May's Most (Un)Wanted - The Spittlebug

Coming Soon to a Tree Near You?

This BIOVEXO blog post will look at the life of a spittlebug – the high-jumping, far-flying, Xylella-spreading bug worrying farmers across Europe. You will be amazed at the way it breeds, adapts, and even how it feeds. You may be curious as to why it got its name – you’re about to find out! After reading this, you’ll be sure to respect this little bug for its athleticism, endurance, and adaptability. You’ll see how they change as they grow and aren’t at all fussy about what they eat. You’ll also see how this threatens the olives and almonds we love to eat too, which is putting the spittlebug on the hit list. Here is the story of Philaenus spumarius.

What do Spittlebugs Have to do With Xylella Fastidiosa?

The Xylella fastidiosa bacteria cannot spread without an organism to carry it from one plant to another (known as a vector). Controlling the populations of the vector is a key part of any strategy to combat the problems Xylella is causing. To control them, you first have to know them: where they live, how they live, and what they eat.

The number of insects that can transmit Xylella is limited – the bacteria can only be spread by insects that are specialised for feeding exclusively on xylem-sap, a trait shared by three types of insects of the order Hemiptera, or ‘true bugs’: Cicadellidae Cicadellinae (a.k.a. sharpshooters), Cicadoidea (a.k.a. cicadas), and Cercopoidea (a.k.a. froghoppers or spittlebugs).

It is one of the last group that has been identified as the primary transmitter of Xylella fastidiosa in Europe: Philaenus spumarius (a.k.a. the meadow spittlebug).

As, depending on the region/climate, spittlebugs reach the adult phase of their lifecycle (the stage in which they pose most threat of transmitting Xylella) between May and June, now is the best time to learn a little more about them and approaches to preventing them from spreading Xylella!

Why is BIOVEXO Targeting Spittlebugs?

Spittlebugs are not usually considered a pest and, as such, are not normally routinely controlled. This means, in suitable regions, its populations are typically large in number.

Spittlebugs can be found in many regions across the world, not just Europe. They thrive in temperate, humid, or moist conditions. Thus, the climate of much of Western Europe and the Mediterranean basin is predicted by models to be highly suitable for spittlebugs and, thus, at potential risk of Xylella.

However, although spittlebugs are one of the most widespread xylem-sap feeders in Europe, they weren’t considered to be a pest until the arrival of the Xylella bacteria in Europe. That is why there has been relatively little research into their behaviour prior to 2013.

As it has been estimated that Xylella could cause economic losses of €5.5 billion per year, preventing its spread is of high importance. One of the ways the BIOVEXO project is seeking to protect crops in Europe from Xylella fastidiosa is through controlling the disease vectors. BIOVEXO is seeking to develop a vector-targeting biopesticide (V-biopesticide) which will target the spittlebug.

In this blog, we will attempt to provide an overview of what is now known about the spittlebug.

The Circle of Life (of a Spittlebug)

“In Mediterranean scrublands, P. spumarius adults emerge during spring and feed on herbaceous ground vegetation. As temperature increases during the season, ground vegetation cover dries, forcing insects to continuously disperse and migrate, looking for succulent woody hosts. In autumn, after the first rains, the ground vegetation starts to grow, and spittlebugs leave their over-summering hosts and return for oviposition.”

Spittlebugs have three life stages: egg, nymph, and adult. Depending on the environment, spittlebugs reach maturity in mid-April to late-May. Around the same time of year, in the Mediterranean region, the herbaceous plants upon which immature spittlebugs feed are dying back. Thus, the new adults seek out larger shrubs and trees, which, with their deeper and larger root systems, are less troubled by summer heat and lack of rainfall.

Spittlebugs, with a host plant list that exceeds 500 different plant species, aren’t fussy eaters. Like all of us, they do have favourite foods: they especially like the olive trees in the Mediterranean region. But they also feed on a wide variety of other species, including almond and oleander. So, as the herbaceous plats die back, the adult spittlebugs head to the trees, where they spend the summer feeding on the trees’ tender shoots.

Depending on the region, the spittlebugs will begin to leave the trees again from September onwards, returning to the ground cover to mate and lay eggs.

In the Mediterranean region, female spittlebugs typically begin to lay eggs at the end of October or early-November, when temperatures drop. They lay their eggs in rows on plant residue (e.g. dead plants, fallen leaves etc.) or on inconspicuous parts of the host plant (e.g. on stems or underneath leaves).

When the eggs hatch, around late February, the juvenile spittlebugs (called nymphs) emerge to look for food. The nymphs mature through five stages (knowns as instars), shedding their skins each time. As they grow, the nymphs change colour, from yellow to green. This means they are well camouflaged but, they can often be found by the tell-tale froth, known as ‘cuckoo spit’ (hence the name ‘spittlebugs’), they produce.

The foam helps prevent the nymphs from drying out and, additionally, it serves as protection from insect predators. These bubbly masses can be up to 2cm across and are made out of the xylem-sap the nymphs feed upon: when they excrete the xylem-sap, they use their breathing tubes to blow air bubbles into it. More than one nymph may share one mass of froth.

Life cycle of Philaenus spumarius in the Apulia region in Italy. Newly emerged adults (green insect) acquire Xylella fastidiosa when feeding on an infected host (red tree or yellow grass). The infected vector can then transmit the bacterium to healthy hosts (green), which will then in turn become infected. Eggs do not carry the vector since transovarial transmission is not possible. Nymphs can also become infected although lose infectivity when moulting (Graphic designed and developed by Infoxylella®, revised by D. Bosco, V. Cavalieri and E. Dongiovanni - Photographs by A. Coti and V. Cavalieri) – EFSA 2019.

Based on understanding of this yearly cycle, and the spittlebugs’ different developmental stages, it is possible to implement various methods to try to interrupt the spittlebug’s lifecycle and, thus, inhibit the spread of Xylella.

Like the adult spittlebugs, spittlebug nymphs feed on a wide range of plants, but they stay preferentially on a limited range of hosts, such as weeds belonging to the group of Leguminosae (e.g. clover, broad bean), Apiaceae (e.g. carrots), and Asteraceae (e.g. thistles, daisies).

As these weeds are relatively easy to eliminate by tilling the soil, this is one factor making the targeting of nymphs easier than adults. Another factor making nymphs easier to target than adults is their relative lack of mobility.

Adult Spittlebugs Are Top Jumpers!

As an adult, the modest, six-millimetre-long bug can spring up to 70 centimetres into the air. Although fleas can jump to a similar height, the spittlebugs’ leaps are particularly impressive as the adult spittlebug is over 50 times heavier than a flea!

Their remarkable athleticism is due to the spittlebugs’ highly-specialized rear legs: these hind legs are so specifically adapted to the job of jumping that the spittlebug doesn’t even use them when walking normally – simply dragging them along behind as it goes. These uniquely adapted legs, which are 1.5 times the length of its other legs, give the adult spittlebug the ability to leap from plant to plant.

Essentially, the spittlebug’s legs act as a catapult system: when it needs to jump, the spittlebug contracts two large muscles (one to power each leg), which it controls via a system of ridges on its legs. Once sufficient force has built up, the spittlebug releases the muscles and springs forward. The spittlebug then accelerates in a millisecond, reaching an incredible take-off velocity of four metres per second.

This means that, when it jumps, the spittlebug experiences a G-force of up to 400 gravities! To put that into perspective, during a rocket launch, an astronaut heading into space may experience around 5 gravities and fighter pilots, wearing special anti-G suits, may deal with G-forces of 8 or 9 G’s.

In an interview with the BBC, Professor Malcolm Burrows, Head of Zoology at Cambridge University, who documented his findings on the spittlebug’s jumping genius as part of his research, explains: “The legitimate comparison is to look at how much force per body weight each animal can generate […] [a spittlebugs] can exert more than 400 times its body weight; a flea can do 135 times its body weight; a grasshopper can do about eight times; and we can do about two to three times our body weight.”

However, spittlebugs do not only move around by jumping – like many insects, spittlebugs can also fly! Research has found that, especially in Spring and Autumn, spittlebugs are capable of flying distances of up to 5.5km. Indeed, as this research was done in a lab, it is possible that, with for example the assistance of wind, spittlebugs could be capable of flying even further.

Spittlebugs' Feeding Habits

“the most fundamental characteristic of vector transmission – especially from a disease control perspective – is efficiency or competence, i.e. how often a vector transmits a pathogen over time or per transmission opportunity”

Basically, the spittlebug feeds by sucking the ‘juice’ out of plants. To help better understand how Xylella is transmitted, a recent study has analysed the precise feeding behaviour of spittlebugs. In that study, the researchers used a technology called an Electrical Penetration Graph (EPG), in conjunction with video recording, to identify different stages of the feeding process.

Then the scientists investigated precisely what was going on inside the plant at different phases of feeding by using CT imaging. In a subsequent paper, the researchers were able to identify unique behaviour in Xylella-infected spittlebugs which they speculated related to the mechanism through which the bacteria was inoculated into the plant.

In order to feed, the spittlebug secretes a small amount of saliva onto the surface of the plant and then inserts its needle-like mouth parts (known as stylets) into the plant until it reaches the xylem tissue. It then begins probing until it reaches the xylem tissue (to learn more about the different types of plant tissue, click here). As it probes, the spittlebug will ‘taste’ to see whether it has reached xylem tissue. It does this using a complex set of muscles, called the cibarium, in its head to regulate the flow of Xylem-sap and transfer the sap into its digestive tract.

As it feeds on an infected plant, the spittlebug swallows most of the Xylella bacteria cells it sucks up; however, some bacteria cells can bind to the spittlebug’s foregut (i.e. the parts of its digestive tract before the gut – in humans this would include the esophagus (throat) and stomach), where the bacteria can multiply, reaching an average of a few thousand cells after 2–3 wks. It is these cells that can then infect new plants. As soon as the spittlebug first carries these cells in its foregut, it is potentially infectious to other plants – there is no incubation period.

The number of probes performed is correlated with efficiency of Xylella transmission. Thus, researchers have concluded that it is likely that the transmission of the Xylella bacteria occurs during the probing phase of feeding. After infection, whenever it probes a new plant, xylem-sap can circulate in the spittlebug’s mouth parts and dislodge the bacteria in its foregut. If the spittlebug then egests (i.e. spits it out, rather than swallowing it) this material, it can spit the bacteria into the plant.

However, as Xylella does not uniformly distribute within a plant (e.g. it may only be found in some parts of a big olive tree), it appears that the likelihood of a spittlebug becoming infectious when it feeds, even on an infected plant, is relatively relatively low. It also appears that, even when they do carry the Xylella bacteria their foreguts, spittlebugs do not always pass it on when they feed. It appears that infected spittlebugs only pass the bacteria on in a small percentage of feeding sessions on olive trees. Although this rate may be affected by season and other environmental factors, and the rate varies between different host plant, it is higher, for example, in oleander. The problem is that:

Even if transmission efficiency is low, spittlebug populations are large;
Olive trees are their primary host in many regions; and,
Regions such as Apulia in Italy have huge concentrations of olive trees.

“P. spumarius carrying X. fastidiosa showed evident difficulties in performing sustained xylem ingestion (ingestion longer than ten minutes), with fewer events compared to healthy insects.”

Xylem-sap is not rich in nutrients, so the spittlebug must ingest a relatively large amount each time it feeds. The spittlebug has to work hard to suck the xylem-sap out of the plant, using its cibarium muscles. Due to the high level of effort involved, the spittlebug will sometimes pause to rest during feeding. Infection with Xylella appears not to be harmless for the spittlebugs, with some evidence that infected insects find feeding more difficult.

Tactics to Combat Spittlebugs

Understanding more about the spittlebugs’ lifecycle, mobility, and feeding habits will enable better planning of efforts to stop the spread of Xylella fastidiosa. There are different tactics available to farmers, targeting different stages of the spittlebug’s lifecycle or behaviour in different ways. These different approaches are not mutually exclusive and can potentially be combined in different constellations depending on the context.

“Effective control of the meadow spittlebug populations could result in a significant reduction in the risk of X. fastidiosa spread.”

Another way to reduce the numbers of spittlebugs is the use of pesticides, such as the V-pesticide the BIOVEXO project is developing. As previously explained, there are a number of practical reasons to target spittlebugs in their nymphal stages. Additionally, targeting the immature spittlebugs with pesticides could be more effective at reducing the spread of Xylella as adult spittlebugs are potentially already transmitting the bacteria. The use of pesticides, even when non-lethal, can impact the behaviour of the target organism and, consequently, the spread of pathogens they may carry.

A further way to reduce the numbers of spittlebugs in a given location is to limit the habitat available to them for breeding.

A paper published in 2022 identifies certain plants that might be grown in or around olive fields to either reduce oviposition (egg-laying) by spittlebugs or increase mortality amongst the resultant nymphs. These plants additionally act as ground cover plants (i.e. to reduce erosion) or to enhance populations of beneficial arthropods (natural enemies and pollinators).

The 2022 paper found that Chervil (Anthriscus cerefolium) increases oviposition, but has a lethal effect on the first nymphal instars of P. spumarius. Perennial wall-rocket (Diplotaxis tenuifolia) is not attractive (and potentially even repellent) for oviposition and is unsuitable for nymphal development. White mustard (Sinapis alba) did not have any effect on oviposition but displayed medium-high cumulative mortality of spittlebug nymphs, as well as other beneficial ecosystem services for olive groves.

The paper also noted that Common Dandelion (Taraxacum officinale) and Lavender (Lavandula angustifolia) encouraged oviposition and nymphal survival and, hence, should be avoided where possible, despite the latter’s provision of other beneficial ecosystem services.

Final Words

We hope this blog has given you a new appreciation of the spittlebug, but also helped you understand why it has become important to control them, as well as some of the ways this can be done. Though not a threat themselves, spittlebugs can transmit a deadly bacterium that threatens to cause devastation across many regions of Europe where olives and almonds form a rich part of the culture and economy.

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