Whiteflies in your greenhouse? Here's how to deal with them
What are Whiteflies?
Glasshouse Whitefly are small, white and moth like. They lay eggs on the undersides of leaves. They are sap suckers so they eat plant material and drop sticky secretions onto leaves below.
Left uncontrolled they will overwhelm the plant by weakening it. Undersides of leaves become populated with Whiteflies adults and white eggs (scales).
Whitefly are fairly inactive in cool temperatures over Winter, but multiply very quickly in warm Spring and Summer weather. They fly about at the slightest disturbance, like tiny moths, and may even create ghostly white clouds in the air.
How to identify Whitefly
They are nearly always moving and grow in numbers quickly. They look like tiny months rather than flies. 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. 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. There are up to 50 different species of whitefly in Europe, but from a gardeners point of view they are all plant pests.
Glasshouse Whitefly on the underside of a leaf
Whitefly Treatments
Encarsia Formosa
These predatory wasps and work by taking over the whitefly eggs. Encarsia works best with temperatures above 21°C but don't like it too hot, above 25°C. They also need 12hr+ of light per day. Generally, this means introducing Encarsia from mid March to August, although it can be earlier or later with extra heat and light. The product need fresh introductions every 2 weeks for areas where whitefly tend to colonise.
Soft Soap
If temperatures don't allow the use of Encarsia then spraying with this non chemical spray 3 times a week should control the outbreak. This can gradually be reduced to fortnightly as a preventative measure.