Been reading about ants in general on the RHS website:
They collect the sweet liquid known as honeydew, which is excreted by aphids and some other sap-feeding insects.
Ants can protect aphids from attack by ladybirds and other predators in order to secure their supply of honeydew. Increased numbers of aphids may result in more damage to plants.
Good to know this ~ I thought ants ate aphids, had no idea they actually protected them! But RHS advice is to leave the ants alone, so I think I’ll zap the aphids instead.