Pest of the Month: Blackfly

We’re definitely experiencing more blackly infestations than normal around here, probably something to do with the cold spring and natural predators not being in such abundance as they normally are. These little aphids can cause quite a lot of stress when infestations get out of control, eventually weakening the plants and stunting growth.

What do they look like?

Clusters of tiny black aphids, measuring no more than 2mm.

Where do they hide?

They usually choose softer shoots, so are often found close to the top of a plant, either on the stem or on the underside of leaves. They are quite easy to spot.

Which plants are most affected?

Broad beans are most commonly affected, but you’ll also find them on globe artichokes (as pictured here) and squashes, as well as other beans and any other veg plant that they take a fancy to!

I’ve got ants crawling around with the blackfly as well – why?

Ants are quite clever at “farming” aphids to harvest the sugary honeydew that these aphids secrete. So, you’ll often see lots of ants where there is a blackly infestation. The ants will also operate their own version of pest control by removing any ladybird eggs and larvae and thus preventing them from feeding on the blackfly.

How do I deal with an infestation?

Many gardeners will advise soapy water, but we’re not really big fans of this for environmental reasons (the ladybirds don’t really want to be eating soapy aphids etc etc) and also we’re not entirely convinced that it is effective.

Our advice is that if you’ve only got a small infestation, then the plant should be able to tolerate it as long as it is kept well watered. So that would be our first port of call – keep those plants well watered and they may well fend off the blackfly themselves.

If you’re facing a large infestation, and especially if ants have moved in to start farming the blackfly, then it’s best to try and remove them. If you can pinch off the affected shoot in its entirety without damaging the plant or ruining its chance of cropping, then this is by far the easiest and most effective way to handle an infestation. You can bury the offending shoot in the compost pile and the blackly will not be able to survive there. If you can’t pinch them off, then try wiping them off with some slightly dampened kitchen paper.