Someone finally got around to fielding a study which indicates that milkweed is necessary but not sufficient for monarch butterfly survival. Other factors contribute to their steep decline. Sure, you can grow milkweed, that's a great thing to do. But don't think it will "save the butterflies." It just slows the loss a little. Other factors lie outside what humans are willing to change, such as pesticides; or able to change, such as climate change. Even on a national scale, responses are limited, because monarchs have transnational migration routes. Serious conservation efforts would require collaboration between America and Mexico, which seems unlikely.