Fungus gnats are those annoying little flies commonly found in groups hanging around our indoor plants and bags of potting mix. They are black or dark grey, about 3mm long and thrive in moist potting mixes or soils. Adults are more than just a nuisance as they can transmit diseases like pythium and fusarium.
The larvae can be hard to spot as they are small (4-5mm long), white to translucent and live within the soil or potting mix. Usually found in the top 10cm of soil the larvae feed on a range of organic matter including plant roots, soft seedling stems and the base of cuttings. They can cause wilting and slow growth of plants especially small soft leaf plants. At greatest risk are seedlings and cuttings which have fewer roots and can be killed outright by the hungry larvae. Larvae also spread various diseases like phytophthora, rhizoctonia, verticillium and others.
Fungus gnats can infest any soil but are most prevalent in potting mixes which are high in organic matter and damp areas outdoors. Commonly seen in indoor plants, seedling punnets, pots with cuttings in them and in greenhouses.
Organic Control Methods for Fungus Gnats
- Apply eco-neem as a soil drench to kill larvae within the soil. A repeat application may be required as adult fungus gnats live for several weeks and may lay new eggs into the soil.
- Allow pots and other areas to dry out as the eggs need continuously moist conditions to survive.
- In general do not overwater indoor plants. Excess watering leads to algal growth in the potting mix and on saucers which fungus gnats will also feed on.