Garden Myths Harmful insects

Yet Another Bogus Fungus Gnat Treatment

Spraying your plants with rubbing alcohol is more likely to cause eye irritation than to kill any fungus gnats. Photo: kiwicare.co.nz

Just when you thought it was safe to go on the Internet, yet another piece of bogus information is being posted. This time, about controlling fungus gnats with rubbing alcohol. 

A reader wanted to know if the “rubbing alcohol treatment” really worked on fungus gnats. I assumed the treatment referred to some sort of soil drench, but I checked online and there they were: not just one site, but dozens of them, all claiming you could control fungus gnats (tiny flies of various genera and species in the superfamily Sciaroidea) by spraying a solution of rubbing alcohol and water on plant leaves. 

On leaves? How is that going to do any good? 

Fungus gnats stuck to a sticky trap
Adult fungus gnats stuck on a sticky trap. Photo: greeneryunlimited.co

Adult fungus gnats may occasionally loiter on leaves, but are just as likely to land on windowsills, walls, potting soil or, indeed, just about any surface. Most of the time, they just flit about our face (they’re attracted to the CO2 our breath gives off) and annoy us. The chances of catching even one on a leaf are pretty slim. And rubbing alcohol has zero residual effect, so if the insect is not on the leaf when you spray, it won’t work. 

The article further suggested if you see any in the air, you could also spray them with rubbing alcohol spray and get them that way. 

Yeah! Brilliant! It’s practically in your face and you’re supposed to apply an alcohol spray? If that spray gets in your eyes, it’s going to sting! And could even damage your eyes. If ever you foolishly follow this advice and get alcohol in your eyes, flush thoroughly with water and if irritation persists, see a doctor.

Don’t you just love the kind of home treatment where you end up in the hospital?

Ignore Them Until They Go Away

Drawing of woman reading while a fly buzzes by.

I must admit I take a fairly hands-off approach to fungus gnats. I just swat any that flit in front of my face and otherwise let nature take its course. Because, in a home environment, they tend simply go away on their own. True enough, eventually they come back for a week or so many months later … but then they disappear again. Chasing after them is just not worth my while.

Especially since they are largely harmless to my houseplants anyway. 

First of all, the adults don’t even eat. At most, they soak up a bit of water or flower nectar (some are even effective pollinators!), but no chewing is involved. They just fly about, looking for a place to lay their eggs.

Ill. pngkey.com & Blaise Sewell, the Noun Project

As for the larvae, they are tiny white to translucent, legless, dark-headed grubs found near the surface of your plants’ soil. As their name, fungus gnat, suggests, they mostly eat fungus and algae found growing invisibly in soil. Indoors, that would be houseplant soil. They also decompose organic matter, so are, in fact, beneficial to our plants in that regard, as this releases minerals plants can use for their growth. They may occasionally nibble on plant roots, but usually only roots rotting due to overwatering … and is that such a loss?

So, you really can ignore them if you want. 

Other Effective Treatments

  • Yellow sticky traps;
  • Letting the surface layer of soil dry out before watering again;
  • Covering potting soil with a layer of sand or fine gravel;
  • Growing insect-catching plants (butterworts [Pinguicula spp.] are fantastically efficient at this);
  • Lighted insect traps;
  • Adding crushed mosquito dunks to houseplant soil;
  • Applying beneficial nematodes such as Steinernema feltiae or mites such as Stratiolaelaps scimitus;
  • Drenching the soil with a solution of insecticidal soap or neem.

Detailed Information

For more detailed information about fungus gnat control, read Keeping Fungus Gnats Under Control

Beware Untrustworthy Sources of Information

And do not follow advice picked up just anywhere on Facebook or the Internet without vetting it first. Visit a serious site you know you can trust. 

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

3 comments on “Yet Another Bogus Fungus Gnat Treatment

  1. I keep a yellow sticky trap out, and you can certainly see if it is successful.

  2. Overwatering, or more accurately, watering too frequently, is a common problem. Fungus gnats are not much of a problem, but might indicate that the soil is too damp. The second recommendation is probably the best, “Letting the surface layer of soil dry out before watering again.”

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