Black Fly and Bees

[sg31]

I have a number of relatively recently planted cherry trees which have a problem with black fly every year. I know black fly are 'farmed' by ants so last year I tried grease bands to stop the ants getting up the trees. Partially successful but eventually the ants built a bridge of dead ants to provide a pathway over the grease. It was then a case of regreasing the trees at regular intervals.

This year I grease banded the trees but also killed the ants nests. The trees still had black fly but numbers were reduced until last week when the population exploded for some reason. As the ants weren't there to protect the black fly lady birds and other predators got busy eating the pest.

One thing I noticed which surprised me was large numbers of bees on the trees. There's no blossom but the bees were very busy crawling inside curled up leaves and basically all over the leaves. I can only assume they were harvesting the honey dew produced by the black fly.

Is this common? Has anyone else seen this happen? Am I right in assuming they are harvesting honey dew?