Aphids are very common sap-sucking insect pests that cause distorted growth and excrete a sticky substance (honeydew) on foliage, which allows the growth of sooty moulds. Some aphids transmit viruses, this is a particular problem on strawberries, raspberries, tomatoes, cucumbers, dahlias, tulips and sweet peas.
What are aphids?
Aphids are sap-sucking insect pests, ranging in size from 1-7mm (¼in or less) 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. These should not be confused with scale insects, mealybug or whitefly. Most aphids feed on foliage, stems and flowers but some suck sap from the roots.
There are more than 500 aphid species in Britain. Some 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.
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
- Most 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 several parasitic wasps. Some of these are available for biological control of aphids in greenhouses (aphid predators). 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.
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, Scotts Bug Clear Gun for Fruit & Veg, STV Time's Up Bug Killer), fatty acids (Bayer Organic Pest Control, Doff Greenfly and Blackfly Killer, Greenfingers Organic Pest 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. Deltamethrin (Bayer Sprayday Greenfly Killer, Bayer Provado Ultimate Fruit & Vegetable Bug Killer ) and lambda-cyhalothrin (Westland Plant Rescue Fruit & Vegetable Bug Killer) are contact insecticides that can be used on ornamental plants and many, but not all, edible plants; including apple, pear, plum, raspberry, strawberry, aubergine, beans, brassicas, lettuce, peas, peppers, cucumber, courgette and tomato. Check the product instructions as there are variable restrictions on how many applications can be made on edible plants and the length of time that needs to be left between spraying and harvesting the particular edible plants.
- 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
- Another systemic insecticide is acetamiprid (Scotts Bug Clear Ultra, Rose Clear Ultra and Bug Clear Ultra Gun). Bug Clear Ultra and Ultra Gun can be used on apple, pear, cherry, plum, potato, lettuce, and glasshouse tomato, pepper and aubergine, as well as ornamental plants. Thiamethoxam* (Westland Plant Rescue Bug Killer Ornamental Plants) is also a systemic insecticide and its use is restricted to ornamental plants.
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 or Vitax Winter Tree Wash).
* Please note this product is being withdrawn. The withdrawal (in effect a ban) comes into force on 30 September 2013, but there is to be a period of grace to use up these materials by 30 November 2013. After this time it will be illegal to use them, any remaining products should be taken to a manned local authority household waste site where they should be handed over to the staff. For information on waste disposal sites see www.pesticidedisposal.org/. It remains legal to use other neonicotinoid-based products that are not affected by the withdrawal. This withdrawal follows concern over their effects on bees.
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 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.
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