I have a shower stall with a drain that is going unused. My first concern is that if the water evaporates out of the plumbing trap below, I could have sewer vapors coming up, so I pour in some water periodically. However my second concern is mosquito larvae growing down inside there since it's so infrequently disturbed i.e. flushed out. I've seen mosquitos in that bathroom, which makes me suspect this problem.

What's the best approach here to prevent both sewer vapors and mosquitos? I've thought about a one way valve for the drain, or maybe pouring oil or human-friendly insecticide down on the water in the trap.

How would you handle this?

  • Does the drain go to a septic tank such that you can't use bleach? Commented Aug 3, 2018 at 19:43
  • It goes to the public sewer. Commented Aug 7, 2018 at 3:28

4 Answers 4


Pour in some heavier liquid, like vegetable oil. Or put a plastic container lid over it, with a little weight on it so it doesn't accidentally move.

  • Vegetable oil sounds like a winner. I'm concerned if I just block off a drain with water in it, I could get mold or mildew growth. Commented Aug 3, 2018 at 17:05
  • 1
    Mineral oil is a better option as vegetable oil may go rancid and smell. Commented Aug 3, 2018 at 17:12
  • Perhaps alcohol (isopropyl) then. Lighter than water so it floats, will not support life growth, and friendly to the environment when finally washed away. Keep it covered to prevent it evaporating.
    – CrossRoads
    Commented Aug 3, 2018 at 17:50
  • @CrossRoads The lighter alcohols (methanol, ethanol, propanol) are totally miscible with water, so will not float on top. However, I agree that in high concentrations (perhaps unfortunately high enough to have an explosive vapour in a confined space) they will act as biocides. Commented Aug 3, 2018 at 19:41
  • Well, maybe a winterizer like is used in RVs & campers & cabins in the north to keep pipes that are not drained from freezing.
    – CrossRoads
    Commented Aug 4, 2018 at 2:00

I was able to purchase pipe plugs with a wing-nut and gasket that you slip into a pipe, turn the wing nut to tighten the gasket and the problem is solved. Try to "GOOGLE" Cherne Industries for an example. I use the yellow ones. I think I bought them at a home store, the orange or the blue store.

  • Those plugs are intended to be used as test plugs to test drain piping. You plug the pipe, fill it with water (usually from the roof at the vent outlet) and make sure the water level doesn't go down. But using it in the way you have is perfectly acceptable, this is more of an FYI regarding their intended use. As an alternative to the plug, you can put some antifreeze down the drain. It won't evaporate and I assume mosquito wouldn't be able to propagate within it. The antifreeze is commonly recommended for vacation homes in cold climates. Commented Aug 3, 2018 at 23:27

After reading online, the approach I decided on was to pour detergent (liquid soap) down into the drain. My plan is to periodically add water to keep the trap filled and avoid vapor backflow, plus add this detergent so that mosquitoes that touch down on the water will be sucked in and drown due to the effect of detergent on the water's surface tension. Soapy water also works as a sort of home remedy flea trap.

I am accepting this answer since it is what I went with, however would like to recognize others who posted answers that may be just as good (or better). If I have problems with my approach I will revisit this question and may choose a different answer. Thanks, all.


I've been wondering about a similar thing for a drain that is used intermittently. I stumbled upon these seal that one can add to a preexisting traps to prevent evaporation and the back-flow of fumes:

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Sure Seal

Quad Close Stink Stopper

Green Drain Super seal

But I swear once upon a time I saw traps that use something like a floating ball instead.


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