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How to control aphids (greenfly or blackfly)
- BBC Gardeners' World Magazine
Published: Friday, 29 March 2024 at 8:00 am
How to tackle aphids (greenfly or blackfly) and when to let nature deal with them for you.
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Aphids, also known as greenfly or blackfly, are tiny, sap-sucking bugs that can occur on most plants, particularly on soft young growth. There are actually more than 500 species of aphid in the UK, varying in colour from green and black to also yellow, brown, and orange, and in size from 1 to 7mm long.
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Aphids need warmth to flourish, so in the garden this limits their lifecycle to spring and summer. However, indoors in the home, greenhouse or conservatory, aphids can occur at any time of year. Many types of aphid feed only on specific plants, so an infestation on one plant doesn’t necessarily mean that it will affect all of your plants.
Learn environmentally-friendly methods for controlling greenfly and blackfly, and other garden pests, with Monty's guide to organic insecticides and how they work, in the video above.
Why are aphids a problem?
Greenfly on a leaf
Greenfly and blackfly are a natural part of the garden ecosystem and a vital source of food for many species of birds, including house sparrows. They're only a problem if they occur in large numbers. Large infestations can weaken plants and cause curling or distortion of affected growth. This can sometimes eventually kill the plant, especially young ones. Some aphid species transfer virus diseases between plants – many viruses cause a range of symptoms including mottled, streaked or distorted foliage, and weak growth. Viruses aren’t curable, so infected plants should be destroyed. An additional problem caused by greenfly and blackfly is the sticky substance they exude, known as honeydew, which can become infested with black mould – this stops leaves photosynthesising and inhibits plant growth.
What should I do if I spot aphids?
A small number of aphids on a garden plant is rarely a problem. Birds will quickly pick them off and feed them to their young. However, in the veg patch, where you might be growing lots of the same plant together, or indoors where there are no natural predators, you'll need to act quickly. Often, simply hand-squashing a small infestation is all that’s required. Be alert for early warning signs and inspect plants thoroughly – greenfly and blackfly can be hard to spot as they nestle in leaf joints and on the undersides of leaves. Often, the first warning sign on indoor and greenhouse plants is the sticky honeydew on the leaves beneath the initial cluster. Ants running up and down plant stems are another giveaway sign of greenfly in the greenhouse, as ants 'farm' aphids and feed on their sweet honeydew.
Encouraging wildlife to control aphids
Ladybird and greenfly
Many wild species eat greenfly and blackfly, and actually rely on them as an important food source. Aphid predators include birds, ladybirds, hoverflies, lacewings and earwigs. Ladybirds, hoverflies and lacewings lay eggs on plants infested with greenfly and blackfly and their young eat them at an astonishing rate – it's thought that one ladybird can consume several hundred aphids over its lifetime. Wasps are voracious feeders of aphids, too, so instead of regarding wasps as villains, leave them alone to help control pests. In spring, aphid populations may rise to begin with as predator populations can be low, but later in the season these predator populations will be rising to get aphids under control.
Controlling aphids without chemicals
Spraying aphids off with water
The chances are that, by the time you've found greenfly or blackfly on your garden plants, the predators have, too. So, bear in mind that removing aphids will likely kill beneficial insects such as ladybirds and lacewings, as well as aphids. These predators are not present indoors, so you will need to control aphids yourself:
- Wash off greenfly with a jet of water from a hose
- Hand-squash small infestations
- Use biological controls, which involve buying live predators to introduce into the home or greenhouse to deal with aphid infestations
How to deter aphids naturally
- Pungent-smelling plants like sage, onions, garlic, and nasturtium, grown alongside susceptible plants, may help deter aphids.
- Create habitats to encourage birds and other predators into your garden – be less tidy in winter so ladybirds can hibernate, encourage birds with climbing plants and dense shrubs
- Aphids target soft young growth, so minimising the use of nitrogen-rich fertiliser helps raise tough-leaved plants (nitrogen is the primary nutrient used to develop leafy growth)
- Thoroughly clean out greenhouses and conservatories once or twice a year. Remove all plants and debris such as fallen leaves; wash surfaces and staging; avoid storing pots and sundries with your plants (they provide hiding places for pests), and fumigate the structure using a chemical-free product.
Using sprays to combat aphids
There’s a wide range of chemical-free (organic) products, those based on synthetic products, and chemicals (insecticides).
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- Organic (chemical-free) sprays are based on plant extracts, plant oils or soft soap. These must be sprayed directly on the aphids in order to work, and often only persist for a short time so may need re-application
- Products based on surfactants or fatty acids have fewer harmful effects than chemical pesticides
- Chemical sprays are based on synthetic pyrethroids. Avoid using sprays containing neonicotinoids due to their harmful effects on bees
- Always follow instructions and do not spray plants in flower, as beneficial insects, bees, and others are likely to be killed. Apply sprays late in the day when insects are less active.
Frequently asked questions
Is there a home-made, organic way to deal with aphids? There are many home-made recipes for killing aphids – the most common is using dishwasher liquid mixed with water and sprayed onto the plants. Bear in mind that, because these are home-made, they are not tested for their effectiveness on aphids or of any side-effects on the plants. To minimise any adverse environmental effects, use only eco detergents and a very weak solution. Also consider not removing the aphids at all – they are a vital source of food for many other species, including house sparrows and ladybirds, which control them naturally for you. If I spray insecticide on aphids will it get into the compost and kill the plants? Chemical insecticides are designed to kill insects, so spraying them on your plants will kill aphids. However, they may also kill ladybirds, hoverflies, and even bees and butterflies if the insecticide gets into the flowers. Residues will be washed off the plants into the soil and could then harm soil invertebrates such as earthworms and beetles. The insecticide itself won't kill your plants but will damage your garden's environment, which will disrupt the ecosystem and make it harder to control pests in future. We recommend letting nature take its course: let birds, ladybirds and hoverflies control aphids unless there is a huge infestation and then remove them by hand or with an organic spray. There should be no need to ever use chemical insecticides in the garden.
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