Fungus gnats are small, delicate, dark grey flies, about 2-4 mm long, mainly Bradysia species in the family Sciaridae. Adults are weak fliers with long legs and a Y-shaped wing vein. The larvae are clear to white legless maggots with a shiny black head, living in the top few centimetres of moist media.
In practice
Adults are a nuisance; larvae do the damage. They chew root hairs and can spread root pathogens such as Pythium, which is why they matter most for seedlings and fresh clones. Constantly wet media and organic-rich mixes breed them. Growers monitor with yellow sticky traps near the medium surface and with potato slices pressed into the top layer, checked after a couple of days for larvae. Letting the top layer dry between waterings, bottom watering and top-dressing with coarse sand help. Biological control uses Steinernema feltiae nematodes, the soil mite Stratiolaelaps scimitus, and Bacillus thuringiensis subsp. israelensis (Bti) drenches.
Don't confuse with
- Shore fly: stockier, with dark wings marked by pale spots, feeding on algae.
- Root aphid: winged forms on traps look gnat-like but have aphid bodies.
- Whitefly: white wings, lives on leaf undersides.
A microscopic tube-like outgrowth of a single root epidermal cell near the root tip that greatly increases surface area for water and nutrient uptake.A coloured card coated in non-drying glue that catches flying insects, used mainly to monitor pest trends: yellow for gnats, whiteflies and aphids, blue for thrips.Bottom watering lets a pot absorb water or solution from a tray beneath it, so the medium wets from the base upward by capillary action.Using living natural enemies, such as predators, parasitoids, nematodes and microbes, to suppress pests and diseases instead of, or before, chemical sprays.An entomopathogenic nematode applied as a soil drench whose infective juveniles enter fungus gnat larvae and thrips pupae and kill them with symbiotic bacteria.A soil-dwelling predatory mite, formerly sold as Hypoaspis miles, that hunts fungus gnat larvae, thrips pupae and other small arthropods in the growing medium.A soil bacterium whose crystal proteins kill caterpillars after they eat treated leaves; subspecies kurstaki and aizawai are the strains used against budworms.Small soft-bodied sap-sucking insects of the family Aphididae that cluster on stems and leaf undersides, excrete sticky honeydew and multiply rapidly by live birth.Stocky dark flies of the genus Scatella (family Ephydridae) that breed in algae on wet surfaces; mostly a nuisance and contamination issue rather than a root feeder.Bacillus thuringiensis subsp. israelensis, a bacterial strain whose toxins kill fly larvae, including fungus gnats, after ingestion; applied as a medium drench.A genus of oomycete water moulds, such as Pythium myriotylum and P. aphanidermatum, that cause brown, slimy root rot, especially in warm hydroponic systems.
