Adult Fungus Gnats stuck to a yellow fly trap. They are often mistaken for fruit flies. Photo by Lauren Sottile.

Fungus gnats are one of the most common houseplant pests we get questions about, and for many people can be one of the more annoying issues in dealing with houseplant care. These gnats families Mycetophilidae and Sciaridae) feed on small bits of fungi and decaying matter in the soil, which is present in any healthy soil medium. Populations of fungus gnats can quickly grow out of control, as their breeding cycle is very short, and although minor infestations are generally not harmful they can interrupt the serenity of keeping an indoor garden. Adult fungus gnats don’t damage plants or bite people but larvae can damage roots and stunt plant growth if the infestation is allowed to continue unabated. Thankfully, there are multiple methods that are effective at dealing with fungus gnats, and with a little care and attention they can be dealt with rather quickly.

 

OUT OF SIGHT, OUT OF MIND

Found around the soil and lower foliage of houseplants, fungus gnats live in the top layer of soil and are only able to thrive in wet or moist environments. Adult fungus gnats only live for about a week, but the eggs they lay will sit in the soil for three days and hatch into nymphs, which will live in the soil for about two weeks before growing wings. Fungus gnats are a pervasive problem precisely because they live most of their lives undetected: they’re almost impossible to see with the naked eye and hidden in the soil, so they can often pass undetected from nursery to plant retailer into your home.

Fungus gnats can also be problematic in the warmer months as they can easily slip in through a window (even a screened window due to their small size). Anyone who has kept plants for an extended period is most likely to have dealt with them at some point, but because they can only thrive under certain conditions they can be easy to treat, so long as the population is not allowed to grow out of control.

 

ATTACK THE SOURCE

To rid a plant of fungus gnats, both the larvae in the soil as well as the adult flies need to be targeted. This is in order to interrupt the life cycle. Simply dealing with the adults will not stop the eggs from hatching or the nymphs from growing into adults, and dealing with the eggs and nymphs in the soil will not stop the adults from laying more eggs.

The eggs themselves are just beneath the soil surface, but the larvae can burrow deep in the pot so a horticultural spray is not recommended as it would be difficult to access the entire pot. The first step to help rid your plant of fungus gnats is to let the potting soil get as dry as possible without damaging the plant. Removing their optimal environment will make it more difficult for fungus gnats to thrive and decrease the number of gnats that make it to maturity.

 

NYMPH TREATMENT: BENEFICIAL BACTERIA 

Bacillus thuringiensis var. Israelensis (BTI) is a naturally occurring bacteria that kills fungus gnat larvae. It is completely safe for people, pets and the earth, and very effectively targets unwelcome pests beneath the soil surface. Dipel Pro DF is a biological insecticide that contains BTI, although it's only sold in larger quantities suited for multiple infested plants. Mosquito Bits, a product marketed for killing mosquitoes also contains the BTI and is sold in smaller quantities. It can be used by soaking the bits in water for 12 hours before applying it to the plant’s soil, only when the plant is ready for watering so as not to overwater. Alternatively you can sprinkle the bits into the soil and the bacteria will release with each watering. This method should be continued until the fungus gnats are completely gone.

 

NYMPH TREATMENT: LAYERING THE SOIL

Another simple method for treatment is applying a coating of pumice to cover the soil, specifically the size commonly sold for the bonsai hobby. Pumice is a lightweight stone that due to its porosity is able to absorb excess moisture. When layered on top of the soil, it makes passage in and out of the top layer of soil much more difficult for the fungus gnats and can deter or prevent them from laying eggs. If using this method, wait for three weeks before removing the pumice to ensure that the fungus gnat life cycle has been completely interrupted.

 

TRAPPING THE ADULT FLIES

In addition to applying a treatment to the soil, the adult flies need to be trapped or killed to prevent them from laying more eggs in the soil. A horticultural spray (like neem oil) can be used to kill the flies on contact. 

However, the best way to capture adults are yellow sticky traps, which are covered in a sweet substance that will attract the flies. The sticky traps are also a great indicator of how bad the infestation is, as there are usually more fungus gnats than there appear to be.

A lone fungus gnat cleaning its hind legs. Video by Lauren Sottile.

 

A fiddle leaf fig tree planted in a sub-irrigated container.

SUB IRRIGATION: CHANGE THE ENVIRONMENT

Lastly there’s a little known method to deal with fungus gnats effectively, which is to eliminate their breeding habitat entirely. Sub-irrigated vessels are containers where there’s a built-in device to be able to water the plant's roots directly, as opposed to watering the top of the planter. Because the top layer of soil dries out completely using this method, there is no place for the fungus gnats to be able to breed. If you’re worried about fungus gnats and want to rule out the possibility of infestation entirely, investing in sub-irrigated planters will ensure you’ll never have to deal with them, as well as being more suitable to the long term health of most indoor plants.

 

FINAL NOTES

It is always important to separate the affected plant from the rest of your collection as soon as you have identified the issue and while it recovers. For future prevention (although not entirely controllable) thoroughly inspect plants before bringing them into your home and avoid overly soggy soil which can attract the gnats.