Royal Horticultural Society

What are aphids?

Aphids are sap-sucking insect pests, ranging in size from 1-7mm (1/16 - 1/4in) long. Aphids are also known as greenfly and blackfly, but other types may 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. Most aphids feed on foliage, stems and flowers but some suck sap from the roots.

There are more than 500 aphid species in Britain. Most species will only attack one or two plant species, but others will attack a wide range of plant hosts.

Almost all plants are affected, including ornamental plants, vegetables and 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 often weakens the plant.
• Most aphids also excrete sugary honeydew on foliage, stems and fruits, which allows the growth of black sooty moulds.
• You may also see white cast aphid skins on the upper leaf surface or the soil beneath where aphids are feeding.

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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, including ladybirds, hoverfly larvae, lacewing larvae and several parasitic wasps. Some of these are available for biological control of aphids in greenhouses. Unfortunately, damaging aphid infestations often build up on garden plants before the natural enemies are active in sufficient numbers to achieve control. Use your finger and thumb to squash infestations on small plants.

Chemical

During the growing season there are many insecticides that give varying degrees of control:

• Insecticides based on natural materials include pyrethrum (Py Garden Insect Killer, Doff All In One Bug Spray, Gem Stop Bugs, Scotts Bug Clear Gun for Fruit & Veg), fatty acids (Bayer Organic Pest Control, Doff Greenfly and Blackfly Killer, Greenfingers Organic Spray), plant and/or fish oils (Vitax Organic 2 in 1 Pest and Disease Control, Scotts Bug Clear for Fruit & Veg), plant extracts (Growing Success Fruit & Veg Bug Killer, Growing Success Shrub and Flower Bug Killer) and mineral lattice/urea (SB Plant Invigorator). These insecticides are contact in action and of short persistence, so thorough treatment, especially of the underside of leaves, is necessary. They can be used on ornamental plants, fruits and vegetables up to one day before harvesting. Aphids protected by curled leaves are unlikely to be controlled.
• Synthetic pesticides generally give a higher level of control. Bifenthrin (Scotts Bug Clear Gun, Doff All in One Garden Pest Killer, Bayer Sprayday Greenfly Killer Plus) is a contact insecticide that can be used on ornamental plants and some, but not all, edible plants; these are apple, pear, strawberry, aubergine, beans, brassicas, lettuce, peas, peppers, cucumber, courgette and tomato. Check the product instructions as there are restrictions on how many applications can be made on these crops. There is a one-day harvest interval for bifenthrin.
• A systemic insecticide, thiacloprid (Bayer Provado Ultimate Bug Killer Ready To Use) can be used on ornamental plants and 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.
• Other systemic insecticides are acetamiprid (Scotts Bug Clear Ultra and Bug Clear Ultra Gun) for use on ornamental plants only, and thiamethoxam (Westland Bug Attack); use of the latter is restricted to ornamental plants growing in pots or other containers.

Even with systemic insecticides, 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.

Overwintering eggs on dormant deciduous fruit trees and bushes can be treated with a plant oil winter wash (Growing Success Winter Tree Wash).

Links

Biological control
Aphid predators

Biology

For much of the year, aphid colonies consist of wingless females that give birth to live young. Winged forms develop when overcrowding or deterioration in the host plant induces a need to 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 different 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, inducing 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 virus diseases when they move from one plant to another. 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, and 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.

Links

Tomato viruses
Tulip viruses
Sweet pea viruses