How do I control whitefly?
Q Is there a way to get rid of whitefly on my pelargoniums under glass?
Chris Smaile, by email
A Greenhouse whitefly ( Trialeurodes vaporariorum) attacks a wide range of indoor plants. Control isn’t easy as whitefly breed rapidly under warm conditions. There are chemical sprays available at garden centres, but you may have a strain that has developed resistance to some insecticides.
An effective alternative is the use of a small parasitic wasp called Encarsia formosa. This breeds in the scale-like whitefly nymphs and kills them. There are a number of mail order suppliers, including Defenders (tel: 01233 813130).
Encarsia only works well during the late spring to early autumn as it needs warm conditions. At other times of the year keep whitefly numbers at a reasonably low level by spraying thoroughly with a fatty acid or plant oil spray and by hanging up sticky yellow traps. These sprays have a short life and so present no danger of toxic residues remaining on the plants when it’s time to introduce the parasite.