I had an issue with fruit flies when i came back from an holiday. Note to myself: don't leave fruits in the fruit basket before leaving the house for a week.
I made a quick print of this thing. I didn't want it to look pretty, I just wanted it to work. It came out surprisingly well and fit nicely on the bottle.
I put 2 orange slices with some beer at the bottom of the bottle. It captured over 100 drosophilas over one day. Problem solved.