Those are black cherry aphids, very common. They need the ants to move them around. I solve the problem by putting tanglefoot around the trunks, then the ants can't get up and the aphids don't multiply. Wrap trunk first with tape then put tanglefoot on the tape. Make sure there are no paths under the tape, if there is one tiny path the ants will find it.

Scott

Yes, tanglefoot works for me as well. The ants farm the aphids and protect them from ladybugs...

I agree that the t-foot is sticky enough to do a great job. If you don't have any at home now and want to promptly put up an available, non-messy barrier to ruin the carpenter ant's lunch plan, sticky masking tape can snag carpenter ants as they try to walk over it to check on their herd of aphids higher up the trunk. A couple of separated rings of 1" masking tape with the sticky side facing outward that are snuggly wrapped around a smooth bark area has proven here that various kinds of ants "can drop in any time you want, but you can never leave", to borrow the song lyrics. Just check each AM to see if any small lizards would be snagged and need to be gently set free. As the sticky glue begins to lose it's stickiness, just replace it with a fresh strip. Or when there are too many bugs on it.........It would be good to hose off the aphids thoroughly to "reboot" the tree before putting the strips.