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Fungus gnats, also known as sciarid flies, are associated with damp composts especially in house plants and seed trays.
Fungus gnat adults
The adults are small flies and can be a nuisance, the compost-dwelling larvae can sometimes damage seedlings and cuttings. These insects also occur out of doors where they cause no damage. Cultivated and wild mushrooms can also be attacked. There are many species of fungus gnats, or sciarid flies, most of which are entirely harmless.
Adult fungus gnats are greyish brown flies that are mostly 3-4mm long. They can often be seen running over the surface of seed trays and pots, or they fly slowly around plants. The larvae are slender white maggots, up to 6mm long, with black heads. Their bodies are semi-transparent and it is often possible to see the dark coloured gut contents. They live in the soil or potting compost. This insect thrives in damp composts containing high levels of organic matter. Potting media formulated for houseplants is the best choice for indoor plants.
Adult fungus gnats do not damage plants but they can cause annoyance when they are flying around indoors. The larvae feed mainly on dead roots and other decaying plant material and associated fungal growth. Some species of fungus gnats may also feed on soft plant growth, such as seedling roots and the base of soft cuttings. Established plants are unlikely to be damaged by fungus gnat larvae.
Fungus gnats are really only of concern if they are causing damage to seedlings or cuttings; established plants are not harmed
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Fungus gnats can breed all year round in greenhouses and houses. The females deposit eggs in the surface layer of the potting compost and these hatch within a few days under warm conditions. The larvae feed on fungal growth and decaying plant material but some species can also damage the roots of seedlings or tunnel into the base of soft cuttings. When fully fed, the larvae pupate in the soil. During the summer the life cycle can be completed in about a month.
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