Philaenus spumarius - The most polyphagous insect herbivore?
You may have seen BIOVEXO’s Blog on Philaenus spumarius a.k.a. the Meadow Spittlebug. In which case, you’ll already know about these little creatures’ lifecycle; their athleticism, endurance, and adaptability; and their ability to transmit the deadly Xylella fastidiosa bacteria to olive and almond trees. You’ll also know that, because they weren’t considered to be a pest until the arrival of the Xylella bacteria, there had been relatively little research into spittlebugs prior to 2013. Since then, spittlebugs have been a hot-topic for researchers and the BIOVEXO Project is pleased to have had the opportunity to interview Vinton Thompson, Claire Harkin, and Alan J. A. Stewart, the authors of a recent paper, The most polyphagous insect herbivore? Host plant associations of the Meadow spittlebug, Philaenus spumarius (L.). The paper both reveals some fascinating new insights into the feeding habits of Philaenus spumarius and offers some great recommendations for methodology for future investigations, so we felt it was worth taking a deep dive! Read on to find out more!
The Background:
The article is at least partially based on the work of the BRIGIT Project, which was triggered by the United Kingdom’s concern that it needed to have a contingency plan in case Xylella was detected in the UK: trying to work out ways to make sure Xylella wouldn’t arrive through any of the potential pathways, and if it did arrive, what would be done about it and would the relevant bodies be ready for it.
The project was based upon citizen science and arose out of a pre-existing recording scheme run by Alan Stewart which allowed members of the public to help collect ecological data by submitting their sightings of various insects, including potential vectors of Xylella, in the UK. The recording scheme already had large amounts of data on the distribution of spittlebugs in the UK.
A key point that the BRIGIT Project was concerned with, was not just the geographical location in which the insects were found, but also the plant species upon which they were found and whether there was evidence of feeding on that plant. The BRIGIT project was extremely successful, collecting more data on British spittlebugs in just 10 weeks than had been recorded in the previous 30-40 years!
Employing citizen science:
It was already known that the spittlebug is highly polyphagous, that is to say that they feed on a large range of plant species – but the headline result of the article was that:
“at 1311 species P. spumarius has far more documented host plants than any other herbivorous insect […]. They include ferns, herbs, shrubs, vines and trees, annuals and perennials, grasses and forbs, plants of the tropics, subtropics, temperate and boreal zones, conifers – just about every imaginable kind of vascular plant except those living submerged in aquatic environments.”
Citizen science turned out to be a really effective way of collecting information, though the authors were careful to point out that they needed to be careful to apply strict criteria to ensure only reliable reports were included. Alan Stewart explained that one reason the list of host plants for spittlebugs nymphs is so much longer than the list for adults is due to this caution: sightings of adults were discounted if there was no firm evidence that they were feeding on the plant upon which they were recorded. Adults don’t produce spittle. That is produced only by the nymphs. Providing evidence of feeding by adults is much harder:
“Adults jump and fly around, so you can’t be completely certain that when you see one sitting on a leaf or a twig that it’s necessarily feeding on it: it might have just jumped off from somewhere else.”
(Alan Stewart, interview)
Vinton Thompson did note that it is likely that, as adults are actually more versatile in their ability to access the xylem stream and plants in general, the number of host plants for adults is an underestimate. Claire Harkin added that one of the biggest challenges of citizen science was the complexity of the message:
“[We] were talking about a plant disease which isn’t currently in the country, that could be vectored by an insect that is in the country, but currently isn’t causing any problem. That was a very complicated message to get across so we had quite a lot of alarming headlines in some of our tabloid media. […] So we had inquiries like ‘should I spray my entire garden with pesticides?’” (Claire Harkin, interview)
(Claire Harkin, interview)
This echoes some of the difficulties encountered in regions, for example those documented in the film The Era of Giants, which charts the political, social, and personal impacts of the devastating discovery of Xylella in Apulia, Italy.
Furthermore, Claire Harkin noted the need to simplify the recording process to enable members of the public to submit their sightings easily – it was swiftly realised that, for many, the initial form was too complex, so a simplified SurveyMonkey form was designed.
Are Xylella and the Spittlebug a perfect storm?
The European Food and Safety Authority (EFSA) have their host plant database, which has confirmed over 500 host species of plants for Xylella. With so many plants now verified as hosts for Xylella’s main vector is this the perfect storm? Is Xylella unstoppable?
Alan Stewart responded that some people think Xylella’s spread to the UK is inevitable – it will come eventually, pointing out that different strains of Xylella have different temperature tolerances, so if one did spread to the UK, it might be suited to the climate. He added that the UK continues to import huge quantities of plant material from Mediterranean countries. But on the brighter side, if you look at Italy, where it was first detected, Xylella is not spreading very fast at the moment.
Vinton Thompson provided the perspective in the United States, which is quite different: Xylella is native to the Americas and, triggered particularly by the introduction of a sharpshooter Leaf Hopper from the Southeastern US to California in the 1990s, has mainly been an issue for grapes. Whilst the situation is being managed in Southern California, it requires constant vigilance. In Northern California the situation is different again, with a potential different set of vectors, the sharpshooter leafhoppers, but also a climate in which so-called ‘Winter cure’ is possible. He pointed out that climate change, as well as the spread of different strains of Xylella, will affect this, both in the US and UK, as well as other parts of Europe and the world.
Some suggestions for future studies
Much previous work on spittlebugs had focussed on their geographic range. Where the plant upon which individuals were found was recorded, but it was not always stated what stage the specimen was in (i.e. nymphal, or adult), nor whether evidence of feeding was present. In addition to recording this information, the paper by Thompson, Harkin, & Stewart points to a paper by Trotta et al. which also made the effort to record plants upon which specimens were not found.
Vinton Thompson praised this effort and called for further studies employing this methodology, noting the value of understanding which plants the Spittlebug avoids for population suppression efforts:
“Spittlebugs are clearly making some choices, and it appears there are a couple of kinds of things that they are either never on or rarely on. If you could find some of those that were adaptable to agricultural situations, that might be quite useful and in controlling the populations of the vector.”
(Vinton Thompson, interview)
Thanks
We hope you have enjoyed the last BIOVEXO Project Blog post for 2023. It was a pleasure to speak to Vinton Thompson, Claire Harkin, and Alan J. A. Stewart. The BIOVEXO Project thanks them for generously giving their time to talk to us and highly recommend their paper to anybody interested in Spittlebugs or Xylella.
The BIOVEXO Project wishes all of its followers all the best for 2024!
Vinton Thompson
Vinton Thompson has studied spittlebugs, including Philaenus spumarius, since the early 1970’s, concentrating on their evolutionary and agricultural ecology, with an emphasis on host plant relationships. He holds a PhD in Evolutionary Biology from the University of Chicago and is presently affiliated with the American Museum of Natural History in New York City. He is President Emeritus of Metropolitan College of New York.
Claire Harkin
Claire Harkin’s research interests are focussed on the ecological impacts of plant-insect interactions, particularly those associated with invasive species and their interactions with recipient communities. She worked as a Postdoctoral Research Fellow at the University of Sussex as part of the BRIGIT consortium, investigating the ecology, phenology, distribution and host plant preferences of spittlebug species that may act as vectors for the plant bacterium Xylella fastidiosa should it arrive in the UK. She currently works as Technical & Training Officer for the Ancient Tree Forum.
Alan J. A. Stewart
Alan Stewart is a professor of ecology at the University of Sussex, UK. His primary research focus is on insect ecology, with a specialist interest in the plant sap-sucking bugs that are collectively known as Auchenorrhyncha and include the spittlebugs. In the early part of his career, he worked for five years on the ecology and population genetics of the Meadow spittlebug, Philaenus spumarius. This was long before this species emerged as the primary vector of Xylella fastidiosa in Europe, putting him in a good position to join the BRIGIT project in 2019, working on documenting the spittlebug’s geographical distribution, host plants and general ecology in the UK.
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