Thanks for your Ash tree caterpillar question.   Numerous caterpillars forage in Ash trees, but basically they are either borers or leaf eaters.  As your “guests” are nesting some 50 feet high, likely they are tent caterpillars.  Western tent caterpillars attack a wide variety of plants including alder, apple, ash, birch among others.  The adult moths are big, hairy dull yellow brown with rows of blue and orange spots.  They overwinter by laying their eggs on twigs and branches in the canopy.  Emerging in spring, they can quickly defoliate a small tree, leaving it vulnerable to stress from extreme weather conditions (like current heat wave).  While tent caterpillars have numerous natural predators in birds and mammals eating the pupa inside the cocoons, it would seem they have been only marginally helpful to you.  Pruning out the nests with offsite disposal seems a difficult option in your tree.  A biological control would be parasitic insects such as the tachinid fly that will lay an egg in the caterpillar and trichogramma wasps who parasitize the eggs.  These parasitic helpers do their work in warmer weather conditions.

Spray in spring after overwintering eggs hatch; control is much more effective if pesticides are applied when larvae are small.  Be vigilant as fluctuating spring temps may result in eggs hatching in waves corresponding to warmer temperatures.  Do obtain an appropriate spray from your local garden center and carefully follow the label directions. 

While most healthy mature trees are able to withstand periodic tent caterpillar visits, it is wise to invite them to depart your tree by the best means available to you.