Honeydew is a sticky substance which is excreted by sap sucking insects, such as aphids. It sticks onto the stem and leaves of plants and encourages the growth of sooty moulds. While honeydew is not harmful itself, an excessive amount of sooty mould on the plant can be unsightly and can reduce the plant’s ability to photosynthesise (because it stops light reaching the leaf or stem) which means the plant can’t produce as much energy for growth.