Iowa State University researchers are learning about the fine-scale movements of monarch butterflies (Danaus plexippus) in unprecedented detail via the use of radio telemetry and transmitters, weighing just 220 milligrams, affixed to the underside of butterflies’ abdomens. (Image originally published Fisher et al 2020, Environmental Entomology)

By Melissa Mayer

When most people hear “butterfly,” images of the beloved orange-and-black monarch (Danaus plexippus) spring to mind. But that iconic fluttering insect has a problem: Its overwintering numbers have plummeted since the 1990s—a situation so serious that the U.S. Fish and Wildlife Service is slated to decide whether to add it to the endangered species list at the end of this year.

Melissa Mayer

There are a few reasons for that steep decline, but the most significant has to do with the butterfly’s obligate relationship with the milkweed plant. Adult monarchs lay their eggs in milkweed because it’s the only thing their caterpillars eat. So, if milkweed sources dry up, monarchs don’t have other options.

“In the Midwest, we know that monarch habitat has declined with urbanization and crop expansion,” says Kelsey Fisher, doctoral candidate with the department of entomology at Iowa State University “Places that used to have milkweed now have parking lots or crop fields. With the introduction of glyphosate-resistant crops, stray milkweed that was in and around crop fields have been eliminated.”

It’s a situation Fisher hopes to change. She’s using VHF radio telemetry—tiny transmitters that track tagged butterflies over surprisingly long, non-linear distances—along with continuous-time movement models to predict monarch flight paths. Eventually, that data could help researchers pinpoint underused areas of land that would make good habitat patches for the beleaguered monarch.

Kelsey Fisher

Fisher and colleagues James Adelman, Ph.D. (now at the University of Memphis), and Steven Bradbury, Ph.D., report on their initial forays with this method and their findings in a study published March 11 in Environmental Entomology.

Tag, You’re It

To track monarch butterfly movements, Fisher and her team collected nonmigratory monarchs from restored prairies and attached transmitters to their undersides. It turns out radio-tagging wriggling butterflies has a bit of learning curve. The team must work carefully to avoid supergluing the wings together and then use dry ice to knock out the butterflies to prevent a fear response upon release. But the whole process now takes less than three minutes.

And don’t worry: Those transmitters are tiny—lighter than a watch battery at just 220 milligrams (mg)—and don’t affect the butterfly’s movements. The research team confirmed this by “sham-tagging” butterflies with actual watch batteries, which weigh in at 300 mg a pop, and observing their behavior. At the end of each run, Fisher recaptures the insects and removes the trackers.

Netting the Data

To acquire her raw data, Fisher released tagged butterflies in a prairie with four radio telemetry operators stationed at the corners, each equipped with a compass and antenna coupled with a radio receiver. For a maximum of 39 minutes, the operators noted the compass bearings for the loudest signal at one-minute intervals.

Then, the team used all that collected data to calculate the insects’ flight paths. It turned out the butterflies were flying much farther than straight-line calculations would suggest. Fisher’s butterflies took small, undirected steps within the prairie and large, highly directional steps when leaving the prairie. Those smaller steps aligned with previous research, but the way the butterflies turned didn’t; Fisher’s butterflies made major 180-degree pivots while foraging.

And those big steps when exiting the prairie—exceeding 50 meters at a time—are larger than any previously reported steps for nonmigratory monarchs. In fact, four of the tagged butterflies had to be tracked down in locations up to 250 meters away.

“We never, with any confidence, have been able to say where monarchs go after they leave a prairie patch,” says Fisher. “Being able to find monarchs in trees over 250 meters away was incredible! Radio telemetry is really opening up doors to ask big movement questions about little critters.”

Iowa State University researchers are learning about the fine-scale movements of monarch butterflies (Danaus plexippus) in unprecedented detail via the use of radio telemetry. Research team members using antennas track butterflies tagged with radio transmitters at one-minute intervals. This map shows the path followed by one tracked butterfly. Small circles indicate estimated locations, labeled by minute of observation. Lines connecting points represent the straight-line steps between the points. Large ovals represent the zone of error calculated for each estimated location. (Image originally published Fisher et al 2020, Environmental Entomology)

New Applications for Familiar Tech

Researchers routinely use radio telemetry to track large, slow-moving animals they can follow on foot; radio tracking tiny flying insects is something else. It’s been attempted recently with migrating butterflies—and now Fisher’s work shows that the method is suitable for collecting movement data at scales useful for analyzing whole landscapes.

That means the gathered information could help researchers make strong recommendations for habitat restoration. “We don’t only need to plant the milkweed,” says Fisher. “We need the monarchs to utilize it. By adding milkweed in places that align with their behaviors, they will interact with milkweed more frequently and lay more eggs. More eggs mean more monarchs.”

Get Involved

For Fisher, being at the center of emerging research is worth the long days in the lab and the field. “We are uncovering new information—and that is really exciting,” she says. “I also really enjoy that I am asking a big research question that can have direct impacts on how we implement conservation strategies for the monarch butterfly. I like working on a project that will help solve a big problem.”

You don’t have to be a professional entomologist to get in on the good feels that come with monarch conservation efforts. Check out these opportunities to serve as a citizen scientist, assisting researchers with data collection, and restoring monarch habitats for the next generation of very hungry caterpillars.

Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: melissa.j.mayer@gmail.com.

Fisher portrait photo by Michael Roth