Insect – Low risk
Gnat
Small flies, 2 to 5 mm long, that flit around potted plants and drains: the black, delicate potting soil fly and the dark gray, hairy drain fly. In Quebec, they thrive year-round in heated homes as soon as potting soil remains damp or a drain becomes clogged.
2 TO 5 MM · SLIM BLACK OR FURRY GRAY
§1Distinctive features
How to identify it
The soil fly has a frail body, long legs, and slender antennae; it flies slowly in a zigzag pattern near potted plants and windows. The drain fly is stockier, dark gray, and hairy, like a tiny moth, with roof-shaped wings, resting on walls near sinks. They are often mistaken for the fruit fly, which is brownish with red eyes.
§2Habitat
Where to find it
Houseplant potting soil, seedlings, cuttings, stored bags of potting soil, and compost bins for soil flies; kitchen and bathroom drains, rarely used P-traps, and pits for drain flies. The larvae develop in the top few centimeters of constantly soggy potting soil or in the gelatinous film lining a drain.
§3Reproduction
Lifecycle
Without intervention, since each female lays up to 200 eggs, the population multiplies within a few weeks.
§4What to look out for
Signs of infestation
- Small black flies around potted plants
- Small, dark gray flies resting on the walls near drains and sinks
- Translucent larvae in the potting soil, visible under a potato slice placed on the surface
- Young plants that turn yellow or wilt for no apparent reason
§5Why act fast
Risks
- Spread of fungal diseases from one plant to another
- Larvae that damage plant roots
- Rapid infestation: a new generation every 3 to 4 weeks
§6Prevention
How to avoid it
- Let the potting soil dry out between waterings
- Scrub drains that are rarely used to remove the organic buildup; pouring cleaning products down the drain alone is not enough
- Cover the potting mix with a layer of sand or dry gravel
- Inspect the plants and bags of potting soil you purchase; store the potting soil in a dry place
§7Pro method
How MW Extermination Operates
Inspection
Thorough inspection to locate sources: plants, drains, compost, and wetlands.
Preparation
Identify the affected plants and drains; test by placing a glass coated with Vaseline over the drain overnight; remove the waterlogged potting soil.
Treatment
Targeted treatment of infested potting soil and mechanical brushing of drains using products approved by Health Canada, followed by yellow sticky traps to capture adults.
Follow-up
Watering and care recommendations to prevent long-term recurrence.
§8We answer you
Frequently asked questions
Why do I have gnats in my house?
All it takes is overwatered potting soil or a clogged drain: these flies lay their eggs in any moist organic matter.
How can you get rid of gnats quickly?
Let the potting soil dry out, brush out the drainage holes, and set out sticky traps. If the infestation persists, professional treatment is necessary.
Are gnats harmful to your health?
They don’t bite and are generally not a health hazard. However, since they come from drains, they can carry bacteria onto surfaces, and the larvae of the soil fly damage the roots of young plants.
Fungus gnats and drain flies almost always trace back to standing moisture: overwatered houseplants, a slow drain, a damp basement. Clear the breeding site and the swarm disappears. That diagnosis is the first step of our pest control service, in Granby, in Sherbrooke, in Saint-Hyacinthe and in Victoriaville.
Sources consulted for this fact sheet: Agri-Réseau, MAPAQ · INRAE Ephytia · Ohio State University, drain fly · UC IPM, potting soil fly.
React quickly, sleep well.
The longer the infestation lasts, the more expensive it becomes.
