At this point, if it is a small tree, it can be sprayed with a product containing carbaryl which will kill the beetle when it feeds. Otherwise, more importantly, keep the tree watered and healthy, and it will be fine for next year. Treating the lawn for grubs should only be done IF YOU HAVE GRUBS. Otherwise it is a waste of money and chemicals to put that stuff down if they aren't even there. The number of adult beetles in your yard has absolutely no correlation to whether there are grubs and vice versa!