ZOMORODI: So you actually link the monarch’s well-being to how we humans grow our food. Can you just explain what that link is, what the connection is between the two?
ALTIZER: Well, so monarchs need milkweed. Milkweed isn’t the only resource that they need. They also need nectar plants. But milkweed is the key resource that monarchs need to reproduce. And it’s an agricultural weed… And so one thing that has become popular since the late 1990s are crops that are genetically modified to resist common herbicides, like Roundup. And the herbicides can be sprayed on crop fields of soybean or corn, and the crops do just fine, but milkweeds and other agricultural weeds that would be providing nectar for monarchs would die.