Do Fungus Gnats Damage Houseplants?

Many houseplant growers like myself have walked into swarms of tiny flying pests. You can see them illuminated by the light through your window. These are likely fungus gnats and they can be a real nuisance. 

Fungus gnats are only about ⅛ long, with dark brown bodies and veiny wings. Their larvae look like tiny translucent white worms with small blackheads. They’re only ¼ inch long and live in your infected houseplants’ soil and root systems. 

If your houseplants are infected with fungus gnats, it could be a cause for alarm. Let’s dive in to see if fungus gnats damage houseplants and what to do about it. 

Marly’s Self-Watering Planter

With its intuitive design, maintaining your plants has never been easier. The self-watering mechanism ensures your green companions receive just the right amount of hydration, eliminating the worry of over or under-watering. Say goodbye to wilted leaves and hello to thriving, vibrant plants.

Buy at Epic Gardening

The Short Answer

The short answer is yes, fungus gnats can damage your houseplants. However, it’s only their larvae that can wreak havoc if they get out of control.

Adults consume water and nectar. They may swarm around your houseplants, but they don’t eat them. Their larvae, however, feed on soil fungi, decaying organic matter, and occasionally root hairs. When the infection is bad, they may even burrow inside the roots.

Over time, a severe infestation can cause wilting, slow down plant growth, and cause the leaves to turn yellow. In extreme cases, your houseplants could die over time.

The Long Answer

As you can see, fungus gnats can be more than just an annoyance. Keep reading below to learn more about these pesky insects and what you can do to get rid of them. 

More About Fungus Gnats

Although these tiny flies resemble fruit flies, they’re quite different. While fruit flies gather around rotting plants and fruit, fungus gnats live in moist soil. 

During their one-week lifespans, they can lay up to 200 eggs. In just under a week, the larvae hatch. During the 12 to 14-day larvae stage, they can eat the root hairs and even the roots themselves. 

After the larvae stage, they mature into pupas and then into adults. Their entire lifecycle lasts for a few weeks. You’ll see multiple generations at once. That’s why, if you see swarming adults, you probably have larvae in the soil. 

Are All Species Harmful?

Fungus gnats make up six families of insects. Most of these families of bugs are not harmful. However, a few members of the Sciaridae family are. 

You won’t know which species is infesting your plants until it’s too late. That’s why it’s important to prevent and treat infestations when they occur. 

How Did My Plants Get Infested? 

The most common reason for infestations is moist soil. These small insects feed off fungi and decaying plant matter that’s common in moist soils. By keeping the top layer of soil dry, they may target easier hosts instead. 

Fall is a common time for these infections to occur. This is because we often move our houseplants outside for the summer and early fall. You’ll see the swarms of flies as soon as you bring your plants indoors. 

The other most common reason is bringing home an infected plant. Nurseries often leave their plants outside in the summer. If the moisture and humidity levels are high, it could create the perfect environment for these pests. 

How Do They Damage My Plants?

Fungus gnats damage houseplants in several ways. While adults feed on water and nectar, the larvae feed on fungi and root hairs. Sometimes, they burrow into the roots themselves, cutting off nutrients and water to the rest of the living tissue. 

Besides damaging the roots, the larvae can transfer deadly pathogens. As they consume fungi, they can spread harmful fungi spores, which cause diseases such as black root rot, Verticillium wilt, and Botrytis blight. 

Which Houseplants Are Prone to Infestation?

Most houseplants can fall victim to these swarms of flies. If you give them their ideal environment, they will quickly make themselves at home. 

Fungus gnats thrive in moist soil. Moisture creates a breeding ground for fungi and encourages plant matter to decay. This is an ideal feeding ground for the gnats and their larvae. 

Any houseplant that needs consistent watering and moist environments is more likely to fall victim. These include flowering plants, such as geraniums, cyclamens, African violets, and carnations. 

Species such as aloe vera, succulents, and cacti thrive in drier soil conditions. These plants are usually safe from infestations unless you overwater them. 

How Do I Prevent Them?

Prevention is the best way to keep these pests away. Like many common pests, fungus gnats prefer overly moist soil. They also don’t like to be disturbed. Wind, rain, and other insects living on outdoor plants can make them find quieter hosts. 

Your houseplants can provide the perfect environments for these flies. This is especially true with water-loving species. To prevent them from hitching a ride on your pants, follow the guidelines below. 

• Keep the top layer of soil dry where possible.
• Consider watering your plants from the bottom using self-watering planters like this one. 
• Use a better draining growing medium to limit moisture buildup.
• Inspect all new plants and treat them immediately.
• Lay yellow sticky traps around your plants. The color attracts the adult gnats and they will be trapped before they lay their eggs.
• Try adding a layer of mulch on the top layer of soil.
• Introduce beneficial insects such as beneficial nematode species. 
• Isolate new houseplants when you bring them home. 

Effective Ways to Treat Fungus Gnat Infestations

Once you notice a fungus gnat infestation, there are a few steps to eliminate them. The first step is to change your watering habits. Water only when necessary, and monitor plants that need consistently moist soil. 

An effective organic method is soaking the soil in a solution of one part hydrogen peroxide and four parts water. You can also use diluted neem oil—just follow the product’s directions. Both these methods kill the larvae. You can also spray the adults with the neem oil solution. 

If all else fails, try using the bacteria Bacillus thuringiensis var. Israelensis in your soil. This bacteria will eliminate the larvae without harming your houseplant.