I went to plane of torment because it seemed to me like a greater number of mobs were there to farm these for poisons.... I cleared the zone more that 5 times total, got every rare spawn for the hunter achievemtnet - got my key to enter Saryn's tower, and killed loads of each mob total number of Plague mites = ZERO... I had fun, best I could do with what was happening - I helped a ranger out with a some tradeskill stuff he needed and he helped me with the Saryn key then he gave me a gift of a Prize: Guise of the Preditor...! A very kind gift I wasn't expecting but will treasure always. So I had a good day in Torment-- your results may vary but I'd suggest farming Plague mites elseware!
Karnors Castle dropping off the drolvargs. And even though it says here used in quests, there is no listing in the quests database here for them, so if anyone knows if they are anything but merchant fodder, it would be nice to know.
I think that these might be used in tradeskills, I am not quite sure though. They started dropping after Planes of Power came out, they might even be used in quests found in Tranquility or Knowledge. But for now, I would assume they are merchant fodder as they drop at an amazing rate.