There is a lot of great discussion on this, but let me add:
Fruitflies are really bad for business and you should concentrate on killing
them off, not moving them out of sight to a different area of the bar.
If your bar is clean, you will not have fruitflies. If I go into a bar with
fruitflies, it usually means they have problems with cleanliness. Pour
screens and plastic wrap help, but do not kill the flies.
If you already notice the flies in bottles - particularly in the nice cognacs,
grenadine or vermouth - you should be worried because this usually means the
flies have begun their breeding process. Not only do you then have to be
really clean - you have to be really clean for a few weeks to kill off the
different generations.
1. Get rid of standing water - no wet towels, no wet bar mats, no margarita
salter, no wet beer drains.
2. Bleach the hell out of all the drains EVERY night. Don't use straight
bleach, mix it with the hottest water you have. If you have the luxury, do it
again in the morning as long as the smell isn't too strong.
3. Cover the beer taps, wines, ice bins, and liquor and spray the living shit
out of the whole bar. Do this for about 2 weeks.
4. Cover or remove all fruit - particularly bananas.
5. Spend some time cleaning the soda guns - do it again. Wrap and soak the
ends.
6. Wipe down the bar top at the end of the night. (Some places let the closers
/ managers hang around late for a few rounds after the bartender goes home -
don't let them trash the place.)
I don't work in places that have flies. It only takes a couple of weeks to
kill them, but you really have to be diligent. I took a summer job with a
place that had a busy outdoor patio and a fly problem. The health inspector
came in and threw away every single bottle that had flies in it. The boss
almost cried as the inspector tossed out a bottle of Grand Marnier, B & B,
Remy VSOP, and a Martel Cordon Bleu. The boss never again said, "well, there
are just a couple of flies."