Blackfly on broad beans

by Jane Moore

Last week I mentioned that my broad beans were mysteriously free from their usual curse of blackfly.

Garden pest - blackflyLast week I mentioned that my broad beans were mysteriously free from their usual curse of blackfly. Strictly speaking, blackfly isn't a black fly, but a 'black bean aphid'.

And while my broad beans may be free from the pest this year, they normally have quite heavy infestations. The tips of affected plants are usually covered with the little blighters, which are carefully tended to by a handful of busy ants. The ants 'farm' the aphids for their sweet, sugary secretions, called honeydew. They push the blackfly to the tips of the plants, where all the fresh, new growth is (apparently blackfly produce the sweetest honeydew from the youngest leaves). This is all quite fascinating if you're something of an amateur entomologist - and most gardeners are!

The best control measure for blackfly is to nip the tips off the plants early in the season. I do this as a matter of course in May, but it rarely stops the little so-and-sos. Before you know it they're everywhere, not just on the broad beans, but on French and runner beans, dahlias and Philadelphus - all my favourites!

With large infestations I resort to my tried and tested weapon of choice - a plant sprayer bottle filled with water and household detergent. The detergent covers the aphids' breathing holes and kills them, so weekly sprayings will usually do the trick. The good thing about using detergent is that it just washes away in the rain - even better if you use a green brand.

Of course, blackfly are naturally preyed on by ladybirds, lacewings and hoverfly and their larvae. The larvae can eat up to 300 aphids before becoming an adult, so ensuring you have plenty of these in your garden will keep aphid populations in check. You can attract ladybirds, lacewings and hoverflies into your garden by growing some of their favourite plants: cosmos, sunflowers, candytuft, lupins and foxgloves.

And blackfly also love nasturtiums, so if you plant them around your broad, runner and French beans, they'll act as a 'sacrificial crop', suffering the onslaught of blackfly while your beans grow untouched.

Discuss this blog post

Talkback: Blackfly on broad beans
Your comment will appear after a quick registration step

Gardeners' World Web User 26/07/2008 at 15:18

I have had an invasion of black fly on my broad beans, Have companion planted and in the end I SPRAYED them and still they stayed. As both sides to me on the allotment have just started, they both had weeds so I put this down to this factor.

Gardeners' World Web User 26/07/2008 at 22:17

I wonder if wasps also eat the blackfly as our beans are clean this year but we have 2 wasp nests one in a concrete bird box & one in an old dovecote.

Gardeners' World Web User 07/08/2008 at 19:47

If I am to continue growing Vegs I must get a way of controlling SLUGS. Would it help if I grew a lot of Garlic on the edges of the beds and in rows in the beds? Help needed

Gardeners' World Web User 15/08/2008 at 15:42

slugs are too greedy - i expect to lose 20% to them but this year they have taken the biscuit - well actually they have taken about 60% of my vegetables - surely this means war!!

Gardeners' World Web User 11/10/2008 at 16:04

I'm interested in the suggestion that a spray of water and household detergent should see off blackfky from recently planted broad beans. I have just planted a crop to mature over the winter and they are under attack! Can any one suggest the best water to detergent mix to ensure the demise of the fly but avoids any problems for the plant or soil. But HURRY!! Please!!

See more comments...