Photos by Michele Walfred and Pat Davis June 10, 2022
UD’s Michael Crossley investigates impact of herbicides and warming temperatures on breeding trends in monarch butterflies
Largely because of well-publicized, diminishing winter colonies in Mexico and California, monarch butterflies across North America have been long thought to be declining as a result of diminishing summer habitat. Previous butterfly research shows that the size of overwintering monarch colonies has fallen across several decades. But what is happening when monarchs breed in the summer was less clear.
In a new research study published in Global Change Biology, lead author Michael Crossley, assistant professor and agricultural entomologist at the University of Delaware, and his collaborators examined trends in breeding monarchs across their entire range, and found there are local regions of decline, but also regions of increase. When considering the entire species range, there was no overall decline, and in fact, even a slight increase in abundance.
“The whole reason that we did this research is because monarch colonies have been declining,” said Crossley. “Up until now, there were real fears that the monarch is in trouble.”
The study’s results suggest that population growth in summer is compensating for losses during the winter. Additionally, findings indicate that changing environmental variables have offsetting effects on deaths and reproduction.
North America’s three largest countries have more in common than playing the 2026 hosts of soccer’s biggest spectacle — the FIFA World Cup. The perception of the monarchs’ decline had prompted the U.S. Fish and Wildlife Service to determine if the species warrants a higher level of protection. Canada’s equivalent agency is strongly considering listing monarchs as an endangered species because of the declines in Mexico.
As the researchers analyzed the data, patterns emerged and were location specific. Portions of the U.S. Northeast and parts of the Midwest revealed population declines. The U.S. Southeast and Northwest were unchanged or increasing, yielding a slightly positive overall trend across the species range. Monarchs in Florida appear to be doing exceptionally well.
“Our question was, ‘Are monarchs declining across their breeding range?’ The key take home message was yes, monarchs are declining in some places, but increasing in other locations,” said Crossley, who conducts research and teaches in the UD Department of Entomology and Wildlife Ecology. “There is no consistent, long-term trend.”
So how is that possible? Near agricultural fields that use glyphosate, which is used in products such as Roundup, sites typically had fewer monarchs. This occurrence was especially evident in the upper Midwest — the corn and soy belt. The researchers also found positive effects of temperature, which is generally increasing faster in northern latitudes, which seems to be counterbalancing glyphosate effects.
“These two effects seem to cancel each other out, especially in the upper Midwest,” said Crossley.
Crossley contends that the different trends in these locations are related to the environment.
“For example, there are populations now that will overwinter in California, Florida or even southern Georgia because it’s become more feasible with increasing winter temperatures,” Crossley said. “We may see fewer migrants because monarchs are doing well, staying in Florida and not joining the migration.”
Since accelerating climate change may bring growing threats, Crossley cautioned against complacency.
The dataset that the researchers used was made possible by the North American Butterfly Association and thousands of citizen scientists — volunteers and butterfly enthusiasts — who collected butterfly counts for the past 25 years at more than 400 sites around the continent.
Another conclusion of the study is that other butterfly species are facing greater peril than monarchs. The authors argue that some of these butterflies — like Poweshiek skipperling (in the upper Midwest), Florida leafwing (in extreme southern Florida) and Hermes copper (in extreme southern California) — deserve more attention. Though monarchs are the most popular among the general public, they are far from the only type of butterfly. The North American Butterfly Association observes more than 450 species. Though it’s facing overall population declines, the monarch is doing comparatively well; 320 species face steeper population losses.
“It seems like breeding monarchs in North America are not in trouble despite winter colony declines,” said Crossley. “This could lead to several explanations about why a disconnect exists between numbers in the breeding range and numbers in overwintering sites.”
The grueling journey
The findings show that monarchs are breeding in strong numbers. Then the insects trek to places like Mexico to overwinter, but these colonies are generally shrinking in size. So, what is hindering the journey southward to the winter colonies? Study co-author Andy Davis of the University of Georgia has investigated parasites of monarchs and found that a protozoan parasite, Ophryocystis elektroscirrha (OE), is on the rise, and could affect the success of the fall migration. As Davis described, making that journey with parasites in tow is equivalent to running a marathon with the flu. And monarchs’ total mileage, which falls between 1,200 and 2,800 miles, far eclipses human’s 26.2-mile mark for a marathon.
“A lot of monarchs are not going to make that southward journey if they are infected,” said Crossley. “One way to reconcile the disconnect [between breeding numbers and overwinter numbers] is to say, well, maybe monarchs have an incredible ability to rebound every spring and summer from relatively small overwintering sizes. And the decline is occurring during this southward migration.”
Besides parasites, other factors are affecting monarchs’ migration. Climate change is making the journey south hotter and drier, making the extended trip more arduous for these small creatures. Additionally, many butterflies use certain valleys as their flyways, which are unfortunately along highways. Cars and trucks are simply running into the iconic insects, something Crossley points out could potentially be amended.
“For other types of species, officials reduce speed limits or reroute traffic during certain times of year, so these animals can cross the road,” said Crossley.
Finally, when you’re on a long trip, you always stop and grab a bite to eat, right? Monarchs feed on nectar. Loss of habitat and nectar resources mean more butterflies lack the energy to fly south. The average person can take action on their own and plant native, fall-blooming goldenrods, asters and perennial sunflowers to fuel the monarchs.
“We certainly can’t go wrong by making more insect habitats. Whether humans or animals, we could all use more,” Crossley said. “We can do that with native plants right in our own backyards.”