Adult Bradysia are small (2-4mm), dark grey to black, slender, mosquito-like silhouette but much smaller and weaker flying. Long legs and long antennae, clear wings with a distinctive Y-shaped wing vein pattern. Larvae are tiny (3-5mm), translucent white with a black head capsule, lying in the top 2-3cm of potting soil. Egg clutches are microscopic and laid on damp soil surface or in soil cracks. Often confused with fruit flies but fungus gnats are darker, more mosquito-like, and hover around plants rather than fruit.
Fungus gnats live entirely around indoor plants. Adults don't travel far from their emergence site — you'll see them in the room with the affected plant. Larvae live in the moist surface layer of potting soil, especially soil that's been kept continuously damp (over-watering, heavy mulched container surface, self-watering planters with inadequate drainage). BC homes with extensive houseplant collections, year-round indoor growing setups (especially propagation stations), and unheated greenhouses all see seasonal fungus-gnat pressure.
- Small dark flies hovering around houseplants, especially when the soil is disturbed.
- Flies walking on soil surface or on nearby leaves.
- Stunted seedling growth or unexplained young-plant die-off (larvae damage roots).
- Yellow sticky traps near plants accumulating small dark flies.
- Visible larvae on soil surface when you lift a plant or water the soil (they respond to moisture).
Human risk: zero. Fungus gnats don't bite, don't carry disease, don't damage non-plant materials. Plant risk: significant for seedlings and cuttings (larvae feed on fine root hairs), negligible for mature established plants. The biggest practical risk is propagation loss — a fungus gnat outbreak in a cutting station can kill 30-50% of seedlings. Commercial nursery and cannabis cultivation operations have additional risk because fungus gnats can transmit plant pathogens like Pythium (root rot) between plants.
Year-round indoors; tightly coupled to houseplant care patterns. Populations explode when ambient humidity rises (summer coastal BC, home-heating-off shoulder seasons) and water management relaxes. New plant introductions are a classic trigger — every new houseplant arrives with an unknown population history. Winter pressure is elevated in BC homes due to closed windows and high humidity from indoor plants.
The entire protocol is water management + targeted biological treatment. Step one: stop overwatering — let the top 2-3cm of soil dry completely between waterings. This alone collapses most populations within 10-14 days. Step two: bottom-water rather than top-water (keeps the surface layer dry — where larvae live). Step three: for active infestations, apply Bacillus thuringiensis israelensis (Bti, sold as Mosquito Bits) to soil surface — a naturally-occurring bacterial treatment that kills larvae without harming the plant. Step four: sticky cards monitor population trend. Yellow cards work; blue cards attract other flying pests. Step five (only if needed): neem-based foliar for heavy infestation. We never recommend systemic insecticides for houseplant fungus gnats — the pest doesn't justify it and the residue is indoor-air exposure.
Residential fungus gnat problems are almost always self-solvable with Bti + watering discipline. Call for professional service if: you're a commercial cannabis cultivator or commercial indoor-garden operation where crop loss matters, you have a persistent population after 4+ weeks of Bti + watering correction, or you need documented IPM protocols for Health Canada cannabis compliance. For commercial cannabis specifically, see our Cannabis IPM page for full-scope programs.
1
Let soil dry between waterings
Top 2-3cm of potting soil should be completely dry before re-watering. Stick a finger in or use a soil moisture meter. The fungus gnat life cycle collapses without continuous surface moisture — this single change resolves most infestations.
2
Bottom-water instead of top
Water into the saucer; let the plant wick moisture upward through the roots. Keeps the soil surface (where larvae live) dry. Standard practice in professional horticulture for a reason.
3
Inspect + quarantine new plants
New houseplants are the primary introduction pathway. Inspect soil surface and check for flies at purchase; quarantine away from other plants for 2-3 weeks after bringing home. Many garden centres unknowingly ship fungus-gnat populations.
4
Apply Bti (Mosquito Bits) to soil at first sign
Sprinkle Bti-containing granular product (Mosquito Bits, widely available) on soil surface and water in. Bti is a naturally-occurring bacterium specific to fly larvae, harmless to plants and mammals. Re-apply weekly for 3 weeks during active infestation.
5
Yellow sticky cards for monitoring
Place yellow sticky cards near plant bases. Count catches weekly to trend population. Catches dropping = program working; catches rising = a breeding site still active.
6
Top-dress with dry material
For chronic plants, add 1-2cm of sand, perlite, or BTi-inoculated mulch to the soil surface. Dry non-organic top layer discourages egg-laying.
The Wild Pest service
Transparent pricing, 60-day return guarantee, same-day response across Metro Vancouver. Every treatment is documented with photos and service notes.
Are fungus gnats the same as fruit flies?+
No. Fungus gnats (Bradysia) are mosquito-like and hover around houseplants; larvae live in soil. Fruit flies (Drosophila) are tan-coloured, smaller-winged, and congregate on overripe fruit or organic waste. Different species, different treatment entirely.
Do fungus gnats bite?+
No. Adult fungus gnats have mouthparts that don't penetrate skin. They're a visual nuisance only — harmless to people and pets.
Will fungus gnats damage my mature houseplants?+
Usually not. Mature, established plants with extensive root systems tolerate fungus gnat larvae without visible damage. Seedlings and cuttings are vulnerable — larvae can kill young plants by eating fine root hairs. Commercial propagation is the main concern.
What's Bti and is it safe for pets?+
Bti (Bacillus thuringiensis israelensis) is a naturally-occurring soil bacterium specific to fly-larvae. Bti is Health Canada-registered for indoor and edible-garden use, is not toxic to mammals, birds, or fish, and is approved for organic agriculture. Pets drinking water with Bti have no known adverse effect.
