Researchers Identify Xylella Vectors on Balearic Islands

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.

The researchers from the university’s applied zool­ogy and con­ser­va­tion research group found Philaenus spumar­ius and Neophilaenus campestris to be the two main car­ri­ers of the deadly olive tree 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.

The pres­ence of Xylella fas­tidiosa was first detected on the arch­i­pel­ago in 2016, and now Formentera appears to be the only island unaf­fected.

Since the project started in 2017, 23 per­cent of vec­tors cap­tured in the Balearic Islands were infected with Xylella fas­tidiosa. Of the two main vec­tor species iden­ti­fied, Philaenus spumar­ius has the high­est infec­tion rate at 23.8 per­cent, while that of Neophilaenus campestris is 21.3 per­cent.

In Mallorca, the preva­lence of infected vec­tors was 24 per­cent; in Menorca, it was 21.5 per­cent; and in Ibiza, 21 per­cent. Due to the eco­nomic dev­as­ta­tion caused by the bac­terium in recent years, the team focused pri­mar­ily on three main crops: olive trees, almond trees and grapevines.

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 autumn. Insects were col­lected biweekly from each plot in Mallorca using a sweep net for adults and a wood frame for nymphs.

On the other islands, only adults were col­lected since nymphs were not present at the time of the sam­pling. Overall, insects cap­tured in almond groves showed the high­est Xylella infec­tion rate, at 25.7 per­cent, fol­lowed by 22.8 per­cent in olive groves and 21 per­cent in vine­yards.

Although no infected spec­i­mens of the vec­tor species were dis­cov­ered on Formentera, both Philaenus spumar­ius and Neophilaenus campestris were present on the island, rais­ing the ques­tion of why the dis­ease has yet to spread there.

The team also observed the bio­log­i­cal cycle of the vec­tors. They found that the nymphs of both species, which are non-infec­tive, are found in ground cover at the begin­ning of March, where their char­ac­ter­is­tic pro­tec­tive foam can be observed. They then go through five nymphal stages until the first adults are observed at the end of April.

Once the ground cover has become 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 point on, the insect remains infec­tive for the dura­tion of its life as the bac­terium repro­duces inside its mouth­parts.

At the end of September, the adults were observed to return to the ground cover, where they laid their eggs to hatch the fol­low­ing year, thus repeat­ing the cycle.

In addi­tion to this field research, a year-long ​“micro­cosm” trial was con­ducted at the university’s exper­i­men­tal plot in Palma, Mallorca. This con­sisted of observ­ing the behav­iors and life­cy­cles of the vec­tor insects within 50 cages con­tain­ing rose­mary, mint, laven­der, basil or mas­tic, and grass.

As there is still no treat­ment or cure for Xylella fas­tidiosa, the researchers believe that under­stand­ing the behav­ior and role of each vec­tor species is essen­tial to devel­op­ing effec­tive con­trol and pre­ven­tion mea­sures.

• Unviersity of the Balearic Islands
• EFSA