I live a mile from one of CA's largest monarch groves in Pismo Beach (also grew up here) [0]. As a child, we'd go to the grove throughout the winter (that's when they migrate here). There were so many monarchs entire trees were orange and you literally had to watch every step so you didn't step on them.
Since moving back here in 2010, it's pretty sad. You have to actually look hard to find them. Looking at the historical numbers [1], it seems to be 1/10th of what it was in the 80s/90s. I always assumed there was some environmental cause–haven't heard the GM theory before.