Today I decided to clean the water-pump filter of my swamp cooler, and there were a couple of larvae swimming in the water reservoir and some mosquitoes flying over it. I am looking for a way to kill the larvae and prevent the mosquitoes from breeding here in the future.

  1. I can't drain the coolers everyday. It is a tedious chore if done manually. And it wastes a lot of water (around 70 liters per unit). Also I suspect that some puddles will remain in the cooler even after draining.

  2. I can't use bleach or insecticides. The water comes into direct contact with air that people will breathe. I tried bleach a couple of years ago and it gave off a horrid smell.

So what other effective steps can I take to solve this problem? Doing some online research, someone suggested that I should add dish-soap to the water. Will that even work?

  • It might be worth looking into mosquito dunks. May 7 '20 at 11:44
  • Dish soap now that would be funny, a mountain of bubbles comes to mind.
    – Ed Beal
    May 7 '20 at 14:57
  • @EdBeal I actually tried it out. Adding too much (1/2 cup maybe) does cause a mountain of giant bubbles which quickly builds up till it starts messing with the fan\blower and sends soapy water into the room. Adding a small amount (10 grams) causes foam at the surface of the reservoir but does not cause any disruption. I don't however know if it is effective against mosquitoes. May 8 '20 at 10:03
  • You might want to read the back label on mosquito dunk/granules. It lists a hazard if breathed. Perhaps that only applies to the concentrated product as packaged. But I would want to be sure it is safe to be breezed coming out of a swamp cooler, for sure, before I used it in my home evaporative cooling system.
    – user122772
    Aug 30 '20 at 15:53

There are biocide tablets for this specific use, large commercial ac systems use this as mandated by law in most areas.

I would rather be protected from legionaries and mosquitoes and bad smells. Ours uses CTT-EC , but look up evaporative cooler treatment, you may find out that untreated water is not legal in commercial facilities, and a health hazard.

If it is the law in many areas it is probably a good idea for home use.


BT ( bacillus thurngensis ) will kill insects . It is not a biocide so will not affect animals.It kills anything with an alkaline stomach and has no affect on anything with an acid stomach like an animal. Commonly available at garden shops.I can't imagine a problem in your system. It can be put into ponds as it does not affect fish. It can be sprayed on vegetables right up to harvest and is safe to eat. I expect you would put in the same dose as recommended for a pond of the same volume.

  • Interesting. Although I can't find this locally, will have to import it. May 7 '20 at 17:46

Peppermint essential oil works great.

  • Welcome to Home Improvement. Could you please edit your post to indicate how/where/when one would apply said oil? If you'll take the tour, you'll note that we tend to like more complete answers, and, as it stands, this is more of a comment than an answer.
    – FreeMan
    Aug 5 '21 at 13:52

Without knowing exactly what the swamp cooler looks like, my first thought is I would take the preventative approach of screening off the areas mosquitos might use to access the water.

Assuming you have a commercial unit with a metal case and vents, I would get some standard fibreglass window screen, cut it to size, and attach with magnets. You could get fancy and sew the magnets into the screen, or just stick them on the outside to hold it in place.

By avoiding additives and taking a preventative approach, you can limit any impact on the functionality of your cooler.

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