Being Driven Gnatty Batty

Sunday February 18, 2007


Mr Pest Control


We service a building that has been having a constant problem with gnats. There are no signs of moisture buildup, no trash, no leaks in drop ceilings, no leaks in the a.c. ducts or drip pan, only 2 ornamental plants inside, and checked the outside perimeter and there was also no sign of plants with moisture buildup. We have been using fly web fly control lamps and they have been catching a lot of them but activity continues. Do you have any recommendations? Please email me at Thanks

Luis, FL

Mr Pest Control


I think it is safe to say that the source of these flies remains healthy and productive, and if that source is inside it probably will continue to produce flies until it is found and corrected. It is always important in fly control to know which species you are dealing with, and the term "gnats" is often applied to any small fly, so it could be drain flies, phorid flies, fruit flies, or fungus gnats. I'd start by getting a firm ID on what you are facing, and this will help you better understand its biology and likely food choices, and thus help you in your search for that source. Focusing on adult fly control, when the production of the flies is indoors, can be a lesson in frustration as you have found out.

Once you determine the species you need to spend a whole bunch of time on the inspection, and perhaps the trapping you have done so far would give you a hint as to where the flies may be coming from, if one of the traps seems to be catching more flies than others are. I know the difficulty in finding the source. I have dealt with fungus gnat populations in private homes that were huge, and while we know the flies must be breeding in some moist habitat I gotta say..... the house was just bone dry. However, these insects have pretty well defined habits, and fungus gnats need moist habitats, fruit flies need fermenting juices, and phorid flies and drain flies need that decaying, wet buildup of organic matter. Somewhere there is such a resource for them in this account, unless...... they are managing to come in from outside. Since you are in Florida your climate could be warm enough to be permitting breeding outdoors at this time of year, so that cannot be overlooked.

Is this a food account of any kind, where waste disposal outdoors is not particularly sanitary? Perhaps filthy dumpsters, grease disposal not adequate, low spots accumulating water and moss. While your situation suggests that the problem is indoors it still pays to make sure that the outdoor environment is not the culprit. If these turn out to be phorid flies is there a chance they could be coming from beneath a slab? I have seen and have heard stories of big problems where plumbing pipes were busted below a slab, releasing lots of gray water onto the soil there and breeding millions of flies. The flies could then find their way up into the structure around expansion joints or junctions of walls. Drain flies like a similar habitat, and of course also the buildup of organic sludge in floor or sink drains. If there is any food service in this account could soda syrups or beer have spilled, or bottles or cans fallen into hidden places.

I'd start with that positive ID, and then read up on the biology and breeding habits of that species to give you more focus on what to look for. Then, take the flashlight and some patience and follow the trail to where the origin is. The use of light traps is good for monitoring the populations and hopefully narrowing the search, but as you can see it will not eliminate the problem if production continues to go on.

Mr. Pest Control

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