GAINESVILLE — 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, an associate curator and program director 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.

"What’s really needed are patches of native vegetation and nectar sources without pesticides," Williams said. "It’s not just for monarchs, but all pollinators.”

In the longest location-based monarch monitoring effort to date, a multi-institute team led by world-renowned monarch expert Lincoln Brower, who died earlier this year, closely followed spring monarch numbers in an herbicide-free cattle pasture in Cross Creek, about 20 miles southeast of Gainesville. The team examined milkweed plants for caterpillars and captured adult butterflies for 37 years, a period spanning more than 140 generations of monarchs.

They found that monarchs’ springtime departure from Mexico is timed to coincide with optimal growth of milkweed in the southeastern U.S. While adult monarch butterflies can feed from a variety of plants, their young depend on milkweed as their sole source of nutrition, storing up the plant’s toxins to ward off predators.

Monarchs lay hundreds of eggs on milkweed over their brief lifetimes, but just over 2 percent of eggs survive to become fully grown caterpillars.

If monarchs get to their breeding grounds too early, they run the risk of their host plants being killed by frosts; too late and the plants may not be able to support their young. To maximize their offspring’s chances of survival, the butterflies must time their arrival in the U.S. within a three-week window, Daniels said, an impressive feat for insects with lifespans between six and eight weeks.

Florida is an important stopover for monarchs returning north from Mexico, as spring breeding in Southern states leads to the butterflies’ recolonization of the upper U.S. and Canada. Monarchs rely on Florida for its abundance of milkweed and warm climate to lay the eggs that will help replenish the eastern population in the U.S., Daniels said.

Daniels said that increasing pesticide-free native milkweed populations in Florida yards and on roadsides is a step in the right direction to prevent monarchs from requiring protection under the Endangered Species Act.

Florida is home to about 21 native species of milkweed. Daniels recommends either Asclepias incarnata, also called swamp milkweed, or Asclepias tuberosa, commonly known as butterflyweed. Asclepias humistrata, or pinewoods milkweed, is also common throughout northern Florida and essential to monarch recolonization.

“It’s not as simple as saying, ‘we plant milkweed and the monarch will be saved,’” he said. “We should think of this as an ecological issue. There are a lot of complexities to any organism and any system.”

The study’s lead author, Brower, died shortly before its publication. A lifelong butterfly expert, Brower was instrumental in finding monarch overwintering colonies in Mexico, the researchers said. This was his final publication.

“He really was the grand old man of monarchs,” said Williams. “Nobody has done more for monarchs.”

Funding from the National Science Foundation and the October Hill Foundation supported the research.