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.

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 Fruit Fly Bait, sprays of kaolin clay, and mass trapping are acceptable for use in an organically certified crop.

Monitoring and Treatment Decisions
There is zero tolerance for damage on table fruit and about 10% for oil olives. The most important aspect of damage levels for oil olives is that the fruit should be harvested early before it begins to rot. Rotten fruit, not just the presence of olive fly larvae leads to off flavors in the oil. Press infested fruit as soon as possible after harvest (24 hours or less) or hold in cold storage until pressed.

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.

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, Olipe 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.

For all trap types, place traps 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. Record numbers of flies trapped weekly.

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. Place two traps 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. Use 3 to 4 yeast tablets per trap and change monthly. In hot weather add water to the trap to replace what evaporates, maintaining correct bait concentrations. 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.)

Olipe trap. 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. 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. 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.

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. Hang the traps in the trees where they should last for five months. Unless fly numbers are very low (isolated populations for oil olives), attract and kill traps should not be used alone to protect olive fruit.

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 County
R. A. Van Steenwyk, Insect Biology, UC Berkeley
M. W. Johnson, Kearney Agricultural Center, Parlier
Acknowledgment for contributions to Insects and Mites:
G. S. Sibbett, UC Cooperative Extension, Tulare County
L. Ferguson, Pomology, UC Davis

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