Aphids, often known as greenfly or blackfly, are a major plant pest.  They can range in size from 1-7mm long and accumulate on shoot tips, flower buds and the undersides of young leaves.  Aphids cause damage by feeding on plant sap.  This leads to stunting of growth, leaf curling and, in extreme cases, plant death.  Once the aphid starts feeding, it also produces the sticky substance called honeydew, on which sooty moulds can develop.  This disfigures plants and reduces plant photosynthesis, leading to poor growth.  Aphids also transmit plant viruses.  

Aphids reproduce very rapidly, leading to damaging infestations.  During their life cycle they moult four times, which results in white skins being left on plants.  These skins are often mistaken for whitefly, when they should seen as a warning of dangerously high aphid numbers.

Infestations of aphids on shrubs and trees can be difficult to treat with conventional beneficial insects, but effective sprays are available.