Pests and Diseases - Aphids, Greenfly, Blackfly etc.

Signs - Usually thought of as greenfly, but can be black, yellow, pink, greyish-white and brown. About 2mm long when fully grown, roundish.

Damage - Heavy infestations can reduce the vigour of a plant and leave it vulnerable to attack by other pests and diseases. Frequently spread virus diseases as they move from one infected plant to another. They also spread virus diseases, these are usually characterised by irregular yellow patches on the leaves which may be wrinkled but otherwise alive.

Treatment - There are a myriad ways of dealing with aphids of various types. They fall into four categories:

Encourage natural predators

Traditional organic remedies

Biological control

Chemical remedies

Encourage natural predators

Ladybug, greenfly predator Lace wing, greenfly predator Such as ladybugs, lacewings and hoverflies, by planting nectar-rich flowering plants which attract them, such as buddleia, calendula, sedum, stocks, sweet William and wallflowers.

If you grow a patch of nettles somewhere in the garden, this will encourage aphids and their predators. Cut back the nettles when aphids appear in other parts of your garden to encourage the predators to seek them out. Ladybug and lacewing houses can be bought to put up in the garden so they have a shelter for the winter.

Traditional organic remedies

Companion planting - Plant garlic cloves (just one or two) among rose bushes. An infusion of garlic crushed into water and sprayed on the aphids will also help remove them. Many herbs, such as hyssop, sage, dill, lavender and thyme discourage aphids if planted near to susceptible plants.

Nasturtiums prevent woolly aphids infesting apple trees if planted at the base (probably more so if encouraged to grow up the tree). If you have all of the nasturtiums eaten by cabbage white caterpillars, just think of the butterflies you're helping to grow.

Nettle spray. The common stinging nettle is a discerning plant that requires high levels of nitrogen in the soil to grow well, using the leaves in sprays of several kinds. As well as using nettles as an activator on the compost heap the organic gardener can use them as a liquid manure and as an aphicide - to kill aphids (greenfly).

Gather 224g (l/21b) young nettles and soak in a bucket of water for a week. Strain and use undiluted as a control of aphids on roses and celery leaf miner. Add the mushy nettles to the compost heap.

Rhubarb spray. The oxalic acid in rhubarb leaves is a safe control agent for aphids, particularly those on roses. Cut 450g (1lb) rhubarb leaves, place in an old saucepan (the oxalic acid may damage one that you still use) with 1.1 litres (2pt) water and boil for half an hour, topping up as necessary. When cool, add 1 dessertspoon of soap flakes dissolved in 275ml (1/2pt) warm water. This acts as the wetting agent when added to the strained rhubarb liquid. Stir the mixture thoroughly and use undiluted as a spray.

Rhubarb soap - shred a couple of pounds of rhubarb leaves into a couple of pints of water and boil for half an hour (don't use your best pan, these leaves contain oxalic acid and whereas it might be ok, I wouldn't risk the cooks wrath!). Strain the liquid, mix in two ounces of soap dissolved in another pint of water. Spray only healthy plants as prevention and affected plants to help get rid of infestations.

Elder spray. This kills aphids, small caterpillars and is useful as a fungicide for mildew and blackspot on roses. The toxic agent is hydro-cyanic acid, so in preparing the spray use an old saucepan.

Gather 450g (1 lb) leaves and young stems of elder prefer-ably in spring when the sap is rising. Place in the saucepan and add 3.3 liters (6pt) water. Boil for half an hour, topping up as necessary. Strain through old tights and use the liquid cold and undiluted. It will keep for three months if bottled tightly while still hot.

Biological control

Another option is to go for biological control. Biological control of a pest relies on introducing a predator species so "fighting nature with nature", if chosen carefully, the predator will stop damage to your plants without damaging the environment. Such biological controls are safe for the user, children and pets. They will not harm other beneficial garden insects and are biodegradable.

They do rely on the predator always having some food though, or they will die out, so like other organic practices it is a question of maintaining a small population of pests to allow the predators to be ready for them, in this case though the balance is skewed away from the problem.

I confess I resort to this when the aphids build up under cover or on my favourite plum tree. Just don't overdo it and spray too often or spray the whole garden.

Reasons to like aphids

Early in the season only females are produced so that when one or two have found a nice tender stem, they can multiply most rapidly to take best advantage of this. Males aren't required at all, a process known as "parthenogenesis" or virgin birth. - well I'm impressed.

During parthenogenesis the baby aphids are born already pregnant, so grandma has daughter and grand-daughter at the same time.

Later in the season, the aphids start to produce males so that they can leave and breed with other females, so spreading the gene pool around and introducing much needed variety to what would become a vulnerable monoculture with narrow genetic diversity.

Aphids have very narrow proboscis's that can penetrate one single phloem tube only that carries sugars and other goodies around the plant. In this way, waste is kept minimized and the aphids are fed under pressure - they don't even have to suck!

Scientists have taken advantage of this and anaesthetize aphids with a blast of carbon dioxide before cutting the proboscis at the end and so getting a fine tube that is inserted in exactly the right place and depth into a single phloem cell.