How To Remove RV Sewer Flies and Gnats

Do you walk into your RV bathroom only to be swarmed with gnats or flies coming from your RV toilet and black tank?

Ugh! Nothing like having to swat all these pests down to take a leak. Or shower. In which case you’ll come out of the shower feeling like you need another shower.

Yes, RV sewer flies are a real problem.

How to remove RV sewer flies and gnats

Disclosure: This page contains affiliate links for various products below. You get the same low prices and we earn a small commission to help us buy more RV tailgating gadgets. Or game tickets if you go on a shopping spree.

What’s Infested Your RV Waste Tanks

The gnats and flies coming out of your toilet are total pests. They are typically slightly lighter in color than your average black fly. And they are fuzzier.

But I doubt you’ll want to spend too much time up close and personal with them to actually identify them. I mean, they did come out of the black tank.

The Good News About Sewer Flies

The drain or sewer flies (you’ll see them referred to as both and perhaps even moth flies) do not bite or sting. They are also poor flyers. They don’t go far from where they started. When they do fly, it is typically only for a few feet at a time.

They like to rest in the shade but are attracted to light. This is why those electric bug zappers work so well.

Unfortunately, it’s also why you feel like you are getting swarmed by the flies when you open up the toilet. They are attracted to the light!

(Kinda) Good News About Sewer Flies

The kind of good news about these sewer flies and gnats is that they aren’t known to spread diseases to humans.

At least directly.

However, they are pests. And since they live in the black tank, where there’s likely to be some unpleasant bacteria, they can be covered in this bacteria. That bacteria can be dangerous to you, especially if the flies land on food and contaminate it. Since the RV kitchen is often right next to the RV bathroom, it is entirely possible.

Also, some people have reported aggravation of asthma from flies and their um, debris.

More Bad News About Sewer Flies

The bad news is that the sewer flies live and breed in decomposing organic material. Yep, that’s pretty much the definition of your black tank. And probably even your gray tank.

More bad news about these flies – they can go through an entire life cycle, from eggs to larvae to adult in as little as a week, although 2-3 weeks is more common. And let me say, that your RV black tank is pretty favorable conditions.

That means that they are replacing themselves as fast as you can typically kill adult flies.

Did I mention that drain flies can lay around 30 to 100 eggs at a time? Oh, and that the drain fly is sexually mature as soon as it hits the adult stage and begins the reproduction stage within a few hours of becoming an adult?

Yep, your sewer flies are having sex right away. (sorry, if I had that mental picture, so do you!)

Prevent RV Sewer Flies

The first thing you’ll need to do is go outside to your RV sewer hookups. Close the tank dump valves.

You should only open the black tank dump when you are actively dumping the RV black tank.

You’ll want the waste to actually build up in the tank – mostly to avoid the dreaded poop pyramid. That’s what happens when you leave the black tank open, the liquids drain but the solids pile up. Over a long period of time, this is going to cause a very big and disgusting problem.

The other issue with leaving the black tank open is that odors from your sewer hookup will drift up. And if odors can come up, so can sewer flies and gnats.

I’ve even heard of sewer rats coming up through the RV sewer hoses!

Seriously, leave the black tank valve closed until you are ready to dump.

What About the Gray Tank Valve?

Some people like to leave the RV gray tank open so they can take long showers, wash dishes and clothes, and what not. If you have a family all in the RV, then you can go through a lot of gray water really fast.

That means that you’ll have to go outside to open the gray tank pretty regularly.

I recommend that you leave both the gray and the black tanks closed until you need to dump the tanks. I think it is worth the extra effort of going outside to open the drain to prevent unwanted sewer-dwelling guests in your RV.

However, if you are going to leave the gray tank valve open, then you need to create a P-trap in the hose. It needs to be a deep enough drop in the hose that the entire hose can fill with water. This will create a barrier to stop the flies and gnats from coming up through the gray tank.

See Also: RV Gray Tanks: The Other RV Waste Tanks

How to Get Rid of RV Sewer Flies

Now that you have closed (or verified that) the black tank valve, it’s time to start getting rid of the insects that have made your black tank their new home and breeding grounds.

Yep, these flies are laying eggs in your black tank and that is why you can never seem to get rid of them once you get them. They lay their eggs in moisture or standing water.

But you just said to keep the water in the tank?!?!

Yes, I did. That’s to keep the flies from getting into your tank in the first place.

Now it is time to get rid of the flies. And really what we are focusing on is the fly eggs and larvae. To stop the reproduction cycle.

First, empty the black tank of all the contents, like you normally would do. We want all the poop and urine out to begin with.

Fill the Tanks

In a sticks and bricks home, bleach typically is not an effective treatment to kill sewer flies. This is because the pipes in your home are typically open all the time. So the bleach flows right on past the eggs.

But since we have a closed black tank valve, we can actually use bleach to our advantage. If you fill the tank and add bleach to the tank, the bleach will actually have enough time to get to the larvae and kill them.

CAUTION: Never mix bleach and ammonia! Combining chlorine and ammonia can create chlorine gas, a potentially fatal situation, especially in the small confines of an RV bathroom.

In fact, do not mix bleach with any other chemicals or cleaners, as many may contain ammonia or other potentially dangerous acids.

How Much Bleach?

I recommend that you go with the same concentration that we use for sanitizing the freshwater system.

Pour down the toilet and into the black tank 1/4 cup bleach diluted into one gallon of water for every 15 gallons of black tank capacity. Then fill up the black tank to the very top.

Why the very top?

That’s actually one of the most likely of places for the fly larvae to be developing. Right near the top where there is organic material to feed on but not enough water and tank treatments to kill them off during normal black tank operation.

Let the Black Tank Marinate

Now let the bleach and water sit for awhile, at least four hours, just like when you sanitize the water system.

But the longer the better. Overnight if you can.

You want to make sure that the bleach has time to actually penetrate the organic film in the tank to get to the eggs and larvae.

Drain and Rinse

After you’ve let the bleach really soak in to kill the eggs and larvae, you need to drain the black tank.

And give it a really good rinse.

Remember, you need to get the bleach out before you start adding your normal RV black tank chemicals back in to prevent the formation of chlorine gas.

If your RV has a backwash system for the black tank, then use that.

Alternative RV Black Tank Rinse Systems

If not, you’ll need to use one the alternative methods. You’ll want one of these methods available for normal operations, so go ahead and invest in one that works for you.

  • Install a Dual Flush Pro with Rinse Valve: this will require a little of DIY skills to install to replace the current valve. Easy to hook a hose to like the built-in systems on the newer RVs.
  • Use a Tank Wand to Rinse the Tank: This will require that you attach a hose from either the bathroom sink or bring the hose through the RV. My old Winnebago – Starter RV – didn’t have a black tank rinse system and I always found that the better water pressure of bringing the hose in through the RV was worth it to get the stuff unstuck from the walls. A hose hooked to the bathroom sink didn’t provide enough water pressure to do much good.
  • Use a Flexible Tank Wand to Rinse the Tank: If the pipe from your toilet to your black tank does not go straight down, and instead has some bends to it, you’ll need to use a flexible tank wand instead of a straight tank wand.

The point is, whether you are using a built-in black tank rinse or one of the alternative methods is to give the black tank the best rinse you’ve had. With lots of water pressure.

You want to make sure that you knock down any remaining organic film and all the eggs and larvae. When you think you’ve done a great job, do it again.

See Also: Make Your Own RV Black Tank Cleaning Bombs

Bleach Alternatives to Killing Sewer Flies in RV Black Tanks

I know that a lot of people are concerned about using bleach in their tanks.

The first concern would be the possible interaction with ammonia to produce chlorine gas.

The second concern is that bleach can be corrosive at high concentrations. This can break down the seals used in the black tank at the valve. And the bleach method leaves quite a bit of bleach in the tanks for hours at a time.

The third concern is debatable, but some claim that the bleach will kill off helpful bacteria in the black tank that helps to break down solids. I say it is debatable because the RV black tank is not a septic tank. It’s a holding tank.

That’s not the debate for today’s conversation. I’m just pointing out that some people have this concern. Since it is not the only reason that one might want to skip the bleach, I’m acknowledging it and moving on.

Enzyme Drain Cleaners

One alternative to bleach to remove gnats and flies from the RV black tank is enzyme drain cleaners.

The enzyme cleaners work because they attack the layer of film of organic material that the flies lay their eggs in and that the larvae use to develop into adult flies. Get rid of that layer and you take away their food source. Then you rinse them all down the drain.

The Zep brand enzyme cleaners are certified biodegradable and are septic system safe. Once the enzymes go through their life cycle, the cleaner breaks down into carbon dioxide, minerals, and water. Because they are formaldehyde free, they are safe to dump in all RV dump stations.

How to Kill RV Sewer Flies with Enzyme Drain Cleaners

You would follow a very similar method that you would with the bleach method above.

  1. Drain the black tank like normal.
  2. Add water and the enzyme drain cleaners. Fill the rest of the black tank with water.
  3. Allow the tank to sit full of water and the enzyme drain cleaners overnight.
  4. Drain the tank.
  5. Rinse and re-rinse the tank.

It may take several applications of the enzyme drain cleaners to get the entire layer of film gone.

Also, you may want to use warm, but not hot, water when you fill your tank. Hot water will kill off the bacteria enzymes. Warm water will provide a better environment for them to get to work than cold water will.

It’s typically not necessary to use an enzyme cleaner in normal RV black tank operations. First, the human body will produce enough bacteria in your waste that the poop begins to break down right away. Second, the waste is typically not in the RV black tank long enough to be very effective. Remember, it is a holding tank and not a septic tank.

But since we are specifically using the enzyme drain cleaners for a longer period of time, they can work here to really help break down the layer of film.

Another bonus: it should help clear up the sensors for your black tank and get rid of the odor.

Mechanical Cleaning

As noted with both the bleach and the enzyme cleaning methods, one area that can be a good breeding ground for drain flies is the area at the top of the tank, where the organic film is likely to build up but not be regularly treated in normal operations.

Another area is right at the valve from the toilet into the tank. Or the pipe from the sink to the tank.

You can often get to these areas and clean them with a drain brush. You know, good old fashioned elbow grease.

I do caution you against using these deep into the drains, especially the shower drain. Even to pull out hair and what not. RV showers and even sinks often have a waterless Hepvo valve instead of a P-trap. Using snakes to clean out the pipes and even long brushes can damage these valves. The waterless Hepvo valves take up much less space than a standard P-trap, which is why they are so useful in RVs. In addition, because they are waterless, you don’t need antifreeze in the valve. There is nothing to expand if it freezes.

If you do use the drain brushes, you’ll want to clear out the film that lives in the pipes, particularly near the top where the bleach or enzyme treatment is not going to reach.

See Also: Winterizing Your RV For Storage

A Note About Treating RV Gray Tanks for Sewer Flies

The flies and gnats can come up through the RV gray tanks as well as the black tanks.

Typically though, you don’t see this quite as much as you do the black tanks. The soaps and shampoos that we use to wash dishes, do laundry (for those with laundry machines), and to take showers do a pretty good job of keeping the organic material to a minimum and cleaning out the gray tank.

This means that the occasional fly that does make it through the gray tank won’t have a breeding ground for future generations.

However, that doesn’t mean that it is impossible to get flies in your gray tanks.

The good news is that the treatment is very similar to the black tanks. The only problem is that most RVs don’t have a gray tank rinse system built in and there is no access to put a tank wand from the inside. To rinse out the gray tank, you’ll need to fill it up and then empty it out again.

For the P-Traps

Many times the RV sewer flies that are coming up through your kitchen or bathroom sinks or the shower are not actually from the gray tank itself.

Rather, the flies are laying their eggs in the P-traps where you can get hair, grease, and other organic material clogs. Once a fly gets in there, it can be quite a nice breeding ground.

Because the eggs and larvae are in a smaller area, you can use a different treatment, rather than the bleach or enzyme methods above. And you probably already have the ingredients in your RV or your home.

Baking soda, salt, and vinegar.

It’s similar to what you use to unclog showers.

  1. Pour a gallon of near-boiling hot water down the drains.
  2. Add 1/2 cup of baking soda down and 1/2 cup of salt the drain and let sit for five minutes.
  3. Pour a cup of household white vinegar down the drain. Enjoy the fizzing action.
  4. Let sit overnight.
  5. Rinse the drains with hot water to remove all traces of baking soda, vinegar, and hopefully any larvae and eggs.

Just like with ammonia, never mix bleach and vinegar together. The weak acid from the vinegar combined with the bleach will create potentially fatal chlorine gas.

Getting Rid of the Flies

So cleaning out the RV waste tanks, whether the black tank or the gray tank, really works to get rid of the eggs and larvae, the early stages of the drain fly life cycle.

But the part you generally see is the adult fly – the one buzzing around being a pest to you.

Fortunately, they only live about 20 days or so and will naturally die off.

But no one wants to wait 20 days for them to die.

Fly Traps

Your best bet is going to be old-fashioned fly traps.

You can get several different versions, like the disposable bag traps or the sticky yellow ribbons.


Another option is to use a contact aerosol containing Pyrethrin.

These often have a really strong odor, so you’ll want to spray and get out of the RV for awhile. Including your pets.

These are great for spraying down the open toilet and then closing the lid. And perhaps in the small bathroom area.

Make sure to ventilate the area well upon your return. And then wash everything down well.

Borax Insect Spray

Another solution for the adult flies is our good friend Borax. You know, the laundry detergent booster that we keep on hand for the RV Black Tank Bombs.

Borax kills insects by poisoning them from the inside if digested and will also damage the skin on some insects. Death can result for many insects.

Anyways, make up a insect spray with 2 cups warm water and 3 tablespoons of 20 Mule Team Borax. Mix well and then spray the flies directly and all around the bathroom or kitchen area. Let it dry.

The borax dust will kill off the flies when they land on the residue.

Caution, although typically not harmful to humans or pets, large doses can cause potentially serious health issues. Use with caution, particularly around children and pets.

One advantage of using Borax as a spray over just spreading the dust is that you are less likely to inhale the dust. Inhalation can cause irritation of the breathing passages. The dust may also irritate your eyes, mouth, and even your skin.

Did you know? In addition to the black tank bombs and killing sewer flies, Borax is also great for killing fleas.

Drown the Flies

Another recommended way to get rid of adult drain flies is to setup a water trap for them. Many of the fly traps that you can buy use a drowning method. But this is a DIY version.

Use a bowl to mix up with equal parts water and apple cider vinegar. Add a small amount of Dawn dishwashing liquid soap. Leave near the flies (next to the sink or in the shower stall is good).

The flies will be attracted to the scent of the apple cider vinegar. And then be stuck by the soap and drown.

This method also works awesome for fruit flies, should they ever be a problem.

Still Having Trouble Getting Rid of RV Sewer Flies?

All of these solutions have really focused in on the tanks and the pipes in your RV. But there are some other breeding grounds for sewer flies in your RV.

For example, if you have a leak in any of the pipes, you can create a new breeding ground. Say under the sink, shower, or around the toilet. Or in your RV basement compartments. Shower pans in particular are prone to leaking and causing fertile breeding grounds in hidden compartments that don’t have easy access to kill the eggs and larvae.

Moist mops can also become a breeding ground. It’s one reason I really like the Swiffer Sweeper Wet Mops and Refills. When you are done cleaning, you throw away the pad and nothing is left for pests to breed in.

Rotting fruits and vegetables are another source for flies to breed in. And if you throw the rotting food into the trash can, the trash container will need to be cleaned to get rid of the flies.

Do you have a washing machine in your RV? If you have wet lint under, behind, or around the washing machine, it can become another favorable breeding ground for flies.

One final spot to find these flies that will be unique to RVers – the sewer hoses. If you don’t properly rinse and drain the hoses prior to storage, the hoses and wherever you store the hoses can become another breeding ground for RV sewer flies. So make sure you rinse and drain the sewer hoses well before you store them away.

Locating the Fly Source(s)

If you are having trouble finding the breeding grounds, then look in the evening when the flies are most active.

And have a light source. Remember how they are attracted to the light? Use that to your advantage to hunt them down! And kill them!

Another way to track them down is to leave pieces of tape in likely areas (sticky side up or out or pointing towards the likely source, of course). This is great for trying to identify specific drains in small areas, like an RV bathroom. Cover the drains with tape for 1-2 days. Come back and check to see if any flies have stuck to the tape. You’ve found your source!

It can also work in smaller spaces like the RV basement if you think that the likely source is from a leak.

Did You Get Rid of the RV Sewer Flies?

How to Remove RV Sewer Flies and Gnats - Kill and prevent future fly infestationsRV sewer flies can be difficult infestations to get rid of. As soon as you think you got them all, more will appear. Partially because there are so many places for them to breed. It may take several attempts and several methods to get rid of all the sewer flies and gnats in your RV waste tanks.

Because many of the methods discussed here involve letting the tank sit and marinate with the treatments in the waste tanks for some time, it is easier for part-time or weekend RVers to treat than it is for someone living full-time in an RV.

Another reason that full-timers especially need to focus on preventing the flies in the first place. Remember, keep those RV black tank valves closed!

Like these tips? Pin for later!

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.


  1. I’ve tried every idea ever posted on the web to get rid of the flying NIGHTMARES!
    When I get finished building my house and move out of this giant fly trap………..I’m gonna burn it to the ground and every ##### one of the flies in it !

    1. Darryl, so sorry to hear about your troubles! It’s so frustrating to get rid of the flies. If you miss just one little fly egg, you’ll get them back in no time. And it’s not like we can stick our heads down the pipe to really check it out to make sure we get them. It’s especially hard if you are living in the RV full time and can’t just fill up the tank to the very top and let a bunch of chemicals just sit forever and kill them all off. I definitely feel your pain here.

      Let me know what you’ve tried already. Maybe we can work together to find some new solutions to try.


  2. Hi so I’m new to the RV world and actually I am now renting and living in an old pull behind RV that is no longer moveable it is hooked up to the owners syptic tank well maybe it’s got PVC pipies running not sure where they run to. But anyways I have been having I assume drain fly larvae/pulp black little catapulir things coming out of toilet how do I get rid of these

    1. Miranda, welcome to the RV lifestyle!

      First, I would make sure that your black and gray tank levers are closed except during the time you are actively dumping the tanks. This will help prevent things from coming up through the sewer hoses. You can leave the hoses hooked up to make it easy, but keep the drains closed.

      Second, you want to make sure that you are dumping and rinsing well. The drain flies will continue to breed until you clear them out. That is where the tank treatments come in. You may need to do it several times, but fill the tank completely full with fresh water. You can add bleach to help kill the existing larvae. But remember, they are often near the top of the tank or around the bottom of the toilet. So when I say full, I mean FULL. Let the bleach and water sit there for several hours – similar to the four hours you would do for sanitizing the fresh water system.

      If that doesn’t work, try Bioclean using the same method as the bleach. Fill up the black tank and then add the recommended dose of Bioclean.

      Finally, make sure that it is the black tank/toilet where the flies are originating. It is also possible that they are coming from the gray tank (sinks, showers). You can do the same treatments for the RV gray tank as you do the black tank, although it is often more difficult to get bleach/Bioclean in the pipes leading to the tanks. It may take several treatments to get all the flies.

      Good luck!

  3. Thank you for this information. I am not clear about the amount of chlorine. If I have a 30 gallon tank, would that be 1/2 cup since you say 1/4 for every 15 gallons or 1/4 cup per gallon for the first 15 gallons, or about 4 cups?

    1. Terry, it would be 1/2 cup total to disinfect the freshwater system. 30 gallons/15 gallons * .25 cup = 0.5 cup

      Just a reminder if you are using chlorine in the black tank: chlorine/bleach will react with ammonia, commonly found in human urine, to produce dangerous gasses. Make sure you dump and flush your black tank before and after using bleach in it to avoid these gasses.


  4. having problem with those flies in a trailer park hooked up to the so don’t know what to do i am new to living in rv full time so i need all the help i can get thank you.

  5. We’re going to try your solutions. One at a time. Question: our RV will be in storage for about a month. What can we leave in the black water tank to prevent further infestation after a good cleaning?

    1. Howard,

      Your best bet here is to make sure that you have cleaned your blank tank as well as you can – deep cleaning. Without a “food source” the RV sewer flies and gnats should not be able to reproduce. Don’t forget the space at the top of the tank, like the pipe from the toilet to the tank. That’s a common area for the flies to leave their eggs.


  6. We’re full timing in our motor home. Had all tanks professionally cleaned a week ago. We Always keep tanks closed and dump when full. Also we rinse after dumping black tank. Now a week past professional cleaning we’re seeing the small flying bugs Again! Help!!!

    1. Cindy,

      It’s likely that you missed some of the bugs at the bottom of the toilet/right at the top of the pipe. I definitely recommend Bioclean Drain Cleaner to help you kill the bugs and clean out the “film” of stuff that they lay their eggs.

      Good luck!


  7. We live in our coach and have been struggling with the nasty little critters for about 6 months. We have Macerator toilets so we don’t believe they are coming from the toilets. We have tried the salt, soda and vinegar. We have taken the pipes under the sinks apart and cleaned them, checked behind the toilets, sprayed the underbelly with chemicals, put thick goo recommended by a blogger down the sinks, can’t find a leak or area where they are coming from. We have an electric mosquito trap with a light that catches some, they seem to be all sizes. We are going crazy. Any suggestions will be appreciated.

    1. One way to check to see if they are coming from the toilets or the sinks is to put tape over the toilet and sinks for a few hours. Then check to see if you have any gnats on the tape.

      I wouldn’t eliminate the toilet just because it is a macerating toilet. There can still be a build up past the macerator that would allow the gnats to breed. And once they start breeding, you have to find where they are laying their eggs to get rid of them in full. Near the top of the waste pipe is common because it is so hard to clean, having to fill the black tank all the way up to let the chemicals soak.

      Don’t forget that they could also come from the shower drain.