Gardeners' World Web User
03/06/2008 at 11:03
Hi Chris, this is perfectly normal. On closer inspection, you'll find that the tips of your broad bean plants are covered with blackfly. Blackfly is a type of aphid, and sucks the sap from the plants, but is relatively harmless in small numbers. In fact, they are beneficial to the garden, as they attract ladybirds and lacewings, who feed off them and help maintain the delicate balance of pest and predator in the garden.
Anyway, back to the ants. Blackfly secrete a sugary substance, called honeydew, which the ants love. The ants 'farm' the blackfly, and push them to the tips of the plant (where growth is newest and lushest, which leads to the production of the sweetest honeydew). Then they drink it.
I wouldn't worry too much, unless you see loads of them on the tips of the plants. But, to reduce blackfly numbers you can pinch the tips out (just nip the tops off the plants), but it's important to leave a few blackfly there to attract ladybirds and lacewings.Good luck, Kate