Q What are aphids?
A Aphids are extremely common, sap-sucking bugs. The group includes greenfly, blackfly, root aphids and woolly aphids. There are many kinds, each with different host plants and life cycles. Most occur in small numbers and do little harm. They spread on the breeze and tend to settle in sheltered sites.

Caption: Aphids attack a wide range of plants

Q What damage do aphids do?
A Aphids weaken and stunt plants by sucking plant sap. They excrete a sugary substance called honeydew, which sticks to leaves that then become colonised by sooty moulds. They also spread virus diseases.

If possible, try to live with them, as they mostly do little damage. Natural predators, parasites and diseases will often get rid of them for you.

Q Do aphids turn up every year?
A Aphids survive as eggs on host plants, as adults in sheltered spots, or on plants in greenhouses until the spring. Winged aphids spread by drifting on the wind. This sounds random, but there are so many aphids that few gardens escape.

Some years are much worse than others. Aphid numbers depend on the weather, how many pests and predators attack them and the effect of diseases.

Q Why do some aphids have wings while others do not?
A Aphids have different forms. When their population is high, or predators are about, or at times of the year when they disperse to alternative food plants, winged forms develop. When they find new hosts, the winged forms lose their wings and give rise to wingless aphids again.

Q Which plant diseases do aphids spread?
A Aphids are very important carriers of viruses. For example, willow-carrot aphid carries carrot motley dwarf virus. When it feeds on parsley, the virus causes serious damage and is more harmful than the greenfly. The parsley turns reddish to start with, but soon goes yellow and becomes unusable.

Q Can aphids be sprayed?
A There is a wide number of sprays available to deal with aphids. These include Bayer Natria Bug Control and Westland Resolva.

Check the pack carefully for the harvest interval and what plants it is approved for.

Some aphids are resistant to insecticides. Peach-potato aphid and the melon/cotton aphid, especially those found in greenhouses, are examples of this. If your sprays don’t seem to be working, then switch to ones based on fatty acids.

Q Can aphids be avoided?
A Early and late sowings often escape the worst of the aphid fly-ins. Carrots sown in June are also likely to escape both carrot fly and willow-carrot aphid.

Q What can organic growers do to control aphids?
A Organic aphid killers work very well if carefully applied to cover the whole of the plant. Repeat sprays will be necessary until the aphids are controlled.

It is possible to exclude aphids from plants by covering with horticultural fleece or insect-proof mesh, but the plants must be completely free of aphids before you cover them.

Fleece captures too much warmth for summer sowings. To avoid cooking your plants, use the better-ventilated insect-proof mesh for these sowings. If you raise your young cabbage-family plants in seedbeds covered by fleece or mesh, you will have strong, pest-free transplants to put out. These covers will also protect against flea beetles and cabbage root fly.

Q Are there any biological controls for aphids?
A Natural pests and diseases often kill great numbers of aphids. Sadly, they often fail to do this before the damage is done.

Predators and parasites can be introduced to control greenfly. They are not equally effective against all aphids. They work best in greenhouses, frames and under cloches, where they are less
likely to stray from the plants that you are trying to protect.

Q How do I use biological controls for aphids?
A Introduce the parasites and predators as soon as you see the greenfly, once temperatures are greater than 10°C. Two introductions of Aphidolytes aphidimyza, a tiny (2mm) midge, should be enough. More will be needed if the plants get heavily infested. Each female lays about a hundred eggs during its two-week lifetime. The tiny larvae eat about five adult or 15 juvenile greenfly during their one to two week development time. They work best in warm, well-lit conditions.

Aphidius is a parasitic midge that is good at hunting down and finding greenfly. They lay eggs inside them and the larvae eat the greenfly from the inside. Use this parasitic midge when you only have a few pests.

If you choose biological controls, only use fatty acid-based insecticides. Other chemical treatments may kill the helpful insects.

Q What should I do with plants with aphids?
A Cabbage aphid-infested brassicas should be chopped up and composted thoroughly before the end of May, or they can be burned. Woody stems can be covered with eggs, so, unless the stems are efficiently destroyed, the eggs can hatch and infest new crops. Carrots, parsnips and parsley, left over in the spring, harbour many willow-carrot aphids. These overwintered root vegetables must also be destroyed before May, by thorough composting.

Identifying common types of aphid

1 Black bean aphid (Aphis fabae) A typical blackfly that appears first on the young growing tips of broad beans and spreads very rapidly if unchecked, eventually weakening the plants. When first seen, in May and June, nip out the growing tip carefully, complete with blackfly colonies. Any stray aphids can be squashed. It can also attack French and runner beans later in the summer, and can spread bean yellow mosaic virus.
2 Cabbage aphid (Brevicoryne brassicae) is blue-grey and forms dense colonies with a mealy look, because of the waxy covering the greenfly produce. They feed on the undersides of leaves of the cabbage family, including weeds such as shepherd's purse, and make the leaves curl and distort. However, they can also infest Brussels sprouts and cabbages more severely, making them inedible.
3 Willow-carrot aphid (Cavariella aegopodii) is an ordinary-looking greenfly that's quite hard to see. It
attacks celery, carrots and parsnips in May and June. The plants become stunted and covered in honeydew, which gets covered with dead greenfly. After July, the greenfly disperse to their winter host plants: willows.
4 Peach-potato aphid (Myzus persicae) looks similar to other greenfly. Seldom builds up to the large numbers seen with cabbage aphids, but it's effective at passing on viruses. Potatoes and lettuces are particularly at risk, so using your own potato 'seed' tubers can be unwise. The virus won't have much effect on the current year's crop, but plants raised from infected 'seed' will be heavily infected and are likely to crop poorly. Bought 'seed' potatoes are raised in cool, northern areas, where aphids don't thrive, and should be virus-free.

Suppliers of biological controls for aphids
Defenders 01233 813130 defenders.co.uk
Green Gardener 01493 750061 greengardener.co.uk
Harrod Horticultural 0333 400 1500 harrodhorticultural.com
The Organic Gardening Catalogue 01932 878570 organiccatalogue.com