If you are seeing large webs filled with catapillers there is an easy solution. The path of least resistance is to cut out the section of the limb that contains the web and dipose in a waste container or trash bag. Just make sure that the container or bag is sealed shut as to not let the worms/gypsy moths to get out and stay on your property. If it is somthing that you feel you can not handle please call a professional so they can be extracted or sprayed if necessary.

What happens if I don't get them out?
If by chance you do not stop the situatuion in time they will chew at the leaves or eat all the leaves of the trees. This is harmful as losing foliage to create photosynthesis can stress a tree significantly. If the tree does survive it will produce new leaves that are less attractive to the insects as a way to defend itself against another invasion.