Greenhouse fungus gnats guide

Identify the fly and its source

Fungus gnats, also called sciarid flies, are small dark insects frequently seen around moist seed trays and potting compost. The RHS fungus-gnat guide explains that many fungus-gnat species feed on decaying matter and are harmless. Some larvae in persistently damp compost can damage vulnerable seedlings and soft cuttings. An adult fly on a greenhouse window is therefore not enough evidence that a mature tomato has a serious pest problem.

Inspect the growing medium of the affected trays. Are flies rising each time you water? Is compost permanently wet below the surface, drainage blocked or a tray holding water? Look for weak seedlings with root or stem problems, but remember that damping off and poor watering can cause similar decline. Do not infer larvae from every wilted plant.

Correct the conditions without harming seedlings

Remove standing water from saucers and trays, clear decaying plant debris and let the medium approach the moisture level required by the crop before watering again. The aim is not to desiccate a new sowing. A small cell tray can dry quickly near a sunny pane; check below the surface. Improve ventilation where it can be done without chilling tender seedlings and space trays so a damp corner is not hidden behind pots.

Use clean containers and fresh commercial seed compost for the next batch. Keep opened compost closed and away from puddles. If a tray is heavily affected and seedlings are failing, isolating or replacing it may be more practical than repeatedly moving adults around the house. The greenhouse propagation guide explains how to separate the humid germination stage from brighter, better-ventilated growth.

Monitor adults carefully

Yellow sticky cards near a suspect tray can indicate continuing adult activity. The RHS explicitly warns that they can catch non-target invertebrates, including predators. Use only a small number where the information will change what you do, and reconsider them during a biological-control release. A card is not a treatment for larvae in compost and will not correct overwatering. Check it alongside the tray rather than interpreting a count in isolation.

Where seedlings are genuinely at risk, the RHS identifies Steinernema feltiae nematodes, predatory mites and rove beetles as biological options for compost stages. These are living agents with application and storage requirements; read the supplier's current instructions and check they match the target and temperature. See the biological-controls guide before ordering. We do not recommend a pesticide merely because one is sold online; the HSE database and exact label govern any pesticide use.

Keep the response proportional

If established plants are healthy and only a few adults are seen, monitor and improve housekeeping rather than trying to make the greenhouse insect-free. The important distinction is whether young roots and cuttings are being harmed. Return to the root-zone moisture pattern after a week, not just the number of flies visible at the door. If the seedlings continue to collapse with few flies present, revisit disease and watering diagnoses instead of escalating gnat treatments.