You can recognise aphid damage by the many white cast off skins left on the leaves or growth points of the plant. Aphids suck the nutrients and sap out of green plants. Every time an aphid bites into the plant, it forces its saliva into the plant cells. This infects the plant with viruses and weakens it. As a result leaves will discolour, wilt or become sticky.
How to prevent (further) damage?
Aphids have several natural predators. One of the most important ones is the ladybird. You can buy special boxes at garden centres to attract ladybirds into your garden. This way you can let nature run its course. Other natural enemies of aphids include the lacewing and the earwig larvae. Make traps with straw or newspaper and hang them in a tree overnight. Earwigs will crawl in. Put these on your plant and they will get rid of the aphids for you. You could also spray your plants with cold water. Repeat this every few days and the aphids will soon leave your garden.
Aphids are usually green, but can also be white, black, yellow, red or purple. Usually they are no bigger than half a millimetre. They look for the growth points of a plant, such as the top of a young stem.