gardening pests

A Spooky Houseplant Pest: Fungus Gnats

By Cristian Acosta, Agriculture Educator, Cornell Cooperative Extension Allegany County

If you’re noticing tiny, ghostly creatures flying around your beloved houseplants, you may be dealing with a spooky houseplant pest: fungus gnats! These little pests are often found haunting plant soil, appearing out of nowhere, and drifting around in eerie clusters. While they’re relatively harmless to people, their presence can spell trouble for your plants, especially young seedlings and newly propagated cuttings.

What are fungus gnats?

Fungus gnats are tiny, blackish-gray flies resembling miniature shadows flitting around your plants. They are often mistaken for fruit flies due to their size and hovering behavior, but fungus gnats are uniquely interested in damp, organic-rich soil, which is an ideal breeding ground for them. Adults are short-lived but highly prolific, laying eggs in moist soil. After hatching, the larvae feast on organic matter and sometimes delicate plant roots, which can lead to stunted growth or even wilting in extreme cases.

Why do they love your soil?

Moisture and decaying organic matter are the perfect combination. Overwatering is the leading culprit in attracting fungus gnats, creating a damp environment that supports both their eggs and their favorite food sources: fungus and decaying organic material in the soil. Poor drainage or soil that retains too much water will further increase the likelihood of a fungus gnat infestation. Humid, indoor conditions, typical for many houseplants, only make matters worse, creating an inviting atmosphere for these pests to thrive.

Tips to control fungus gnats from your “haunted” soil:

If fungus gnats have taken up residence in your plants’ soil, don’t worry! There are several effective low-cost ways to manage them and prevent future infestations. Here are a few tricks to evict these unwanted guests:

  1. Let the soil dry out
    Fungus gnats are particularly attracted to consistently moist soil. By allowing the top 1-2 inches of soil to dry out between waterings, you’ll make it much harder for fungus gnats to lay eggs and for larvae to survive. If you’re worried about your plants drying out, switch to bottom-watering methods for a while, this will keep the surface layer drier and less attractive to gnats. Some plants don’t tolerate the dry soil, even with the bottom-watering method, for those dry-susceptible ones check the next control methods.
  2. Sticky Traps
    Bright yellow sticky traps are an easy and effective method to catch adult fungus gnats. These traps, available at most garden centers, lure the adult gnats and trap them, preventing them from laying more eggs in your plants’ soil. Place a trap near the affected plants and watch as the adult gnats meet their sticky end.
  3. Hydrogen peroxide solution
    A mild hydrogen peroxide solution can help kill fungus gnat larvae without harming your plants. Mix one part 3% hydrogen peroxide with four parts water and pour it over the soil. This mixture will fizz as it attacks the larvae. Be sure to let the soil dry out afterward to prevent the gnats from returning.
  4. BTI
    There is a great control method, and organic, called Bacillus thuringensis israelensis. Probably you have heard about BT, a popular organic insecticide to control pests’ larvae. This is a group of bacteria used to control larvae of many pests. Now, the serotype israelensis has been studied and approved to be commercially distributed to control various species of mosquitoes, blackflies, and fungus gnats. You can find products containing BTI in hardware stores or online, just read the label and follow the instructions. In the first days probably you won’t notice anything because this bacteria doesn’t kill the adults, so they will be flying around, but it will kill the larvae, so, after 4-5 weeks you’ll notice how the fungus gnats' population is reduced until they finally go away. And every time you see new ones flying around you can apply again. It won’t hurt you or your animals at home, so it’s safe to use indoors.

Keep your plants spook-free year-round!

With a little care and a few simple steps, you can make sure fungus gnats don’t stick around to haunt your plants this Halloween or any other time of the year. Don’t let the fungus gnats ruin your indoor garden. Arm yourself with these control methods and keep your plants thriving and pest-free all year round. And remember, if you have any questions about this or any other pest or gardening topic, feel free to contact your local Cornell Cooperative Extension office, we’re always happy to assist you with your gardening projects. Happy gardening!

Contact

Cristian Acosta
Agriculture Educator - Master Gardener Volunteer Coordinator
cfa34@cornell.edu
585-268-7644 ext 14

Last updated October 31, 2024