Aphid Attack!

12 June 2009, written by Jeremy Dore

It is around this time of year that pests seem to appear from nowhere in the garden. Of all the insects that suddenly spring up, aphids are the ones that always take me by surprise – within a matter of days plants can be covered with one of the various varieties of them: whitefly, greenfly, blackfly, mealy aphids and root aphids. So just how serious is it when your garden comes under attack and what can be done to counter such an invasion?

Aphids are interesting pests because there is such a diversity of opinion on them. There are a few facts that everyone agrees on:

Most aphids live on or under the leaves of plants, piercing them and extracting sap, which can cause leaves to deform or curl up .
Grey-white root aphids, on the other hand, live in the soil and can attack plants causing them to suddenly wilt and die.
Aphids can transmit mosaic virus to plants, which turns the leaves a mottled yellow colour and stunts growth.
They excrete a sticky honeydew on plants that can be home to sooty moulds, blocking the leaf pores. This is why ants actually farm aphids, protecting their eggs over winter and carrying the hatched aphids to plants so that they can feed from the honeydew they produce.

Despite these facts there is far less agreement on how serious aphids attacks are, or how to treat them:

Some organic gardeners believe that aphid attacks are simply a sign of weak plants and that good organic cultivation should prevent most serious consequences. Charles Dowding writes in his book ‘Salad Leaves for all Seasons’: ‘Severe infestations of aphids often point to plants that are unhealthy and are an indicator to the gardener that something is not quite right, for example that the soil is too dry, or the plant is growing out of season. He recommends simply using water to reduce their numbers and encouraging predators such as ladybirds.
Others suggest spraying aphids as soon as they are spotted with insecticide such as pyrethrum, derris (now banned under EU regulation) or soft soap solutions (although insecticidal soap is also banned). To be effective these sprays have to actually land on the aphids themselves and this is the main problem – aphids usually populate the most inaccessible parts of the plants: in between and underneath leaves. It is therefore very difficult to actually treat all the aphids and numbers can rapidly increase again.
Simply picking aphids off plants, or squashing them, can be a simple but effective means of keeping numbers down. I have even heard of gardeners who use a vacuum cleaner to suck them off leaves but that might be taking things a bit far!

I tend to agree with the ‘let them be’ approach – in a well managed garden where crops are rotated aphids can usually be controlled by good companion planting to encourage predators such as lacewings, hoverflies and ladybirds. I don’t mind having to wash off a few aphids before eating produce from my garden and I am prepared to lose the odd plant in preference to using sprays.

However, there are occasions when the unexpected happens: we recently had over a week of dry weather with high temperatures and little rain which was quite unusual for the end of spring in Northern England. Unfortunately the companion plants I usually plant to draw in predators had not yet flowered - poached egg plant (Limnanthes douglasii), marigold (Tagetes) and dwarf morning glory (Convolvulus tricolor). Suddenly I got an attack of several aphid varieties in my salad bed: greenfly and whitefly on the leaves and three or four lettuces just suddenly wilting from root aphid attacks. So currently I am spraying them with plenty of water and keeping a close eye on the situation.

In my opinion organic pest control is all about balance. It is always tempting to get out a spray or take drastic action but I think it is more important to try and prevent attacks with good garden management and concede the occasional loss to nature. Aphids are one of those grey areas – they rarely kill plants so I can live with a few of them, work to control them and take measures to ensure that they don’t get out of control. To my mind that is much better than spraying the salad which will end up on my plate in a few days’ time. Aphids are resilient pests and using nature’s own predators to fight them is the best weapon available to the organic gardener. I just wish those ladybirds would hurry up and do their job of guarding my garden...

If you have any tips for dealing with aphids then please do add them below...