(This article was written by Angela Mason Foster, Extension Master Gardener℠ Volunteer)

If you’ve ever noticed tiny, mosquito-like insects flitting around your houseplants, you’re not alone. These are fungus gnats—one of the most common pests of indoor plants. While their presence can be annoying, understanding their life cycle and impact on houseplants is the key to managing them effectively.

What Are Fungus Gnats?

Fungus gnats (Bradysia spp.) are small, dark-colored flies with long legs and antennae. They are often mistaken for fruit flies, but unlike fruit flies, they are more attracted to moist soil than to ripening fruit. Fungus gnats thrive in the damp conditions often found in houseplant pots, where their larvae feed on organic matter, fungi, and sometimes plant roots.

Do They Harm Houseplants?

Adult fungus gnats are mostly a nuisance, as they do not bite or harm plants directly. However, their larvae, which live in the top layers of soil, can be problematic. In small numbers, they help break down organic matter, but in large infestations, they may feed on delicate root hairs, potentially weakening young plants or cuttings. Established plants typically suffer little harm, but seedlings and plants with weak root systems can experience stunted growth. In extreme cases, a severe infestation can lead to root rot, as the larvae create wounds that allow harmful pathogens to enter the plant.

How Do They Get Into Houseplants?

Fungus gnats typically enter homes through contaminated potting soil, newly purchased plants, or open doors and windows. They are particularly common in overwatered houseplants, as damp soil provides the perfect environment for egg-laying and larval development. Potting soil that remains consistently wet can become a breeding ground, especially if it contains decomposing plant material, algae, or excessive organic matter that supports the larvae.

How to Control Fungus Gnats

The good news is that fungus gnats can be managed using simple and effective strategies. Here are several ways to control and prevent these pests:

1. Let Soil Dry Between Watering

Since fungus gnat larvae thrive in moist soil, allowing the top inch of soil to dry out between waterings will disrupt their life cycle. Most houseplants, except for moisture-loving varieties, benefit from this approach. If possible, water plants from the bottom rather than the top to avoid leaving excess moisture on the soil surface where gnats lay their eggs.

2. Use Sticky Traps

Yellow sticky traps, available at garden centers, can capture adult gnats, reducing the number of eggs laid in the soil. Placing traps near plant pots helps to monitor and control infestations. Regularly replacing the traps will ensure ongoing effectiveness.

3. Apply Biological Controls

Beneficial nematodes (Steinernema feltiae) and Bacillus thuringiensis subspecies israelensis (Bti) are natural enemies of fungus gnat larvae. These biological controls, available in garden stores or online, effectively target larvae without harming plants or humans. Bti is commonly found in mosquito dunks and can be mixed with water and applied directly to the soil.

4. Top-Dress Soil with Sand or Gravel

Covering the soil surface with a layer of coarse sand, gravel, or diatomaceous earth creates a barrier that discourages adults from laying eggs and helps dry out the soil surface. This method works best when combined with other control strategies.

5. Repot and Refresh Soil

If an infestation persists, consider repotting the affected plant with fresh, well-draining soil. Discard old, heavily infested soil and ensure new soil is not overly moist. Sterilizing pots before reuse can also help prevent reinfestation.

6. Control Moisture Levels

Using pots with drainage holes and avoiding excessive water retention in saucers can significantly reduce fungus gnat problems. Improved air circulation around plants can also help prevent excess moisture buildup. If a plant requires frequent watering, consider using a well-aerated potting mix that drains efficiently to reduce excess moisture accumulation.

Final Thoughts

Fungus gnats are more of a nuisance than a serious threat to most houseplants, but their presence indicates excess moisture and potential soil health issues. By adjusting watering habits, using natural controls, and keeping an eye on plant conditions, you can keep your indoor garden free from these pesky flies. Prevention is key—regularly inspect new plants before bringing them indoors and maintain proper soil conditions to discourage fungus gnat populations from growing.

For more information on fungus gnat control and houseplant care, consider these books:

• The House Plant Expert by Dr. D.G. Hessayon
• The Well-Tended Perennial Garden: Planting & Pruning Techniques by Tracy DiSabato-Aust
• Gardening Indoors with Soil & Hydroponics by George F. Van Patten

By taking proactive steps, you can enjoy thriving houseplants without the annoyance of fungus gnats. Happy gardening!

If you are having trouble with growing in your home landscape, call the Extension office at (252)946-0111 or email Gene Fox at gene_fox@ncsu.edu. Don’t forget to check out the Beaufort County Master Gardeners Facebook page for additional resources. Until then, Happy Gardening!