At Long Lake Prov. Park (Halifax), the majority (>3/4)
of Beeches had most leaves eaten away or severely damaged.
Some trees were nearly bare. Unaffected trees were mostly
isolated from other beeches, typically young, scattered trees.
In a forested area in Cape Breton (Lime Hills), nearly all
(>95%) of the beech trees in the forests were dying or
dead. Those were beech dominated hardwood forests (other
dominant species were yellow birch and sugar maple), and the
area was some 1 km from the next road (i.e., spread of the
miner was unlikely by people).
I think the leaf miners have been around for at least 5
years. I think to assess the impact, one would have to choose
a sample area, and do a bit of counting (# trees in area / #
of trees affected / # of trees healthy) and follow for a few
years.