There’s A[n Unwelcome] Party In My Plants

Dear Home-Ec 101,

I have a dilemma in that I looked up the tiny flies that are strolling around my plant soil. I found that they are called fungus flies and live on the decaying soil matter. I’ve sprayed the soil with Home Defense both bug spray and soapy water (nope), put a dish of apple cider vinegar with a drop of dish washing liquid in it to bait them (nope), and most recently, put coarse sand on top of the soil so the “babies that hatch” can’t crawl up and out and die (yech). Needless to say, they are no longer visible on that plant but the fliers have moved on to my other plants. I will never buy that type of soil again (with small wood chips or something) because I’ve never seen this before.

Is there any other way to rid myself of these without buying enough sand to put on all of my plants? They don’t damage the plants (I’m told), but I can’t stand bugs.

Signed,
There’s a Party in My Plants and They Won’t Go Home

Heather says

Fungus gnats, fruit flies, and drain flies are all pretty annoying, but mostly harmless pests. I say mostly because they are aggravating as all get out.

Cider traps don’t work for fungus gnats like they do for fruit flies because fungus gnats don’t eat rotting plant material, they eat what grows on the material. Fungus gnats smell cider and say, “Sorry, I’m just not that into you.” Okay, maybe not literally, but close enough for our purposes.

The solution for getting rid of the fungus gnats has three parts; the most important being perseverance. Due to the life cycle of the flies, it’s going to take weeks to get rid of the little  buggers (ha ha) once and for all.

Dry out the fungus the gnats feed on.

First, you’ll need to ensure that the top two inches of soil are as dry as your plants can tolerate.  Those two inches will need to stay dry for as long as they will tolerate. If possible, practice what is called bottom watering.

To start bottom watering, you’ll need to set the plant’s pot in a container of water. Ensure that the water level in the container does not rise above the top two inches of soil. Let your plant hang out in the container until you begin to feel moisture along the wall of the container in the top two inches.  You’ll want to avoid setting the wet pot on a surface that can be damaged by water until the container itself is dry.

Allowing the top two inches of the soil to dry will reduce the amount of food supply the larva have available.

Capture the breeding adults.

Unlike immature humans, fungus flies can’t breed until they are fully grown. This handy evolutionary trait allows you to implement step two – hopefully before the procreation happens.

Find sticky traps like these Gnat Stix. You should be able to find them in the garden section of most big box stores or your local nursery. You may want to give them a call first to make sure, though. You’ll want to place at least one sticky trap in each of your plant’s container. Replace the traps when the stickiness wears out or you can’t stand looking at the little carcasses any longer.

Keep at it.

Here’s where the perseverance part comes into play. You’ll need to keep using both of these techniques for a few weeks after the fungus gnats first appear to be gone.

Why?

The next batch of eggs and larva are hanging out in the soil and are just waiting for you to water your plant from the top and ring the bell signaling dinner is ready at the fungal buffet.

Another option is to get a medium the gnats don’t want to hang out in and cover the soil in each plant, this is similar to the sand technique you mentioned but with a different material. I haven’t tried this technique so I can’t vouch for the effectiveness.

I hope this helps.  Thank you for writing in.

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