Sciarid Flies are small midge like flies or gnats on composts of houseplants - indoor plants - lay eggs in the compost and the small maggots that emerge are happy to live on the decaying matter of peat based composts, or even attack the small roots of the pot plant. The midges - flies - are often seen flying about near houseplants.
The small grey black fly is just an eighth of an inch long - 2 - 4mm - and is normally seen hopping about on the surface of the compost, or flying around the pot plant. Often referred to as midges.
The maggot feeds just below the surface of the compost, but can sometimes be seen on the surface.
The maggots of the Sciarid Flies in the compost are translucent appearance - with a slight orange or white colouring and a black head. However, bearing in mind the small size of the grub, the head may not be noticeable.
Peat based composts are favoured by the Sciarid Fly Gnat - providing a plentiful supply of decaying organic matter (Peat) for the young maggot to feed upon. Damage to fine roots is probably incidental in the maggot's quest for food, as the fine roots will be embedded in the peat compost.
The Sciarid Fly is also known as the Fungus Gnat, because it has been proven to carry various fungus spores such as Pythium, Botrytis and Fusarium. Each of these fungi diseases are harmful to plants. Sometimes plants collapse or at least lose vigour and health, because of the fungal disease attacking the fine roots, causing the plant to die.
In the glasshouse, small seedling can be affected, as the Sciarid Fly lays its eggs in the compost used for seed sowing. By eating into the fine root hairs of the young seedling, the plant wilts. Damping Off disease is often blamed, whereas it is actually physical damage by the maggots which is the real cause.
As well as being a pest of house plants, the flies can be a pest where plants are grown by Hydroponics, as they are happy to lay their eggs - and therefore maggots - in the Rockwool material often used in Hydroponics.
The Sciarid Fly, Gnat, Midge, does not directly harm the plants - but are a pest when flying around indoors.
Pot grown plants and greenhouse - glasshouse - plants normally have to be watered regularly, and this provides the perfect environment for the fly., which loves the humid conditions artificially induced.
Treatment and Control
Sciarid flies can be controlled by a contact spray such as Bifenthrin, but also by a soil drench. the latter is effective if the adult flies are seen, because it almost certain that the eggs will have been laid in the compost.
Sticky Yellow Traps are a good idea in the greenhouse - also indoors if practicable, for they do trap a large number of the gnats - also other flying insects that may be a trouble, such as whitefly.