How to Manage Pests
UC Pest Management Guidelines
Olive Fruit Fly
Scientific Name: Bactrocera oleae
(Reviewed 3/14, updated 10/14)
In this Guideline:
Description of the pest
Olive fruit fly causes serious economic damage to California's table olive and large-fruited olive oil orchards. Olives grown by homeowners for home curing or oil are equally at risk. A native of eastern Africa, it is considered the most damaging pest of olives in southern Europe, North Africa, and the Middle East. 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.
The adult olive fruit fly is about 0.2 inch (4–5 mm) 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. Mature larvae pupate in fruit in summer; in fall they leave the fruit and pupate in the soil under the tree. Larvae produced during late fall pupate in the soil, where they spend the winter.
Although the olive fruit fly does not have a true diapause, development is sufficiently slowed during the winter that pupae produced in late fall do not emerge until the following spring. Olive fruit fly also overwinters as larvae in fruit and to a lesser extent as adults and eggs.
In spring, early-emerging adults lay eggs in unharvested fruit from the previous year’s crop, whereas later-emerging flies (May–June) can lay eggs directly into new fruit. Olive fruit flies that do develop in unharvested fruit from the previous year emerge to mate and lay eggs on the new olive crop (July and August.) It is not necessary to have unharvested fruit on trees, however, to get considerable damage by mid-summer. It is believed that at least three, possibly four, generations of olive fruit flies could develop in various areas of California. In southern and coastal areas such as San Diego County, development may be continuous throughout the year.
Olive fruit fly larvae are the main stage causing damage and feed exclusively in olive fruits. Olive fruit fly damage includes egg laying "stings" from female flies on the fruit surface, fruit drop, or direct pulp destruction by larvae that renders fruit useless for canning. Larval feeding allows microorganisms to invade the fruit, causing rot and lower oil quality.
In areas of the world where 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.
Surveying fruit for infestation can give some indication of the severity of an infestation. Looking for maggots infesting fruit that has fallen from trees in late winter and spring is useful, as it will give some indication of overwintering olive fly numbers. Adult fruit flies can be monitored with McPhail, Olipe or yellow sticky traps. Plastic 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. However, yellow sticky traps baited with a pheromone lure or ammonium bicarbonate may be easier to use.
Native parasites have been observed attacking olive fruit fly pupae, but these wasps appear to be generalists that are only present when fly numbers are high. Paphiopedilum concolor, a non-native parasite that can be raised in culture has been released for other fruit flies including the Mediterranean fruit fly. Preliminary releases have been attempted in California with limited success to date.
Sanitation is important in reducing overall fly densities. Remove old fruit remaining on trees following harvest and destroy all fruit that are on the ground by either burying at least 4 inches deep or taking 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 spring by using a chemical such as naphthaleneacetic acid (NAA; Olive Stop) or destroy fruit on the ground in fall to reduce this invasion pathway. An areawide approach is needed to reduce olive fly densities where commercial plantings are near ornamental or unmaintained trees.
Olive fruit fly adults feed on honeydew. Reducing black scale populations may reduce a food source needed during high summer temperatures.
Organically Acceptable Methods
Cultural controls, the use of GF-120 Naturalyte Fruit Fly Bait (spinosad), sprays of kaolin clay, and mass trapping are acceptable for use in an organically certified crop.
Monitoring and Treatment Decisions
Traditionally, there is zero tolerance for damage on table fruit and about 10% for oil olives. But after sensory evaluation it was determined that the 10% threshold is very conservative. Orchards can tolerate damage up to 100% if the damage is caused by stings or tunnels in unripe fruit and there is less than 1% fruit rot. Olives can be used for processing into oil as long as the fruit is not rotten (brown, soft, or showing decay). The most important aspect of damage levels for oil olives is that the fruit should be harvested early to avoid rot. Olives damaged by fruit fly larvae develop ripe fruit flavors even when fruits are still green. This makes it possible to harvest fruit early, but still produce tasteful olive oil. Rotten fruit, not just the presence of olive fly larvae, leads to off flavors in the oil. Flavor is affected when between 1 to 3% of the fruit is spoiled. Process infested fruit as soon as possible after harvest (24 hours or less) or hold in cold storage until processed.
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 (when less flies should be caught).
Mass trapping and monitoring devices
Adult fruit flies can be monitored with McPhail, Olipe or yellow sticky traps.
Although traps reduce damage, they are more useful as monitoring devices. Traps should not be used as a stand-alone treatment, in part because of their inconsistent ability to attract and trap adult flies and because traps can not compete with olive fruit as the fruit becomes more attractive.
McPhail traps are plastic or glass containers with a reservoir for liquid baits. Yellow McPhail traps appear to be superior to red or clear traps. Flies enter from the bottom of the trap through an opening and drown in the solution. Recommended baits to use in these traps are torula yeast or NuLure bait. The addition of pheromones in liquid traps does not increase trap catches. Torula yeast catches more flies earlier than ammonia-based lures.
Olipe traps are made with 1.5 to 2 liter plastic non-food bottles, with several 4 to 5 mm sized holes drilled or melted at the top, and baited with 3 to 4 torula yeast tablets per liter of water. Pheromones may or may not increase trap catches.
Yellow sticky traps
Yellow sticky traps are baited with a sex pheromone (spiroketal), an ammonium bicarbonate attractant, or both. The sex pheromone attracts the males whereas ammonium bicarbonate attracts both males and females. Both lures can be combined in one trap, but pheromones may not increase trap catches.
Attract and kill traps
The Magnet OL trap attracts flies with a food lure on every trap and sex pheromone on every fourth trap. The traps contain a pyrethroid insecticide, which kills the flies when they land. The traps are designed to be effective for about five months (usually May through September for table olives and July through November for oil). For mass trapping, large numbers of traps are needed (up to one trap per tree). They are most effective when fly numbers are very low in oil olives and where orchards are isolated from nearby untreated trees.
Bait and deterrent sprays
Research indicates that bait and deterrent sprays are very effective in managing olive fruit fly. Both spray treatments provide consistent damage reduction and can be used as a sole management tool. If used in conjunction with a mass-trapping device they can provide exceptional control of the olive fruit fly.
Bait and insecticidal sprays
Applications of bait sprays (GF-120 Naturalyte) are very effective and should begin when trap captures begin to increase in early summer (late June in the Central Valley). If flies are captured in spring and the orchard has a history of damage, begin spinosad bait treatments at pit hardening or when fruit reach about 10 mm in length (the point when flies begin to sting fruit and larvae can develop). Once initiated, continue to apply bait sprays according to label directions to protect the crop until harvest. Apply at least to every other tree row at biweekly intervals.
If fly captures begin to increase in late summer but few fly stings on fruit are found, continue treatments with spinosad, kaolin clay, or both. However, if the number of stung fruit is increasing or if greater than normal numbers of flies are being captured in traps, treat with fenpropathrin (Danitol). Fenpropathrin is a relatively inexpensive pyrethroid, which may tempt a grower to use it early and often, but early season or excessive use of fenpropathrin can lead to a treadmill effect that can increase mite or scale numbers. Therefore, the routine use of fenpropathrin as a cover spray should be avoided unless significant damage is anticipated.
Kaolin clay sprays
Apply kaolin clay sprays (Surround WP) when olives become attractive to the fly, which is usually in early June or when the fruit reaches pea size. Completely coat leaves and fruit. After the spray dries it turns into a white powdery coat, which acts as a repellant to olive flies. Repeat every 5 to 6 weeks, which is approximately three times during the season. It can be used successfully in orchards with high fly numbers. Because of the need to completely cover fruits and leaves, which makes this treatment expensive, one application just before most olive fruit fly damage occurs (late in season) is probably most practical, most economical, and may be sufficient for oil olives, especially if the fruit is harvested early. The clay will need to be rinsed from the fruit before processing.
UC IPM Pest Management Guidelines: Olive
Insects and Mites
F. G. Zalom, Entomology, UC Davis