Small black flies are drifting over the trays. The logical response is to reach for an aerosol and treat the air. The flies drop, and a week later there are just as many. The reason is that an adult gnat lives 7-10 days and barely affects the plant, while the damage is done by the larva sitting in the top few centimeters of the substrate, where an air treatment never reaches. To bring the population down you work on the substrate and on its moisture.
Quick glossary: Fungus gnats (sciarids) - small flies of the genus Bradysia, most often Bradysia coprophila and Bradysia impatiens; adults about 3 mm, black, with long legs and antennae and a characteristic Y-shaped wing vein. Larva - white and translucent with a black head capsule, up to 8 mm long, living in the top 2-3 cm of moist substrate. Shore fly - a lookalike neighbor in the greenhouse, genus Scatella: stouter body, very short antennae, five light spots on dark wings; its larva feeds on algae and never touches roots.
The Cycle: Why Treating Adults Changes Nothing
A female lays eggs in moist substrate, up to 200 over a lifetime, and lives 7-10 days doing it. Eggs hatch in about 4 days. The larva then passes through four instars and grows to 8 mm, then pupates, then the next generation emerges.
The speed is set entirely by temperature. At 72°F (22°C) the full cycle takes about 21 days; at 61°F (16°C), about 40 days. In a typical grow room at 64-75°F (18-24°C), egg to adult takes three to four weeks.
The consequence matters. At any given moment most of the population is eggs and larvae in the substrate. Treating the air removes the adults currently flying and touches neither eggs nor larvae. A few days later a new wave emerges and the picture returns to where it started. An ordinary yellow card works the same way: it counts the population rather than reducing it.
Diagnosis: Gnat, Shore Fly, or Fruit Fly
Three different flies in a greenhouse call for three different actions, so identification comes first.
Fungus gnat: slender body, long legs and antennae, the overall look of a miniature mosquito, with a forked Y-shaped vein visible on the wing. It flies slowly and reluctantly and more often runs across the substrate surface. Its larva lives in the substrate and damages roots.
Shore fly: compact, stout body, very short antennae, five light spots on each dark wing, no Y-shaped vein. It flies fast. Its larva eats algae and leaves roots alone, so the shore fly itself is not a plant pest. Its presence means something else: plenty of algae and permanently wet surfaces, which are exactly the conditions feeding the gnat.
Fruit flies stay near harvested produce and organic waste rather than near substrate, and have nothing to do with the root zone.
Adults are counted on yellow sticky cards, and for gnats placement matters: the card lies flat on or just above the substrate surface rather than hanging at leaf height. The placement rate is 1-4 cards of 8x13 cm per 90 m², inspected twice a week. The general logic of routes and thresholds is covered in the article on pest monitoring.
Cards will not show larvae. For those there is the potato test: pieces of potato about 6 mm thick are laid cut-side down on the substrate and pressed in slightly. Larvae move to them within a few days, and the count per piece shows the real state of the tray. It is the one simple way to see what you are actually dealing with.
What They Actually Damage
Young larvae eat root hairs, fungi, and algae in the top layer. Older ones bore into roots, into the stem at substrate level, and into leaves lying on the surface. On microgreens and on plug trays, where the entire root system is that top layer, the effect shows immediately: seedlings lag, some collapse, and the picture resembles damping-off.
The second mechanism matters more than the first. A damaged root is an open door for pathogens, and the gnat itself carries their spores. Adults have been documented transporting Fusarium oxysporum f. sp. radicis-lycopersici from culture and from diseased plants onto the roots and hypocotyls of healthy tomato and bean (Gillespie and Menzies, 1993). Larvae of Bradysia impatiens ingest propagules of Pythium aphanidermatum (Gardiner, Jarvis and Shipp, 1990), and a larva takes up roughly 21 times more spores than an adult does. The spores survive the insect's digestive tract and come out in the frass somewhere else.
The honest limit of that claim: larval transmission of Pythium has been shown under laboratory conditions, and how much it contributes in a working greenhouse compared with water and substrate remains an open question. The practical conclusion holds either way: a gnat population in a system that already has Pythium or Fusarium works for the pathogen.
Control: Moisture First, Biology Second
A dry top layer is the strongest lever for control. Eggs and young larvae live in the top 2-3 cm and do not survive drying. Letting the top 2-5 cm dry out between waterings breaks the generation with no product at all.
For substrate systems that means moving from watering on a schedule to watering on substrate status - how to measure that instead of judging by feel is covered in the article on volumetric water content. On microgreens during germination drying is not an option, and there bottom watering replaces overhead watering; see watering microgreens.
Remove algae and standing water. Green film on substrate, on mats, on the floor and in trays is a canteen for larvae and shore flies alike. Clearing water off benches and floor removes half the food supply.
Bacillus thuringiensis subsp. israelensis (Bti) is a bacterial product accepted in organic production, applied as a substrate drench and active against larvae. It needs repeats because it does not affect eggs: a series of drenches at an interval that covers each new emergence.
Entomopathogenic nematodes, Steinernema feltiae, are applied by drench, drip irrigation or substrate spray. Larvae die three to four days after contact. The nematodes need moist substrate and a temperature of 61-90°F (16-32°C); in dry substrate they cannot move, which is why nematodes and top-layer drying are used in sequence rather than together.
Predators in the substrate: the mite Stratiolaelaps scimitus (formerly Hypoaspis miles) and the rove beetle Dalotia coriaria. Both work on larvae in the top layer and hold a background level rather than stopping an outbreak. The logic of introducing beneficials is covered in the article on beneficial insects.
What does not work: diatomaceous earth and sand as a barrier layer on the substrate surface showed no effect in trials.
Three Mistakes That Cost the Most
Fighting adults instead of larvae. Aerosols, foggers and sticky tape near the ceiling remove the visible part of the population and change nothing in the substrate. A yellow card is not a control method either; it is a measuring instrument.
Keeping the substrate permanently wet "so it does not dry out". Steady surface moisture is the precise condition under which the gnat lays eggs. Most grow-room outbreaks trace back to watering on a schedule under a closed lid with weak ventilation.
Applying nematodes and drying the substrate right afterward. Two correct measures used together cancel each other: Steinernema feltiae moves only through water in the pores. First the nematode drench and a few days of normal moisture, then the drying regime.
Signs That Fungus Gnats Are Under Control
- Adult counts on yellow cards fall over two consecutive readings and then stay low
- The potato test shows a handful of larvae or none
- The top 2-3 cm of substrate dries between waterings, no green film on the surface, no standing water in trays
- Seedling stands even out and unexplained losses stop accumulating
If the cards are clean and seedlings still collapse, gnats are not the cause, and the next place to look is seedling disease - see microgreen diseases: mold, damping-off, bacterial rot.
If you want to go deeper: IPM in Controlled Environments: Pest Management as a System, Not a Spray Schedule shows how to build drying, nematodes and predators into one plan instead of reacting to each outbreak on its own.