Kill those gnats, or at least try

Semi-serious gnat problem in our bathroom.

I figured I’d experiment some.

Round #1.

Ordered pure 99.8% isopropyl alcohol. Put into spray bottle. When gnats landed on large mirror, I sprayed 'em.

Very gently scooped off the alcohol-wet gnat with 3x5 card. Placed gnat onto white tissue. Placed glass over top.

Gnat rested for around five minutes. Then started moving its legs. And came too, fully. Even able to fly around inside the glass.

Retried the experiment many times. Same result.

Round #2.

Ordered acetone. Did all of the above the same. Same results.

Sort of incredible.

What say you? Are these special super-tough gnats?

What spray, if any, will fairly expeditiously kill the gnats?? Uhh, without killing me as well.

Thanks a million,
Nicholas Kormanik


in my anecdotal experience, flies etc. are remarkably resistant even to full immersion in ethanol. I would have imagined acetone to be a tad deadlier, but I’m not surprised.

As a kid (and an adult when I had some very annoying large fly around the house) I found that house detergents, such as kitchen cleaners in spray bottles, are remarkably effective insecticides. Big flies stun dead after being hit by a single squirt.

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Fumes from many volatile solvents can have a chloroform-type knocking-out effect on many animals. Being so volatile, they overwhelm quickly but also dissipate quickly (especially acetone). They are, after all, the same group of compounds that send us giddy and peculiar (not just ethanol but countless compounds favoured for so-called solvent abuse or ‘glue sniffing’) only to come round relatively rapidly and usually none the worse for wear.

In any case, I would be very cautious of spraying acetone around the place. As it damages the majority of plastics, it could cause untold permanent damage around the house and even to clothing etc.

If I wanted to kill gnats by spraying, I would be inclined to experiment with a spray along the lines of vinegar mixed with a dash of liquid soap.

Flies definitely do seem very hardy. RAID Fly & Wasp Killer spray wipes out wasps in seconds, but seems to take minutes to have any effect on most flies, if any at all. I would be interested to know the reason for this.


I kinda doubt thise are gnats, but anyway they’re in your bathroom only because there’s constant wet, if you solve that problem you will have no insect multiplicating in your bathroom.


The most common indoor fly problems are drain gnats (bleach the relevant drain), fungus gnats (put bt in the relevant planter) and fruit flies (take out the garbage/compost more). As to their robustness, I speculate that animals that feed on rotting stuff are more tolerant of toxins.


If they are attracted to the light, turn off all the lights. Shine a flashlight (I used my cell phone) into the toilet or a sink filled with water. The gnats will fly into the water and get stuck. After a lot of them are in there, flush or drain the water.

Usually I don’t kill anything, but we were at a beach bungalow in Albania and we didn’t notice that the door was slightly open at dusk, and millions of these gnats came in, attracted to the light. We didn’t want to sleep with millions of them in the room, so we invented this system I described. It was 100% successful.


as noted above, more than likely they are fungus gnats…if you have house plants they will breed and reproduce in the damp soil of the house plant.
We used diatomaceous earth to get rid of them in our bathroom…


I don’t know if they’re effective, but seems like a good excuse to get household sundews if you have a spot that gets a lot of light. :)


What’s BT in this context?

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Bacillus thuringiensis


Sticky traps work well. But the best way to eliminate the problem will always be to eliminate whatever they are feeding and reproducing on - as per @schizoform’s suggestions.

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Slightly offtopic since this won’t affect the bathroom species, but for anyone with a fruit fly problem, you can make a kitchen trap with a small jar of cider vinegar with a few drops of dish soap in it (cuts surface tension, flies sink on landing). Optionally fashion a funnel out of paper, place it in the jar neck to keep them inside better once they enter the jar.

Source: Relative who worked in a yeast research lab with a fruit fly research lab on the same floor. ;)


As someone who had thousands of fruit flies in the kitchen — it works, but it never catches them all, you need to find where they lay their eggs and eliminate that possibility for them.


Psychodidae, also known as moth flies. They breed in, and feed on, the residue that collects in the drain, e.g. bits of hair, dirt washed off your hands, and of course the bacterial biofilms that form on the same.

Which disproves the old saw about attracting more flies with honey than with vinegar, doesn’t it?


I’ve found that leaving out a bowl of soapy water and putting a chunk of overripe banana in it works very well. They all drown themselves, and even if they lay eggs you just throw out the fruit before any can hatch.


70% isopropyl alcohol could be more effective than 99.8%. The 70% concentration is recommended for disinfectant purposes (i.e. for killing bacteria and viruses). I believe the issue is that higher concentration solutions tend to make cell walls impermeable, so they protect the organism against taking up more than a small amount.

But vinegar and/or liquid soap could be a better approach overall.


Areosolized alcohol or acetone is the setup for a flamethrower.

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That’s one way of ensuring they won’t reanimate, I guess!

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Welcome, @nkormanik! That is a serious problem. I would recommend an Electric-Flyswatter/Racket. I do not know how effective it would be against gnats, but it definitely helps me against regular flies and mosquitoes!

Figure out where they’re coming from, and plug up or otherwise remove that. Killing them on sight will work very slowly, if at all.