All over the valley large healthy trees are being attacked and destroyed by web worms. At first limited to Lombardy Poplars and cottonwoods, they can now be found on sumac, pecans, fruit trees, ashes, and in general most broadleaf deciduous trees.
Most affected branches are very high up on the trees and hard to reach with sprayers or pruners.
The ability of trees to survive difficult conditions is amazing. The insect you described is the fall web worm which appears to be doing tremendous damage. I was in Las Cruces and saw first hand what you have described. Dr. Carol Sutherland, NMSU Extension Service Entomology Specialist, assured me that the web worm only feeds on leaves and doesn't harm the bark or the buds. While there is a little growing season left, early loss of leaves at this time does not harm the tree excessively.