The olive fruit fly epidemic was especially bad this year. Trying to use fewer chemicals, in June I set out simple bottle traps filled with a yeast and water mixture. This system lures fruit flies to a watery death and is extremely effective. If you have not used traps for pest, look on this website for ideas, it provides an excellent tutorial for making your own traps in various forms.
Then, in August, I started bi-monthly applications of fruit fly chemicals. Well, failure is a great teacher. After watching my crops slowly being consumed by fly larva, the decision was to pick early and save some of the crop. As stated repeatedly, always ask for help if this is an unfamiliar process to you. Upon calling olive growers in northern California, the advice was to use the same fly spray I had used starting in August but to start spraying for Olive fruit fly in early June and repeat weekly until harvest. (Check UC Davis website for recommended olive fruit fly sprays.)
I know UC Davis is currently doing research on predator wasps to combat olive fruit flies. They have an excellent article on how to manage garden pests including olive fruit flies here. According to their research, yeast tablets dissolved in water is proven to be very effective as bait. Regardless of the method you choose, as in all natural prevention processes, it takes time to truly be ridden of unwanted pests.
Your olive crop will need to be safeguarded against its most severe threat, and this will require time and care. If you are growing olives for curing, fruit fly damage is even more critical. When pressing olives for olive oil, you can live with 10 % damage. However, the outcome of a quality olive harvest is delightful and rewarding, and harvest day with good friends can be a party in itself. In any event, I sure enjoy dipping a good slice of artisan bread into my homegrown olive oil.