Fungus Gnats on House Plants

Fungus gnats (scientifically known as Bradysia species) are small, dark-colored flies often mistaken for fruit flies. They are about 1/8 inch long and have delicate, translucent wings. They thrive in moist environments, so houseplants are a perfect breeding ground. Adult gnats lay their eggs in the soil, and when the eggs hatch, the larvae feed on organic matter, including fungi, plant roots, and decaying organic material.

Adult female fungus gnats live about 7 days and can lay up to 1,000 eggs during their lifespan. Once the eggs are laid, they will hatch in 4-6 days. After hatching, the larvae begin tunneling through the soil in search of food. The larval stage lasts 10-14 days. Once the larvae are old enough, they spin a cocoon in the soil and enter the pupa stage which lasts 4-7 days. At the end of the pupa stage, the adult exits the cocoon and begins mating and laying eggs. Then, the cycle starts over. A full cycle takes 25-34 days to complete.

Fungus gnats are drawn to the moist, well-fertilized soil of houseplants, where they can find abundant food sources. Overwatering or poor drainage increase the likelihood of a fungus gnat infestation; these conditions create an ideal habitat for the gnats and their larvae. The increased humidity caused by waterlogged soil attracts these pests.

1. Root Damage: The most significant impact of fungus gnats is on the plant’s roots. The larvae feed on the roots, which can lead to stunted growth, yellowing leaves, and poor overall health. If the infestation is severe, the roots may be damaged enough to cause plant death.
2. Weakening the Plant: Even without direct damage to the roots, the presence of fungus gnat larvae can weaken the plant, making it more vulnerable to other pests and diseases.
3. Unpleasant Flying Adults: While the adult fungus gnats do not harm the plants directly, their constant presence can be a nuisance. They tend to fly around the plant, especially when disturbed.
4. Fungal Infections: As their name suggests, fungus gnats are often associated with the growth of fungi in the soil. The larvae feed on fungal spores, which can contribute to an increase in fungal growth, further compromising the health of the plant.

There are several effective ways to control fungus gnats and prevent future infestations. Here are some common methods:

1. Reduce Watering: Allow the top inch of soil to dry out between waterings to make the environment less attractive to gnats. Avoid letting plants sit in excess water, as this encourages them to lay eggs in the soil.
2. Use Well-Draining Soil: Ensure that your houseplant’s soil has good drainage. Adding sand or perlite to the soil mix can help reduce moisture retention and discourage fungus gnats from breeding.
3. 

   Yellow Sticky Trap

   Sticky Traps: Yellow sticky traps are an effective way to capture adult gnats. These traps are coated with a sticky substance that attracts and traps flying insects. Place them near your infested plants to reduce the adult population. You can purchase these from Home Depot or your local garden centre.

4. Trap Plants: Some carnivorous plants will attract and trap adult gnats. Good choices are are Butterwort (Pinguicula) and Sundew (Drosera). Place them near the infested plants. Butterworts are available in some greenhouses and I’ve seen them at plant shows like EHS’s Garden Festival.
5. 

   Butterwort plant

   Soil Treatments: Introducing biological controls, such as nematodes (microscopic worms that prey on fungus gnat larvae), can help reduce the gnat population in the soil. Alternatively, sprinkling a thin layer of diatomaceous earth on the soil can help kill the larvae by damaging their exoskeletons. I picked up a big bag from Apache Seed, who carry all sorts of pest control items.

6. Hydrogen Peroxide Solution: A solution of one part hydrogen peroxide and four parts water can be watered into the soil. This solution helps to kill fungus gnat larvae while being safe for the plant. Avoid overusing this treatment, as it can also harm beneficial microbes in the soil
7. Repotting: If the infestation is severe, repotting your plant with fresh, sterile soil can help eliminate most of the larvae and eggs. Be sure to clean the roots thoroughly and remove any affected soil before replanting.
8. Insecticidal Soap: Insecticidal soap can be used as a last resort to control adult fungus gnats. This soap suffocates the insects. Use it carefully, as it can also affect beneficial insects.
9. Bleach Spray: Lastly on my list, you may find fungus gnats flying around your kitchens and bathrooms, even if you have no plants in these rooms. They love dark humid places and what’s darker than drains? Especially in the overflow part of the sink, that little hole on the backside that you don’t normally notice. Spray both the main and overflow drains with a bleach solution at least once a month to keep them clear and prevent bacteria build-up.

To sum up, if you have fungus gnats, don’t panic! They are a common pest that thrives in the soil of overwatered houseplants. Though they are not typically a serious threat to the overall health of your plant, they can cause significant damage to the roots and contribute to fungal problems, if not treated. The best way to prevent and control fungus gnats is by managing soil moisture, using physical traps, and applying treatments when necessary. Regular inspection and maintenance of your plants will help keep these pesky insects at bay, ensuring your houseplants stay healthy and thriving.

Happy gardening everyone!