A research team from the University of the Balearic Islands has identified two species of insects as the main vectors of Xylella fastidiosa on the Mediterranean archipelago.
Researchers from the university’s Applied Zoology and Conservation Research Group found that Philaenus spumarius and Neophilaenus campestris were the two main carriers of the deadly olive pathogen.
The study is the largest of its kind in Europe and took place over three and a half years on the islands of Mallorca, Menorca, Ibiza and Formentera.
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The presence of Xylella fastidiosa was first detected on the archipelago in 2016, and now Formentera appears to be the only unaffected island.
Since the start of the project in 2017, 23% of vectors captured in the Balearic Islands were infected with Xylella fastidiosa. Among the two main vector species identified, Philaenus spumarius has the highest infection rate with 23.8%, while that of Neophilaenus campestris is 21.3%.
In Mallorca, the prevalence of infected vectors was 24%; in Menorca, it was 21.5%; and in Ibiza, 21%. Due to the economic devastation caused by the bacteria in recent years, the team focused primarily on three main crops: olive, almond and grapevine.
Three organic farms of each crop were selected in Mallorca for annual monitoring. The islands of Menorca, Ibiza and Formentera were sampled twice a year, in summer and in autumn. Insects were collected every two weeks from each plot in Majorca using a sweep net for the adults and a wooden frame for the nymphs.
On the other islands, only adults were collected since the nymphs were not present at the time of sampling. Overall, insects caught in almond trees showed the highest Xylella infection rate, at 25.7%, followed by 22.8% in olive groves and 21% in vineyards.
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Although no infected specimens of the vector species were found in Formentera, Philaenus spumarius and Neophilaenus campestris were present on the island, raising the question of why the disease has not yet spread there.
The team also observed the biological cycle of the vectors. They found that the nymphs of both species, which are not infective, are found in ground cover in early March, where their characteristic protective moss can be observed. They then go through five nymphal stages until the first adults are seen in late April.
Once the ground cover becomes too dry, these adults migrate to nearby trees and vines. At this stage, infection occurs when they feed on plants in which Xylella fastidiosa is already present. From this moment, the insect remains infectious throughout its life because the bacterium reproduces inside its mouthparts.
At the end of September, the adults were observed to return to the ground, where they lay their eggs to hatch the following year, thus repeating the cycle.
In addition to this field research, a year “microcosm” was conducted at the university’s experimental site in Palma, Majorca. The aim was to observe the behaviors and life cycles of insect vectors in 50 cages containing rosemary, mint, lavender, basil or mastic and grass.
As there is still no treatment or cure for Xylella fastidiosa, researchers believe that understanding the behavior and role of each vector species is key to developing effective control and prevention measures.