Identifying Olive Fly Damage

This is the time of year when a lot of people with a few landscape trees or hobby orchardists see a crop ripening and decide to try to make oil with their fruit. Frequently tree owners are unfamiliar with the existence of the olive fly, the effects of the fly on the quality of olive oil, and how to identify olive fly damage to fruit. This post is not about how to control the fly or the effects of fly damage as that has been covered in great detail many places including on our site.

This post is intended for new comers to making olive oil so they can take a look at their fruit and identify infestation. At this point, in California, I can say with 99% certainty that if you are not doing fly mitigation, your orchard is infested, but this is always a tough sell. So, take a look at this and then go take a look at the fruit on your trees. At its most early detection when the fruit is still forming, what you will see on damaged fruit is a slight pit where eggs have been laid inside the fruit, sometimes with a little discoloration, sometimes not. Usually there will be nothing more than a slight indentation with or without a gray/brown spot smaller than a pin head at its center. There may only be one or there may be many pits and the fruit will actually become deformed proportionally to how many times eggs have been laid in it. The photo below shows early fly damage on the olive in the middle.

As the season progresses, portions of the fruit will begin to turn black where damage has occurred. Note that this discoloration will be completely inconsistent with the natural coloring of the fruit as it ripens. Often times when you cut damaged fruit open, you will actually see the worms developing inside the fruit. Depending on the maturation of the process, you may not see anything at all.

Eventually, usually once the fruit is very ripe, you will see the exit holes in the fruit where the flies have left the fruit.