Get Rid of Drain Flies in Basement

Drain flies in your basement may be coming from a floor drain, sump basin or a main sewer line break under your slab. There are a few methods of treatment, including drain gels, growth regulators and insect growth regulator aerosols.

If you start to see small black flies buzzing around your basement, take note of where they appear. Drain flies tend stay near their breeding site, as they do not fly well. If they are in the vicinity of a floor drain or sump pit, start your inspection there.

Drain Flies in Sump Basins

On occasion, we’ve heard of drain flies breeding in sump pits. Fully enclosed basins might have thousands of flies inside, but only a few escape and catch your attention. This typically happens when:

  • Sewer water is leaking into the pit
  • Pump has a poor quality float switch
  • Broken sump pump
  • The pump does not trigger often
sump pump basin in basement

The water in sump pits can get stale very quickly if left sitting. Many models of pumps do not trigger when the float switch is below a specific level. This reduces the number of cycles and could increase the longevity of the pump. However, this may lead to a scenario where the water sits for months at a time, becoming a breeding ground for bacteria, insects and drain flies.

Broken sump pumps and lower water tables are also to blame, since the water does not drain on a semi-regular basis.

types of sump pumps

Getting Rid of Flies in Your Sump Pit

Step 1) Drain the Standing Water

The first step to getting rid of flies in your pit is to drain the standing water. The easiest way to do this is by triggering the pump. If it is broken, you can either replace it yourself, or have a plumber do it for anywhere from $200-350. Otherwise, you will have to remove the water manually, which is time consuming.

Step 2) Clean the Basin

Next, use a stiff brush to scrub the basin walls. This will help remove any buildup where flies might breed.

Step 3) Use an IGR to Stop Breeding Cycles

Finally, you will need to ensure the breeding cycle is stops, or the problem may restart in a few days. The issue is that you cannot pour insecticides down the drain due to environmental regulations. However, you can use an insect growth inhibitor, such as Gentrol.

Gentrol IGR Aerosol Spray

An insect growth inhibitor effectively breaks the lifecycle by eliminating the insects ability to breed. Adults eventually die off without reproducing.

Floor Drains

Another area where flies may breed are basement floor drains or sinks. This is usually occurs when the sink or drain is not used. If you confirm either a sink or floor drain is the cause, follow our drain fly removal guide to ensure complete eradication. This method involves manual cleaning of the drain, followed by the use of a microbial drain gel to remove organic matter.

Sewer Line Breaks

A sewer line break beneath your home may go unnoticed if it is small. Insects are drawn to the area, and will start to feed and breed. If you have a french drain internal style water management system (common with sump pits), then the flies could make their way up into the basement between the small gap between the wall and floor.

As the crack worsens, or contaminated wastewater makes its way into the sump pit, you may begin to notice a foul rotten egg smell. This can be either outdoors, or in your basement.

If you suspect this issue, the only way to properly diagnose it is to call a plumber. They will run a camera down the line and look for breaks. Fixing a sewer line break can be difficult and costly. We recommend that you speak to an exterminator if the plumber confirms the issue.

You can also use florescent plumbing dye to track if water is leaking from sewer lines into the pit.

Simply pour the dye into a toilet, flush and then check the pit for the dye after 10 minutes. We recently did this on a job and used a black light to trace results. It worked incredibly well and will let you know right away if your suspicions are correct!

sump pit water filled with dye
Water filled with fluorescent dye leaching into the sump pit

An exterminator will work together with the plumber to ensure the insects are eradicated. You don’t want to seal up the floor, yet still have thousands of flies breeding in the contaminated mud beneath your property!

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