Whiteflies can damage plants in various ways. First of all, they prick the leaves to extract nutrients. This causes the leaves to turn yellow and eventually fall off. Whiteflies also have toxic saliva that can cause a lot of damage to plants. Additionally, the insect can spread viruses. Finally, whiteflies secrete honeydew on the leaves. This makes for an excellent breeding ground for fungi.
How to prevent (further) damage?
It can be hard to get rid of whiteflies. The most common type (the greenhouse whitefly) has developed a resistance to pesticides. You could try spraying water onto the plant to get rid of them. If you use this method, make sure to also get rid of the larvae. You could also make use of the braconid wasp. This natural predator lays its eggs inside the larvae of the whitefly.
Whiteflies are easy to see with the naked eye. The small, white flies are about one to three millimetres long. They are mostly found on the young leaves at the top of the plant. If there are a lot of them and you shake the plant, you will see a white cloud rising up.