Above: Olive Fruit Fly (B. Oleae)

Above: Olive punctured by Olive Fruit Fly (B. Oleae)

Above: Maggot (before it has evolved into an Olive Fruit Fly) living inside Olive

Larvae hatch within a few days in warm weather and eat their way through the fruit as maggots before emerging in late summer or fall where they may find other foods besides olives to consume. Some overwinter (in soil or in fallen fruit – so it’s important to remove any remaining fallen fruit each year). Infestations are as high as 80% in the Mediterranean. California’s infestation is dramatically lower. In 1999, olive fruit fly was found in California though and is now present in most California olive-growing areas.

The most visible sign of infestation (and the time of greatest opportunity to damage fruit) is when the fruit begins to soften and turn color, from September to November. Of course, by late fall the damage is long completed. Hence, it’s imperative to take preventative measures earlier in the growing season.

Many producers are unaware they have an infestation until they harvest and/or begin the curing process for olives meant for food production. It is then they discover the problem.

When harvesting young olives in the early ripening stage, the maggots may not yet have matured, but they have destabilized the fruit, ensuring they won’t withstand the curing process. Maggots feeding within the olive will destroy its value as a table olive because the fruit may break apart during the curing process.

In Europe there is some tolerance for infestation in the food safety chain. But in the U.S., tolerance of infestation is virtually zero.

First, the key in the future is preventative measures. Second, how do you mitigate current infestation circumstances and reduces losses?

Although your fruit may no longer be usable for table olive production, it may be usable for olive oil. Olives that will be pressed for oil may have from 10-30% olive fruit fly infestation without a problem unless fruit pressing is delayed following harvest. However, olive oils extracted from olives with high infestation levels are very difficult to commercially classify as virgin olive oils, due to both sensorial and chemical quality parameters.

Damaged fruit that are stored for longer than a few days may have increased levels of acid within them that will alter the flavor of their oil. (There is a direct correlation between acid accumulation and the build-up of microorganisms [e.g., bacteria (Xanthomonas), yeasts (mostly Torulopsis and Candida), and molds (mainly Fusarium and Penicillium)] that develop in damaged fruit.)

The percent infestation by olive fruit fly maggots is not indicative of the amount of flavor change due to microorganisms. Total phenol content in olive oils is inversely related to the olive fly infestation level of the olive fruits.

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Identification

Insecticide

Kaolin Clay

Temp & Location

Regardless of the preventive measures taken, you can see that this is an annual, ongoing process to mitigate. Develop a plan to inspect fruit, use preventive measures and have processes to reduce losses.

Here are additional resources online to learn about the olive fruit fly:

• https://cisr.ucr.edu/invasive-species/olive-fruit-fly
• Controlling the Olive Fruit Fly at Home
• https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/olive-fruit-fly
• https://www.oliveoilsource.com/page/olive-fly-control
• Olive Fruit Fly Handout – UC Davis
• Sticky Traps for Olive Fruit Fly Trapping
• https://blog.agrivi.com/post/manage-olive-fruit-fly-by-following-protection-measures

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