Just copied and pasted this from the web to save typing.
"Traps baited with food lures can be used to reduce adult fly densities by mass trapping. One such trap was developed in Spain and is called an OLIPE trap (See References for additional information.). It is a homemade trap used in Spain to suppress olive fruit fly populations in organic olive groves and in sensitive areas near homes and natural parks. The OLIPE trap is made from a 1- to 2-liter plastic bottle with.20-inch (5 mm) holes melted into the shoulder. Current research in California suggests that Torula yeast tablets dissolved in water are very effective as bait. Traps are hung in the shade, and flies are attracted to the trap, crawling inside and dying.

McPhail-type traps also can be used for mass trapping olive fruit flies. If used for this purpose, traps should be checked weekly during the summer to ensure that the water-based lure has not evaporated. The lure should be changed regularly to maintain maximum attraction. Several companies currently sell McPhail-type traps. Contact your local cooperative extension office for a source, or try a vendor such as Great Lakes IPM (www.greatlakesipm.com) or ISCA Technologies Inc. (www.iscatech.com). These and other mass trapping devices have been shown to reduce olive fly damage by 30% to as much as 100%. Mass trapping effectiveness also is quite variable and may not provide adequate control when used as the only control approach.nd pasted this from the net to save me typing it out."

Yup, it's American.  The OLIPE trap would seem to be a cheap way of reducing the numbers.  I think there's some powder available from Sol D'Ebre that can be dissolved in water to put in the traps.  I'll check it out and let you know.