Drosophila melanogaster is a fruit fly, a little insect about 3mm long, of the kind that accumulates around spoiled fruit. The bane of any bartender with a sense of pride in their work and workplace; nothing is worse than walking into the bar in the morning and disturbing a cloud of bar flies thanks to infestation. Every bar deals with them to varying degrees.
Bar flies are attracted to sugary substances, as well as sticky and moist areas. The most common occurance of this behind the bar are beer taps / dried beer, fruit, drains, pipes, cloths, mops, rags, and sweet fruity liquers such as Midori, Cointreau, and Vermouth. Fruit fly larvae can only survive in moist, decaying matter.
Cleaning Up Your Act
Bar flies have a life cycle of 48 hours, so interrupting their breeding pattern is paramount to stopping them, or reducing their numbers. The best way to prevent bar flies is to keep your bar clean and sanitised. Set up a cleaning roster so that all of those places no one likes to clean (dishwasher area, drains, pipes, fridge seals, corners of the bar) get cleaned on a regular basis.
Clean any area you can find that could possibly have a buildup of alcohol or sugar. This includes wiping down your bottles and backbar, as well as soda guns and holsters, the beer taps, and the underside of ledges. After cleaning the bar, make sure that any wet areas are dried, and kept reasonably dry afterwards.
If you have cloths / sponges lying around, put them into a container and store them overnight in the fridge; this removes a source of humidity in the bar. Any mops or brooms in the bar should also be stored away from the bar itself. Any other sources of standing water should also get the chop.
Worst Case Scenario
The worst case scenario is that you find yourself with an infestation. The first thing to do is clean the bar as described above, and take action to make sure that any breeding areas are neutralised. The flies themselves are an annoyance, but if you eliminate their breeding areas, the flies will be gone after two to three days at the most. Focusing on the flies themselves is only a temporary solution and will not solve the problem.
Some bars have had success with a vinegar and water wash of the bar, though large amounts of vinegar do tend to attract fruit flies, thus this method might not be for you. Others recommend pouring bleach down the drains, and keeping drain holes covered overnight, as these are prime breeding grounds for barflies. A final resort is using fly strips, but this will only help with the current batch of flies as they are hatched, rather than destroying the problem at it's source. Using fly spray / insecticide in the bar is against the Health regulations of most US states and international countries, since you're spraying a poison into the air around a food / drink preparation area.
Keeping Them Away
Bar flies can't live in a clean and sterile environment, so you should maintain a cleaning roster and take proactive measures. You may have the odd bad day, but with a solid cleaning schedule you should be able to keep the scourge of the bar world at bay.