The Olive Fruit Fly: A Persistent Pest in a Changing Climate

The olive fruit fly, Bactrocera oleae, was recorded attack­ing olives in ancient times and has long been a for­mi­da­ble pest in the Mediterranean basin.

The lar­vae are monophagous, feed­ing exclu­sively on olive fruit of the genus Olea, includ­ing O. europaea (cul­ti­vated and wild), O. ver­ru­cosa and O. chrys­o­phylla. As such, the olive fruit fly’s dis­tri­b­u­tion is pri­mar­ily lim­ited to regions where cul­ti­vated and wild olive trees are found.

Today, the olive fruit fly is reported through­out the Mediterranean basin, south and cen­tral Africa, the Canary Islands, the Near and Middle East, California and Central America.

Reports of Bactrocera species col­lected on wild olives in China bring into ques­tion the fly’s pres­ence in Asia, but it has not yet been reported in South America or Australia.

Measuring the dam­age of the olive fruit fly

On both cul­ti­vated and wild olives, females lay their eggs in ripen­ing fruit, in which the newly hatched lar­vae feed upon the pulp and either pupate in the olive or exit to pupate on the ground.

Economic dam­age results from the adult fly’s egg inser­tion on the fruit sur­face of table olives; the fruit drop caused by feed­ing lar­vae (and sub­se­quent reduc­tion in oil pro­duc­tion); low­ered qual­ity and value of pressed oil due to increased acid­ity (result­ing from microor­gan­ism growth); and the direct pulp destruc­tion from lar­vae that can ren­der fruit use­less for can­ning.

The olive fruit fly con­stantly threat­ens crop yields in the Mediterranean basin, where about 95 per­cent of global olive oil pro­duc­tion occurs.

In Italy, for exam­ple, losses due to the olive fruit fly have been esti­mated to reach up to 30 per­cent of the crop in some areas. In Spain, the cost of con­trol­ling the olive fruit fly has been esti­mated to exceed €100 mil­lion annu­ally.

The sit­u­a­tion is not much bet­ter for olive grow­ers in California. Since the olive fruit fly was first detected in the state in the 1990s, its pres­ence has caused sig­nif­i­cant finan­cial losses for grow­ers. According to a study by the University of California, losses due to the olive fruit fly in the state can reach up to $100 mil­lion (€94 mil­lion) per annum.

Olive fruit fly ori­gins and ini­tial efforts to curb its spread

Despite its abun­dance and noto­ri­ety on cul­ti­vated olives, the olive fruit fly most likely orig­i­nated in regions of sub-Saharan Africa where wild olive vari­eties are found and domes­ti­cated cul­ti­vars were derived from.

Exotic pests often thrive in new areas because they lack their usual preda­tors and face inef­fec­tive local preda­tors. Classical bio­log­i­cal con­trol involves using nat­ural ene­mies to reduce the inva­sive pop­u­la­tion.

Several species of par­a­sitic wasps have been used in the Mediterranean region as bio­log­i­cal con­trol agents, as they lay their eggs in the eggs or lar­vae of the olive fruit fly, killing them before they can cause dam­age.

However, using bio­log­i­cal con­trol meth­ods can be chal­leng­ing, as they require pre­cise tim­ing and envi­ron­men­tal con­di­tions to be effec­tive.

Successful pro­grams yield pos­i­tive eco­nomic returns, but they must be care­fully exe­cuted to avoid releas­ing preda­tors that harm non-tar­get ecosys­tems.

For inva­sive plant-eat­ing pests, this involves under­stand­ing the preda­tor’s impact on its native habi­tat, its biol­ogy, host pref­er­ences and poten­tial inter­ac­tions in the local food chain. Environmental fac­tors also play a role in select­ing the proper preda­tor.

In the early 1900s, the first sig­nif­i­cant effort to con­trol B. oleae pop­u­la­tions by intro­duc­ing co-evolved par­a­sitoids occurred.

Natural ene­mies were explored in Africa and brought to Italy by Filippo Silvestri. Various bra­conid species were found in Africa, but Silvestri could not cul­ture them suc­cess­fully, and only a few were released.

P. con­color from Tunisia was sub­se­quently intro­duced mul­ti­ple times in olive groves across the Mediterranean but only estab­lished itself in some south­ern areas and was not effec­tive. Nevertheless, there is still inter­est in mass-rear­ing and releas­ing these species.

As work in clas­si­cal bio­log­i­cal con­trol con­tin­ues, grow­ers rely upon a com­bi­na­tion of more estab­lished con­trol meth­ods to mit­i­gate the effects of the pest.

Producers turn to new meth­ods to con­trol the olive fruit fly

Insecticides are the most com­mon method used to man­age the olive fruit fly. However, the overuse of insec­ti­cides can lead to the devel­op­ment of resis­tance and can also neg­a­tively impact non-tar­get species and the envi­ron­ment.

The effec­tive­ness of insec­ti­cide-based pro­grams is also lim­ited by the abun­dance of road­side and res­i­den­tial olive trees in many olive-grow­ing regions, which serve as reser­voirs and con­tribute to the fly’s rein­va­sion of treated groves.

Integrated pest man­age­ment (IPM) prac­tices are often rec­om­mended to improve the out­comes of insec­ti­cide pro­grams and reduce the asso­ci­ated risks. IPM involves using mul­ti­ple con­trol meth­ods, includ­ing cul­tural, bio­log­i­cal and chem­i­cal con­trol, to effec­tively man­age pests with min­i­mal neg­a­tive impacts.

Cultural con­trol is one of the most effec­tive ways to con­trol the olive fruit fly through prac­tices that dis­rupt its life cycle. These include remov­ing infested fruit from the tree and ground, prun­ing the tree to reduce dense foliage and har­vest­ing the fruit early to pre­vent it from becom­ing over­ripe.

Education and out­reach also play a crit­i­cal role. Growers must be informed about the pest, its life cycle and effec­tive man­age­ment strate­gies. Extension pro­grams and work­shops can give grow­ers the knowl­edge and skills to man­age the pest effec­tively.

In addi­tion, pub­lic edu­ca­tion cam­paigns can help to raise aware­ness of the olive fruit fly and its impact on the envi­ron­ment and the econ­omy. Outreach pro­grams can also pro­mote sus­tain­able and envi­ron­men­tally friendly pest man­age­ment prac­tices, reduc­ing the adverse effects of insec­ti­cides on non-tar­get species and the envi­ron­ment.

New tech­nolo­gies to mon­i­tor and detect the olive fruit fly

Monitoring and early detec­tion are crit­i­cal com­po­nents of any effec­tive man­age­ment pro­gram. Regular mon­i­tor­ing of olive trees can help to detect the pres­ence of the olive fruit fly before it causes sig­nif­i­cant dam­age, allow­ing for timely inter­ven­tion.

In addi­tion, early detec­tion can help to pre­vent the spread of the pest to neigh­bor­ing groves, reduc­ing the over­all impact of the pest on the olive indus­try.

Researchers and grow­ers are con­stantly explor­ing new tech­nolo­gies and inno­va­tions to improve the man­age­ment of the olive fruit fly. One such devel­op­ment is using pheromone traps to attract male olive fruit flies and pre­vent them from mat­ing with females. This can reduce the over­all pop­u­la­tion and decrease the need for insec­ti­cide use.

Another recent inno­va­tion is using the ster­ile insect tech­nique, which involves releas­ing large num­bers of ster­ile male olive fruit flies into the grove to mate with wild females. This results in a reduced pop­u­la­tion of olive fruit flies, as the eggs pro­duced by the wild females fail to hatch.

The ster­ile insect tech­nique involves the use of radi­a­tion to ster­il­ize insect pests. The method has been used since the 1950s and has been instru­men­tal in erad­i­cat­ing dis­ease-car­ry­ing mos­qui­toes from numer­ous world regions.

Because it requires no genetic mod­i­fi­ca­tion and has min­i­mal envi­ron­men­tal impact, the process is not sub­ject to the strin­gent reg­u­la­tory restric­tions of more advanced bio­log­i­cal con­trol meth­ods.

The future of olive fruit fly con­trol

Genetic engi­neer­ing is one of the most recent areas of research being explored, not for its poten­tial to mod­ify the pest itself but to develop olive trees resis­tant to its pre­da­tion.

Investigations are under­way to intro­duce genes from other plants or insects that pro­duce com­pounds that repel or kill the olive fruit fly into the olive tree genome.

However, using genet­i­cally mod­i­fied organ­isms is a con­tentious issue, and there is sig­nif­i­cant debate around the safety and ethics of using genet­i­cally mod­i­fied plants.

Although only one of many threats to olive groves around the globe, the olive fruit fly remains among the most seri­ous, as it has for mil­len­nia.

With increas­ingly large-scale envi­ron­men­tal and cli­mate change, effec­tive and sus­tain­able man­age­ment meth­ods will only become more crit­i­cal.

• CABI
• Molecules
• Sci Rep