How to deal with greenfly, blackfly and other aphids

Certain seasons produce a preponderance of particular pests and this summer certainly seems to be the one for blackfly and aphids. On a recent trip to the Weald & Downland Living Museum in West Sussex, I was amazed to see masses of blackfly (black bean aphid) on the broad beans. I don’t think that I’ve seen as many since I was a child and my Uncle Jim’s broad beans were plastered with them one year.

Left to their own devices, such a population can suck out a huge amount of sap from the plants and cause blackening of the foliage as a result of the ‘honeydew’ they produce. Honeydew is the polite work for it. Like all aphids, they feed by sticking their ‘stylet’ or pointy mouthparts into the stem or leaf and allow the natural water pressure in the plant to push the sugar-rich sap through their bodies and.. well… out of the ‘other end’, under pressure.

This honeydew then gets infected with a fungus (a secondary infection) called sooty mould. Such an infection is common on the lower foliage of plants or on other species growing underneath aphid-infested plants. It blocks light from penetrating the leaf surface and looks unsightly.

Lifecycle of aphids

But it is not just black bean aphids that are a problem in some seasons. All aphids are able to reach ‘plague-like’ proportions. There are many species of aphid, often with their own favourite host plants and particular life cycle. Greenfly and blackfly are the most familiar, but there are also yellow, red, orange and brown aphids. Greenfly on roses in 2021 have been just as bad as the blackfly on beans!

The different sorts of sphids usually spend the summer on one plant, moving to a different host plant to survive the winter. They breed at an alarming rate. As well as lay eggs to overwinter, adult females can give birth to live young when only a week old. Left unchecked each of these offspring can then produce their own live young after a further week. It’s easy to see, therefore, how numbers escalate so quickly.

Looking out for aphids

Young tender growth is the most vulnerable to aphid attack. In severe cases, growth becomes distorted, leaves curl up, the plant weakens and can die. But it is best to look out for the problem before it gets out of hand and causes this sort of severed damage. We’re generally not very good at keeping an eagle eye out for the first signs of such insects. But once out of hand they can be much less easy to cope with.

Checking young shoots and developing flower buds is best. And not just a cursory glance, but looking closely with your glasses on, if necessary! Picking off 2-3 aphids by hand in mid-late spring has the potential to reduce the summer population by at least a few thousand.

How to control aphids

Natural predators, such as birds, earwigs, ladybirds, lacewings, hoverflies, ground beetles, spiders and parasitic wasps are our best allies in limiting numbers of aphids. Some of these are available as biological controls, however, they need time to tackle the problem, so don’t expect them to hoover up the aphids overnight. Ladybirds are favourites with children thanks to their cheerful colour and rounded shape – but their voracious appetite for aphids makes them welcome residents in the garden. The larvae of ladybirds are less attractive and endearing than the adults, but each one can consume up to 400 aphids before it pupates and takes on the familiar rounded body shape.

Just as useful are the larvae of hoverflies. The adult flies look like wasps, but are smaller and, as their name implies, they hover in one position for prolonged periods. Don’t swat them, they can’t sting. The eggs they lay will hatch into greenfly-munching larvae. It seems that aphids have plenty of enemies as  the larval stage of lacewings also eat them hungrily. Even the much maligned wasp and its close relatives will help control pests, either eating them or, in some instances, lay their eggs inside them.

Birds, in particular members of the tit and flycatcher families, are great at picking aphids from the young stems of a whole host of plants. And, in winter and spring, you’ll often see them dangling from stems, eating either the aphid eggs or freshly hatched larvae. Such high-protein food is good for the birds and their young. In addition, swallows, house martins and swifts will take aphids on the wing, when they are flying off to find new plants to infest.

And don’t forget the vast array of garden spiders that catch innumerable aphids and other winged insects. Don’t be afraid of them and leave their webs intact. None of our native spiders will harm you, but they’re bad news for pests. They are particularly useful at mopping up any remaining aphids at the end of the summer and on into the autumn, so that there are fewer of them to lay overwintering eggs that will hatch the following spring.

Meanwhile, small clusters of aphids can be squished with your fingers or washed off with a strong jet of water. Insecticidal soap sprays are effective, but must come into contact with the pest to work. It’s also crucial to avoid feeding plants with nitrogen-rich fertiliser, as it encourages soft, sappy growth that’s especially vulnerable to aphid attack.

But of course, to rely on such natural predators, we much expect a small time lag between an infestation occurring and their predators starting to control numbers. This can be anything up to 14 days. So it is important to resist reaching for the insecticide sprays as these could kill the very pest populations that other species rely on for food.

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