What is whitefly?
Whitefly (Trialeurodes vaporariorum) is an insect related to aphids that particularly affects plums, greengages and damsons. Adult females – small, white moth-like insects about 1.5 mm long – lay eggs onto the underside of leaves when the temperature warms up in spring. The eggs hatch in 5-10 days, and the nymphs start to feed on the sap in the leaves, excreting a sticky honeydew substance that coats the leaves, inhibits photosynthesis, and encourages the formation of black sooty mould. The nymphs undergo another three nymphal stages, and then pupate, after which more adults emerge. Every season there are several generations.
How to treat whitefly on plums
In order to control whitefly on plums, ensure that the tree is properly pruned, to ensure good light entry and air circulation in the crown. Clear the surroundings of weeds which may be a secondary home for whitefly, and remove any fallen leaves and debris. Ants farm whitefly, and their activity can be reduced by means of grease bands. Encourage the natural predators, ladybirds, lacewings and hoverflies, by not using pesticides in the garden, and growing insect-friendly plants such as nettles, marigolds, nasturtiums, foxgloves, sedum and others. Small birds can also help in the task of reducing aphid population, and they can be encouraged with nesting boxes.
Like other aphids, whitefly can be combated by spraying with dilute washing-up liquid (a few drops in a litre of water). Spray upwards onto the underside of the leaves, repeat every 2 or 3 days for a couple of weeks. Garden centres sell insecticidal soaps that work in much the same way. There are sticky yellow lures that help reduce the number of adult whitefly.