How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Olive fruit fly poses a serious threat to the California table olive and olive oil industries. 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 (May-June) flies 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.

DAMAGE

Olive fruit fly larvae are the main stage causing damage and feed exclusively in olive fruits. Damage by olive fruit fly includes oviposition "stings" on the fruit surface, fruit drop, or direct pulp destruction rendering fruits 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.

MANAGEMENT

Removing and destroying fruit left on the tree following harvest is somewhat important in managing this pest. Examine fallen fruit in late winter for the presence of olive fruit fly. Monitor populations in spring with McPhail or yellow sticky traps and apply bait sprays when traps indicate populations are increasing in early summer.

Biological Control
No native parasites are known to attack olive fly at this time. 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.

Cultural Control
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 like "Fruit 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.

Organically Acceptable Methods
Cultural controls, the use of GF-120 Fruit Fly Bait, sprays of kaolin clay, and mass trapping are acceptable for use in an organically certified crop.

Monitoring and Treatment Decisions
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 densities. Adult fruit flies can be monitored with McPhail traps or with 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.

Hang traps as high as possible at least 12 inches from foliage on the south side of trees from October to April and on the north side of trees from May to September. Place traps in trees that have a good fruit load or in trees with an abundance of fallen fruit on the ground where larvae and pupae are expected to be present. Adult olive fruit flies can be identified in the traps by their yellow or white scutellum and the distinct, single black spot at the tip of each wing.

Preliminary research indicates that applications of bait sprays should begin when trap captures begin to increase in early summer (late June in the Central Valley). Once initiated, continue to apply bait sprays according to label directions to protect the crop until harvest.

McPhail traps. McPhail traps are plastic or glass containers with a reservoir for liquid baits. 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 with or without a pheromone. To count trapped flies, 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.)

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. Replace the yellow sticky traps once a month or more often if they get wet, contaminated with non-target insects, or dusty such that they are no longer sticky. Replace spiroketal lures every 4 months and ammonium bicarbonate packets every 2 weeks. The spiroketal lure must be pierced with a pin (e.g., a small map pin or insect pin) before using. Some types of ammonium bicarbonate packets must be pierced with something larger than a pin to produce an opening of at least 1 mm so that sufficient vapors will escape. Ammonium bicarbonate packets made by PaCoast have a peel-off cover that exposes a release area on one side. Examine the packets before using to make sure that they do not have broken seals on the sides and are leaking powder—these packets should be thrown away because the amount of ammonium bicarbonate remaining is unknown.

PUBLICATION

UC IPM Pest Management Guidelines: Olive
UC ANR Publication 3452
Insects and Mites
F. G. Zalom, Entomology, UC Davis
P. M. Vossen, UC Cooperative Extension, Sonoma Co.
R. A. Van Steenwyk, Insect Biology, UC Berkeley
Acknowledgment for contributions to the insects and mites section:
G. S. Sibbett, UC Cooperative Extension, Tulare Co.
L. Ferguson, Pomology, UC Davis

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