Sciarid flies constitute the family of Sciaridae. The species that cause most damage to crops belong to the genera Bradysia and Lycoriella. Lycoriella species are mainly found in commercial mushroom cultivation where they can cause enormous damage.
Sciarid flies are usually found in damp, humid environments, and appear very commonly in greenhouses throughout the world. Although they generally do little harm to healthy plants, sciarid flies are particularly problematic in cuttings and other young growing plant material.
The life cycle of sciarid flies has the following stages: egg, four larval instars, a pupal instar, and the adult. The adult insects are 1 to 5 mm long, grey-black flies with long antennae. They have relatively long legs, and the wings show very clear venation. The fly has a small head equipped with sucking mouth parts, but they hardly feed at all during their short life. The males are usually smaller than the females. They are not fast fliers, and prefer damp areas in dark places, with a lot of organic material. They occur the whole year round.
The eggs are minute (0.1 to 0.25 mm) and yellowish white in colour. They are laid on the ground surface close to plant roots. The larvae can grow to 5 to 12 mm in length and 0.5 to 1.5 mm in diameter. They are legless and have a conspicuous black head. The head is equipped with biting-chewing mouth parts. Pupation takes place in a small hole in the ground. Pupae are white, but later become yellow to brown. The posterior end of the pupa remains mobile.
The larvae of sciarid flies feed mainly on organic remains and the fungi that grow on them, although some species can also eat living plant material. The adults prefer humid habitats and are attracted to a broad range of pathogenic and non-pathogenic micro-organisms for oviposition.
Indirect damage is caused by the larvae and adults transferring fungal diseases, such as Fusarium, Botrytis and Verticillium, from diseased to healthy plants.
Direct damage is the immediate result of larvae feeding on the roots. The larvae feed mainly on decaying plant material and on algae and fungi present in the soil. They, however, can also feed on root hairs, rootlets, and tender root, stem and leaf tissue. Later instars may even feed on plant stems. The injuries caused by feeding also provide invasion routes for various pathogenic fungi. Because the larvae move very little, plant death is generally local. Young plants that are kept humid and well-watered are particularly at risk.