Aphids

Symptoms
Infected leaves often feel sticky, and the sap can be colonised by black sooty mould. Bad infestations can leave a plant vulnerable to attack by other pests and diseases (e.g. sooty mould). Virus diseases can be spread by aphids as they move from one infected plant to another.

Spot them
Usually small, fat, green insects. However, aphids can also be black, yellow, pink, greyish-white and brown. They are all about 2mm long, round and full of sap.