Hybrid hornet/bumblebees are eating my lilac trees!!?

hi, i live in plymouth m. i have noticed a new type of bee in my yard. it looks like a cross between a bumblebee and a hornet, but bigger than both. they dont seem to be bothersome to people (yet), but they are killing my lilac trees!! there are three trees in my yard and the bees are eating the bark!! they arent doing a thing to the leaves, only chewing on the bark.. i havent seen where they come from or where they go to and they are ONLY eating the lilas trees! anyonw know what they are and how to permanetly get rid of them?