How Do You Get Rid of Fungus Gnats in Houseplants?

Get rid of fungus gnats by letting the surface of the potting soil dry between waterings, trapping the adults on yellow sticky cards, and treating the soil with Bti, a biological insecticide that kills the larvae. The University of California Statewide Integrated Pest Management Program notes that Bti does not persist indoors, so an infestation may need repeated applications at about five-day intervals.

The repeat treatments matter, and the life cycle explains why. According to UC IPM, at 75 degrees, fungus gnat eggs hatch in about 3 days, the larvae feed for roughly 10 days, and adults emerge about 4 days after that. A full generation takes about 17 days, and generations overlap, so eggs, larvae, pupae and adults are all present in the same pot at once. One treatment catches only the stage it reaches. This post explains how to confirm the problem, why it started, whether the plants are in danger, and the steps that end it.

What are fungus gnats?

Fungus gnats are small flies whose larvae live in moist potting soil. UC IPM describes them as nuisance flies that commonly infest potted plants. The adults are about one-sixteenth to one-eighth of an inch long, with light gray to clear wings. 

The life of a fungus gnat is spent mostly out of sight. UC IPM notes that most of it passes as a larva and pupa in the soil, feeding on fungi and organic matter. The flying adults are the short-lived, visible part of a population that is mostly underground.

How do you confirm you have fungus gnats?

Look at where the insects gather. UC IPM says adults rest on plants, soil, windows and walls, and that because they are attracted to light, people often first notice them flying near windows indoors.

Two simple tests help:

• Sticky cards. Place a yellow sticky trap at soil level in the suspect pot. Gnats on the card within a few days point to that plant.
• Potato chunks. UC IPM describes setting chunks of raw potato in the pot with the cut side down on the soil. Larvae are attracted to the potato. After a few days, lift the chunks and look underneath.

The larvae have a shiny black head and a long, whitish to clear, legless body. In very wet pots with heavy populations, UC IPM notes that larvae can leave shiny slime trails on the soil surface that look like the tracks of small snails. 

Why do fungus gnats show up?

Moisture. UC IPM states that fungus gnats thrive in moist conditions, especially where there is plenty of decaying plant material and fungi, and that females lay their eggs in moist organic debris or potting soil. 

In a house, that describes an overwatered pot. Soil that never dries at the surface, a saucer that stays full, a decorative outer pot with no drainage, and dead leaves left on the soil all make a container more inviting. 

The wettest pots are the place to look first.

Do fungus gnats hurt houseplants?

The adults do not. UC IPM is clear that adult fungus gnats do not damage plants or bite people, and that their presence is primarily a nuisance. 

The larvae are a different matter in large numbers. They feed mainly on fungi and organic matter, and they also chew roots. UC IPM notes that heavy populations can damage roots and stunt growth, particularly in seedlings and young plants, and that significant root damage and even plant death have been observed in houseplants when high populations were associated with moist, organically rich soil.

One practical point follows from that. UC IPM observes that a wilting houseplant may not need water at all. The wilting can be a sign of damaged or unhealthy roots, and watering a plant in that state makes the conditions better for gnats.

How long does it take to get rid of fungus gnats?

Plan on several weeks of steady effort, because the population has to be interrupted across more than one generation.

UC IPM's figures at 75 degrees: 

• Eggs hatch in about 3 days.
• Larvae develop for approximately 10 days.
• Adults emerge about 4 days after the larvae pupate.
• A generation, from female to female, takes about 17 days.

Warmer conditions speed that up, and UC IPM notes that fungus gnats have many overlapping generations each year.

The adults on the sticky card today came from eggs laid two to three weeks ago. Stopping the next round means dealing with the soil.

What are the steps to get rid of fungus gnats? 

UC IPM states that the most effective control methods target the immature stages in the soil, since the adults are mobile and short-lived. The steps below work together.

1. Let the soil surface dry. UC IPM's first recommendation is to avoid overwatering, provide good drainage, and allow the surface of container soil to dry between waterings. Empty saucers and outer pots after watering. 

2. Clean up the pot. Remove dead leaves and other organic debris from the soil surface, since that is where larvae feed.

3. Trap the adults. Yellow sticky traps can be cut into small squares, attached to skewers and set in the pots. They catch adults and show whether the numbers are falling. 

4. Trap the larvae. Raw potato chunks set on the soil attract larvae away from the roots. UC IPM suggests removing the infested chunks after a few days and replacing them with fresh ones.

5. Treat the soil with Bti. Bacillus thuringiensis subspecies israelensis is a biological insecticide sold for home use under names such as Mosquito Bits and Gnatrol. UC IPM notes that these products are readily available at garden centers. Mix with water and apply as a soil drench, following the label. Because Bti does not reproduce or persist indoors, UC IPM says repeated applications at about five-day intervals may be needed.

6. Consider beneficial nematodes for a longer fix. UC IPM lists Steinernema nematodes as a biological control and notes that Steinernema feltiae is more effective against fungus gnats than other commercially available species. Nematodes are also applied as a soil drench.

Why do traps alone not solve the problem?

Because traps only reach the adults. A sticky card or a dish of bait can catch a great many flying gnats while the larvae in the soil go on developing.

That is the reason UC IPM points control efforts at the soil. Drying the surface removes the conditions the eggs and larvae need. Bti and nematodes kill the larvae directly. Traps are useful for monitoring and for cutting down on egg-laying, and they work best alongside the other steps.

Prevention is the same list in a lighter form. Water when the plant needs it, not on a fixed schedule. Use pots with drainage holes. Keep the soil surface free of debris. 

FAQ 

Do fungus gnats bite? 

No. UC IPM states that adult fungus gnats do not bite people or damage plants.

How long do fungus gnats live? 

A generation takes about 17 days at 75 degrees, according to UC IPM: about 3 days as an egg, 10 as a larva and 4 as a pupa before the adult emerges.

What is Bti? 

Bti is Bacillus thuringiensis subspecies israelensis, a biological insecticide used against fungus gnat larvae. It is mixed with water and poured onto the soil.

How often should you apply Bti for fungus gnats?

UC IPM notes that Bti does not persist indoors and that repeated applications at about five-day intervals may be needed. Follow the product label.

Will letting the soil dry out get rid of fungus gnats?

It helps. Fungus gnats thrive in moist soil, and UC IPM recommends allowing the surface of container soil to dry between waterings. Combine it with traps and a soil treatment for an established infestation.

What do fungus gnat larvae look like?

They are legless and whitish to clear, with a shiny black head. They live in the potting soil.

Houseplant Help From Roots to Petals in Oakmont 

Roots to Petals is an independent, woman-owned florist and plant studio at 527 Allegheny Avenue in Oakmont. The Plant Studio stocks hundreds of houseplants and a wide variety of pots, and the shop's horticulturists help customers match plants to their lighting and lifestyle. Plants from the plant collection can be delivered within the shop's floral delivery area. For more on keeping plants healthy indoors, see the shop's guides to the best house plants for Pittsburgh homes and apartments, to choosing houseplants at a plant store, and to flowers, plants and workshops around Pittsburgh. 

Visit Roots to Petals or call 412-822-6023 today. Contact us for more information.