Do you have small black flies living in your house plant compost? Or hovering around your house plants? These are fungus gnats, also known as house plant flies and sciarid flies. They’re mostly harmless – adult gnats cause little or no harm to plants, but they can become a nuisance in the home. Their tiny worm-like larvae live in the top 5-8cm of compost, where they feed on algae, fungi and plant roots. Healthy house plants usually tolerate this minor root damage, but the larvae can harm seedlings or weak plants.