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Fungus Gnats #682491

Asked March 03, 2021, 12:47 PM EST

Hi - We have fungus gnats in some of our houseplants. I've tried: A dish soap, cider vinegar, H20 trap Peroxide and H20 drench Cinnamon Neem oil spray Mosquito bits Yellow sticky traps Drying out the soil My next step is to try nematodes. The question is: will the cinnamon and mosquito bits that are already on top of the soil affect the nematodes from working? Thank you.

Washtenaw County Michigan

Expert Response

Hi Lindsey, 

Thank you for contacting MSU Extension!

A short answer to your question, yes, cinnamon and mosquito bits will almost certainly be lethal for nematodes. 

Are you certain you are dealing with fungus gnats? What houseplants are infected? How often do you water? Do you allow water to collect in the dish under the pot? 

Deirdre HopePronouns: she, her(s)MSU Extension Program InstructorMSU Extension Master Gardener Coordinator Replied March 04, 2021, 4:12 PM EST
Hi!  Thanks for answering.  

I think they're fungus gnats (?).  Attached is a photo of a sticky trap in the rex begonia pot.

I don't have traps in every pot, but the ones that do have a trap with gnats stuck on them are:  foxtail fern, senecio crassissimus, sedum makinoi, a couple alocasia plants, yucca, fiddle leaf fig, monstera, and a rex begonia has the most stuck on 3 sticky traps.  All are adult plants.

I haven't watered in a month and don't let water collect in the dish under the pots. 

Should I dig out the cinnamon and mosquito bits?  Do I need to repot with new soil?  Nothing has worked so far except for trapping the adults on the sticky traps.  Ugh.  So annoying!

Thank you!


On Thu, Mar 4, 2021 at 4:12 PM Ask Extension <<personal data hidden>> wrote:
The Question Asker Replied March 04, 2021, 4:50 PM EST

Thank you for the photo and more background information. 

The photo shows that your soil appears to be wet. Do all of your containers have drainage holes? If they do have drainage holes, are those holes plugged? You may need to add more drainage holes. Would you like to send me a photo of a couple containers? 

Peroxide and cinnamon applied in high doses can damage the plant and we don't recommend them for this purpose. 

The best way to control fungus gnats is to reduce soil moisture. At this point, I suggest that you repot with fresh indoor potting mix. Dispose of the current potting mix, bag it up and toss. If you chose to use mosquito bits again, follow the label for the correct amounts and mix in to the top few inches of soil. If you chose to use neem oil, it needs to be applied as a soil drench. I would avoid cinnamon, hydrogen peroxide, and other home remedies. 

In nearly all instances of fungus gnat issues, can be eradicated by water reduction, bleach in drains, no standing water, and water/dish soap traps near plants. 

Deirdre Hope

Deirdre HopePronouns: she, her(s)MSU Extension Program InstructorMSU Extension Master Gardener Coordinator Replied March 09, 2021, 11:45 AM EST
Hi - Thanks for the information and suggestions.  The soil in the photo I sent is completely dry.  I haven't watered now in 5 weeks.  Not all the pots have drainage holes.  I guess I'll be drilling some and start repotting.  Thank you again.  Lindsey

On Tue, Mar 9, 2021 at 11:45 AM Ask Extension <<personal data hidden>> wrote:
The Question Asker Replied March 09, 2021, 11:59 AM EST

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