Nov 14, 2012 – Little Flies, Big Annoyance


I have a restaurant that recently called about a small fly infestation. When I arrived I noticed a lot of tiny black flying insects that seem to stay up towards the ceiling and stored items on their shelves. I believe they are fruit flies. The room is 5 feet wide by 10 feet long and 8 feet high with the dish washing sink, water heater, two fridges and cleaning supplies. I placed vinegar traps and Gold Sticks and also placed Bio-Gel down the drain. It helped a little, but we still have some flying around. Any suggestions.


It is very important for any kind of pest problem to identify exactly what pest you are facing, and in fly management this often becomes even more critical. Knowing only that it is some small flying gnat is not sufficient to tell you what its likely breeding sources and food attractions are, and until you can find the source of the problem - the place these flies are coming from - you will be likely to have a continuing problem. And, in a restaurant the presence of small flies usually indicates that a sanitation problem exists that needs to be addressed. It is the presence of filth such as spills, plumbing leaks, food buildup on equipment, or dirty drains and floors that small flies often breed in and which should not continue to exist in a food service account. 

So, in a sense we ought to thank these little flies for pointing out that a more important problem may be present. That is my first suggestion, and if you have already captured some of the flies on a sticky trap you could examine them under high magnification to determine just what kind they are. The choices most often are fungus gnats, phorid flies, drain flies, vinegar flies, and dark-eyed fruit flies, but of course other small flies might also be present, so don't close the door to something else. Even the use of a small hand lens magnifier should be sufficient to show you the details needed for proper ID, and I strongly suggest that one of these ALWAYS be with you in the field. One supplier of a variety of low cost models is BioQuip, and you can find them online. 

All of these small flies are going to be associated with moist situations, but if it turns out to be fungus gnats, for example, you might be looking for wet soils in potted plants, water leaks under sinks where mold may be growing, or even an exterior source that is feeding the gnats to the inside when doors are opened. But, since you feel you are seeing them primarily in upper areas around the shelving these gnats don't seem as likely. Phorid flies are easily ID'd by their annoying habit of landing and then running a short distance on the surface, which may be your face or arm. These breed where there is a buildup of organic crud, so dirty drains, buildup of food debris around floor edges or within equipment, and broken plumbing under slabs are some of the sources for them. 

Also commonly breeding in drains and wherever food debris accumulates in very wet settings could be drain flies, and if your treatment of the drains seems to have helped a bit you might take your inspection further to see what else exists. Dark eyed fruit flies are drawn to the same kinds of fermenting food materials as are vinegar flies, but also to the same kinds of wet filth as the phorid flies, making them a double whammy. Another common problem within food accounts is the grease trap, which may not be properly maintained or emptied, allowing large numbers of flies to find it and breed in it. 

But, bottom line is that you need to find the SOURCE, and finding it will be easier if you know what kind of fly you are dealing with. ID first, inspect to find the source second, clean up that sanitation problem third, and then if necessary do a mop up operation to eliminate the final adult flies that are lingering in the area. Univar now carries many biological cleaners that can be used by our industry to clean surfaces, drains, grease traps, and most other places within a restaurant that might accumulate filth. I would take my focus away from the adult flies and spend my next visit there with a flashlight and the cooperation of the customer, working to find out where the problem is originating. 

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