canadianplant - I suspect Hokkaido is much colder in the inland areas than PEI. I just googled 'record low Hokkaido" and came up with -41c for Asahikawa in the northern interior.
Here is an interesting article that says Sasa kurilensis occupies 89% of forested areas in Hokkaido so one might expect to find it around Asahikawa, infested or uninfested. That pic of the advancing front of Sasa into snow meadows is pretty scarey eh?
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3287379/
I have been corresponding with a doctor in Ishikari on the west coast for about 16 years and he gets mountains of snow and dips to to -20c's once in awhile. He collects up in the mountains and I see Sasa in his photos, the wind up there is brutal but again lots of snow if it doesn't blow away and it guess it could considering the terrain. But how do we explain a Zone 7 plant like Skimmia japonica being native there? It is just hardy here and only with careful placement.
I think tc said the mites had already come through one winter. Let's hope your rid of them now tc but I'd keep at them and assume the worse and hope it pays off. Your bamboos are doing so well there and the property and soil are to die for - go at it tooth and nail!
+5c, missed the wind last night