Tips for combating pesky fungus gnats

By Melanie Mathieson
The Gardening Guru

Fungus gnats are those little hopping bugs you see when you water your indoor plants.
They are small, with black or gray bodies and clear wings, and look like a cross between a black fly and a fruit fly.
Although not very strong fliers, they do fly around the house but most are often found close to the plant where they originated from the soil.
The adult stage lasts only about one week, but the female will lay between 100 and 150 eggs during that time. The entire life cycle—from egg to larvae to pupae and back to egg—lasts only four weeks.
The larvae are one-quarter inch long, white-bodied maggots with black heads and can be found just under the surface of the soil.
Adult fungus gnats do not cause any damage to healthy, mature plants but are a nuisance inside the house. The larvae feed on dead roots and leaves but when their numbers become high, they sometimes will feed on the very tender root hairs on the plant.
Plants damaged by gnats will lose vigour and may show higher than normal yellowing of leaves and leaf shed.
Fungus gnats are most commonly found on tropical plants, but can spread to other plants when at the nursery, greenhouse, or store. You often do not notice them in a plant until you bring home with you and see the flies flying around the house.
I’ve had fungus gnat infestations most recently from new potting soil that I have re-potted some plants in. I’ve had nothing but frustration on this because as I always say, buy good-quality potting soil, and I have been.
I most recently purchased a leading brand you all will recognize yet I developed both mould and fungus gnats in my soil. So I switched to another reputable brand a leading big box store carries and the same thing.
I don’t currently have a solution on this one, but I may complain to both companies as I wonder if they are not sterilizing their soils in order to cut costs.
Anyway, so you already have fungus gnats. Now what?
1. As fungus gnats feed on dead plant material, make sure to keep plants clean of fallen leaves and debris.
I groom my plants regularly at watering time.
2. Remove any areas of soil that develop mould on the surface.
3. Do not over-water your plants and allow the soil to remain soggy as this creates ideal conditions for mould and fungus gnat larvae.
Instead, let soil dry out between watering because the larvae cannot survive in dry soil.
4. When purchasing a new plant, I always re-pot into fresh soil as soon as it arrives home.
I always knock off as much excess soil from the plant as I can without disturbing the roots, then I plant into fresh sterilized soil (or so I think!)
It is okay to add the old soil to your composter.
5. There are a few pesticides on the market you can find at your garden supply store that are effective in killing the larvae in the soil.
One is made of pyrethinsis organic and still available (just make sure you follow the directions carefully).
6. Spraying the leaves and soil surface with a mixture of one tsp. of Sunlight soap to one litre of warm water or insecticidal soap will kill the adults.
Sometimes a good shower in the sink or tub will wash most of the adults away.
7. If you can find the yellow sticky tape strips for insect control, they will work well to trap the adults.
8. I also have seen an online tip indicating to cover the soil of all your houseplants thoroughly with a good layer of sandbox sand (sold in bags).
I’ve never tried this but it will look nice, too. I suppose aquarium rock also would work.
Be patient as it may take a few weeks to clear out all of the bugs, depending on the concentration of them.
But with perseverance and good housekeeping, you will win the war!

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