The leaves on 3 once healthy oak trees (20 yr. or so old, 30 ft tall) have withered and darkened: we only noticed this 2 days after the recent frost. Upon inspection there are some tiny insect holes on some leaves. We live north of Kingston On.
Determining the cause of of plant ailments is always a bit of a guessing game.
You mentioned the frost – that would indeed caused blackening of the leaves but not the holes in the leaves. The good thing about frost is that, while it will weaken the tree, those withered leaves will likely grow back in the warm weather.
If your trees are afflicted with disease or an insect infestation- that is a little trickier to diagnose. Diseases or insect activity will often have similar symptoms. Checking under the bark for insect activity (for example tunnelling through the bark) or to see if there is discolouration (black and sooty looking) will help to give a clearer indication of what you may be dealing with. If you do find evidence of insect activity or disease, contact an arborist to determine if further action is needed.
See below for further reading -this link in particular may be helpful for determining if your tree is diseased.
You can search for a certified arborist through the site: