The Flying Killer - Feltiella acarisuga
The "Predatory Gall Midge" is a native British species. The adults fly, but the young are like tiny worms or caterpillars ("larvae"). Commercial Growers release them in glasshouses because the larvae eat red spider mite.
The adult's life is brief - they fly around looking for a place to deposit their eggs where there are mites to eat.
The photo of the larva has been supplied by BCP Certis and it shows the larva attacking the two-spotted mites (another name for glasshouse spider mite).
They will devour large quantities of red spider mite, and are active in spring and autumn when light levels are lower. They will be active from 15 degrees, although they will act quicker then warmer.
Like most midges they are fine in moist conditions and for that reason the midge is recommended for introducing at the beginning of the season, before the red spider mite gets out of control. However they do need some spider mite to feed on, so they are only worth using if you know you had a problem last year and you fear that spider mite eggs and young may have overwintered.
This control is not recommended for the hot dry conditions of mid summer. You should also take account of the fact that the cocoons and the midge larvae are visible to the naked eye, so this is not a solution for ornamental plants (flowers).
The Predatory Gall Midge is supplied in a tub of cocoons. Adult female midges emerge from the cocoons, mate inside the tub then fly off to lay their eggs amongst spider mite infestations. The treatment is more expensive than the phytoseiulus but if you have a known problem on a sizeable crop then one tub is excellent value because if you introduce the midges in the right conditions they will establish and breed just from one tub.