New Study Tracks Populations and Movements of Major Xylella Carrier

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).

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.

“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.

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.

“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.

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.”

“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.

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.”

• Nature