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New Study Tracks Populations and Movements of Major Xylella Carrier

Tracking spittlebug movements throughout their lifespan, researchers have made recommendations to help curb the spread of the pest.

Foam produced by a spittlebug on a vascular plant
Dec. 13, 2019
By Daniel Dawson
Foam produced by a spittlebug on a vascular plant

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A land­mark study on the phe­nol­ogy of spit­tle­bugs may help olive farm­ers and local gov­ern­ments combat the spread of Xylella fas­tidiosa in the Mediterranean basin.

Over a period of three years, a team of researchers from Italy’s Institute for Sustainable Plant Protection, the University of Turin, the University of Brescia and the University of Bari, stud­ied the repro­duc­tive traits and pop­u­la­tion move­ments of three species of spit­tle­bugs at two dif­fer­ent sites in Italy: one in Puglia (south­ern Italy) and one in Liguria (north­ern Italy).

Any con­trol mea­sure applied after the fourth instar peak could poten­tially target the whole nymphal pop­u­la­tion before the onset of the adults, thus achiev­ing the max­i­mum effi­cacy (in pre­vent­ing the spread of Xylella fas­tidiosa).- authors of the study

The spit­tle­bug is the only proven vector of Xylella fas­tidiosa ST53 – the pathogen respon­si­ble for the mas­sive olive tree die-off in Puglia.

The researchers believe that what they have learned during the course of the study, which ran from 2016 to 2018, will help farm­ers make informed deci­sions when taking mea­sures against the spread of Xylella fas­tidiosa.

See more: Xylella fas­tidiosa News

“The present work pro­vides a large amount of data on the life cycle of the spit­tle­bugs within an olive agroe­cosys­tem that can be used to design effec­tive con­trol pro­grams against these vec­tors in infected areas and to assess the risk of the estab­lish­ment and spread of Xylella fas­tidiosa to Xylella-free areas,” the researchers wrote.

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Prior to the pub­li­ca­tion of this study in the jour­nal Nature, very little was known about the repro­duc­tive habits and life cycles of spit­tle­bugs.

Based on their obser­va­tions, the researchers found that spit­tle­bug nymphs begin to emerge in the second week of March, with the vast major­ity hatch­ing by mid-April, which also coin­cided with the fourth instar (fourth phase of devel­op­ment) for many of the nymphs. The emer­gence of the first adults was only recorded after this peak.

This find­ing led the researchers to rec­om­mend apply­ing insec­ti­cides or any other con­trol mea­sures in mid-April in order to elim­i­nate as many of the nymphs as pos­si­ble before they develop into adults.

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“Any con­trol mea­sure applied after the fourth instar peak could poten­tially target the whole nymphal pop­u­la­tion before the onset of the adults, thus achiev­ing the max­i­mum effi­cacy,” the researchers wrote.

By the end of May, the vast major­ity of the adult spit­tle­bugs were counted, gen­er­ally being found on the herba­ceous cover sur­round­ing the olive trees as well as on the trees them­selves.

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Starting in late June, many of the spit­tle­bugs began to migrate from the olive trees to other wild woody host plans, mainly conif­er­ous trees and shrubs (how­ever, some remained behind in the olive groves, most notably one spe­cific species of spit­tle­bug in Liguria). The researchers hypoth­e­sized this was due to the lack of water inside the olive trees com­pared to other woody and vas­cu­lar plant species.

The researchers also the­o­rized these wild trees and shrubs, some of which are known reser­voirs for Xylella fas­tidiosa, were likely where spit­tle­bugs become infected with the pathogen.

By the end of the summer, the spit­tle­bugs began to move back to the olive groves, where the females laid their eggs. This is the moment in time when spit­tle­bugs are most likely to infect olive trees with Xylella fas­tidiosa, although the danger of trans­mit­ting the dis­ease remains con­stant once the spit­tle­bugs have matured.

“The period imme­di­ately after adult emer­gence is the cru­cial moment for both Xylella fas­tidiosa acqui­si­tion and trans­mis­sion to olives by insect vec­tors,” the researchers wrote. “It is worth point­ing out that, once infected, vec­tors are per­sis­tently infec­tious.”

Spittlebug

Insecticides tar­get­ing the adult stages should be applied timely to the olive canopy, mainly in this period, in order to pre­vent the spread of the dis­ease during the year,” the researchers added.

From the end of October to early November, the team began to notice the decline in the spit­tle­bug pop­u­la­tions, with very few spit­tle­bugs sur­viv­ing through the winter.

Along with observ­ing the move­ments and pop­u­la­tion dynam­ics of the spit­tle­bugs, researchers also noticed how dif­fer­ences in the land­scapes impacted the insect pop­u­la­tions.

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In the part of Puglia in which the study took place (a part that has so far remained Xylella fas­tidiosa-free), the researchers found that spit­tle­bug pop­u­la­tions thrived in olive groves that were largely undis­turbed and nat­ural.

“Different levels of insect dis­tur­bance, as a result of agro­nomic mea­sures, such as soil tillage – which is usu­ally car­ried out in summer in Puglia – can have an impact on the adult pop­u­la­tion, thereby deter­min­ing mass move­ments to olive trees and other woody hosts,” the researchers wrote.

For this reason, researchers warned that less cul­ti­vated olive groves, such as those that were observed in Liguria, pose the high­est risk for being infected and allow­ing the spread of Xylella fas­tidiosa.

While this research is a good first step in build­ing up the col­lec­tive knowl­edge about spit­tle­bugs, the team acknowl­edged that a lot more work needs to be done in order to better under­stand the rela­tion­ship between spit­tle­bugs and the spread of Xylella fas­tidiosa.

“There is still a gen­eral lack of infor­ma­tion on both the abi­otic and biotic fac­tors that influ­ence the com­po­si­tion of xylem-sap feeder com­mu­ni­ties in olive groves and on species abun­dance,” the researchers con­cluded.

“Further stud­ies on vec­tors in olive, almond and other agroe­cosys­tems poten­tially at risk to the spread of Xylella fas­tidiosa in Europe are urgently needed to improve con­trol efforts and to con­tribute towards lim­it­ing the spread of Xylella fas­tidiosa epi­demics.”