Occasionally wash out the cup because it will turn into a graveyard for the pesky flies. ;)
Homemade Fruit Fly Trap https://traditionalcookingschool.com/food-preparation/recipes/techniques-tutorials/free-video-homemade-fruit-fly-trap/?utm_campaign=autoblog&utm_source=blog&utm_medium=bloglink&utm_content=Free+Video%3A+Homemade+Fruit+Fly+Trap