Drain Fly Inspection Guide

Drain flies do not come up from your pipes or drains, rather they come from outdoors to the drain when they smell the organic matter they need to breed. Drain Flies get into your home through tiny holes.

Drain flies are most active in the evening when they congregate around sinks and drains. Before treating for drain flies, you'll need to make sure your infestation is really coming from the drain rather than some other hidden source of moisture or decay. Fruit flies and Phorid flies are other types of small flies found in kitchens and bathrooms that may be easily mistaken for Drain Flies, and in that case the source is probably mold or rotting fruit, and the treatment would be different.

To inspect a drain for drain flies we recommend performing a simple test:

1. Right before bedtime, thoroughly dry off all sinks, bathtubs, and floor drain areas.
2. Place a strip of clear sticky tape or a small glue board across the center of each drain, with the sticky side facing down towards the drain. Do not cover the drain opening completely, or else the flies will not have an updraft on which to emerge.
3. Leave the sticky tape or glue boards in place overnight or over the weekend if possible.
4. If you find flies stuck to the tape or glue board, you know that drain is indeed one of the breeding grounds for drain flies. If you do not find flies, repeat the test for at least 4 more nights to provide for variations in the breeding cycle.

You may also find drain flies resting on walls in rooms with infested drains. There will usually be several drain flies in close proximity of each other.

Pro Tip

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In some situations drain flies can breed and develop in odd locations such as under a loose floor tile, in the tank of a toilet that is not used often, under a sink where a leaky pipe is located and other odd locations. If you have positively identified that you do in fact have drain flies but the drain inspection did not turn up any flies, you can place glue boards around the infested room. Check the glue boards daily and note which glue board has the most fly activity. Start searching in that area for moisture that can be conducive to drain flies.