Olive fruit fly back, threatens crops

As if news of a disastrous olive crop wasn’t bad enough for growers, it appears the olive fruit fly is back and making a real pest of itself this summer.

There are considerably more olive fruit flies this year than at this time last year, said Bill Krueger, UC Cooperative Extension farm advisor for Tehama and Glenn counties.

“When it comes to the olive fruit fly we really don’t understand everything we need to know, and this is just one example,” he stated.

Last year, only three of the flies were trapped in 18 McPhail traps placed throughout both counties.

This year, 100 olive fruit flies have been trapped already.

“Theoretically, the reason last year’s numbers were so very low is because of the good, hard frost we had in December 2009, but that is just speculation,” Krueger said.

His office has been working on the olive fruit fly since 2003, and it was in 2004 a good number of the flies were discovered in both Tehama and Glenn counties.

Krueger said they began trapping the destructive pests in 2006.

“We believe there are several factors involved in this years higher numbers. Of course this is just speculation, but we have had a fairly cool summer, a lot of fruit was left on the trees because the crop came in so late last fall, and I believe there has been less spraying for the fly this summer because of the poor crop,” he explained.

To help fight the olive fruit fly, both Tehama and Glenn counties formed pest management districts.

Local grower Gary Strack, who is a member of the Tehama County pest management district, said the district is very active in trapping the flies in both orchards and landscape trees, gathering statistics on fly counts, and providing traps to growers.

He said funding for the program comes from district fee assessments.

“The districts are working on controlling the infestation,” Krueger said.

If uncontrolled, olive fly populations can build rapidly to extremely high levels and have the potential to infest 100 percent of an area’s fruit.

Such is nearly the case in the Oroville area, according to Krueger.

“Growers in that area have not taken as much care to control the olive fruit fly and the numbers in there are much higher than in Tehama and Glenn counties,” he said. “It has been bad enough that canners wouldn’t take their fruit.”

He doesn’t want to see this area turn into an Oroville situation, although the potential is there and could greatly damage the area’s olive industry, further opening the doors to foreign olive imports, which have already caused financial frustration to California olive growers.

The Tehama County olive crop, for both table and oil olives, had a gross value of $24,439,200 in 2010, the county’s annual crop report states.

In Glenn County, the gross value was $24,705,000, according to the crop report.

Those numbers do not include the economy generated by olive oil and other related products.

The olive fruit fly, Bactrocera oleae, found throughout the Mediterranean Basin and southern Africa, has successfully colonized in all of California, were is was first discovered in the Los Angeles area in 1998.

Olive fruit fly causes damage to the fruit when first generation females, which have over-wintered in an area, emerge in March or April and lay eggs in any olive fruits which are left on the tree.

In certain cases, mature fruit on the ground may also be attacked.

Second generation adults emerge later in the spring and attack any of the remaining fruit from the previous years crop and the new crop’s olives as they develop from June through August. Larval feeding causes premature fruit drop and destroys the pulp of the fruit causing it to rot and making the olives unsuitable for canning.

Feeding of the maggots also increases the acidity of the oil, which lowers olive oil quality.

Mature larvae tend to pupate in the fruit during the summer but leave the fruit to pupate in the soil under the tree during the fall. The number of generations per year will vary depending on local conditions. At least four generations are expected in the olive growing regions.

Krueger strongly encourages anyone with any olive producing trees on their property to spray for the pest, whether or not the trees are harvested or not.

“My observations at harvest were that most of the higher levels of infestation were coming from unsprayed or minimally sprayed orchards, or trees that were next to unsprayed orchards,” he said.

Krueger emphasized the need for growers to have traps in their own orchards, to help in making treatment decisions.

The most common method of prevention is to spray for the olive fruit fly, and sanitation which includes removing old fruit remaining on trees following harvest and destroying all fruit that are on the ground by either burying at least four inches deep or taking to the landfill.

People with ornamental olive trees can help with the problem by using the appropriate chemical in the spring or destroying fruit on the ground in the fall to reduce the olive fruit fly invasion pathway.

“It is hard to estimate the damage caused to this year’s crop, but because of the light crop, some growers aren’t even picking their fruit. This in itself could lead to greater olive fruit fly numbers next year if we don’t have a hard freeze this winter and have another mild summer next year,” Krueger said.