There are three opuntias in Canada. I live in the southern interior of BC where there is one of them (opuntia polycantha), another one lives on the prairies (opuntia fragilis) and the third is opuntia humifusa, which I am somewhat envious that you have seen, because they are endangered and beautiful with a quite limited range, and Southern Ontario is one of the parts of this country I would like to see. The only other cactus native to Canada is escobaria vivipara, which is a small, round plant with really nice flowers that grows on the prairies, mostly in Saskatchewan.
Having eaten a bunch of the local prickly pears as vegetables, I can say that these ones aren't very active, and although I don't know what mescaline feels like enough to know what a threshold dose would do, given the fact that there are a number of opuntias used as food, I think that the possibility of any one of them causing any kind of psychoactivity distinguishable from placebo to be fairly unlikely. The presence of mescaline in most opuntias that have had it show up in them is pretty trace to my knowledge.
I read in Jonathan Ott's Ayahuasca Analogues that shamans in South America have added certain opuntias to caapi containing brews. But in reading that book, one realizes that many, many medicinal and psychoactive plants are added to ayahuasca/yage from region to region, and a lot of things seem to be added for more medicinal purposes than psychedelic purposes.
