Hey, remember when they said that all of the bees on Earth were mysteriously dying? Remember when all the schlocky apocalypse-themed movies and shows from The Happening, to Knowing, to Doctor Who used it as a plot point? Remember when everyone inaccurately quoted Einstein in saying that the death of the bees would quickly lead to the death of humankind? Remember when Cracked speculated that the bees were probably dying because of pesticides?
Brushy Mountain Bee Farm remembers.
But Really ...
They figured it out back in 2014. The reduction in bee populations, formally known as "colony collapse disorder," has conclusively been found to be a direct result of the increased result of a class of pesticides called "neonicotinoids" in recent years.
We still don't know entirely why this happens, because exposure to neonicotinoids doesn't kill bees directly (they suspect that it actually makes bees more susceptible to disease, so it's more of a bee AIDS thing) but the research is nevertheless pretty clear. It's not global warming, or aliens, or the collective hive-intelligence of the bees who have decided to relocate to Mars due to pollution. It's just the poisons we've been spraying on our plants behaving like poisons.