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Fungus Gnats

Household Pest Control: Fungus Gnats

by Rob Sproule


If you have houseplants, then I bet you’ve had fungus gnats. They are a winged household fixture in late winter but thankfully they look like more trouble than they really are.

The good news is that Fungus Gnats are harmless to you and your plants. They’re passionate about the decaying organic matter in soil; the plant being there is happenstance.

That being said, no one wants gnats flitting about during dinner. They’re easy to get rid of and usually don’t require chemicals.


Fungus Gnat Life Cycle

Like most tiny varmints, Fungus Gnats reproduce fast and build populations with astonishing speed. It takes about 2 weeks of gorging on a fungus, algae, and rotted leaf buffet before the larvae pupate into adults.

During their week-ish long life span, females will lay about 200 eggs in the tiny cracks between soil particles. They prefer peat moss based mediums but anything moist will do.


Recognizing Fungus Gnats

Gnat populations tend to peak indoors in the fall and later winter. They often arrive as stowaways in the fall when we rescue tender plants from frost.

Reduced sunlight hours in wintery northern climates slows down metabolisms of all the critters who living there, including us and our plants. When our metabolisms slow, we get droopy and feel lethargic. When our plants’ metabolisms slow, they get droopy and take up less water.

Less water uptake means that the top layer of your potting soil stays consistently moist. This is especially if you dote over your green babies and keep watering at the same tempo as in the summer. On top of that, f you haven’t repotted in a while your medium will have degraded; old potting mediums lose drainage and stay wet longer.

Old, damp pots of dirt are like new condo complexes to Fungus Gnats. The live in plants’ potting medium (soil) and feed on algae, fungus, and decaying leaves in the top 2-3 inches. They ignore the plants completely.

These prolific pests are weak flyers and tend to flit through the air in weak, zig-zag patterns. They never stray far from home, so when you see one look for the nearest potted plant as the probable source.


Killing Them

People ask me everyday what they need to spray their plant with to kill fungus gnats. I tell them to save their money and put the watering can away; the secret is drying the plant out.

Water stressing a plant (ie. making it droop) is never healthy, but if you have gnats than the plant’s probably seeing more water than it needs anyway. Cut back the watering and, when the top few inches of the soil are at their driest, gently scrape it out, being careful not to disturb the root systems on overgrown tropicals.

Replace the old soil with bagged, sterilized soil that’s available almost anywhere. Make sure it’s well draining. The new medium will be free of old organic matter and will drain faster than the old.

If you want to be sure, spread an inch or two of decorative pebbles across the surface. You can also hang yellow “sticky strips” from the plant stems to bring down their numbers.

If the gnats persist after a couple of weeks, reach for a pyrethrum based insecticide. Spray outdoors if possible and in a well ventilated room if not. The pyrethrum persists on whatever it touches so keep pets and children away a while.

Sprays like insecticidal soap and neem oil work on contact with adult bugs (not eggs and rarely larvae), so while they less potent than pyrethrum you’ll need to reapply every few days for a couple of weeks to nip their life cycles.


Read through our Growing Guides for tips to enrich your garden! 

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