Keep on top of greenfly (or whatever colour they are)…
Aphids are a major pest, especially under glass or in hot summers outside. Autumn cleaning is vital to kill eggs or insects, which will breed in frost-free conditions. 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 – even woolly! 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. Apart from weakening plants, some can introduce viruses. For more on aphids and their hosts, the RHS has this informative page.
Tell-tale signs that aphids are about
- Obvious colonies on shoot tips, flower buds and the underside of young leaves.
- Stunted growth with curled or distorted leaves, weakening the plant.
- Sticky ‘honeydew’ on the plant and surroundings, sometimes 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 from 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 on. 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.
Aphid predators
Thankfully, aphids have a large number of predators – that’s why they breed so quickly. If you can put up with colonies outdoors, it can encourage these welcome species into your garden. Birds, earwigs and predatory beetles will all feed on aphids. Those listed below can be bought as biological controls.
Hoverfly larvae: Of the UK’s 260+ hoverfly species, just under half of them have larvae that feed on aphids. The legless, semi-transparent maggots are up to 12mm long.
Lacewing larvae: Up to 8mm long with curved jaws and tapered rear ends, some larvae camouflage themselves by sticking aphid skins on their back bristles.
Ladybirds: The UK is home to more than 50 species of ladybird, with spots varying from none to 24 on yellow, brown, red and black backgrounds. Most ladybird larvae prey on aphids.
Predatory midge larvae: Yellowish orange maggots up to 3mm long, which suck out the aphids’ soft innards. The adult tiny flies feed on honeydew.
Parasitoid wasps: Adults lay single eggs inside aphids. After hatching, the small grub feeds on the host’s body, kills it and pupates.
Ancestors’ remedies to kill aphids
Here’s a sample of weird and wonderful archaic plant remedies*. Do also check out my Plant Folklore page for more on this topic.
- 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 by David & Charles, 1977.
Tomato leaf spray for aphids
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 it in a spray bottle.
Caution: If you are allergic to members of the nightshade family, don’t use this spray.
Potent (but non-selective) garlic oil spray
Garlic contains sulphur, which is toxic to pests and is an antibacterial and antifungal agent. This spray, as it contains soap, 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.
