Fungus gnats
Small dark flies that hover around houseplants; their larvae live in damp potting soil and can damage roots and seedlings.
Also known as: Sciarid flies, Soil gnats
Symptoms and identification
Tiny black flies running across the soil surface or flying around plants, especially when pots are watered. Clear, thread-like larvae with black heads live in the top few centimetres of soil. Seedlings and cuttings may wilt or collapse as larvae feed on fine roots.
Causes
Constantly wet potting soil rich in organic matter, such as peat or compost, gives the larvae ideal conditions. Overwatering, poor drainage and saucers left full of water are the usual causes.
Organic treatment
Allow the soil to dry out as much as the plant tolerates. Use yellow sticky traps to catch adults. Water the soil with a solution of Bacillus thuringiensis israelensis (Bti), or with 1 part 3% hydrogen peroxide to 4 parts water, to kill larvae. Repeat weekly for three weeks.
Prevention
Let the top 2–3 cm of soil dry out between waterings, empty saucers after watering and use pots with drainage holes. Cover the soil surface with a layer of coarse sand or fine gravel, and store opened bags of potting mix sealed.
Chemical treatment (use with care)
Chemical treatments are rarely needed. If an infestation persists, a soil drench labelled for fungus gnat larvae can be used according to the label.