These common sap-feeding pests mainly affect houseplants and greenhouse plants. They are a sub-tropical pest that was introduced into the UK by accident, and now over 1400 species of whitefly have been recorded worldwide, 56 of these have been found in Europe. They thrive in the warm environment of greenhouses.
Yellowing and often disfigured leaves are caused by whitefly feeding on the plant cells. Whitefly excretes sticky honeydew deposits as they feed which fall on to the surrounding foliage. This causes a dark sooty mould to develop on the leaves.
Watch out for your greenhouse vegetables like cucumber, melon, tomato, and peppers. Outdoor plants are also affected but not as badly as greenhouse plants.
How to spot
Whitefly are about 1.5mm (about 1/16in) long and they can be easily seen especially if a plant is disturbed – you will see clouds of small white-winged insects flying up.
They lay their eggs in a circular or crescent pattern on the undersides of leaves. Eggs are cone-shaped and are from dark gray to burnt orange. When the eggs hatch in 4-12 days what emerges is a white, legless, flat, oval shaped nymph.
Whiteflies weaken plants and make them vulnerable to further attacks from other pests and diseases.
How to treat
Squash the eggs if you see them on the leaves. Try tapping the leaves of infected plants and, using a vacuum cleaner, suck up the clouds of whitefly as they fly up. You can purchase yellow sticky strips which attract the flies and causes them to stick to the card.
Tiny parasitic wasps search for fresh whitefly larvae and lay their eggs amongst them. The Encarsia larvae develop inside the scales, killing the Whitefly larvae and turning them black. It is therefore easy to see if it is working.
If you use this method then don’t trim off any plant material with black scales on it or you will be throwing out valuable parasites. And, avoid spraying insecticides when using this biological control, as most insecticides will kill off the Encarsia parasite.