We seem to all be having issues with these critters this year. If you go outside anywhere near trees, be prepared to be rained on with caterpillars! They are very hungry at this time and are busily munching away on any leaves they can get to.
With the tiny caterpillars, you can just squish them against the tree trunk with your rubber gloves. Be sure to not let them touch your bare skin as various people are having very itchy allergic reactions to them this year!
In May, June and July you can wrap your trees with burlap to trap the larger caterpillars, then collect them daily and drown them in soapy water. Make sure you check under both layers of the burlap as they will hide anywhere there is shade. We did this on our property last summer and destroyed thousands over the summer which has reduced the numbers this year...thank goodness! We are about to wrap our trees this week.