Glasshouse whitefly are, as their name implies, mainly a pest of greenhouses and other protected spaces such as conservatories, coldframes and porches but in a hot summer and especially in warmer parts of the country, they may sometimes attack plants in sheltered spots in the garden. This may not be something we’ve much chance of seeing so far this year, but I’m getting an increasing number of enquiries about them in greenhouses so do keep an eye out for them. They breed really rapidly so regular checks are essential if the situation is not to get out of control.
The adults are tiny white moth-like creatures which feed by sucking the plant’s sap.
Their young bear no similarity to their parents and look like 1mm long elliptical slightly raised greenish yellow flattened blobs on the lower leaf surfaces (so very easy not to recognise!) Hanging a yellow sticky trap or two in a greenhouse makes it easy to spot when they arrive or just check frequently, you’ll find that as soon as the plants are disturbed the whitefly fly off the plants, quickly returning to feed as soon as you leave!
Plant sap has an extremely high sugar content, so high that the whitefly cannot digest it all so their excreta is rather like sugar-solution and, when they flick it out the foliage often becomes sticky. Not so pleasant but far worse, this sugary excreta, known as ‘honeydew’ , soon attracts black fungi called sooty moulds and , in no time at all, your plants look as if someone has sprinkled soot on them.
There are chemicals available which you can spray on most ornamental plants but far better to introduce a tiny parasitic wasp, Encarsia formosa, which will zoom around the greenhouse killing off the young whitefly for you and not leaving any unpleasant chemical residues behind. You can find out more and order this from www.pippagreenwood.com/products/protect-your-plants