Plant care guide
Gnats on Monstera: Dry the Mix and Break the Cycle
Updated
Why does my Monstera deliciosa have fungus gnats?
Small dark flies that run across a Monstera's potting mix or hover just above it point to fungus gnats. They breed in mix that stays damp. The adults are a nuisance; large numbers of larvae can feed on roots.
What to do first
Let the top quarter to third of the mix dry before watering again, empty the saucer, and trap adults with yellow sticky cards. If larvae persist, drench the mix with a Bti product labeled for houseplants and repeat as the label directs.
Tiny black flies rising from a Monstera pot are irritating, and they can make you worry about the roots. In most homes they are fungus gnats, and the fix starts in the potting mix, not with a spray. University of Maryland Extension calls fungus gnats annoying but harmless to indoor plants, and links them to overwatering, which feeds the fungi their larvae eat.
Confirm they are fungus gnats
Clemson HGIC’s houseplant pests factsheet describes adults as delicate flies about 3 mm (1/8 in) long. They run across the mix or fly weakly near it, and collect on windows when numbers are high.
Check for:
- Larvae in the top of the mix. The UC IPM fungus gnat Pest Note describes a shiny black head and a whitish to clear, legless body. Clemson says larvae grow to about 6 mm (1/4 in).
- Slime trails on the surface. UC IPM notes these in very moist, heavily infested pots.
- A potato test. Push chunks of raw potato, cut side down, into the mix. UC IPM says larvae are drawn to them. Check after a few days.
- The real source. UMD suggests covering the plant, or a nearby drain, with screen or cheesecloth for several days to see where the flies emerge. Iowa State Extension adds condensation trays and other wet spots to the list.
Why a Monstera pot can breed them
NC State Extension advises watering Monstera deliciosa thoroughly, then letting the top quarter to one-third of the mix dry before the next watering.
UW–Madison Extension’s Monstera page says to water less in winter, and notes that leaves “sweat” when the medium stays too moist. Clemson adds that fungus gnats are most often a problem in peat-rich mixes that are overwatered. Iowa State calls them a good indicator of overwatering.
How much harm they do
UC IPM says adult gnats do not damage plants or bite. Larvae in large numbers can damage roots and stunt growth, mainly in seedlings and young plants. UC IPM also stresses that wilting is far more often caused by watering problems, poor drainage or root decay than by gnats. If your Monstera is drooping or yellowing, check the roots as well; see our Monstera root rot guide.
What to do
- Let the mix dry down. Wait until the top quarter to one-third is dry, as NC State advises. Clemson says dry conditions kill the larvae.
- Empty the saucer and outer pot. Clemson and Iowa State both warn against leaving pots standing in water.
- Trap adults. UC IPM suggests cutting yellow sticky traps into small squares and mounting them on sticks in the pot. Iowa State notes traps show you the trend but do not stop new adults hatching.
- Treat larvae with Bti. UC IPM, Clemson and UMD all list Bacillus thuringiensis subsp. israelensis (Bti) as a soil treatment for larvae. UC IPM says repeat applications at about five-day intervals are commonly needed. Bt sold for caterpillars is a different subspecies and does not work on gnats.
- Consider nematodes. UC IPM names Steinernema feltiae as the most effective nematode. Apply it as a drench to moist mix at 16–32 °C (60–90 °F).
- Skip the sprays. UC IPM says insecticides are rarely warranted for fungus gnats in homes. Iowa State adds that sand or gravel top layers are unnecessary once watering is right.
UC IPM gives a generation time of about 17 days at 24 °C (75 °F). Expect some adults for a few weeks while larvae already in the mix finish developing.
Keep pets in mind
The ASPCA lists Monstera deliciosa as toxic to dogs and cats because of insoluble calcium oxalates. Signs include mouth irritation, drooling, vomiting and difficulty swallowing. Keep fallen leaves, traps and treatment products out of reach while you work.
Prevent a return
- Use bagged, pasteurized potting mix. UC IPM warns that incompletely composted organic matter often carries gnats.
- Keep an infested pot apart. UC IPM advises against moving it where emerging flies can reach other pots.
- Repot into fresh mix rather than reusing mix from an infested pot.
Our fungus gnat guide covers the life cycle and treatments in more depth. White fuzz on the surface often comes with the same wet conditions; see moldy soil on Monstera. More care pages are on the Monstera deliciosa hub.
References
- UC Statewide IPM Program: Pest Notes: Fungus Gnats (Publication 7448)
- University of Maryland Extension: Fungus Gnats on Indoor Plants
- Clemson Cooperative Extension HGIC 2252: Common Houseplant Insects & Related Pests
- NC State Extension Gardener Plant Toolbox: Monstera deliciosa
- UW–Madison Extension: Swiss-Cheese Plant, Monstera deliciosa
- Iowa State University Extension: Diagnosing Houseplant Problems Caused by Insect Pests
- ASPCA Animal Poison Control: Cutleaf Philodendron (Monstera deliciosa)
How this guide was put together: UC Statewide IPM Program, University of Maryland Extension, Clemson Cooperative Extension, NC State Extension, UW–Madison Extension, Iowa State University Extension and ASPCA guidance.