How can I treat these foliage destroying caterpillars rat are mnchung their way through every leaf on our weeping copper beech tree, oak tree and Japanese maple trees.
It is hard, without a full description of the caterpillars in question, to make a positive identification of what is eating your trees. Below you will find a link to a website page that shows pictures of many of the spring caterpillars in Ontario which should help you, in situ, make the identification yourself.
Many of these caterpillars have years where they are not very prevalent and other years, they seem to be eating everything in sight. Once you know what is eating your trees, determine if this event cycle occurs frequently or infrequently and then decide on a course of action. It may be that nothing can be done and you just have to let nature take its course or it may be wise to intervene and stop a potentially devastating irreparable deforestation.
I would recommend contacting a certified arborist who is familiar with your area to help you with this problem. If you need help finding such help, contact Landscape Ontario or the International Association of Arboriculture (Ontario division) – see links below.