The caterpillars in your photos are Gypsy moth caterpillars.  The relative amount of potential harm to trees depends on the caterpillar population, species of tree, and weather conditions.  If they are not numerous for more than a year or two, little permanent harm will result to most deciduous trees.  Evergreen trees, like spruce or pines, can be more seriously harmed in a single year of feeding.  Gypsy moth caterpillars are not normally found on spruce or pine, but in years of high populations they may end up feeding on these trees.  See:

https://www.canr.msu.edu/ipm/Invasive_species/Gypsy-Moth/index