Buddlia Problem

I have noticed ants going up the trunk and the leaves are going yellow and falling off.   I cannot see any aphids. Any problem solutions please, regards,   Mrs Bouquet

No dry, but I think I have found the problem.  Apparently they can get Red Spider Mite.   Took out the big magnifying glass, and there they were, tiny.   Have now sprayed entire tree with the appropriate organic spray.   Thanks for your reply,  Mrs Bouquet