Aphids

Aphids are very common sap-sucking insects that can cause a lack of plant vigour, distorted growth and often excrete a sticky substance (honeydew) on foliage which allows the growth of sooty moulds. Some aphids transmit plant viruses which can be a problem on strawberries, raspberries, tomatoes, cucumbers, dahlias, tulips, sweet peas and many other plants. 

Mottled arum aphid on dahlia

Quick facts

Common names Aphids, greenfly, blackfly, plant lice
Scientific name Various - many species
Plants affected Most plants are susceptible
Main symptoms Poor and distorted growth, sticky honeydew and sooty moulds
Most active Spring to late summer on garden plants; all year round indoors
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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 fluffy white waxy secretion and can be confused with some scale insects, mealybug or whitefly. Most aphids feed on foliage, stems and flowers but some suck sap from roots.

There are more than 500 aphid species in Britain. Some species only attack one or two plant species, but others attack a wide range of plant hosts. Almost all plants can be affected, including ornamentals, vegetables, fruits, greenhouse plants and houseplants.

Symptoms

You may see the following symptoms:

  • It is usually possible to see aphid infestations with the naked eye, and they tend to colonise shoot tips, flower buds and the underside of younger leaves
  • Aphids cause stunted growth with curled or distorted leaves. This can weaken the plant
  • Many aphids also excrete a sticky honeydew which allows the growth of black sooty moulds
  • White cast skins of aphids can accumulate on the upper surface of leaves 

    Aphids secrete honeydew on foliage, stems and fruits, which attracts the growth of sooty moulds. Here you can also see white cast aphid skins.

    Control

    Non chemical

    Aphids have many natural enemies (aphid predators), including ladybirds, hoverfly larvae, lacewing larvae and parasitic wasps. Some of these are available for biological control of aphids in greenhouses. Unfortunately, out of doors, damaging aphid infestations can build up in spring before the natural enemies are active in sufficient numbers to achieve control. However, by mid-summer aphid infestations are often kept in check. Where practical infestations can be squashed by hand. Pest control products based on natural compounds or with a physical mode of action are less likely to have serious effect on natural predators.

    Chemical

    During the growing season there are many insecticides that can be used against aphids. It is only feasible to control aphids on plants that are small enough to be sprayed thoroughly. Aphid infestations on tall trees have to be tolerated. Always read the label use pesticides safely. Plants in flower should not be sprayed due to the danger to pollinating insects.

    Pesticides based on natural compounds and/or with a physical mode of action
    These pesticides are contact in action and have short persistence, so thorough spray coverage, especially to the underside of leaves, is necessary. They can be used on ornamentals and edibles up to one day before harvest. Whilst good control can usually be gained of aphids feeding exposed on stems and leaves those protected by curled leaves are unlikely to be controlled. Products include: natural pyrethrum (organic e.g. Bug Clear Gun for Fruit & Veg, Py Insect Killer Powder and Pyrol Bug & Larvae Killer, ecofective Bug Killer (also contains fatty acids)); Fatty acids (organic e.g. Bayer Organic Bug Free,  Doff Greenfly and Blackfly Killer, Doff Universal Bug Killer); Plant/fish oils (organic e.g. Vitax Organic Pest and Disease Control, Bug Clear for Fruit & Veg Gun!), Plant oil winter wash can be used to treat overwintering aphid eggs on dormant deciduous fruit trees and bushes (organic e.g. Growing Success Winter Tree Wash).

    Synthetic pesticides contact action
    These usually have more persistence than those based on natural materials and so can give longer lasting control but will have limited effects on aphids within distorted leaves. Products include the pyrethroids Deltamethrin (e.g. Baby Bio Houseplant Bug Killer, Bayer Provado Ultimate Bug Killer, Bayer Sprayday Greenfly Killer, Bayer Provado Ultimate Fruit & Vegetable Bug Killer), lambda-cyhalothrin (e.g. Westland Resolva Bug Killer) and cypermethrin (e.g. Py Bug Killer). These are contact insecticides that can be used on ornamental plants and listed edibles; which in some products includes apple, pear, plum, raspberry, strawberry, aubergine, beans, brassicas, lettuce, peas, peppers, cucumber, courgette and tomato. Check the product instructions as there are restrictions specific to the crop on how many applications can be made and the length of time that needs to be left between spraying and harvesting (harvest interval).

    Synthetic insecticides systemic action
    In addition to some contact action these are absorbed into plant tissues and may have an effect on aphids hidden within distorted leaves. Products include the neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra and Rose Clear Ultra). In addition ornamental plants Bug Clear Ultra products can be used on  apple, pear, cherry, plum,  potato, lettuce, and glasshouse tomato, pepper and aubergine. Read the manufacturer's instructions regarding restrictions on the use of these products and harvest intervals.

    Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

    Downloads

    Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

    Biology

    Aphid lifecycles are variable, 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 usually 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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