Olive Day at CLFP | The Olive Oil Source

Olive Day at CLFP

Source: The Olive Oil Source
March 02, 2004

Sacramento: Every year the California League of Food Processors (CLFP) has their equipment and services show in Sacramento. "Olive Day" is a morning set aside for olive growers and producers.  Two years ago there was mention of the olive fly at the 1/2 day presentation.  Last year there was concern and a brief presentation.  This year the only item on the agenda was the fly, a pest which could ring the death knell for the California ripe olive industry.

While growers who have olive oil varieties can tolerate infestation in up to 10% of their olives, ripe black olive growers have zero tolerance.  The fly has hit harder in the cooler coastal valleys, moving through the state from South to North in the past 2 years. Lecturer Hanah Nadel reported counting up to 30 fly "stings" in a single olive in a Santa Barbara grove.  Several enthusiastic presentations from U.C. researchers this year give hope of understanding and controlling the olive fly.  Following are salient parts from their presentations.  Investigators who presented or whose work was mentioned include:

Hannah Burrack

Louise Ferguson

Kim A. Hoelmer

Marshall Johnson

William Kreuger

Hanah Nadel

Mark Robertson

Robert Wharton

Lynn Wunderlynch

Frank Zalom


In order to find out where the fly is and what it is doing and effectiveness of control measures, monitoring is essential.  Traps used for monitoring can be different from those used for control.  The simplest but least productive method of monitoring is visual inspection.  With heavy infestation the fly can be seen on the olive trees but even in orchards with 200% infestation (2 grubs/olive), the fly may not be noticed.  The fly is small, may not be active and can resemble similar species.  Pupa in the soil are difficult to detect. 

Visual inspection of fruit can yield information about the number of times eggs were laid, number of larvae which subsequently developed and number of flies which hatched. Not all ovipositor "stings" results in a grub.  Stings cause holes and a moon shaped blemish on the olive.  Other insects and mechanical damage can cause similar confusing marks.

The two most common types of traps are the sticky panel trap and the McPhail plastic sphere which has a liquid reservoir.  Flies are attracted to panel traps by their yellow color, a bait and a pheromone attractant.

Panel traps were originally used more extensively but Hanah presented convincing research that the McPhail trap is superior.  It attracted 10 times more flies in some comparisons.  Collection involves pouring the bait liquid and drowned flies through a sieve then pouring new bait into the trap.  Flies are in better condition for examination that those glued to a sticky panel trap.  The McPhail trap attracts a greater percentage of females, the more revealing species for control studies, and fewer beneficial species. 

Fly habits

Research on olive fly habits was presented.  The olive fly has been well characterized in Europe where it is endemic but not a native. How it behaves in California where climate and ecology are different may not coincide with European findings.  Careful observation of fly habits throughout the year in Spain allows for effective control measures.  Farmers spray depending on climate and monitoring findings which are plugged into formulae which predict when the files will reach reproductive state and become active.  These predictive formulae are the result of years of observations.  It is hoped that similar observation here will yield information on when and how to spray, etc.

Post graduate student Hannah Burrack with others is developing a degree-day table. By consulting such a table predictions can be made about fly behavior in the orchard and control measures can be timed.  Hannah's team raises adult flies in the lab which are then introduced into plastic cages on olive tree branches with susceptible olives multiple times during the season.  This field caging is being done in Amador, Butte, Tulare, Solano, Sonoma, Ventura and Yolo counties.  Mission and Manzanillo groves are used - two trees per site, six cages per tree.  The flies are inserted for four weeks after which the olives are stripped and examined.  Stings, eggs, tunnels, larvaee, pupa and exit holes where adult files have hatched and emerged are counted. The research will show what the flies are doing in relation to the weather, season, and degree of olive ripeness.  They have found that activity is low during the hot months in mid summer.  A degree day model should be developed by the end of  2004 and validated during 2005.

Olive fruit Phenology

The degree day models require knowledge of olive conditions and how they correlate with weather conditions.  Untreated Olive trees, mostly Manzanillo or Mission, were sampled at 8 sites in a variety of climates.  Other cultivars will be investigated at two of the sites - Sevillano, Arbequina, Leccino, Frantoio, Koroneiki and Aglandau.  Weather was recorded and weekly fruit samples were made. Olive length, diameter, color, flesh and pit hardness and oil content were determined.  Samples were checked for larvaee and eggs.

Cultural preference

Preliminary studies by Zalom, Burrack and Kreuger show that the fly has definite variety preferences.  This could be important for oil producers who have multiple varieties to choose for planting.  Olives from different varieties were picked from an experimental grove (Wolfskill) near Winters which has 137 varieties of olives.  The olives were stored in nitrogen until sorted into ripe and unripe batches, then exposed for one week to lab reared flies.  Olives were dissected every two days to see the degree of infestation. Below is a table showing percent of olives infested - very preliminary data!

VarietyAttack rate














Fruit skin thickness

The female fly must puncture the fruit with her ovipositor.  It has been postulated that fruit which is easier to puncture will attract more flies.  Machines have been used to determine the force needed to puncture different varieties.  Manzanillo has been found to have the most fragile skin.

Post harvest Management

Bill Kreuger in Tehama has studied the effect of bare ground vs. winter cover crops on fly over wintering.  The olive fruit fly must leave ripe olives in the fall to avoid being consumed along with the ripe olive by birds and foragers.  The larvaee exit dropped olives and burrow one inch into the soil to pupate and over winter. Cover crops seem to decrease fly levels, presumably by encouraging pupa predators.  Flaming, mowing, disking, flailing and fruit destruction will be also studied.

Irrigation practices

Traps in sprinkler irrigated groves had ten times more flies caught than traps in non-irrigated groves. It is not know if the flies are attracted to the fruit on irrigated tress or whether cooler temperatures in irrigated groves offer a better refuge.   The observation that flies are often found in riparian areas where no olives are present would seem to support the latter. 

Olive Fly Range

Several investigations are ongoing to assess how far the flies spread.  A "flight mill" was used to determine how far a fly can go on a given amount of food.  The flies were glued to an outrigger and flew in circles till exhaustion.  Laps were counted and speed and distance calculated.  Females flew faster but males flew further.  Flies fed a protein rich diet tended to disperse less; energy may have gone into reproductive vs. flying organs.

A field test was also done to assess fly dispersal.  Flies marked with a fluorescent dye were released in a Fremont orchard.  Traps placed in a radial pattern were checked for wandering flies.  Interesting observations were that females left olive orchards for a nearby walnut orchard where presumably shade and cooler conditions prevailed. The adult flies may congregate in areas where sucrose is available from aphid infestations.


Control of the fly in orchards will depend on what happens in ornamental and abandoned trees. Hanah is investigating roadside trees in 10 sites in the state between Fresno and Porterville.  These sites are not sprayed or picked.  Fly populations in these sites will be compared to treated orchards.


The olive fly has few natural enemies here and in Europe.  Researchers hope to find a predator in the fly's native habitat in Africa.  Parasitic wasps are one hope for bio-control. The wasp larva lives inside and consumes the fly larva, hatching out of the pupa.  U.S. investigators are working with French and African researchers on natural predators. 

Bio-control has a successful history in California.  In the 1950s olive groves were nearly destroyed by the olive scale.  A parasitic wasp from Persia and Pakistan was introduced which has made the scale a minor nuisance.

Kim A. Hoelmer and others have collected Braconid wasps (Bracon celer, Psyttalia Lounsburyi, and Utetes africanus) for the USDA insectary in France on three occasions in the South Africa provinces of East Cape, West Cape, Gauteng, Northwest and Mpumalanga. These species are relatively specific to the olive fruit fly.  Researchers have successfully maintained colonies of Psyttalia lounsburyi (Africa via France), and Fopius arisanus, Psyttalia concolor, P. humilis and Diachasmimorpha krausii all from Hawai) on olive fruit fly in California for possible introduction.

Care must be taken that an olive fly predator doesn't also parasitize beneficial species.  P. humilis and D. kraussii attacked and reproduced on a fruit fly (Rhagoletis sp.) from bitter cherry, but did not reproduce on Chaetorellia succinea, an important biocontrol agent of the plant pest yellow star thistle, or on the walnut husk fly. Fopius arisanus did not reproduce on the bitter cherry fly or walnut husk fly.

How the wasp parasitizes the fly larva differs. In one species the wasp lays its egg in an infested olive  The wasp's grub finds and enters the olive fly larvae.  Another species of wasp actually penetrates the olive flesh with its ovipositor to deposit its egg within the larvae. Studies are ongoing to determine if the ovipositor in these wasps is long enough to reach larvae in the large fruit varieties common in California.  Other questions are whether the wasps can tolerate our climate. and what are the best times to release the wasp for optimum fly control.  California native parasitic wasps (Pteromalus nr. sp. myopitae) have been found to parasitize the olive fruit fly but not in numbers adequate for control.

If a wasp is found then there is the problem of rearing large quantities for release.  The wasps must be reared on a special lab-acclimated olive fruit fly which is different from the wild type. Mark Robertson has developed an artificial diet for OLF, which we have used to rear P. concolor. Researchers feel that no single species of wasp will prove to be the silver bullet.   For wasps to succeed, they must have a natural reservoir of olive fruit fly in the wild.  Abandoned and ornamental trees may actually be useful for this purpose as the wasps will have a year-round host.

Another form of biocontrol is sterile fly release.  Marshall Johnson is spearheading ongoing research in this area.

Control District

William Kreuger of Glenn County explained the activities of a control district. Owners of abandoned groves are encouraged to spray or remove their trees.  Untreated orchards are treated and an assessment made against the owner.   Roadside or ornamental trees are sprayed.  Eradication activities are coordinated for homeowners who have backyard trees. To pay for this, growers are assessed per tree.

Most Olive Day participants reported leaving with the feeling that something substantial was beginning to be done to combat this serious threat to the industry.

Links: Marshall Johnson's presentation on PowerPoint

California Olive Committee Report