Plant care guide
Fungus Gnats on a Peace Lily: Dry the Mix, Break the Cycle
Updated
Why does my Peace lily have fungus gnats?
Small dark flies around a peace lily are usually fungus gnats breeding in damp, organic potting mix. Adults are a nuisance; large numbers of larvae can damage roots. The moist mix that peace lilies like is what lets them build up.
What to do first
Let the top of the mix dry between waterings, empty saucers, trap adults with yellow sticky cards, and drench the mix with a Bti product on a repeat schedule if larvae persist.
Tiny black flies drifting up from a peace lily pot are irritating, and they tend to show up just when you are trying to keep the plant well watered. Clemson Cooperative Extension lists fungus gnats among the pests of peace lilies. The good news is that the fix is mostly about the mix, not about sprays.
The tricky part is balance. A peace lily wants its mix moist, while fungus gnats breed in mix that stays wet. The steps below dry the surface enough to break the gnat cycle without letting the plant collapse.
Confirm they are fungus gnats
University of Minnesota Extension describes fungus gnats as slender, dark flies, 1.5–3 mm (1/16–1/8 in) long, that fold their wings behind the back at rest. They are found around houseplants in over-watered mix or mix high in organic matter.
The UC IPM fungus gnat Pest Note adds two checks:
- Larvae have a shiny black head and a whitish to clear, legless body. They live in the top of moist potting mix.
- Potato test. Push a few chunks of raw potato into the mix. Larvae are attracted to them. Check after a few days, then replace them.
If the flies are white and rest on leaf undersides rather than the mix, they are not fungus gnats. Identify them before treating.
How much harm they do
Minnesota Extension notes that fungus gnats do not harm people and typically do not harm plants. UC IPM adds that larvae can damage roots and stunt growth when present in large numbers, and very high populations can kill houseplants.
UC IPM also cautions that watering problems and root decay fungi are more common causes of wilting than gnats. If your peace lily is drooping, check the root zone as well as the flies.
Why peace lilies attract them
NC State Extension advises keeping peace lily mix moist but not soggy, allowing it to dry between waterings, and using a potting mix high in organic matter. Those are the conditions gnats like when the “moist” side wins.
Virginia Cooperative Extension’s watering guide lists fungus gnats among the signs of overwatering. A steady gnat population is a useful early warning that the mix is staying too wet.
Break the cycle: step by step
- Let the surface dry. UC IPM advises letting the surface of container soil dry between waterings. Check 2.5 cm (1 in) or more down with a finger; water when the top feels dry but before the plant wilts.
- Fix drainage. UC IPM recommends good drainage. Empty saucers and decorative outer pots after watering. Make sure the inner pot has an open drainage hole.
- Water from below if it helps. NC State notes that watering from below works very well for peace lilies. Lift the pot out once the mix is moist, and do not leave it standing longer than 60 minutes, as Virginia Cooperative Extension advises.
- Trap adults. UC IPM suggests cutting yellow sticky traps into small squares, mounting them on sticks, and placing them in the pots. They catch adults and show you whether numbers are falling.
- Treat larvae with Bti. UC IPM recommends Bacillus thuringiensis subsp. israelensis (Bti) products as a drench or spray that targets larvae. Repeat applications, at about five-day intervals, are commonly needed. Follow the label.
- Consider nematodes. UC IPM lists Steinernema feltiae nematodes, applied as a soil drench when the mix is moist and temperatures are 16–32 °C (60–90 °F).
- Remove breeding material. Clear fallen leaves and spent flowers from the mix surface. UC IPM notes that decomposing organic matter is a favorite breeding site.
UC IPM says insecticides are rarely warranted for fungus gnats in homes. If you do use one, UC IPM notes pyrethrins give only temporary, fast-acting control.
How long it takes
UC IPM reports that at 24 °C (75 °F), a full generation takes about 17 days, from egg to adult. Expect adults to keep appearing for a few weeks while larvae already in the mix finish developing. Keep traps up until catches stop.
Prevent a return
- Use pasteurized potting mix. UC IPM warns against incompletely composted organic matter in potting media.
- Repot with fresh mix rather than reusing old mix from infested pots.
- Keep new plants apart from others until you know they are clean. UC IPM advises not moving infested plants where flies can reach other pots.
- Repot in late winter. NC State recommends repotting peace lilies, when necessary, in February or March. Clemson suggests a pot only 2.5–5 cm (1–2 in) larger, which avoids excess wet mix.
For more on gnat control, see our fungus gnat guide. The peace lily moldy soil guide covers the same wet-mix conditions, and the peace lily hub lists our other pages.
Related guides
Other peace lily problems we cover: sunburn treatment, drooping, yellow leaves, brown tips, drooping recovery conditions.
References
- UC Statewide IPM Program: Fungus Gnats, Pest Notes
- University of Minnesota Extension: Fungus gnats
- Clemson Cooperative Extension, Home & Garden Information Center: Peace Lily
- NC State Extension Gardener Plant Toolbox: Spathiphyllum
- Virginia Cooperative Extension: Properly Watering Container Houseplants (SPES-804)
How this guide was put together: UC Statewide IPM Program, University of Minnesota Extension, Clemson Cooperative Extension, NC State Extension, and Virginia Cooperative Extension guidance.