Adult whiteflies as their name suggests are white in colour. They resemble tiny moths and are about 3mm long. When they are not flying (which they do profusely when disturbed) they can usually be found on the undersides of leaves.
Young whitefly look like clear scales and can also be found on the undersides of leaves.
NB. Aphid cast skins are sometimes mistaken for whitefly! However, whitefly will fly away when disturbed, these cast skins will fall off or not move. Just shake the plant to find out which you have!
The whitefly eggs are laid in a horseshoe shape on the undersides of leaves. Once hatched the young nymphs move a short distance so that they are equally spaced and then moult into legless scales that spend the rest of their development feeding by sucking sap. These scales excrete a sticky “honeydew” which falls onto the leaves and fruit below.
Sooty moulds often develop on this “honeydew” and plants under attack appear to lack vigour.
Sooty moulds can reduce the amount of light reaching the leaves which can lead to leaf yellowing and plant death.
Whitefly Life Cycle
Whitefly go through several growing stages between egg and adult. The whole cycle takes about 30 days at 20ºC (68ºF). The length of the adults life depends on the plant on which it was raised.
Freshly hatched adults from the lower leaves of plants fly up to the younger leaves of the plant to lay their eggs.
Whitefly can over-winter in a greenhouse on crops or weeds and the scales can withstand the odd frost.
Biological Control of Whitefly
During the warmer months whitefly can be controlled biologically in greenhouses or conservatories by using their natural enemies such as Encarsia formosa or other specialist whitefly parasites and predators.
You can monitor whitefly populations with Yellow Sticky Traps.