Greenfly, blackfly and aphids

Aphids are small, sap-sucking insects that attack just about every plant – both outdoors and in the greenhouse and home. They range in size from 1-6mm (up to ¼in) long and, although commonly referred to as greenfly and blackfly, some species are yellow, white, pink, brown and even bicoloured.

There are more than 500 different species of aphids – some attack a wide range of plants, whereas others only attack one or two plant species, such as the black bean aphid and lupin aphid – one of the largest aphids known. Some, such as woolly aphid, cover themselves with a white, waxy, fluffy secretion that disguises them from predators and prevents them being killed by some chemical insecticides. This covering often means they are confused with mealybugs.

There are even root aphids, which attack the underground parts of plants.

In late summer or autumn, winged females fly off to lay batches of brown or orangey eggs in the bark of plants or in other protected areas. These hatch in spring to start the whole lifecycle all over again.

• Trees – including fruit trees
• Shrubs
• Herbaceous perennials
• Annuals
• Vegetables
• Houseplants

Being sap suckers, aphids extract sap from the plant by using their piercing mouthparts. Large infestations can weaken small and young plants or even kill them. Even quite small infestations can cause reduced or stunted growth. Large well-established plants are rarely severely damaged. But their feeding also causes the plant parts they are feeding on to become curled, twisted, or otherwise distorted and disfigured, and even take on a discoloured or mottled appearance.

Some aphids also transmit plant virus diseases in their saliva, resulting in plants that then succumb to the virus disease. Such plants should be destroyed, as there is no cure for plant viruses and to prevent the disease spreading to other plants.

As they feed, aphids excrete a sugary, sticky substance, known as honeydew, which drops on to the leaves below where they’re feeding. This not only makes the leaves sticky, but the sugar attracts sooty mould, which can cover the whole leaf with a black deposit that reduces or prevents light reaching the leaf and so reducing the plant’s overall vigour.

Ants feed on honeydew, and even “farm” aphids, moving them to younger parts of the plant. So signs of ant activity may be a symptom of aphid attack.

Aphids breed very quickly – producing live young – and one aphid can produce millions of aphids over several months. Once numbers build up, control becomes more difficult. When large numbers are present, more damage is done to the plants, and this will affect your plants more severely. So, it is important to check plants regularly, going out on Pest Patrols, and dealing with any you see – as soon as you see them. They tend to congregate on the underside of leaves, on plant stems and on and around flowers.

You can easily control a small numbers of aphids by either squashing them or taking off and destroying the plant parts where they are congregating and feeding.

Aphids have a lot of natural predators – including ladybirds, hoverflies, lacewings and numerous species of birds. So always do what you can to encourage these into your gardens. Incorporating plants that are high in nectar into your planting schemes is certainly one way to encourage them, as is providing secure overwintering sites – even putting up “bug hotels”.

Unfortunately, early in the year, aphid infestations often build up to damaging levels before these natural predators are active enough to control them. Some suppliers of biological controls also sell ladybirds and lacewings – either as the adult or the more voracious larvae – that you can release into your garden to help the endemic population.

You could try blasting large colonies with a strong jet of water from a hosepipe. However, those aphids not killed by this method could be dislodged and end up on other plants close by, so spreading the problem.

There are also biological controls for aphids, such as Agralan. But these can really only be used under cover, such as in greenhouses and conservatories.

There are numerous chemical insecticides available that will kill soft-bodied insect pests, such as aphids. The majority of these are what are known as “contact” insecticides. That is, you have to spray the actual insect (make contact with it) to kill it. If you miss some, or others land on the plant after spraying, these will not be affected and will carry on feeding, breeding and building up the aphid numbers. Also, most are based on “organic” or “naturally-occurring” materials and have very little persistence once sprayed on to the plant. This means you have to spray regularly – which could be as often as every couple of days in a warm summer – to stand any chance of controlling them.

Contact insecticides are unlikely to penetrate the waxy coating on woolly aphids nor get to the aphids that cause leaves to curl up – such as plum leaf-curling aphid – since they hide and are protected within the curled-up leaf.

A systemic insecticide (BugClear Ultra) – which also kills on contact – is absorbed by the leaves and moves all around inside the plant, so will control future attacks from insect pests, for up to two months.

Use plant protection products safely. Always read the label and product information before use. Plants in flower should not be sprayed due to the danger to pollinating insects. Either spray early in the morning or late in the evening when pollinating insects are less likely to be active.

Not all insecticides have approval for use on all edible crops. Check the label before spraying.

You can protect your plants from attack by covering them with a physical barrier – such as horticultural fleece. Just bear in mind that plants must be pest-free before covering – otherwise you’re providing a protected environment for them to carry on feeding and breeding – and, as aphids are small, they can easily get through any breaks in the barrier.

• Horticultural fleece
• Contact insecticides
• BugClear Ultra
• Bug hotels