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Landscape trees are sanctuaries for olive fruit fly

With gray-green leaves, textured bark and interesting shape, olive trees make striking landscape plants. And for many gourmets, there is nothing like the taste of home-cured olives. But these simple pleasures are beginning to frustrate olive farmers.

The olive fruit fly, introduced into California in 1998, is living and reproducing in untreated olive trees, increasing populations that make olive fruit fly control more challenging in the state's commercial olive groves, according to UC Davis emeritus entomologist Richard Rice.

Rice and his colleagues recently completed a project that found olive fruit fly is present and active year-round in coastal California and the Central Valley. Because the pest is so new to the state, more research is needed on the relationships between the fly and climate, fruit development and olive variety susceptibility.

However, scientists agree that olive fruit fly, currently found in at least 41 California counties, poses a serious threat to the livelihood of the state's olive farmers. The olive fruit fly is considered the most devastating insect pest of olives in the Mediterranean region, where it has plagued olive growers for more than 2,000 years. The female olive fruit fly lays eggs inside immature olives. In doing so, she also introduces bacteria. As the olives mature, the bacteria break down the flesh of the fruit, enabling her hatchlings to feed better. After six to seven weeks, the decay makes the fruit completely unusable for table consumption or oil.

Must control

"If we don't get that fly under control, there won't be an olive industry in California," said Jan Nelson of the California Olive Board.

The California Department of Pesticide Regulation approved an emergency exemption - known as a "Section 18" - for olive growers to spray a natural pesticide called spinosad mixed with bait on commercial olive farms. It is still illegal for homeowners to use spinosad on olives, so the thousands of olive trees in abandoned orchards, planted in parks, along roadsides and in home and business landscapes go untreated.

In response, commercial olive farmers are organizing pest-control districts to keep a handle on olive fly populations near their farms.

In Tulare County, the state's No. 1 olive producer, farmers were to vote on Feb. 26 whether to form a Tulare County Olive Fruit Fly Pest Control District. If it passes, the district will assess growers $25.50 per year to manage the pest countywide.

Nelson acknowledges that growers are expressing mixed feelings about the new tax.

"It's not a good time for olive growers," Nelson said. "Olive prices are down."

In Butte County, the state's No. 2 olive-producing county, with thousands of acres of abandoned olive trees, growers are also considering the formation of a pest-control district. Glenn and Tehama counties already have pest-control districts in place. In Glenn County, olive farmers are assessed 7 cents per tree. In Tehama County farmers also assessed themselves 7 cents per tree, however, the assessment will be re-balloted due to an election error last year.

Homeowners’ part

Homeowners can do their part to reduce the population of olive fruit fly. If olive trees appear in the landscape for esthetic reasons only - the owners have no intention to harvest the crop - fruit development may be prevented altogether by spraying a plant growth regulator, such as FruitStop or Florel. The products are available in most garden stores. Timing is very important, said UC Cooperative Extension horticulture farm advisor Ed Perry.

"The treatment must be applied at bloom time, generally in March or April," Perry said. "The blooms on olive trees are not very showy, however, if homeowners watch their trees carefully, they can see the blooms developing in the spring. Once the fruit sets, it is difficult to do anything about the fruit."

For homeowners who wish to harvest the fruit, there are few options. Perry recommends picking up and removing all old olives in the tree and on the ground at the end of each growing season to reduce the number of olive flies that spend the winter near the trees.

Sticky traps treated with bait and pesticide are available in some counties to attract and kill adult flies. Information is available from the agricultural commissioner or at garden stores.

Sonoma and Marin counties UCCE farm advisor Paul Vossen is testing a homemade trap that was developed in Spain to suppress olive fruit fly populations in organic olive groves and in trees near homes and parks. The trap is made from a 1- to 2-liter plastic bottle with quarter-inch holes melted into the shoulder. The bottles are filled with torula yeast tablets - available from some garden chemical suppliers - dissolved in water for bait. The traps are hung in the shade on the south side of the tree. In theory, the flies will be attracted to the trap, crawl inside and drown.

Although scientists have been successful with the homemade trap in Spain, Vossen has not been able to get any measure of olive fly control in California backyard settings.

Trap potential

"The traps have potential," Vossen said. "We're trying to fine tune them and will try it again this summer with the torula yeast and a commercial bait called Nulure."

Vossen said the very effective spinosad bait spray that farmers are now using will eventually be available for home use.

"It's very easy to use, affords excellent control and even organic growers can use it," Vossen said. "We're hoping that by this June it will have gone through its rigorous regulation process so it will be available for homeowners to use, but it may take longer."

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