Mysterious African Insects Are Infesting Portuguese Olive Groves

Unexplained damage to olive trees over a six-year period in Portugal has been confirmed as the effects of an obscure insect never before observed in Europe.
Armored scale insects
By Simon Roots
Feb. 8, 2023 20:02 UTC

Melanaspis cor­ti­cosa, a poorly-described armored scale insect, has been iden­ti­fied as the species infest­ing olive groves in the Algarve region of south­ern Portugal.

Previously unknown out­side sub-Saharan Africa, the new pest has proven resis­tant to insec­ti­cide treat­ment.

First detected in the final months of 2016 from the unusual scale on the branches of orna­men­tal olive trees, exten­sive dam­age has since been reported from sites across the Algarve, from pri­vate gar­dens and urban trees to agri­cul­tural land and com­mer­cial olive groves.

See Also:Research Reveals How Deadly Pathogen Infects Olive Trees

However, the insect respon­si­ble remained uniden­ti­fied until 2022, when sam­ples were sub­jected to a com­bi­na­tion of mol­e­c­u­lar and mor­pho­log­i­cal analy­ses.

These analy­ses con­firmed the pest as Melanaspis cor­ti­cosa, a species first described in the Cape region of South Africa in 1919 and later in Guinea, Mozambique and Zimbabwe.

In its native envi­ron­ment, the insect is polyphagous, feed­ing on a diverse range of hosts from the indige­nous African coral tree and Cape lilac to intro­duced species such as the peach tree.

For rea­sons yet to be deter­mined, its pres­ence in Portugal appears lim­ited exclu­sively to the olive tree.

The research team that car­ried out the genetic sequenc­ing noted that there was neg­li­gi­ble dif­fer­en­ti­a­tion between COI (a mito­chon­dr­ial DNA encoded sub­unit) sequences, which they believed sug­gests that the species has expe­ri­enced very lit­tle selec­tion pres­sure in adapt­ing to its new envi­ron­ment. This could imply that the organ­ism is already well-suited to this envi­ron­ment.

Of the affected olive trees observed, severe dam­age was found in many. This included dieback of branches, leaf brown­ing, and leaf abscis­sion (shed­ding).

In most of the col­lected sam­ples, both branches and shoots were cov­ered entirely by massed indi­vid­u­als, includ­ing adult females and nymphs. This equates to level 4 (the high­est level) of the pro­posed Kosztarab clas­si­fi­ca­tion sys­tem of infes­ta­tion by scale insects: gen­eral or lay­ered infes­ta­tion (scales com­pletely cover the infested parts of the plant).”

Although so far only reported in the south­ern coastal region, the pest has affected a range of habi­tats in at least 15 sep­a­rate loca­tions from Vila do Bispo in the extreme south­west to Cabanas de Tavira in the east, some 15 kilo­me­ters from the bor­der with Spain. It has also been observed dur­ing all sea­sonal peri­ods.

See Also:In Portugal, Xylella Infection Spreads to More Species

The obscure nature of Melanaspis cor­ti­cosa means that nei­ther the poten­tial scale of the prob­lem nor any con­trol strat­egy can be pro­posed at this stage. Only five nat­ural preda­tors are known to exist, all of which are par­a­sitic wasps endemic to Africa and about which almost as lit­tle is known as about the pest itself.

The inter­na­tional European and Mediterranean Plant Protection Organization notes that phy­tosan­i­tary treat­ments with insec­ti­cides approved against scale insects in olive trees have been applied but with lim­ited effec­tive­ness.

In addi­tion to their analy­ses, the team respon­si­ble gen­er­ated a DNA bar­cod­ing sequence” to aid future iden­ti­fi­ca­tion. They also slide-mounted a total of 25 adult female spec­i­mens, micro­scope images and details, all of which were sent to the uni­ver­si­ties of Catania and Padua in Italy.

As this present out­break is the first of its kind in Europe and, indeed, out­side Africa, they hope that these resources will pro­vide the foun­da­tion for fur­ther study and the devel­op­ment of con­trol mea­sures.

Worldwide, the olive tree is host to some 100 vari­eties of scale insects, approx­i­mately 43 per­cent of which are found in the Mediterranean basin. Many of these are non-native species, such as Saissetia oleae, which, like Melanaspis cor­ti­cosa, is believed to be native to South Africa.

Saissetia oleae is one of the most eco­nom­i­cally impor­tant global olive pests and is of par­tic­u­lar con­cern in the Mediterranean region, where it is con­sid­ered one of the olive’s three main pests, along­side the olive fruit fly and the olive moth.


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