Experts in Italy Offer Advice for Olive Growers Combating the Fruit Fly

The Tuscan Regional Phytosanitary Service has released tech­ni­cal guide­lines for mon­i­tor­ing and con­trol­ling the olive fruit fly pop­u­la­tion by grow­ers and tech­ni­cians work­ing on organic and inte­grated farms.

Considered one of the most harm­ful olive tree pests due to the dam­age they cause to both the quan­tity and qual­ity of the fruit, this dipter­ous insect is found in the Mediterranean basin, South Africa, Central and South America, China, Australia and the U.S.

The instruc­tions, pro­vided by the experts focused on the sit­u­a­tion in Tuscany can be adapted by farm­ers accord­ing to the devel­op­ment cycle of the fly, which can vary depend­ing on the soil and weather con­di­tions of the olive grow­ing area.

“In European coun­tries, the chal­lenge aris­ing from the ban on Dimethoate requires a new approach in the con­trol of the olive fly,” said Massimo Ricciolini of the Tuscan Regional Phytosanitary Service. ​“Yet, con­sid­er­ing the wide­spread need of sus­tain­abil­ity, we believe that not only phy­ti­atric reli­a­bil­ity but also tox­i­co­log­i­cal and envi­ron­men­tal safety should be at the base of any effi­cient strat­egy against this pest.”

The mar­ket with­drawal of the sys­temic organophos­phate insec­ti­cide Dimethoate, which was used against the lar­vae of the fly, has led experts to con­sider the adult stage of the insect as the main goal of the fight.

“Prevention should be the main focus of an effec­tive and sus­tain­able approach,” Ricciolini said. ​“There is no alter­na­tive in organic farm­ing at this time, so while we wait for the research results on new valid cura­tive treat­ments (i.e. against eggs and lar­vae), it is nec­es­sary to imple­ment tech­niques to kill or repel the adults.”

“It is impor­tant to note that in our region the fly com­pletes its first annual gen­er­a­tion in spring,” he added. ​“The insect uses the olives that remain on the plants, due to incom­plete har­vest­ing or aban­doned olive groves, as a repro­duc­tive sub­strate and food source. Hence, between the end of June and early July, usu­ally, the sec­ond flight of the year, which is larger than the first one, occurs.”

The females deposit their eggs in the olives of the cur­rent year, which are already recep­tive and usu­ally at the begin­ning of the stone lig­ni­fi­ca­tion process.

“From these eggs, the sec­ond gen­er­a­tion of the year, which is the first of the sum­mer, emerges,” Ricciolini said. ​“The green, grow­ing fruits are then dam­aged by the activ­ity of the lar­vae which, pass­ing through three stages, develop at the expense of the pulp, dig­ging a tun­nel in the meso­carp that is at first super­fi­cial and thread­like, then deep and with a larger sec­tion, and, finally, sur­fac­ing at the ellip­ti­cal sec­tion.”

“According to the sea­son, the mature lar­vae drop to the ground to pupate or, when the pupal stage is com­pleted, the adults eclose [emerge from the pupal case],” he added.

During the warmer months, peri­ods of high tem­per­a­tures (above 30 to 33 °C — 86 to 91.4 °F) and low lev­els of rel­a­tive humid­ity (below 60 per­cent) can cause the death of sub­stan­tial parts of eggs and young lar­vae pop­u­la­tion, with con­se­quent poten­tial harm reduc­tion.

The fly pop­u­la­tions gen­er­ally increase con­sid­er­ably in September and October, caus­ing a risk of pro­gres­sive dam­age until the har­vest, due to both fruit drop and oxida­tive processes affect­ing the holed olives. In order to pre­vent ovipo­si­tion and lar­val devel­op­ment, grow­ers should carry out an early har­vest, which is effec­tive espe­cially in years of high infes­ta­tion.

“In Tuscany, with all due excep­tions, the risk of attacks is usu­ally greater along the coast, and tends to decrease toward the inland areas, high hills, and the Apennines,” Ricciolini said. ​“In the last 15 years, increased knowl­edge about olive fly biol­ogy and the set­ting up of an exten­sive agrom­e­te­o­ro­log­i­cal and demo­graphic data­base have made it pos­si­ble to define a cli­mate-based infes­ta­tion risk fore­cast model.”

“It showed that, in our ter­ri­tory, low tem­per­a­tures in win­ter act as a lim­it­ing fac­tor for this insect and that the sur­vival rate of its pop­u­la­tions in win­ter influ­ences the pop­u­la­tions of the spring gen­er­a­tion,” he added.

The sug­ges­tion is to mon­i­tor both the adult pop­u­la­tion dynam­ics, start­ing from the first annual flight, and the olive infes­ta­tion trend, start­ing from the sec­ond flight of the year.

The flight mon­i­tor­ing should be car­ried out, on a weekly basis, with chro­motropic or pheromone traps (one to three traps for a stan­dard one-hectare/2.5‑acre plot with 280 olive trees); the infes­ta­tion mon­i­tor­ing should be car­ried out, on a weekly basis, sam­pling of 100 olives per olive plot (con­sid­er­ing an aver­age one hectare/2.5 acre with 280 olive trees).

If the infes­ta­tion exceeds the thresh­old of five per­cent (given by liv­ing eggs, first and sec­ond age lar­vae) or 10 per­cent (given by liv­ing eggs and first age lar­vae), it is pos­si­ble to pro­ceed with the use of the allowed lar­vi­cide prod­ucts.

Within this frame­work, based on the knowl­edge of the ter­ri­tory and the harm­ful­ness of attacks in terms of fre­quency and inten­sity, the experts stress the impor­tance to imple­ment a deter­rent and/or killing action against the first sum­mer adults.

“We must con­sider that some devices and prod­ucts per­form best in vast orchards,” Ricciolini said. ​“Others tend to be more effi­cient in small plots.”

Large olive groves (more than five hectares/12.4 acres) require devices or bait prod­ucts with an ​‘attract and kill’ action which aim to lure males and females adults to a food or pheromone source and then kill them by inges­tion (of the poi­soned bait) or by con­tact (with the active sur­face of the device).

Pheromone and insec­ti­cide traps avail­able on the mar­ket, as well as hand­made traps con­tain­ing pro­tein baits are exten­sively used and effec­tive; more­over, the nat­ural insec­ti­cide, Spinosad, is allowed in sev­eral coun­tries.

In small plots it is rec­om­mended to use prod­ucts with repel­lent action against males and females and with anti-ovipo­si­tion effects against females, such as cop­per, kaolin, other min­er­als such as zeolith and ben­tonite, and a com­pound based on fun­gus, Beauveria bassiana. Research is ongo­ing on the lat­ter two treat­ments.

Growers in inte­grated farm­ing can use, where allowed, insec­ti­cides based on Phosmet (organophos­phate), Acetamiprid (neon­i­coti­noid) and Deltamethrin (in Italy, this pyrethroid ester can be used only in the traps).

“In all cases, the aim is to pre­vent ovipo­si­tion,” Ricciolini said. ​“In our region, this implies act­ing against the adults of the first sum­mer flight, that occurs in late June to early July. We should con­sider as crit­i­cal para­me­ters the first cap­tures of adults in the traps, the very first ovipo­si­tion holes and the pit hard­en­ing in the fruit.”

The Tuscan Regional Phytosanitary Service offered final gen­eral sug­ges­tions:

“From the sec­ond sum­mer flight on, pre­ven­tive inter­ven­tions can be decided by tak­ing into account the dura­tion of action of the prod­uct used, the com­ple­tion of the pre­vi­ous preimag­i­nal (i.e. devel­op­ment stage that imme­di­ately pre­cedes the adult) stage of the insect, the first catches of adults of the pre­vi­ous gen­er­a­tion, and the very first ovipo­si­tion holes of the new gen­er­a­tion,” Ricciolini said.

• AgroAmbiente Regione Toscana