Biologists are debating the cause of the decline. Theoretical culprits include more frequent summer droughts, invasive parasites and habitat degradation. Griffiths looks to milkweed, the only plant on which monarchs will lay their eggs.
Milkweed thrives in the Central Valley’s hot, dry conditions. But the valley is also prime ag land, and some farmers have planted crops that are genetically modified to tolerate the popular weed-killer Roundup. The resulting blanket herbicide applications have likely killed much of the valley’s native milkweed, she says. Ranchers also target the plant, which is toxic to cattle.
The past two dry years foretell another low monarch turnout this winter, but Griffiths hasn’t lost all hope for P.G.’s airy mascot. “They have the ability to reproduce quickly,” she says, “provided they have the right conditions.”