When a few plants were kept too damp at the Ogden Publications headquarters, many were asking “How do I get rid of gnats in my potted plants?” Although there are over 2,000 species of fungus gnats worldwide, they all have one thing in common: they’re annoying. While fungus gnats don’t pose a threat to humans, they can become a large nuisance in homes and greenhouses. The larvae stage feeds on organic matter and roots and can cause damage to plants. The larvae and adults can spread plant pathogens like black root rot, Botrytis blight, Fusarium wilt, Pythium blight, and Verticillium wilt.
Fungus Gnat Lifespan
To get rid of fungus gnats you will need to consider the fungus gnat lifespan, which includes eggs, four stages of larvae, pupa, and adult. Depending on the species, adults can lay up to 200 eggs. Eggs are laid where fungus is growing, which usually corresponds to damp soils. The larvae feed for around two weeks. When temperatures are low, this stage can last longer. They pupate near the soil surface for 3 to 7 days. Adults live a little over a week.
Fungus Gnat: Dry Soil Treatment
“We manage pests by using a combination of non-chemical solutions to control and prevent pest problems,” Lauren Fordyce says. Fordyce is an Urban and Community Integrated Pest Management (IPM) Educator at the University of California. She helps clients with houseplants and indoor and garden pests. IPM is an integrated approach to solving pest problems. The goal is to sustainably manage pests.
Fordyce explains, “When we’re looking at a fungus gnat issue, our first thought isn’t what we can spray on this, but to take a holistic approach of understanding why this is happening in the first place.”
For fungus gnats, the main driver is excess moisture. Spraying a pesticide isn’t going to get rid of the excess moisture that’s allowing the fungus gnats to live there. Fordyce suggests a simple solution: allow the top couple of layers of soil to dry out.
“This is really the best thing that you can do. That kills the larva that’s living in the soil,” Fordyce says. “While you’re trying to dry the soil out, you could think about switching your potting soil to something that has better drainage. If you have the house plant in a pot that doesn’t have a drain hole on the bottom, switch it to a pot that does and put a saucer underneath. Then water the plant from that saucer, and it can pull the water up through the roots, so the top of the soil isn’t constantly remaining wet.”
With this method, fungus gnats will not have an opportunity to lay their eggs. Since adults only live around 8 days, your problem will “dry” up quickly.
Faculty Advisor for the Oregon State University Organic Growers Club, James Cassidy agrees. “We really clamped down on our watering in our greenhouse and were able to manage it. In the home environment, people tend to over water.”
Fungus Gnat Predators: Carnivorous Plant Treatment
In addition to drying out the greenhouse plants, one of Cassidy’s students, Holly Golightly, thought about incorporating fungus gnat predators. She added close to 20 carnivorous plants to the 700-square-foot greenhouse and soon noticed the plants capturing dozens of tiny pests.
“When Holly suggested the idea, I said, ‘This doesn’t sound like it’s going to work’. And then I thought, what am I saying? I don’t know. She wants to do it and now it’s like her whole life is kind of revolving around it. She’s become scientifically engaged in a way that she wouldn’t have been if I hadn’t said yes. I learned a lot from that situation, more than just about the plants.”
While Golightly added various carnivorous plant species into the greenhouse, the best at catching fungus gnats are the sundews (Drosera species) and butterworts (Pinguicula species). Both groups of plants have adhesive leaves that trap and kill tiny nuisance pests, like yellow card sticky traps.
While sundews and butterworts offer a fun organic solution to fungus gnats, a green thumb is needed for growing them. Carnivorous plants need pure water like rainwater or distilled water, with well water and tap water often killing them due to their mineral and chemical content.
Although carnivorous, these are still plants and require sunny conditions and may not be suitable for all indoor environments.
Fungus Gnat: Bacillus thuringiensis Subspecies israelensis Treatment
“If people do want to go the route of pesticides, Bacillus thuringiensis (Bti) is a naturally occurring soil bacterium that’s used as a pesticide. You can find it in just about any store now,” Fordyce says.
The most important part of treating fungus gnats is getting the correct subspecies. While there are other subspecies of Bacillus thuringiensis, they do not work on fungus gnat larvae. Subspecies israelensis targets larvae stages of certain dipterans (flies) which fungus gnats belong to. Other subspecies are also larvicides but target moths and butterflies.
“You just have to continually apply Bti until the problem is gone,” Fordyce says. One way to incorporate Bti into your fight against fungus gnats is to purchase it in a powder form or as mosquito dunks and dissolve them in water. Then use that water for your plants.
Fungus Gnat: Diatomaceous Earth Treatment
Diatomaceous earth (DE) is a naturally occurring soft rock that is crumbled into a powder. It is often used as an insecticide because it pulls moisture away from many tiny creatures and dehydrates them. However, the efficacy is very low in damp environments or high humidity.
“I’ve definitely heard of using DE for fungus gnats. You hear of a lot of things on the Internet,” Fordyce says. “There is really nothing that suggests that DE would be effective at killing fungus gnats.”
Fungus Gnats: Neem Oil Treatment
Cassidy has used neem oil in the OSU’s greenhouses to get rid of fungus gnats. “It generally doesn’t kill them. It just makes them irritated, and they’re not as functional and therefore less likely to multiply.”
“Neem is not going to really solve the problem,” Fordyce explains. “It’s a two-part problem, but just killing the adults isn’t going to kill the larvae in the soil and it’s not going to solve the issue of having too much moisture in the soil, so it’s going to keep happening.”
She adds that if folks choose to go the route of using a pesticide, it is important to check to see if the product is labeled for indoor use.
When searching “how do I get rid of gnats in my potted plants” one interesting, proposed treatment was cinnamon powder or spray. The idea is that cinnamon is a fungicide and therefore it kills the fungus that the larvae want to eat.
“That is an interesting conclusion,” Fordyce says. “There is no evidence that it works, however.”
Homemade pesticides do not have warning labels or calculations of how much to use. Some can even be dangerous.
“With cinnamon, I’m not sure there’s a huge risk there, but with something like hydrogen peroxide, there is the risk of burning your skin. Hydrogen peroxide, especially if you’re using high enough concentrations, can cause serious burns. And you also risk burning the plant and the roots.”
Hydrogen peroxide was also a common treatment recommendation for getting rid of fungus gnats. However, the percentage needed to be applied to the soil would not only kill the fungus gnats but also the plants you are trying to save.
“Homemade pesticides don’t tell you to wear gloves. We’ve had people trying to use 47 percent vinegar and that could really harm you if you inhale it and if it gets on your skin. We’ve got pictures of folks who thought that spraying dish soap and vegetable oil on their plant to solve their pest problem would be helpful and it cooked the plant and burnt the entire foliage. There are lots of stories of how that could go wrong. It could be as light as hurting your plant and as extreme as hurting yourself or your pets or your children.”
The best way to get rid of gnats is to keep the top few inches of substrate in your potted plants dry. If you look for additional options, sticky traps or sticky carnivorous plants are a good way to get rid of adults. Bti is the best and safest larvicide route.
Kenny Coogan earned a master’s degree in Global Sustainability and has published over 400 articles on pets, livestock, and gardening. He lives on a 1-acre homestead with a flock of Pekin ducks, managing a permaculture-style landscape. Coogan also runs a successful carnivorous plant nursery in Tampa. Listen to Coogan co-host podcasts by visiting Mother Earth News and Friends Podcast.