Aphids - Greenfly, Blackfly etc. - Plant Pests

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. these are usually characterised by irregular yellow patches on the leaves which may be wrinkled but otherwise alive.

  picture used permission of User: Lviatour - published under GNU Free Documentation License, Version 1.2

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

  • Encourage natural predators
  • Biological control
  • Traditional organic remedies
  • Chemical remedies

Encourage natural predators

Such as ladybirds, lacewings and hoverflies, by planting nectar-rich flowering plants which attract them, such as buddleia, calendula, sedum, stocks, sweet william, lilies 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. Lacewing houses can be made or bought to put up in the garden so they have a shelter for the winter, see here.

Adult and larval ladybirds (ladybugs) will eat 50 or more aphids a day, they will help with heavy infestations of aphids but will go elsewhere if there's not enough for them.

Lacewings will eat as many as 100 or more aphids a day during the larval stage, they will also take other plant pests that they encounter. The adults feed on nectar and pollen but will lay up to 200 eggs a day that will hatch into those hungry larvae. Picture courtesy of Charlesjsharp published under CC 3.0 Attribution Share and Share Alike Unported license.

Biological control

Aphid Treatments : Natural Pest ControlSimilar to encouraging natural predators, but giving them a helping hand by introducing them as well as just hoping that they will happen along of their own accord.

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.

It is possible to buy-in aphid predators for use in greenhouses and conservatories, used outdoors they are less effective as they tend to disperse.

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.

    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 litres (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.

Pest Control

Insect hotelInsect hotel / house
A sheltered insect house to help a range of beneficial insects to over-winter in your garden. If they are over-wintering there, they are in an ideal position come the spring and summer to wake up and help to combat any pests you may have. Amongst others, good for ladybirds, lacewings and solitary wasps which will help keep your unwanted garden visitors down. Place in a sheltered place outdoors.

biological controlBiological Pest Control - Organic living pest control for a variety of problems, red spider mite, whitefly, sciarid fly, thrips, mealy bug, vine weevils and more.

These are parasites or predators that will specifically infect the pest they are brought in to control without harming any other living things and without the use of chemicals. A small population of pests needs to be maintained so that the control agent doesn't die out.

pesticideChemical insecticides - If your plants get a heavy infestation, then I think it's acceptable to use a chemical insecticide as a "smart missile" just on that particular plant. I resort to this when the aphids build up under cover on my favourite plum tree, or other plant/s. Don't overdo it and spray too often and don't spray the whole garden "just-in-case", buy a small 1L or thereabouts sprayer so you don't get carried away.

Not pest specific.

Q. Hello, and help I have an old hedgerow at the end of my garden it's been there gods knows how long, it has the usual plants that you find in hedgerows allover Ireland Hawthorn, Honey suckle, Elder. My Problem is the Elder tree is covered with small black flies the leaves are falling and the tree has developed a scruffy appearance what can I do to remedy this as I really love the hedge and so do the birds. Can you help?

It sounds like your elder has a bad attack of blackfly. Elder are susceptible to this but some years are worse than others. You could spray with a systemic insecticide though I would advise against a chemical spray as it is part of a hedgerow with associated wildlife.You could spray an organic remedy on it based on natural soaps (should be available commercially down the garden centre) though I haven't found these of much use myself. I'd approach it in one of two ways:

    1/ It's a part of natures cycle and may die or will survive and next year won't be as bad (or it will die then).

    2/ Cut off the worst affected parts with insects attached and destroy them or remove down the tip, a sort of modified version of "1" above.

We expect our gardens to be full of healthy plants with no signs of nature. Blackfly and elder are as much a part of nature as are the birds that enjoy your hedgerow, they're both native species - depends on how you see your local ecosystem.

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, the daughters are genetically identical to the mother.
  • During parthenogenesis the baby aphids are born already pregnant, so grandma has daughter and granddaughter 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 diversifying the gene pool, introducing much needed variety to what would become a vulnerable monoculture with narrow genetic diversity. We wouldn't want them dying out would we?
  • Aphids have very narrow proboscis's (probosci ?) that can penetrate a single cell of a single phloem tube 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 to a single phloem cell.

  • The sticky "honeydew" that many aphids secrete are plant juices that are surplus to needs. Aphids have only one source of food and that has relatively too many dissolved sugars and not enough other things - specifically amino acids - so the poor old aphid has to drink more sap that it wants, and dump some of the sugars to make sure it has enough amino acids. The discarded honeydew can make your car sticky and also allow fungus to take hold on plants as it gives the leaves a nice sugary coating to get the spores started.

I know they're not really reasons to like aphids, but maybe by first understanding them....

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