A 37-year survey of monarch populations in North Central Florida shows that caterpillars and butterflies have been declining since 1985 and have dropped by 80 percent since 2005. This decrease parallels monarchs' dwindling numbers in their overwintering grounds in Mexico, said study co-author Jaret Daniels, program director and associate curator of the Florida Museum of Natural History's McGuire Center for Lepidoptera and Biodiversity.
"It's alarming in a number of different ways," said Daniels, who is also an associate professor in the University of Florida's department of entomology and nematology. "This study shows the tight connection between monarchs and milkweed and highlights very dramatic losses in abundance in Florida that further confirm the monarch is declining."
While the drivers of the decline are not clear, the researchers said shrinking native milkweed populations and a boost in glyphosate use in the Midwest are part of the problem. Glyphosate, an herbicide often applied to agricultural fields to eliminate weeds, is lethal to milkweed, the monarchs' host plant. Less milkweed means less habitat for monarchs, said study co-author Ernest Williams, professor emeritus of biology at Hamilton College in New York.