I have no experience with mass outdoor fruit flies but i can tell you how I get rid of indoor fruit flies. In a mason jar I mix 1 part apple cider vinegar to 2 parts water then add a few drops of dish soap to remove the surface tension of the water and just let it sit in the vicinity of the problem area. In 2-3 days I have usually killed most of the flies. You could try this on a slightly larger scale and tie a few jars off on some of your trees or wait for someone much smarter then myself to give you a solution.

Much Gratitude for the info John Donovan :)