Problem focus: Blackfly

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Aphids are extraordinary creatures. Most of us reach for just one kind – greenfly – but in fact there are about 500 different aphid species living in our gardens and the wider countryside, each with narrowly-defined preferences.

Blackfly clustered around a broad bean shoot tip

There’s woolly aphid, making froths of cotton wool on apple trees and pyracantha, the giant willow aphid like a grey living carpet on the trunks of willows: and black bean aphid, also known as blackfly, which you will find mainly on the bean family – runners and climbing French beans, but right now spreading like a black stain down the stems of your broad beans. Their other favourite plants are, inexplicably, dahlias, and nasturtiums – of which more in a minute.

There are many amazing things to say about aphids of any kind. They are born already pregnant, for one thing: no blokes required, for they are capable of parthenogenic (female-only) reproduction.

Their prolific rate of baby-making also means they can triple a population within a matter of days: one of the reasons why you spot one or two blackfly on a Monday and return at the weekend to find a devastating infestation.

Ladybird on the hunt

But it also means they are among the most important prey species in the garden. They feed bluetits, sparrows and other birds; ladybirds (of course), hoverflies and lacewings; wasps, midges, rove beetles and spiders. In fact pretty much everything wants to eat an aphid. If I were a blackfly I’d have a serious persecution complex.

This means two things: first, if you have a good enough level of biodiversity in your garden, your aphid problem will stay more or less in balance – enough to feed the predators, not so much that they start to impact on plant survival. But second, this means a few aphids is a good and indeed desirable thing: it’s a little counter-intuitive, but if you don’t have aphids you don’t have the predators to eat those aphids either.

Plant survival is a pretty low bar, mind. When you’re trying to grow food, you want something a little better than that: and the problem with bean aphids in particular is that they will insist on colonising the developing flowers, destroying the beans that would otherwise be forming. They can really trash your crop if they get out of hand.

So it is, as so often, a balance: you want just enough blackfly to keep the predators nearby, not so much they start to affect your harvest.

Here are a few things you can do to adjust the balance: