Aphids

Aphids are common sap-sucking insects, they have many predators and are the basis for many food chains.  The sap sucking can cause a lack of plant vigour, distorted growth and often excrete a sticky substance (honeydew) on which sooty moulds can grow.  Some aphids transmit plant viruses.

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Mottled arum aphid on dahlia

Quick facts

Common names Aphids, greenfly, blackfly, plant lice
Scientific name True bugs (Hemiptera) in the Aphididea
Plants affected Most plants are susceptible
Main symptoms Presence of aphids, poor/distorted growth, sticky honeydew and sooty moulds
Most active Spring and summer on garden plants; all year round indoors

What are aphids?

Aphids are sap-sucking true bugs. They range in size from 1 to 7mm (¼in 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 from foliage, stems and flowers but some feed from roots.

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.

Symptoms

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

Control


Aphids form the basis of many food chains in the garden and it is not unusual to have some of these animals in a healthy balanced garden ecosystem. On tall trees aphids can be considered part of the biodiversity they support, natural enemies will normally reduce numbers by late summer, in addition artificial treatments are only likely to be successful if the entire plant can be reached. To reduce serious damage to plants check susceptible plants frequently from spring onwards so action can be taken before a damaging population has developed. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.

Non-pesticide control

  • Where possible tolerate populations of aphids
  • Use finger and thumb to squash aphid colonies where practical
  • Encourage the natural enemies of aphids in the garden, such as ladybirds, ground beetles, hoverflies, parasitoid wasps and earwigs. Be aware that in spring aphid populations often build up before natural enemies are active in sufficient numbers and then give good control. Indiscriminate use of pesticides can reduce the numbers of useful predators. 
  • Some natural enemies can be purchased for use as biological control of aphids in greenhouses, including hoverfly larvae, lacewing larvae and several parasitoid wasps. More information about these can be found on the ‘aphid predators’ page.

Pesticide control

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.
  • Little can be done to deal with aphids on tall trees as treatment is only likely to be successful if the entire plant is sprayed
  • ​On fruit trees and shrubs. Overwintering aphid eggs can be destroyed by using a plant oil winter wash (organic e.g. Vitax Winter Tree Wash). This can be used when the buds are fully dormant in November-early February on a dry frost-free day. Plant oil winter washes are less likely to be detrimental to natural enemies and can mean that spring sprays are unnecessary 
  • In glasshouses it is possible to use glasshouse fumigants. Glasshouse should be sealed and instructions on the product label must be followed. Products based on the synthetic pyrethroid permethrin are available as DeadFast Greenhouse Smoke Generator, DeadFast Greenhouse Smoke Fumigator.
  • Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer), fatty acids (e.g. Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of aphids. These pesticides have a very short persistence and so may require reapplication to keep aphid numbers in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults
  • Plant invigorators combine nutrients to stimulate plant growth with surfactants or fatty acids that have a physical mode of action against aphids (e.g. Ecofective Bug Control, RHS Bug and Mildew Control, SB Plant Invigorator and Westland Resolva Natural Power Bug & Mildew). These products contain some synthetic ingredients and so are not considered organic
  • More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
  • A systemic containing the active ingredient Flupyradifurone (Provanto Smart Bug Killer) is available for use on ornamentals and selected edibles
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
Follow label instructions when using pesticides. On edible plants make sure the food plant is listed on the label and follow instructions on maximum number of applications, spray interval and harvest interval.
Plants in flower should not be sprayed due to the danger to bees and other pollinating insects
Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener

Downloads

Pesticides for gardeners (link downloads pdf document)
Biological control suppliers (downloads pdf document)

 

 

Biology

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.

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