This small white, moth-like pest must never be allowed to breed unchecked in a greenhouse for more than a week, ignore this rule at your peril, as whitefly will then become almost impossible to eliminate.
They breed throughout the year on house plants and in glasshouses and have a three week life cycle.
The white winged adult whiteflies are about 1mm long and take flight when plants are disturbed.
Placing yellow sticky cards in convenient places around you plants may help in catching them when they are in flight.
Outdoor species (cabbage whitefly) tend only to attack brassicas.
The symptoms are seen as a sticky honeydew (whitefly excrement) and sooty mould on the leaves.
Any found on the underside of the leaves should be squashed between the finger and thumb and any scales (eggs) present should be given the same treatment.
Insecticides have limited effect on whitefly.
Indoors: the use of a parasitic wasp encarsia formosa is generally accepted as the most effective biological control.
These should be introduced into the greenhouse in early spring where they will remain active from mid-spring and a repeat dose may be required in mid-autumn.
Outdoors: it may be better to use a suitable systemic insecticide as it may be too cold for the parasitic wasp to survive, or indeed, remain in the garden.
As a preventative measure use several suitable sprays with different base chemicals to the manufacturer's instructions.