What Causes Drain Flies?

A partially clogged or slow-emptying drain is the most common cause of drain flies. In the wild, drain flies are drawn to stagnant water because the bacterial buildup in these areas serves as a food source. In your home, a slow-draining sink or bathtub encourages bacteria to grow on the walls of your pipes. This attracts drain flies. They will lay their eggs in the bacterial buildup, causing an ongoing infestation. Luckily, you can eliminate these infestations by clearing your drains and killing the bacteria inside your pipes with an enzyme-based drain cleaner.

What causes drain flies?

What Attracts Drain Flies to Your Home?

Drain flies are drawn to the scent of bacteria and decaying organic matter in water. The main food source for drain flies is this polluted water. If drain flies have invaded your home, it’s because your drains contain bacteria or bits of other organic matter (such as food). This typically occurs when a drain is partially clogged and drains slowly. The scent of dirty water trapped in the pipes draws drain flies from outside.

  • Drain flies are drawn to the scent of stagnant water that contains bacteria and decaying organic matter.
  • A slow-draining kitchen sink may trap food in stagnant water, providing the perfect feeding and breeding ground for drain flies.
  • Drain fly larvae thrive in any partially clogged drain, including bathroom sinks, bathtubs, and showers.

Even if there is no food in your drain, drain flies might still invade your home. Standing water in slow-draining bathtub plumbing can still cause bacterial buildup. Drain flies will then lay their eggs in this buildup. So, drain flies are attracted to your home in search of a place to feed and lay their eggs.

How Do You Find Where Drain Flies are Coming From?

To find where your drain fly infestation is coming from, begin by searching your home for sources of standing water. Although drains are the most common place for drain flies to congregate, there are other locations that can attract them. Inside your home, a basement sump pump, potted plants, and unused toilets are breeding grounds for drain flies. Standing water outside your home can also attract flies that later find their way inside.

  • Search for indoor areas where drain flies breed: sinks, overflow drains, shower/bathtub drains, sump pumps, unused toilets, and potted plants.
  • Check outside your home for standing water that may attract drain flies. Buckets, tarps, and planters can hold water.
  • In the daytime, drain flies will remain stationary near their feeding area.
  • In the evening, drain flies will fly around their food source.

Once you’ve identified possible places where drain flies may be coming from, observe these locations in the daytime and in the evening. During the day, search for drain flies sitting still on surfaces near their feeding area. For instance, if you see drain flies on your shower walls during the day, they’re probably coming from your shower drain. In the evening, drain flies will buzz and hover around their food source. Wait until sunset and check your home. Drain flies hovering around your potted plants means they’re feeding on the excess water in the pots.

Can Drain Flies Come From Your Toilet?

It is very rare for drain flies to come from a toilet that is regularly used, flushed, and cleaned. Drain flies require dirty or stagnant water. Flushing a toilet regularly means flies won’t have much luck laying their eggs there. An unused toilet or one that is left in a dirty condition can attract drain flies, however.

  • It is very rare for drain flies to come from a toilet that is regularly flushed and cleaned.
  • In some cases, drain flies will breed in the toilet tank, provided the tank is accessible and dirty.
  • Toilets that are not flushed for weeks can become infested with drain flies.

Because it is rare for drain flies to breed in a toilet bowl if the toilet is regularly flushed, it’s best to check the tank. If the toilet tank is dirty and the flies can find a way in, they may breed there. This is still rare. The most common cases of drain flies coming from toilets occur when the toilet is seldom used. A toilet in the basement that is not flushed for weeks or months can attract drain flies and provide a place for them to lay their eggs.

Do Drain Flies Mean You Have a Plumbing Leak?

It is not common for leaky pipes to cause drain flies in your home. In most cases, the opposite is true. A drain that is partially clogged with food, hair, or any other substance will trap water in the pipes. This creates a slimy bacterial buildup on the inside walls of your pipes. Drain flies will lay their eggs in this buildup. So, the presence of drain flies usually indicates a clogged drain, not a leaky pipe.

  • Drain flies are not commonly caused by a water leak in your home.
  • Standing water from a partially clogged drain is more likely to cause drain flies than a leak.
  • Clearing drain clogs with Drano or other methods will help prevent drain flies.

Because drain flies flock to poorly functioning drains, Drano is great for killing drain flies. The cleaner and faster draining your plumbing is, the less likely you are to get a drain fly infestation. So, regular drain cleaning and maintenance are key for preventing sewer flies.

Are Drain Flies Seasonal?

Drain flies are a seasonal nuisance in most climates. Temperatures below 70℉ (21℃) cause drain flies to be less active. If temperatures drop below this point, drain flies will go into hibernation until things get warmer. So, if you live in a region with cold winters, your drain fly problems will lessen during the colder months.

  • Drain flies become more active in warm temperatures—70℉ (21℃) and higher—so they are at their worst in spring and summer.
  • In many climates, drain fly problems will lessen in winter.
  • In warm regions, drain flies can remain active all year.
  • If drain flies have invaded your warm home, they may continue to breed all winter.

In warm regions, drain flies can be a year-round nuisance. Plus, drain flies that have invaded your home will be subjected to far less cold temperatures. So, a bad drain fly infestation can continue into the winter months if the drain flies have gotten a strong foothold in your home’s drains.

Why Do Drain Flies Keep Coming Back?

Drain flies will continue to plague your home if you do not find the source of the infestation and kill the drain fly larvae. You may be frustrated if you found and killed dozens of the adult flies, only to see more a few weeks later. This is because drain fly larvae take 4 weeks to mature. So, even if you kill the adults the fly larvae in your drain will transform into adults and repeat the cycle.

  • Drain flies come back if you kill the adults but do not kill the larvae.
  • Drain fly larvae take 4 weeks to mature into flies, so they can continue the infestation even if you’ve killed all the adults before.
  • Identify where drain flies are breeding so you can kill the adults and larvae.

Using a fly swatter or fly traps will kill the adult drain flies, but it won’t end the infestation. So, it’s essential to search your home for places where drain flies gather and breed. Then, make a plan to kill the whole fly population. 

How Do You Permanently Get Rid of Drain Flies?

The best way to permanently rid your home of drain flies is by identifying where the flies are breeding, then taking steps to kill the adult flies and larva. You can use apple cider vinegar traps and fly paper to kill the adult flies. Then, pour an enzyme-based drain cleaner into your drain to kill fly larvae and the bacteria they feed.

  • For the best methods of getting rid of drain flies, check our comprehensive guide for killing drain flies.
  • Simple fly traps and fly paper will kill adult drain flies.
  • Treat your drains with this enzyme-based drain cleaner to kill drain fly larvae.
  • Drain standing water from potted plants and spray the plants and pots with this insecticidal soap.
  • Drain, clean, and cover sump pumps to stop a drain fly infestation in your basement.

If drain flies are infesting your potted plants, drain any excess water from the overflow portion of the pot. Then, follow up by spraying the plant and pot with insecticidal soap. Other sources of standing water—such as sump pumps—should be drained of water, cleaned, and sealed to kill off drain flies in every stage of life.

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What Causes Drain Flies in the House?

Drain flies commonly invade your home for the following reasons:

  • Standing water outside your home.
  • A partially clogged or slow-draining sink, shower, or bathtub.
  • Indoor potted plants with excess water in an overflow portion of the pot.
  • Unused toilets that contain standing water.
  • Sump pumps with standing water.

Search your home to see where drain flies are gathered. During the daytime, they will sit still near their food source. In the evening, they will fly and hover near the source of the infestation. Once you’ve pinpointed where they’re coming from, you can take steps to quickly kill drain flies and their larva.

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