Actually it turns out there was no extinction event or population collapse. There was a mass migration, and literally NOBODY knew where the bees went, and now they're back.
Monarch butterflies do mass migrations across almost the entire US, and scientists went through a lot of effort tagging them to track them. It's new behaviour in bees so we don't know where they went. It's possible that- if the migration isn't annual- bees may have been doing this the entire time and we just haven't been tracking bee populations long enough to know that.
