The olive fruit fly poses a serious threat for all olive growers. A native of eastern Africa, there are records of infestations in fruit going back to the third century BC. It is considered the most damaging pest of olives in southern Europe, North Africa, the Middle East, and now, in California. The olive fruit fly was first detected in North America infesting olive fruits on landscape trees in Los Angeles County in November 1998. It can now be found throughout the state. It commonly infests ornamental olive trees in urban and landscape locations that are not actively managed. These non-commercial olive trees may serve as an important source of invasion for commercial groves.
Following is some basic information about the olive fly and possible control measures. For more in-depth information, we recommend the book and articles from Paul Vossen, as well as the book from Louise Ferguson and G. Steven Sibbett, listed in our Sources below. There is a lot of research being done on the olive fly; we stay current with the latest findings as much as possible, but make sure to contact your local farm adviser for the most up-to-date information. The UC Integrated Pest Management information on the olive fly listed in our Sources below is also a useful resource.
TAXONOMY, DESCRIPTION, LIFE CYCLE, AND LOCATION
The olive fly’s scientific name is Bactrocera Oleae. Following is some basic information about its taxonomy, description, life cycle, and location.
|Arthropoda||Millipedes, centipedes, spiders, scorpions, crustaceans and insects|
|Hexapoda||While crustaceans are the dominant group of arthropods in marine environments, hexapods, including insects, rule the land|
|Pterygota||veined wings on the second (meso-) and third (meta-) thoracic segment|
|Neoptera||have the ability to fold the wings back over their abdomen|
|Endopterygota||insects with complete metamorphosis Four of the five largest orders of insects belong to this group (Coleoptera, Hymenoptera, Lepidoptera, and Diptera).|
|Diptera||true flies with two wings|
|Tephritidae||fruit flies, the most agriculturally important family of flies|
|Oleae||Species - the olive fly|
The adult olive fruit fly is about 3/16” (approximately 5 mm), reddish brown in color, with large reddish eyes and small antennae. The body is long with clear wings containing dark veins and a small dark spot at the wing tip. The head, thorax, and abdomen are brown with darker markings, and the thorax has several white or yellow patches on each side. The end of the male fly's abdomen is blunt, whereas females have a large black ovipositor at the end of their abdomen that is visible to the naked eye. Larvae are yellowish-white maggots with a pointed head.
First, the most basic concepts: the female olive fly mates and lays eggs into the olive fruits. Each egg hatches into a tiny larva (maggot) that feeds throughout the olive and develops into a pupa (pupates) in a hollow area just beneath the outer skin. The adult fly emerges from the pupa. And the cycle starts again. The olive fruit fly has three, and perhaps as many as six, generations per year depending on local conditions. In southern California and coastal areas such as San Diego County or Santa Barbara County, development may be continuous throughout the year.
And now, more details: the olive fly can overwinter as an adult; alternatively, it can spend the winter as a pupa in the soil or in fallen fruit. Overwintered adult populations decline to low levels by February or March. First generation new adults from overwintered pupae start emerging in early spring. The first females can lay eggs in un-harvested fruit from the previous year's crop; later emerging flies can lay eggs directly into new fruit. Olive fruit susceptibility begins at the time of pit hardening, in July or earlier depending on climactic conditions.
The second generation represents offspring from overwintered adults and pupae and emerges between June and August, depending on regional temperatures. These adult flies mate and lay eggs on the new olive crop. During the summer, the olive fly can complete a generation in as little as 30 to 35 days at optimum temperatures. Eggs hatch in 2 to 3 days, larvae develop in about 20 days, and pupae in 8 to 10 days in the summer. Adult flies can live from 2 to 6 months depending on the temperature and food availability (honey dew, fruit juices, bird feces, etc.) A female can lay from 50 to 400 eggs in a life time. Additional generations of flies are produced during the late summer and fall months into December, depending on fruit availability.
In the fall most of the last generation larvae leave the fruit and pupate in the soil under the tree for several months. Adults can also overwinter in protected areas, especially areas with mild temperatures. Olives left on trees after harvest can produce high populations of flies from late fall to early spring.
Olives are the only breeding host plants. The larger olive varieties, e.g. Manzanillo, Sevillano, or Mission, are preferred for oviposition by the female. All olive varieties, however, are susceptible. Flies have been trapped in other plants or crop orchards where the adults are looking for food or refuge.
Olive flies survive best in cooler coastal climates, but are also found in the hot and dry regions of Greece, Italy, Spain, Mexico, and California. The fly activity threshold is approximately 60°F (15°C). The optimum temperature for development is between 68° and 86°F (20° and 30°C). Temperatures above 100°F (38°C) are detrimental both to the adult fly and to the maggot in the fruit. The flies are very mobile, however, and have the ability to seek cooler areas. Reports of fly movement range from 650ft (200m) in the presence of an olive host, to as much as 2.5 miles (4km) to find hosts.
In most cases, the greatest damage occurs as the fruit begins to soften and turn color, from September to November.
As we saw above, the adult female can lay 50 to 400 eggs, generally one in each fruit. Olive fly eggs are small and difficult to see, embedded under the fruit surface. The tiny larvae hatching from the eggs are also quite difficult to see until they feed for a while and get larger. The first damage sign is an oviposition "sting" on the fruit surface. This looks like a small indentation or lump on the surface of the fruit. Sometimes there is discoloration. While feeding, the larvae tunnel through the fruit, destroying the pulp and allowing entry of secondary infestation of bacteria and fungi that rot the fruit and greatly lower the quality of oil. The oil’s free fatty acid level (“acidity”) increases considerably. If the damage is extensive enough, it may cause premature fruit drop.
In areas of the world where the olive fruit fly is established and not controlled, its damage has been responsible for losses of up to 80% of oil value because of lower quantity and quality, and in some varieties of table olives, this pest is capable of destroying 100% of the crop. Some European districts cannot grow table olives because control of olive fruit fly is not economical. The expense of treatments and the likely crop damage have the potential for eliminating olive culture in home orchards or as a viable commercial industry in California.
For commercial table fruit processed in Europe, the damage threshold is 1%, but California table fruit processors have zero tolerance for olive fly damage. For olive lovers wanting to cure olives for personal uses, the fruit can be sorted by hand.
The European damage threshold for olive oil production is 10%. Research in Spain showed that, even with 100% of the fruit showing stings, high quality oil could be produced as long as the fruit was not rotten. This is our experience at our mill as well, although the oil quality is definitely lower than with healthy olives and the oil acidity is quite a bit higher. The real problem occurs when larval feeding introduces rotting organisms that create off flavors. This usually happens toward the end of the larvae feeding cycle when the maggots get quite large. Consequently, early harvest may help.
When olives are damaged by olive fruit fly, the fruit is more sensitive to oxidative and microbial breakdown, therefore the time from harvest to milling should be kept as short as possible and every effort should be made to handle the fruit properly to limit further damage (see our Harvesting section).
As seen above, olive fruit flies in the larval stage cause the most damage. This damage is not always obvious to people unfamiliar with olive fly infestation signs. When we ask new growers whether they use olive fruit fly control measures for their trees, before accepting the fruit for milling, they often respond that they “don’t have the fly.” Once we tell them exactly what to look for, they invariably call back to say that in fact they did find damage.
In order to detect fruit fly damage, fallen winter fruit and fruit on the tree should be visually inspected for oviposition stings, maggots, or tunneling and decay. Adult flies can best be discovered by trapping. The olive fly can be monitored with McPhail, Olipe, or Yellow Sticky Traps. McPhail traps have proven to be more effective than yellow sticky traps in catching larger numbers of olive fruit flies and catching them earlier in the season.
While there is no relationship between fruit damage and the number of insects found in traps, surveying trap catches can evaluate treatment efficacy by comparing trap catches before and after treatment.
For all trap types, the traps should be placed in fruiting trees before March 1 in the second tree row or further in to reduce dust accumulation in the traps. Hang the traps mid-canopy, in the shade (north side of the tree), and in an open area to avoid leaves blocking the trap. The number of flies trapped weekly should be recorded. Preliminary research indicates that applications of bait sprays should begin when trap captures begin to increase in early summer.
McPhail traps are used extensively in Europe, primarily for monitoring, but sometimes for mass trapping. They are plastic or glass containers with a reservoir for liquid baits, containing a 4% solution of ammonium salts (ammonium bicarbonate or ammonium phosphate) as bait attractants. Flies enter from the bottom of the trap through an opening and drown in the solution.
The UC Integrated Pest Management website recommends Torula yeast or NuLure bait, with or without a pheromone, as baits to use in these traps. Two traps should be placed for each 5 to 10 acre block of trees to evaluate treatment efficacy. More traps per block are necessary to evaluate fly activity or density. 3 to 4 yeast tablets should be placed per trap and changed monthly. In hot weather, water should be added to the trap to replace what evaporates, to maintain correct bait concentrations. The easiest way to count trapped flies is to empty the trap contents into a sieve so that the liquid drains out and the flies can be identified and counted. (Be sure to remove the used liquid from the orchard.) These traps also work in non-breeding host orchards (e.g. citrus, cherry, plum, and nectarine.)
Olipe traps are made with plastic non-food bottles of approximately 0.4 to 0.5G (1.5 to 2L), with several 0.15 to 0.2” (4 to 5mm) sized holes drilled or melted at the top, and baited with 3 to 4 Torula yeast tablets per liter of water. Flies attracted to the bait crawl into the bottle through the holes at top, and drown. Change the bait solution monthly. Use the same method to count trapped flies as with McPhail traps.
Yellow Sticky Traps
Yellow sticky traps are baited with a sex pheromone (spiroketal) and/or ammonium bicarbonate attractant. The sex pheromone attracts the males whereas ammonium bicarbonate attracts both males and females. Both lures can be combined in one trap. Traps are placed on the south side of the tree in the winter and on the north side in the summer. Hang the trap in open shade, with 8-10” (203 to 254mm) of clearance from foliage, and for trees with fruit, inside the canopy.
The yellow sticky traps should be replaced once a month or more often if they get wet, contaminated with non-target insects, or dusty such that they are no longer sticky. The spiroketal lures should be changed every 4 to 6 months and ammonium bicarbonate packets every 2 to 6 weeks. The flies should be removed weekly when the traps are monitored. Read the instructions carefully about how to handle and place the bait and pheromone.
Attract and Kill Traps
The Magnet OL trap attracts flies with a food lure in every trap and sex pheromone in every fourth trap. To kill them, the traps contain a pyrethroid insecticide. The traps are hung in the trees where they should last for five months. Unless fly numbers are very low, attract and kill traps should not be used alone to protect olive fruit.
When it comes to controlling the olive fly, being vigilant, diligent, and systematic is critical. Keep in mind that the fly is most likely present in your orchard even though you may not see either the fly or the damage without careful attention. The fly population can multiply rapidly, as we saw above. It is important to keep on top of the control program, and not miss steps. The best approach is to monitor populations in the spring with McPhail or yellow sticky traps, apply bait sprays when traps indicate populations are increasing in early summer, and get rid of all fruit at harvest time, including fruit on the ground.
The olive fly is attacked by a number of parasitoid species in the Mediterranean and also in sub-Saharan Africa, where the fly is thought to have originated. No native parasites are known to attack the olive fly at this time in California. Preliminary releases of P. Concolor, a parasite that can be raised in culture and has been released for other fruit flies including the Mediterranean fruit fly, have been attempted in California with limited success to date. Research is ongoing.
Sanitation is important in reducing overall fly densities. Remove old fruit remaining on trees following harvest and destroy all fruit on the ground by either burying it at least 4 inches deep or taking it to the landfill. Extremely high fly populations can occur in fruited varieties of landscape trees and in unmaintained ornamental situations. These can be a significant source for invasion of commercial groves. Prevent fruiting on landscape trees in the spring by using a chemical like "Fruit Stop" or destroy fruit on the ground in the fall to reduce this invasion pathway. An area wide approach is needed to reduce olive fly densities where commercial plantings are near ornamental or unmaintained trees. This is often difficult to implement, however. Fruitless varieties should be used for landscaping purposes. They have the additional advantage of producing less pollen that may aggravate people’s allergies.
Olive fruit fly adults feed on honeydew. Reducing black scale populations may reduce a food source needed during high summer temperatures.
Kaolin clay (brand name Surround) is a particle film. The product is mixed with water and applied with a high-pressure sprayer. The solution dries to a white powder that repels the olive flies. The exact mechanism by which this happens is not known. The mode of action is thought to be tactile or visual in nature. Kaolin clay has no nutrient value for the plant. It is not toxic for the insects. The first application should be a week or two before pit hardening, and it should be re-applied every 5 to 6 weeks.
Most insecticidal controls are applied as a bait and insecticide mixture, although cover sprays are also used in some European commercial production areas. The baits attract adult flies that must feed to prolong life and produce eggs. Ground application (from a tractor or shoulder pack) is recommended for bait sprays rather than aerial application, which results in droplet sizes that are too small to be effective. Droplets 1/6” to 1/4” (4 to 5mm) in diameter will best resist evaporation. Presently, the only insecticide registered in California is GF-120 (Naturalyte), a bait containing the insecticide spinosad. It is available both for commercial operations and for private growers. It is approved for organic orchards. Spinosad is a fermentation by-product from the actinomycete bacteria called Saccharopolyspora Spinosa. The bait is a formulation of hydrolyzed protein.
Depending on the extent of the fly damage, the tree variety, and environmental conditions, the GF-120 should be diluted 1 part of product with 1.5 to 4 parts of water (e.g., with 4 gal of product, use from 6–16 gal water for a total of 10–20 gal spray solution.)
The diluted solution should be applied at a rate of 1 to 3 fluid ounces per tree in a coarse spray or stream to a small portion of the tree. There is no need to cover the whole tree, because the adult flies are attracted to the bait, feed on it, and die. It should stay wet as long as possible, so very early morning or very late afternoon application, on the shady side of the tree, is best. The most common method is to spray every other row each week. Light infestations might get by with applications every two weeks. In order to achieve adequate control in very heavily infested orchards, however, weekly spraying of every tree is needed from late spring until harvest. GF-120 cannot be applied more often than once a week.
Organically Acceptable Methods
Cultural controls, the use of GF-120, sprays of kaolin clay, and mass trapping are acceptable for use in an organically certified crop.
Louise Ferguson and G. Steven Sibbett: Olive Production Manual, University of California.
Paul Vossen, Lucia G. Varela, and Alexandra Devarrenne: Olive Fruit Fly
Paul Vossen: The Spanish Olipe Trap for Olive Fruit Fly
Paul M. Vossen: Organic Olive Production Manual, University of California.
Marshall Johnson: Olive Fruit Fly: Biology, Control and Research Update 2006
Paul Vossen, Lucia G. Varela, and Alexandra Devarrenne: Controlling Olive Fruit Fly at Home