Getting rid of greenfly (or blackfly)

Aphids are a major pest, especially under glass or in hot summers. Cleaning in autumn is vital to remove eggs or hidden insects, which will continue to reproduce under frost-free conditions during winter.

So what are these vile creatures? They’re sap-sucking insects, also known as greenfly and blackfly, although they can be yellow, pink, white or mottled. Most feed on foliage, stems and flowers, but some suck sap from roots.

There are more than 500 aphid species in the UK and almost all plants can be affected by one type or another.

Aphid colony
Aphid colony
Aphid symptoms
  • Obvious colonies on shoot tips, flower buds and the underside of younger leaves.
  • Stunted growth with curled or distorted leaves, weakening the plant.
  • Sticky ‘honeydew’ on plant and surroundings, often with black sooty mould on it.
  • White cast skins on the surface of leaves and soil.

The first step is vigilance. Inspect your plants closely at least twice a week and squash any you see. On stronger plants, blast them off with a hose. Killing one or two sends off a chemical signal that may warn other aphids to stay away.

Quick anti-aphid tips

  • Ants ‘farm’ and guard aphids to harvest their secreted, sweet honeydew – put sticky traps on tree trunks to prevent ants reaching and safeguarding colonies.
  • Attract aphid predators such as lacewings and ladybirds with mint, fennel, dill, yarrow and clover.
  • Alliums (the onion family) are said to drive aphids away.
  • High levels of nitrogen encourage weak, sappy growth which aphids love to feed off. Prevent this by using slow-release organic fertiliser to avoid a nitrogen spike.
  • Use yellow sticky traps to capture flying aphids and monitor their arrival.

Ancestors’ remedies

Archaic plant remedies*:

  • The smell of foliage on elder twigs drives away aphids.
  • A solution of chopped elder leaves, soap and water works well on infestations of plums and greengages.
  • A similar solution of rhubarb leaves, which contain oxalic acid, is also recommended.
  • Burning of laurel leaves in a greenhouse is supposed to kill aphids, thrips and red spider mite.
  • In autumn, fill a large metal can with oak leaves and let them smoke. Apparently, the smoke kills the aphids. Not recommended for conservatories!

*From The Gardener’s Folklore, Margaret Baker, published David & Charles, 1977.

Tomato leaf spray

Greenhouse tomatoes
Harvest tomato leaves to make an anti-aphid spray

Tomato leaves contain toxic compounds called alkaloids, released when they are chopped. When diluted with water, they make a spray that is toxic to aphids, but safe around plants and humans.

Take 2-3 handfuls of chopped leaves and steep them overnight in a pint of water. Strain the leaves out, add another pint of water to the liquid, then use in a spray bottle.

Caution: If you are allergic to members of the nightshade family, don’t use this spray.

Make a garlic oil spray

Garlic makes a potent insect killer but it is non-selective

Garlic contains sulphur, which is toxic to pests and is an antibacterial and antifungal agent. The soap in also breaks down the bodies of soft-bodied pests. Take 3-4 cloves of garlic, mince them and add two teaspoons of rapeseed oil. Leave for 24 hours.

Sieve out the garlic, and add the liquid to one pint of water. Add one teaspoon of liquid dish soap/Fairy liquid. This mixture can be stored and diluted.

When you need to spray, use two tablespoons of the mixture added to one pint of water in a spray bottle.

To use, test by spraying an inconspicuous part of the plant to see if your mixture harms it. If there are no signs of damage after a day or two, it is safe to use.

If there is leaf damage, dilute and try the test again.

Warning: Garlic oil is a non-selective insecticide, which means that it will kill beneficial insects as well.