A BUTTERFLY CONUNDRUM
Florida’s ‘cesspool’ of infection may leave monarchs too weak to migrate
Every year, swarms of majestic monarch butterflies take wing in a massive autumn migration from the northern United States to a small region near Mexico City — except for a wayward population that diverts to Florida.
Why some butterflies break off toward the Sunshine State is one of the many mysteries of monarch behavior, but an emerging scientific debate revolves around thousands of South Florida enthusiasts who have planted butterfly gardens to help revive an iconic and at-risk species. Some scientists believe that the Florida diaspora, which has naturally existed for a long time, is being unnaturally coaxed into loafing year-round instead of migrating because of a widely imported tropical plant.
What’s alarming is that when monarchs don’t migrate, they begin accumulating a wing-deforming parasite known as OE.
“South Florida is a cesspool of infected monarchs,” said Andy Davis, an assistant research scientist of monarch ecology at the University of Georgia. “Take any given monarch there and it will have OE.”
There is hope that a switch to planting native milkweeds may change the region from a parasite peninsula to a natural butterfly vacation spot.
‘A COMPLICATED STORY’
The full impact of tropical milkweed, an import prevalent in South Florida gardens, is still undergoing research and is only one of several questions about the monarchs that flit about the state — some of them the subjects of considerable uncertainty and ongoing debate.
“The role of Florida in monarch migration is a complicated story,” said Jaret Daniels, regional expert and curator of lepidoptera (i.e. moths and butterflies) at the Florida Museum of
Natural History at the University of Florida. Much like the human population of South Florida, he said, monarchs seem a mishmash.
There are ‘South Florida residents’ that never migrate and spend their days drinking nectar in Milkweedville year-round. There are “true migrants” that merely pass through Florida on their way to Mexico. There are some that “fall off” migration and form butterfly enclaves. Others elope to the Caribbean, and scientists don’t think they come back. What’s especially perplexing, Daniels said, is why some adopt these separate lifestyles and how much interaction there is between the groups.
Here’s one telling measure of the Florida monarch mystery: Karen Oberhauser, director of the University of WisconsinMadison Arboretum and chair of Monarch Joint Venture’s steering committee, is a foremost authority on their migration patterns. “When we run monarch population models, we actually often cut off Florida from the panhandle below,” she said. South Florida is just too confusing to include.
TROPICAL MILKWEED WOES
Monarchs flying to Florida is not new. But their constant and sickly state of laying eggs on non-native milkweed might be.
Florida actually has plenty of native milkweed species, but less than a handful of the 20 or so in the wild are available in commercial nurseries. Instead, tropical milkweed from Central America has exploded in popularity because it’s easy to grow, attracts tons of monarchs and blooms a lovely burst of orange and red flowers.
“According to recent research, when monarchs encounter tropical milkweed, they tend to stop migrating,” said Daniels. The theory is that native milkweed flowers disappearing in winter and reappearing as temperatures warm are like bells chiming the monarchs to sleep and wake for migration.
But tropical milkweed doesn’t die back in Florida. Instead it flowers yearround, so monarchs don’t get the signals to enter hibernation or migrate north in spring.
While Mexican monarchs spend their winter celibate and huddling for warmth in chilly mountains, Florida monarchs have a ceaseless bacchanal of milkweed, sex and sunshine.
It sounds good, but it isn’t.
“The South Florida population has been around for a long time,” said UMadison’s Oberhauser, “but it’s probably unhealthy.”
She’s referring to high infection rates of OE, the parasite on milkweed. OE is actually ubiquitous on both native and non-native plants. However, nature has a way of purifying it from the population: migration. Slower infected monarchs can’t survive the long flight, leaving mostly healthy specimens at the end. “Trying to migrate with OE is like trying to run a marathon on one foot,” said UGA’s Davis. The clean marathon winners at the end get to reproduce in prairie country up north and in the Midwest.
But non-migratory monarchs in Florida aren’t cleansed. Instead, they constantly pass on parasites to each other. Worse, they may be spreading OE to the rest of the monarch population, too.
“Florida wasn’t always a disease hotbed,” said Davis. “But OE is spreading like wildfire because of tropical milkweed planting. Now southern Georgia is becoming a hotbed, and eastern Louisiana. It’s a problem that’s growing, and it’s being led by Florida.”
There are some who argue that tropical milkweed is in fact totally fine for monarchs, pointing to unresolved contradictions. For example: Some native milkweeds in Florida also flower year-round just like tropical milkweed, yet they don’t seem harmful. Also, tropical milkweed in northern Mexico supports monarch migration just fine. Perhaps the issue is restricted to the Southeast, where the combination of heat, humidity, tropical milkweed and distance from Mexico encourages the butterflies to laze around in parasite country.
After asking 12 experts their opinions, Davis came to this conclusion: “No reputable monarch scientist recommends that people plant or use tropical milkweed.”
“If there’s one thing I could tell people in Florida, it would be that we need to break the desire to have monarchs hanging around in winter. Often, people will see caterpillars on tropical milkweed and think, ‘Wow, this is great. I’m helping!’ Then this big jerk of a scientist comes and tells them not to plant it. But the problem is, you have to look at the whole life cycle, at the science.”
ARE THEY SUPPOSED TO BE HERE?
A migration that spans 2,more than 000 miles and crosses several countries is usually reserved for large animals or soaring birds, not an insect that weighs as much as a dollar bill. That alone has made these orange marvels one of the most beloved insects in the world.
But with legions of Florida monarch lovers planting milkweed to support the species, the question naturally arises: Are monarchs supposed to be here? After all, Mexico City is a far cry from Miami.
“Most monarch research is in the Midwest,” said UF’s Daniels. “Florida has not been focused on much at all. We don’t know the story of what’s happening here.” Only recently have more studies been trying to illuminate why some monarchs end up so far away from their ancestral grounds in Mexico.
A 2018 study by Hannah Vander Zanden, assistant professor of animal movement and biology at the University of Florida, shed some intriguing light. Her research involved taking the wings of sacrificial Florida monarchs, setting them on fire, and using ions and magnets to read the smoke and determine where they were born. To the untrained, that may sound like pagan sorcery, but it is a scientific technique called “isotope ratio mass spectrometry.” While the study was only exploratory, it suggested that a lot of Floridian monarchs actually come from outside of Florida, possibly from the Midwest.
Even with this new data, “We don’t know how or why they come to Florida,” Vander Zanden said.
Why would a small gang of monarchs, some from the east coast and maybe some from the Midwest, choose Florida over Mexico? Scientists have speculated about that for at least four decades.
Fred Urquhart, co-discoverer of the monarch winter colonies in the 1970s, called monarchs headed to Florida “aberrant” and lost trying to find Mexico. Lincoln Brower, another pioneering monarch scientist, argued instead that the monarchs intended to come to Florida. “And we still don’t have an answer to this day,” said Davis.
One reason: There is not much historical data.
Ray Moranz is a pollinator ecologist with the insect conservation nonprofit Xerces Society. He studied under Lincoln Brower at the University of Florida in the ’90s. “Twenty or thirty years ago,” said Moranz, “people just thought ‘Monarchs are headed to Mexico, why tag them in Florida?’ And now we have almost no clue how many used to come here.”
However, for decades scientists have observed a large annual migration of monarchs east of Tallahassee and southbound, at a time when the rest of the population is reaching Mexico. The Atlantic coast is probably a normal migration route for beachloving wanderers. Perhaps they haven’t much of a choice: New England monarchs might get sandwiched between Appalachian mountains and the ocean, funneling them into Florida.
As for vagrant midwestern monarchs, “We still don’t know if they take a wrong turn on their way to Mexico,” said UMadison’s Oberhauser.
“There were never enough people tagging in Florida,” said Xerces Society’s Moranz. “And there still aren’t enough.”
Scientists have hinted that the near future will see an impressive harvest of new monarch studies, slowly fitting the Sunshine State into the bigger picture more comfortably.
IN THE END, FLORIDA MAY MATTER
With monarchs just waitlisted for the Endangered Species Act last year, understanding their decline is only gaining national attention. That also makes resolving some of the mysteries surrounding the longignored Florida population more important.
Since the North American monarch population has declined by more than 80% in the last few decades, hosting sojourners in Florida may be important for recouping the species as a whole. As long as it doesn’t burden them with parasites or discourage them from migrating.
Luckily, there is an easy solution for those who have already planted tropical milkweed: Just cut it back or bring it indoors during winter. “I too grow some tropical milkweed just because it is easy and a beautiful plant,” Anurag Agrawal, professor of environmental studies at Cornell University who specializes in the relationship between monarchs and milkweeds, said on his website last year. “Simply bringing [tropical milkweed] in at the end of the season is probably just fine. Nonetheless, I always do get concerned when people promote planting lots of it to help the butterflies. From the perspective of conservation, I simply recommend general habitat protection and planting native species.”
To anyone sticking to their tropical milkweed guns, UF’s Daniels recommends that they grow it from seeds or cuttings, because the commercially sold plants are often tainted with lethal pesticides.
“We’re not saying don’t help [Florida monarchs],” said UGA’s Davis. “We’re just saying help them differently. Planting natives is something the whole state of Florida needs to do together.”
You can find more information on tropical milkweed and native species through the Xerces Society and the University of Florida. You can also read why Native Nurseries in Tallahassee stopped selling the non-native plant.
Krishna Sharma, who recently earned a master’s degree in ecology from the University of Georgia, is reporting for The Herald as a Mass Media Fellow with the American Association for the Advancement of Science (AAAS).