Advice
RHS Help & Advice
Aphids (various species)
Plants affected
A wide range of ornamental plants, vegetables and fruits.
Symptoms
Sap-feeding insects, ranging in size from 1-7mm long, infest plants, especially on the shoot tips, flower buds and the underside of younger leaves. The foliage may be sticky due to the sugary honeydew that aphids excrete. A black sooty mould often develops on the honeydew. White cast aphid skins often accumulate on the upper leaf surface or soil beneath where the aphids are feeding. Aphid damage can result in stunted growth with curled or distorted leaves.
Biology
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 the foliage, stems and flowers but some suck sap from the roots. For much of the year, aphid colonies consist of wingless females that give birth to live young. Winged forms develop when overcrowding or a deterioration in the host plant induces a need to move to another plant. The egg-laying form of aphids only develops at the end of the aphids’ period of activity, usually autumn, but for some species this is in the summer. 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 woody 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. Infestations on the winter/spring host plant die out during the summer but there will be a return migration in the autumn, when both males and females occur. After mating overwintering eggs are laid. 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 fruit, 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 daffodils, dahlias, lilies, pelargoniums, tulips and sweet peas. Virus-affected plants should be destroyed to prevent the disease being spread to other plants.
Specific aphid problems
Cherry blackfly
Currant blister aphid
Cypress aphid
Green spruce aphid
Lupin aphid
Mealy cabbage aphid
Plum leaf-curling aphid
Waterlily aphid
Woolly beech aphid
Control
Aphids have many natural enemies, including ladybirds, hoverfly larvae, lacewing larvae and several parasitic wasps. Some of these are available for biological control on aphids in glasshouses. Unfortunately, damaging aphid infestations often build up on garden plants before the natural enemies are active in sufficient numbers to achieve control.
Overwintering eggs on dormant fruit trees and bushes can be treated with a winter wash. 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), rotenone* (Bio Liquid Derris Plus*), fatty acids (Bayer Organic Pest Control, Doff Greenfly and Blackfly Killer, Greenfingers Organic Spray), plant and 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, 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. Imidacloprid is a systemic insecticide that is absorbed into plant tissues and is taken in by aphids as they feed. This means that aphids feeding in curled leaves can be controlled. The only edible plants that imidacloprid (Bayer Provado Ultimate Bug Killer) can be used on are glasshouse-grown tomato, peppers and aubergine. These edible plants can be treated up to four days before harvesting. Another 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 glasshouses, apple, pear, cherry, plum, almond, hazel, walnut, strawberry, bilberry, blackberry, blueberry, cranberry, gooseberry, black/red/white currants, raspberries and hybrid cane fruits, lettuce, leafy brassicas and herbs. Read the manufacturer's instructions regarding restrictions on the use of this product and harvest intervals. 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.
* Rotenone/derris-based insecticides are likely to be withdrawn from sale in September 2008 (date to be confirmed). Gardeners will be able to continue using up any rotenone/derris-based pesticides they have for 12 months after the final sale date.