Garden Pests and Diseases: Mimosa Trees under Attack - what critters are responsible?!, 1 by Readerwoman

Readerwoman wrote:
Duplicate post from the one posted on the mimosa page:

What I am seeking is some information on what might be eating our mimosas! We live in NW Arizona, about 3,000 ft. elevation, and have three of the trees. Two are being attacked, in a pattern that looks to be from the top down. They were planted about a year or so ago, and are about 7' tall. They were fine in the early summer, with lovely blossoms and sturdy growth. Our immediate yard is fenced, and we have started closing the driveway gates to keep the burros and larger animals out - but they couldn't reach that high anyway, unless they are/were pulling at the branches to lower them for nibbling! Branches are disappearing, as well as leaves, but they seem to be leaving the pods alone. HELP!
They don't seem to be invasive here, the climate prevents it, I think, and we keep the runners trimmed off.
Any ideas as to what pest/critter could be attacking these trees, and how to prevent it? Thanks (oh, and the third tree is in a different location - about 20 yards away and does not seem to be affected)