How to Identify, Treat, and Prevent Fungus Gnats on Indoor Plants

Have you noticed tiny flies buzzing around your houseplants or hovering near the soil? If so, you may be dealing with fungus gnats. These small pests are a common nuisance around indoor plants, particularly when potting soil remains consistently moist. In this guide, we’ll cover how to identify fungus gnats, recognize the signs of an infestation, manage these persistent pests, and help prevent them from returning.

What Do Fungus Gnats Look Like?

Fungus gnats are tiny, dark-colored flies with slender bodies, long legs and antennae, and clear wings. Measuring around 1/16 to 1/8 inch long, they can resemble miniature mosquitoes and may be difficult to spot individually. You’ll often notice them hovering around your houseplants, seemingly oblivious to your efforts to shoo them away, or moving across the soil surface. They may also gather around nearby windows.

Fungus gnat larvae look quite different from the adults. Found in the soil, the larvae are small, wormlike, and whitish to translucent with dark heads.

Fungus Gnats vs. Fruit Flies

Because of their small size, fungus gnats can easily be mistaken for fruit flies. One of the easiest ways to tell the difference is to look at where they’re gathering. Fungus gnats are commonly found around houseplants and moist potting soil, while fruit flies are more likely to gather around overripe fruit and other fermenting organic matter. Fruit flies also tend to be lighter brown and may have noticeable red eyes.

Fungus Gnat Life Cycle

Fungus gnats go through four stages: egg, larva, pupa, and adult. Adult females lay eggs in moist growing media, where the larvae hatch and feed mainly on fungi, decaying organic matter, and sometimes plant roots. The larvae eventually become pupae before emerging as the small adult flies you see around your plants.

Since most of this life cycle happens below the surface, getting rid of the adult gnats you see may not eliminate the entire infestation. Targeting the larvae is also an important part of breaking the cycle.

Signs of a Fungus Gnat Infestation

Seeing adult gnats flying around your plants is often the most noticeable sign of an infestation, but there are other clues to watch for. While fungus gnats typically cause little damage to established houseplants, large numbers of larvae can affect plant health. Keep an eye out for the following signs:

1. Yellowing or wilting leaves: Fungus gnat larvae can feed on plant roots, interfering with the plant’s ability to take up water and nutrients. This may contribute to yellowing leaves or wilting, although these symptoms can have many other causes.
2. Slow or stunted growth: As an infestation becomes more severe, affected plants may show stunted growth or a general decline in vigor. Seedlings and young plants are particularly vulnerable to fungus gnat damage.
3. Larvae in the soil: Finding fungus gnat larvae in the pot is a clear sign that adults have laid eggs and the infestation is continuing beneath the surface.

How to Get Rid of Fungus Gnats

Getting rid of fungus gnats often means targeting both the adults flying around your plants and the larvae developing in the soil. Depending on the severity of the infestation, try one or more of the following methods:

1. Allow the soil surface to dry: Fungus gnat larvae thrive in moist conditions. Wait until the top layer has dried as much as your plant can tolerate before watering again to make conditions less favorable for larvae and egg-laying adults.
2. Use yellow sticky traps: Place yellow sticky traps near affected plants to capture adult fungus gnats. Sticky traps can help reduce the number of adults and allow you to monitor the infestation, but they won’t eliminate larvae already developing below the surface.
3. Apply Bti: Bacillus thuringiensis subsp. israelensis, or Bti, is a biological control that targets fungus gnat larvae. Look for products containing Bti and follow the label directions for application. More than one treatment may be needed.
4. Try beneficial nematodes: Beneficial nematodes, particularly Steinernema feltiae, can be added to the soil to target fungus gnat larvae. Because they’re living organisms, be sure to follow the product directions for proper storage and application.
5. Repot the plant: For persistent infestations, repotting the plant in fresh potting mix and a clean container can help remove larvae and infested material. Gently remove as much of the old soil from the roots as possible before repotting.

How to Prevent Fungus Gnat Infestations

Once you’ve dealt with fungus gnats, a few simple plant-care habits can help make future infestations less likely:

1. Use well-draining soil: Choose a well-draining potting mix and a container that allows excess water to escape. Good drainage helps keep the soil from staying overly wet, creating less favorable conditions for fungus gnats.
2. Try bottom watering: Instead of watering from the top, place the pot in a shallow tray of water and allow the soil to absorb moisture through the drainage holes. This can help keep the soil surface drier and less attractive to egg-laying fungus gnats. Self-watering planters that deliver water from below can offer a similar benefit.
3. Remove plant debris: Clear fallen leaves, dead plant material, and other debris from the pot. Decaying organic matter can provide a food source for fungus gnat larvae.
4. Quarantine new plants: Before adding a new plant to your collection, keep it separate from your other houseplants for a couple of weeks and watch for signs of fungus gnats or other pests.
5. Use fresh potting mix: When potting or repotting houseplants, start with fresh, clean potting mix and avoid using garden soil indoors. Store unused potting mix in a closed container to help keep pests out.
6. Inspect plants regularly: Check your plants periodically for adult gnats or other signs of an infestation. Catching a problem early can make it much easier to manage.

Fungus gnats can be frustrating, but they’re much easier to control when you address the problem quickly. By adjusting your watering habits, monitoring your plants, and taking action when needed, you can help protect your houseplants and reduce the chances of another infestation.