Glasshouse whitefly are small white flying insects which lays eggs on the undersides
of leaves. Whiteflies suck sap from their host plant and drop sticky secretions
onto leaves below. Left uncontrolled they will overwhelm the plant, weakening it
and flying around in huge numbers. Undersides of leaves become populated with Whiteflies
adults and white eggs (scales).
Click here for whitefly treatments
Remember whiteflies are nearly always moving. If you see white threads static on tops of the leaves, they are not whitefly but something else - very likely to be shed aphid skins dropped down from developing Aphid under leaves above. Or they could be thrips larvae. They are not "whitefly" unless body and wings are a ghostly white.
The glasshouse whitefly (Trialeurodes vaporariorum)is the most common. You can recognise
them by their heart-shape profile when looked from above (see picture in the middle
below). Tobacco whitefly (Bemisia tabaci) have parallel wings which are longer,
and when folded you can still see a strip of their back between the wings. (see
photo on right) There up to 50 different species of whitefly in Europe, but from a gardeners point of view they are all plant pests.
The most effective treatment for whitefly is the parasite "Encarsia". It is supplied on small cards which can be hung up around your plants. For more information about Encarsia click here
Encarsia works best with temperatures above 21°C with 12hr+ daylight hours. Generally, this means introducing Encarsia from mid April to August, although it can be earlier or later with extra heat and light.
If you have tobacco whitefly, or if the temperatures are very high in midsummer, then there is another parasitic wasp called "eretmocerus" which is recommended.