[Grubbypaws]

When I see ants on the branches of my apple tree I know to go looking for blackfly. Question. Do they bring them to the tree from elsewhere or do they just re-arrange ones that are already on the tree? Can the blackfly flourish without the attention of the ants?

If it is that they bring them to the tree then I could maybe stop the ants from climbing the tree with a grease band or something sticky. However if the blackfly are already on the tree and just grouped by the ants stopping the ants would be a bad thing as I would never find the blackfly without them 

[Lardman]

Both. Break the cycle, grease band to stop the ants playing about in the tree and thumb squish or spray with whatever you feel comfortable with - even water to knock them back. Once the ants stop guarding them the natural predators will arrive, the ladybird larvae have a field day on my trees