What are these viruses?
Impatiens necrotic spot virus (INSV) and Tomato spotted wilt virus (TSWV) are closely-related viruses. They are transmitted by thrips species, particularly the western flower thrips (Frankliniella occidentalis). This species first arrived in the UK in the late 1980s and, in addition to causing direct feeding damage, created huge problems by infecting plants with INSV and TSWV.
Western flower thrips (WFT) does not overwinter well outdoors in the UK, so the problems it causes are found mainly on plants grown in glasshouses, polytunnels, conservatories, etc. Where the viruses are found on plants such as dahlias and impatiens growing outdoors, these will usually have been infected when they were raised under protection prior to planting out. There may, however, be localised spread of WFT outdoors during summer.
Plants on which one or both of the viruses can be found include Begonia, chrysanthemum, Cineraria, Cyclamen, Dahlia, Gloxinia, Hoya, Impatiens, Pelargonium and tomato.