DETECTION

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

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

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 dust 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.

CONTROL MEASURES

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.

Biological Control

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.

Cultural Control

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.

Barrier Control

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.

Chemical Control

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 foliage. 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.

SOURCES

G. Steven Sibbett and Louise Ferguson: Olive Production Manual, University of California.

Paul M. Vossen: Organic Olive Production Manual, University of California.

Paul Vossen, Lucia Varela, and Alexandra Devarenne: Olive Fruit Fly.

Paul Vossen: The Spanish "Olipe" Trap for Organic Control of Olive Fruit Fly.

UC Integrated Pest Management Program.

Marshall Johnson: Olive Fruit Fly: Biology, Control and Research.

Olive Fruit Fly Integrated Pest Management for Home Gardeners and Landscape Professionals.

Paul Vossen and Alexandra Devarenne: Controlling Olive Fruit Fly at Home.

UC IPM How to Manage Pests in Gardens and Landscapes - Olive Fly.