Thrips (Heliothrips haemorrhoidalis, Frankliniella occidentalis) are thin yellow or dark brown insects less than 1.6mm long that live mainly on the upper surfaces of leaves and on/in flowers, flying poorly they often prefer to jump. They feed by sucking sap and rasping surfaces causing a dull green or silvery discolouration of the foliage, which is also marked by minute black spots caused by the thrips' excretions. Can also be found on the underside along veins. Leaves may curl, and in extreme cases plants wither, brown and die.
Control with soft soap, then Amblyseius predatory mite should be introduced. All plant material should be removed from greenhouses/rooms with several days of heating to starve out thrips.