Aphids are sap-sucking true bugs. They range in size from 1 to 7 mm (¼ in or less) long. Some aphids are known as greenfly or blackfly, but there are species that are yellow, pink, white or mottled. Some species, like woolly beech aphid and woolly aphid on apple, cover themselves with a white waxy secretion and can be confused with some scale insects, mealybug or whitefly. A few species such as the Pemphigus aphids on poplar (Populus) cause plants to produce galls inside of which they live. Most aphids suck

Sap is a fluid that circulates through a plant's vascular system, in a similar way to blood moving through our veins. Phloem sap carries the sugars produced in the leaves by photosynthesis down to roots and other storage organs, as well as carrying minerals and hormones. Xylem sap is watery and transports nutrients absorbed by the roots upwards to the rest of the plant. Some plants leak sap from wounds or pruning cuts, and this is known as bleeding.

There are more than 500 aphid species in Britain. Some feed on only one or two plant species, but others can be found on a wide range of plant hosts. Many have lifecycles that involve more than one host plant. Almost any plant can be a host to aphids, including ornamentals, vegetables, fruits, greenhouse plants and houseplants.

You may see the following symptoms:

• It is usually possible to see aphid colonies with the naked eye, many species colonise shoot tips, flower buds and the underside of younger leaves
• Aphids can cause stunted growth with curled or distorted leaves and can weaken the plant
• Many aphids excrete a sticky honeydew on which black sooty moulds can grow
• White cast skins of aphids can accumulate on the upper surface of leaves 
• Ants may be found climbing plants with aphid colonies, they tend the aphids obtaining honeydew as a reward. The ants will remove aphid predators

The life cycles of each aphid species differ in general for much of the year, aphid colonies consist of wingless females that give birth to live young. Winged forms develop when overcrowding, deterioration in the host plant or seasonal changes induce a move to another plant. Most aphid species overwinter as eggs but some can remain as active aphids, particularly in mild winters or on indoor plants.

Many aphids, especially those on fruits and vegetables, go through an annual cycle that involves two or more host plants. The plant on which overwintering eggs are laid is often a tree or shrub. In the spring, the eggs hatch and the aphids feed on the young foliage. By early summer, the foliage has grown older and tougher, this combined with increasing temperatures and day-length induces winged forms of the aphid that migrate to the summer host plant. This is usually a non-woody plant with soft, succulent foliage. Some aphids, however, spend the whole year on one type of plant, although they may be active for only part of the year.

Some aphids can transmit plant viruses. This is a particular problem on soft fruits, such as strawberry and raspberry, and some vegetables such as tomatoes and plants of the cucumber/marrow family, as well as on some ornamental plants, such as dahlias, lilies, pelargoniums, tulips and sweet peas. Virus-affected plants should be destroyed to prevent the disease being spread to other plants.