Fungus Gnats

Fungus gnats are a family of small, dark flies commonly seen hovering around pot plants, seedling trays and propagation areas, especially where potting mix is kept consistently moist.

Adults are slender, mosquito-like flies about 2-4mm long, dark grey to black, with long legs, long thin antennae and a single pair of clear wings held over the body. They are weak fliers and tend to run or hop across the soil surface and pot rims rather than fly far, often seen resting on the inside of glasshouse glass or pot walls. The larvae live in the top few centimetres of growing media: they are legless, thread-like maggots up to about 5-8mm long, translucent to whitish with a distinctly shiny black head capsule, and are usually found wriggling through moist soil, mulch or drainage holes rather than on foliage.

They breed in organic matter such as damp potting mix, compost, mulch, decaying roots and algae growing on the soil surface. Populations build quickly in warm, humid conditions where the growing medium is kept too wet for too long, which is why they are such a common problem in glasshouses, nurseries, indoor plants and overwatered seedling trays across Australia. Life cycle from egg to adult can be completed in a few weeks in warm weather, so numbers can increase rapidly if conditions stay favourable.

Management is mostly about moisture control: allow the top layer of potting mix to dry out between waterings, improve drainage, remove algae and decaying plant matter from pot surfaces, and avoid overwatering seedlings. Yellow sticky traps placed near pots help monitor and reduce adult numbers. Larval feeding on fine root hairs is usually only a real problem for young seedlings or weakened plants, where root damage can slow growth or increase susceptibility to root rot pathogens.