How to control aphids (greenfly or blackfly)

How to get rid of aphids on house plants

Why are aphids a problem?

Greenfly and blackfly are a natural part of the garden ecosystem and a vital source of food for many species of bird, 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 – there are many viruses that 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, so you'll need to act quickly. Often simply hand-squashing a small infestation is all that’s needed. 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, and 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 is another giveaway sign of greenfly in the greenhouse, as they ‘farm’ aphids and feed on their sweet honeydew.

Encouraging wildlife to control aphids

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

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 into the home or greenhouse to deal with aphid infestations

Learn environmentally friendly methods for controlling greenfly and blackfly, and other garden pests with Monty's guide to organic insecticides and how they work. Monty also talks you through how to make your own, organic pesticide:

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 use of nitrogen-rich fertilizer helps raise tough-leaved plants (nitrogen is the major 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).

• 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