One of the most beautiful events in nature is the annual migration of millions of monarch butterflies from Canada and the US to an area near Mexico City. Butterfly experts have gathered there each year to witness and chronicle the event, and have noticed something disturbing — the number of monarchs has been dropping dramatically, and this year’s total was a record low.

To explain why, I invited Lincoln Brower, the world’s foremost authority on monarch butterflies, to join me on America Weekend. He explained the impact of pesticides, genetically modified crops, and climate change — and why we should even care about these tiny creatures. Listen, then click here to subscribe to these podcasts via iTunes!