Fungus gnats can be a particularly pesky houseplant or greenhouse pest. They might be difficult to spot early on and multiply quickly. Fungus gnat larvae feed on the roots of plants, especially potted ones, which can cause plants to wither and die if the infestation isn’t addressed. Separately from the issues they cause beloved plants, they are just plain annoying to have around living spaces. If you’re looking to get rid of fungus gnats, you’ve come to the right place.
What are fungus gnats?
Fungus gnats are tiny brown or black two-winged insects commonly mistaken for fruit flies, drain flies, or mosquitoes. They have a slimmer, longer body and longer legs and antennae than fruit flies. Fungus gnats are smaller in size than mosquitoes, and do not bite humans. They are particularly pesky because they don’t target the leaves or stems the way other plant pests might. Fungus gnat larvae feed on the roots, so they may be more difficult to identify during a quick visual scan.
Why do they appear?
Fungus gnats are attracted to just that: decaying organic matter. Simply put, they are particularly attracted to damp soil and standing water near plants. If you are a chronic over-waterer when it comes to your houseplants, they will plague your plants over those cared for with benign neglect every time.
What damage do fungus gnats cause plants?
Adult fungus gnats don’t cause any damage themselves – to plants or humans. The larvae are the big problem. Fungus gnat larvae feed on the roots of plants, which can limit growth and even cause plants to die if the infestation isn’t addressed. Early on, your plant may stop growing. It may start to droop. It may start to abnormally lose leaves. And finally, it may die.
Younger plants are at a higher risk of irreversible damage. Since sprouts and young plants don’t have many roots, they are more vulnerable to damage from fungus gnat larvae. Older plants have more roots and this sheer volume can make them more resistant to permanent damage from fungus gnats.
How to know if you have a fungus gnat problem
Plants suffering from a fungus gnat infestation will show the following signs early on:
- Fungus gnats crawling on the soil surface or pot
- Fungus gnats hovering around the plant
- Abnormal drooping or shedding of leaves
- When repotting, you see tiny white specks (larvae) wriggling around the roots
To identify an infestation, visually check for the above signs and look first to the plants that are damp most often.
Plants such as jade plants are particularly susceptible because their soil will be frequently moist from regular watering. However, don’t discount plants that aren’t watered as frequently. Any plant that has damp soil or a plant saucer that frequently holds standing water will be a magnet for fungus gnats.
How to prevent fungus gnats in the first place
To prevent fungus gnats in the first place, mind your plant P’s and Q’s.
When purchasing plants, select healthy plants with vibrant colors. Avoid plants that are droopy or wilted. Do not select ones that have brown spots on leaves or stems. Unhealthy plants attract pests such as fungus gnats.
If you can visibly see plant roots before purchase, scan for tiny white fungus gnat larvae wriggling around the roots. If a plant already infested with fungus gnats is brought home, it’s only a matter of time before the infestation spreads to other plants.
Encourage conditions that are prohibitive to fungus gnats. Know your plants well and care attentively for their specific watering needs. Ensure potted plants have soil that drains properly and opt for pots with drainage holes over ones without. A 1-2” layer of sand over potting soil can also provide defense against fungus gnat larvae since it dries quickly and won’t have the tasty organic matter that fungus gnats love.
Avoid overwatering plants and err on the side of benign neglect where possible. When watering plants, use a towel to blot and soak up water in any saucers that catch water or water plants in a sink, wait until they are done draining, then move them back to their saucer. Excess water around the base of a plant is a neon sign advertising free-for-all buffet for fungus gnats.
How to get rid of fungus gnats
If the infestation is not that bad, simply drying plants more than usual might address the issue effectively. If it is a bigger issue, you may want to turn to hydrogen peroxide soil soaks, yellow sticky traps, or a combination of both. It can take several days for the full life cycle of a fungus gnat infestation to die out, so be patient.
A hydrogen peroxide soil soak will address the root problem of fungus gnats: the larvae. Hydrogen peroxide cleanses the soil and the larvae along with it. A 4:1 ratio of water to 3% hydrogen peroxide would be safe for plants and effective for killing off fungus gnat larvae. You can simply water the soil with your 4:1 water-hydrogen peroxide solution, being careful to avoid wetting the leaves, or set the base of the pot in the solution where the soil can be reached and allow the soil to soak it up.
For an in-depth explanation about proper hydrogen peroxide soil soak, check out this video:
Yellow sticky traps such as those found here (https://amzn.to/2D7bThM) are another good way to get rid of fungus gnats by targeting the adults. Yellow sticky traps attract the fungus gnat adults and they are unable to fly away to lay more larvae eggs. Cutting yellow sticky sheets to size and laying the traps across the surface of the soil is an effective method at getting rid of an infestation over time since fungus gnats will inevitably crawl onto them. This targets the adults, not the larvae.
For a tutorial on making your own yellow sticky traps out of paper and rubber cement or petroleum jelly, check out this video:
Fungus gnats are a persistent pest that can damage greenhouse and indoor potted plants, especially young ones that don’t have many roots. They can be mistaken for other household pests like mosquitos, but don’t cause physical harm to humans. Drier plant conditions will avoid making a comfortable home for fungus gnats and can prevent them from taking residence in the first place. If you already have an infestation, hydrogen peroxide soil soaks and yellow sticky traps are excellent methods to get rid of fungus gnats. Happy hunting!