candidate for such are role.

Astrid Cruaud then detailed their study in Corsica, France, in which Xylella was found to be far more widely present than previously thought. Having established the efficacy of using of Philnaenus spumarius as sentinels, she explained that they then set out to try to understand the potential effects of climate change on the epidemiology of generalist vector-borne diseases, describing their subsequent 4-year investigation which showed “a positive correlation between the proportion of vectors positive to X. fastidiosa and temperature” (Farigoule et al. 2022). This suggests that climate change will make the problem of Xylella more acute. The relationship between temperature and transmission efficacy from vectors to host plants remains one of a number of areas for further research. She also noted that other vectors besides Philaenus spumarius remain under researched.

Isabel Rodrigues from the XVectors.pt project explains her project to BIOVEXO experts Stéphane Compant and Pasquale Saldarelli ©RTDS Group

In addition to the presentations in this session, a number of posters in the poster exhibition focussed on vectors. One project which is seeking to address the lack of knowledge on vectors other than Philaenus spumarius is the Xvectors project, which recently began in Portugal. This is an EFSA-funded project which is investigating the biology, diversity, and abundance of insect vectors and prospective vectors in various agrosystems, including urban and semi-natural settings, cork oak woods, and olive groves, in various agroclimatic conditions throughout Portugal.

Another poster described work being done in Switzerland to understand the phenology and ecology of Philaenus spumarius in the climate of the southern Alps. The project aims to: identify the developmental periods, the host plants, and their specificity for the nymphs; provide estimates of the population density in different agroecosystems; and, test different management strategies for Philaenus spumarius.

Yet another poster detailed the results of surveys undertaken over a three year period with the goal of monitoring for the presence of Xylella and better understanding potential vector distribution and populations in Montenegro.

The session closed with a Q&A session, which can be viewed in the video of session 3.

Slides from the presentations are available on the #xylella23 Conference website.

We hope you found this summary of the Latest findings on Xylella fastidiosa resistance and control interesting and informative. Our summary of the final session of the 4th European Conference on Xylella fastidiosa, Tools for Xylella fastidiosa surveillance and detection, will be published soon.

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