By Kelly Miller
Fungus gnats are among the most common houseplant pests. They’re unsightly and annoying, and in large numbers they can be harmful to plants.
Fortunately, there are several simple, effective methods for dealing with a fungus gnat infestation—without using harmful chemicals. Below, we’ll cover some easy ways to eradicate fungus gnats from your home.
What Are Fungus Gnats?
Fungus gnats are small flies that are attracted to moist soil, where they lay their eggs. Gnats have short lifespans. The adult phase lasts only around 10 days, during which time a single gnat may lay over 200 eggs. Eggs hatch after around four days, releasing larvae that feed on fungus and organic matter in the soil, before maturing into adults.
Fungus gnats are harmless to humans. They don’t bite and they aren’t significant disease spreaders. However, they can be an unattractive nuisance. And, although they usually don’t cause major harm, the feeding larvae can damage plant roots.
Using a magnifying glass to see more closely, you can identify adult fungus gnats by these traits:
- No more than 1/8 inch (3 mm) in length, similar to a fruit fly.
- Dark gray bodies and gray, see-through wings.
- Long mosquito-like legs and antennae.
Fungus gnats are delicate, weak flyers. Around houseplants, they’re often seen walking on the soil or taking short flights around the edges of pots.
Life Cycle of a Fungus Gnat
Understanding the life cycle of any insect pest can be helpful for managing an infestation. The entire life cycle of a fungus gnat is only around 3-4 weeks. Fungus gnat eggs hatch in 4-6 days and they remain in a larval stage for about 2 weeks. Then, for a period of about 3-7 days they are in a pupal stage, transforming into adults. The adults then emerge and become noticeable as they fly freely around your houseplants and elsewhere. Adults generally live around 7-10 days and during that time a female fungus gnat lays her eggs and the cycle repeats.
How Do Fungus Gnats Find Your Houseplants?
Fungus gnats enter your home through soil, as eggs or larvae. This can occur when you bring an outdoor pot indoors or when you buy soil. Even potting mixes labeled “sterile” may harbor gnats.
It’s also wise to “quarantine” houseplants that you bring indoors by keeping them apart from your other plants for two or three weeks. This includes new houseplant purchases or gift plants you might receive. During that time, if a pest or disease is detected, you don’t have to worry about it spreading.
Whenever you want to check a potted plant for fungus gnats, use fresh potato slices. Put a few slices on the soil surface, then wait a few days. If larvae are present, they’ll gather to feed on the tasty potato starch.
Mechanical Pest Control Methods for Fungus Gnats
Mechanical pest control involves physical, non-chemical methods for eradicating pests or blocking their access to plants. Examples include fencing, traps, and picking pests off plants by hand.
Here are four easy and effective mechanical controls for indoor fungus gnats.
Drying Out the Soil
Fungus gnats thrive in moist conditions, and they love overwatered houseplants. So, when dealing with an infestation, drying out the soil is a good first step.
Additionally, a gnat infestation is a good motivator to reconsider your watering habits. For many houseplants, it’s appropriate to let the soil dry out between waterings, at least enough to make the surface dry to the touch. Many houseplant owners get impatient and tend to overwater. By letting the soil dry out for a few extra days, you can create a less attractive environment for gnats.
Replacing the soil can be helpful, especially if it has become broken down and compacted over time. Most houseplants prefer well-draining soil. If the soil surface remains damp for several days after watering, it may be low-quality soil, compacted, or a mix that wasn’t meant for houseplants. In this case, it’s helpful to replace some of the soil with a high-quality potting mix or to stir in some perlite to improve drainage.
Sand or Gravel
Fungus gnats lay eggs in the top inch of soil. Therefore, adding a layer of gravel or sand on top of the soil creates an effective barrier that discourages egg laying. Use cactus sand, aquarium sand, or a small-particle gravel mix. Note that sand and gravel don’t offer any nutrients and they can trap moisture in pots. So, this method may do more harm than good for some sensitive plants.
Diatomaceous Earth
Diatomaceous earth (DE) is a white powder made from the fossilized remains of tiny aquatic organisms. DE works as a pest control because its individual particles are very sharp. When insects touch the powder, they suffer cuts that eventually lead to dehydration and death. Meanwhile, DE is safe for plants and doesn’t harm earthworms or beneficial microorganisms.
To use DE, simply spread a thin layer on top of the soil. The powder works against both adults and larvae.
Sticky Traps
A sticky trap is a card that’s coated with glue on one side. They are typically yellow since the gnats are attracted to the color yellow. When bugs land on the trap, they get stuck. To use sticky traps, simply place them (sticky side up) on the soil surface, or propped along the rim of a pot. This is a highly effective method for killing adult fungus gnats and breaking the life cycle before they are able to lay more eggs.
Biological Pest Control Methods for Fungus Gnats
Biological pest controls use living organisms to combat infestations. Here are the two best biological treatments for fungus gnats.
Mosquito Dunks
A Mosquito Dunk® is a small donut-shaped disc, packed with the bacterium Bacillus thuringiensis israelensis (Bti). When placed in water, the disc dissolves and the Bti releases a toxin that kills the larvae of mosquitos and fungus gnats.
To use a Mosquito Dunk®, soak it in water, then water your plants with the solution (following the package instructions). In the soil, the larvae will ingest the Bti, then die soon after. Meanwhile, Bti is nontoxic and harmless for humans and plants.
Nematodes
Another biological control option for fungus gnats is the use of beneficial nematodes. A nematode is a tiny worm (about 0.5 mm long) found in soil around the world.
The nematode species Steinernema feltiae, when added to houseplant soil, is a deadly parasite for fungus gnat larvae. The worms enter larvae through body openings, then release a toxin that kills the hosts within a day or two.
Nematode treatment is more expensive than other methods, but it’s an effective solution for a gnat infestation.
Insecticidal Soap for Fungus Gnat Control
Insecticidal soap can be used to contact-kill adults, but it also offers a way to kill fungus gnats in the larval stage. Fungus gnats lay their eggs in the top layer of soil where moist conditions are ideal for their larvae. To combat this issue insecticidal soap can be applied directly to the affected areas.
Choose a quality insecticidal soap that is safe for indoor plants. If the plant is in need of water, be sure to do that before application. Spray the soil surface with the insecticidal soap solution, making sure to cover not just the top layer but also slightly below where gnat larvae may reside. Repeat treatments may be necessary. This approach is more effective if used in combination with sticky traps.
More Tips for Managing Houseplant Pests
Fungus gnats are a nuisance, but most houseplant owners achieve good results using any of the methods described above. Learn more about identifying and combatting pests in our guide to Common Houseplant Pests.
0 Comments