Also Known as Fungus Gnats, Mushroom Fly.  They feed on mycelium in the soil and are generally of no importance. However they occasionally attack garden plants. 

 Small black flies up to 3-4mm long.  They are attracted to potting composts and are often seen in greenhouses.

Biology
Adults are attracted to potting media especially if the organic content is high, like peat as an example. Females lay 100 eggs each which hatch in 1 week.  Larvae feed for about a month.

Not all sciarids affect plants so treatment is only necessary if damage to plants occur.

Symptoms
White larvae, up to 10mm long with black or brown heads live in potting and rooting media.  They feed on root hairs at the base of cuttings of chrysanthemums and other plants propagated by soft tip cuttings, like pelargonium, carnation, orchids and many others. They also attack seedlings.

Treatment
Imidacloprid watered into the compost will give long lasting control often for up to 1 year.

Sciarid Fly Larvae