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Butterfly battle: scientists disagree on how milkweed affects a parasitic spread in monarchs

A display of monarchs are laid out. Some are wilted and some are normal.
Jordan Adams
These butterflies were found at Ormond Beach, Florida. The ones infected by the parasite can be spotted by their smaller size, crippled wings, and dull colors.

An open letter to Floridians about a parasite in monarch butterflies has become controversial as some researchers suggest removing milkweed entirely will prevent the spread; others suggest that people should plant more.

A naturally-occurring parasite that has been spreading among monarch butterflies in Florida has kicked over a virtual hornet's nest in the monarch research community.

According to Monarch Health at the University of Georgia, the parasite, Ophryocystis elektroscirrha or OE, can cause deformations, small size, impaired mating, and decreased flight endurance in monarchs.

This has caused controversy, as some researchers suggest removing milkweed entirely to prevent the spread, while others suggest that people should plant more.

Jaret Daniels is a professor in the Department of Entomology and Nematology at the University of Florida.

He said that habitat loss is a real issue in the state, and removing the milkweed just removes other organisms from their habitat.

“We have to look at all alternatives to rebuilding resources for wildlife,” Daniels said. “These landscapes are critical for the future of biodiversity on this planet.”

Florida’s growing population, which according to the Florida Chamber of Commerce increased by 1.6% in 2023, has already contributed to major damage to habitats.

“The monarch habitat is really habitat for a wide range of other organisms, other pollinators, other insects. So it's more than just about the monarch,” Daniels said.

He added that planting more milkweed can help prevent the spread of the parasite.

“The distribution of milkweeds across the landscape and the more milkweeds in the landscape, the lower the proportion of infected monarchs, it's just sort of a nature of scale of that situation,” Daniels said. “So having more milkweeds broadly across the landscape, distributionally and numerically, will ultimately reduce the amount of infection that's out there.”

The infection spreads more when people start using non-native milkweed, known as tropical milkweed.

Tropical milkweed, or Asclepias curassavica, has exploded in popularity as its simplicity and commercial availability makes it an easy option for gardeners.

Daniels said that because the plant is also attractive to monarchs, they can be concentrated into smaller spaces, increasing the spread of the parasite.

“We should discourage people from purchasing and utilizing tropical milkweed,” he said. “This plant has a number of downstream negative effects for the monarch, in addition to not dying back and influencing an increase in the infection rate of OE.”

A closeup image of a yellow and red plant.
Yard and Garden News
University of Minnesota Extension
Tropical milkweed flowers may appear orange at a distance, but upon close examination, petals are deep red contrasting with bright yellow hoods.

However, the scientist who wrote the original letter regarding the removal of all milkweed suggests otherwise.

Andy Davis has studied monarchs for over 25 years and said that because of misinformation and emotions, Floridians are speeding up the spread.

“My solution is not really a statewide solution, my solution is more for the backyard gardener,” he said. “Really, the only thing you can do is remove all the milkweed from your backyard and then they won't be transmitting this disease to the next generation."

He said it won’t solve the problem entirely in Florida, but could possibly solve it in someone’s backyard.

Davis added that Floridians can even test whether a butterfly is infected with a microscope.

“There's no better motivator than learning that all of your monarchs are infected,” he said.

One thing that both scientists agree on is that non-native milkweed should be avoided. They urge gardeners to learn more about milkweed before they begin planting.

“I just think we have to really try to realize that we are profoundly affecting the natural world and we have to come up with reliable solutions for the monarch and all these other organisms of which we share this planet,” Daniels said.

Kayla Kissel is a WUSF Rush Family Radio News intern for spring of 2024.