Ask the Expert: Question of the Week

Q: I recently got called out to a major small fly infestation in a first floor apartment and figured out that phorid flies are the problem.  What’s the easiest way to determine the source?

A: Phorid flies breed in moist organic matter, which can be plentiful and available in numerous locations in and around an apartment building.  The fact that only one apartment appears to be infested will help narrow it down, unless neighboring units are empty, non-responsive to such issues, or the apartment in question just happens to be the easiest way for the flies to surface.  If you put fly boards around the unit, you can probably narrow down the general point of entry.

Consider all possible breeding sources and try to tick them off the list starting with the most likely and easiest to determine.  Are there any obvious sources of decaying organic matter in or around the unit like visibly dirty drains, compost containers, overwatered plants, animal carcasses, dumpsters, garbage and recycling cans, grass clippings, organic debris under or around appliances, or a downspout causing an abundance of water outside the unit?  The most difficult phorid fly problems are associated with plumbing problems like broken drains pipes under slab floors.  Of course, these aren’t easy to check but there are a couple of tricks that can help you determine if that’s the source of the breeding material.  Put pieces of clear tape sticky side down over drain openings or floor cracks while leaving some space in between them so air can still flow.  This will catch adult flies as they are traveling between the breeding source and the rest of the apartment to help you pin point what is sustaining the infestation.