Aphids are very common sap-sucking insect pests that can cause distorted growth and often excrete a sticky substance (honeydew) on foliage allowing the growth of sooty moulds. Some aphids transmit viruses which is a particular problem on strawberries, raspberries, tomatoes, cucumbers, dahlias, tulips and sweet peas although many other plants can be affected.
What are aphids?
Aphids are sap-sucking insects, ranging in size from 1-7mm (¼in or less) long. Aphids are also known as greenfly and blackfly, although they may also be 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 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.
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 on foliage, stems and fruits, which allows the growth of black sooty moulds
- White cast skins of aphids 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.
Aphids have many natural enemies, including ladybirds, hoverfly larvae, lacewing larvae and parasitic wasps. Some of these are available for biological control of aphids in greenhouses (aphid predators). Unfortunately, out of doors, damaging aphid infestations often build up on garden plants before the natural enemies are active in sufficient numbers to achieve control. Where practical infestations can be squashed by hand.
During the growing season there are many insecticides that can be used. 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.
- 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. Aphids protected by curled leaves are unlikely to be controlled. Products include: Pyrethrum (e.g. Py Garden Insect Killer, , Bug Clear Gun for Fruit & Veg, Py Bug Spray, Growing success Frit & veg Bug Killer, Growing Success Shrub & Flower Bug Killer, Vitax House Plant Pest Killer and Pyrol Bug & Larvae Killer); Fatty acids (e.g. Bayer Organic Bug Free, Bayer Natria Bug Control, Doff Greenfly and Blackfly Killer, Doff Universal Bug Killer, Just Green Savona Concentrate); Plant/fish oils (e.g. Vitax Organic 2 in 1 Pest and Disease Control, Bug Clear for Fruit & Veg), Plant oil winter wash can be used to treat overwintering eggs on dormant deciduous fruit trees and bushes (e.g. Growing Success Winter Tree Wash or Vitax Winter Tree Wash); Mineral lattice/urea (SB Plant Invigorator).
- 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: Deltamethrin (e.g. Bayer Sprayday Greenfly Killer, Bayer Provado Ultimate Fruit & Vegetable Bug Killer ) and lambda-cyhalothrin (e.g. Westland Resolva Bug Killer) are contact insecticides that can be used on ornamental plants and listed edibles; including 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. Thiacloprid (e.g. Bayer Provado Ultimate Bug Killer Ready To Use) can be used on ornamentals and some edibles including tomato, peppers, aubergine, courgette and cucumber in greenhouses, apple, pear, cherry, plum, almond, hazel, walnut, strawberry, bilberry, blackberry, blueberry, cranberry, gooseberry, black, red and white currants, raspberries and hybrid cane fruits, lettuce, leafy brassicas and herbs. Bayer Provado Bug Killer Concentrate 2 can be used on ornamental plants and the above mentioned glasshouse vegetables, potatoes, beetroot and Swiss chard. Read the manufacturer's instructions regarding restrictions on the use of these products and harvest intervals. Acetamiprid (e.g. Bug Clear Ultra, Rose Clear Ultra and Bug Clear Ultra Gun). Bug Clear Ultra products can be used on apple, pear, cherry, plum, potato, lettuce, and glasshouse tomato, pepper and aubergine, as well as ornamental plants. Read the manufacturer's instructions regarding restrictions on the use of these products and harvest intervals.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
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.
Common names Aphids, greenfly, blackfly, plant lice
Scientific name Various - many species
Plants affected Most plants are susceptible
Main symptoms Distorted growth, sticky honeydew and sooty moulds
Most active Spring to late summer on garden plants; all year round indoors