A new University of Ottawa study shows that even a small rise in temperature can reduce the sugar content and amount of flower nectar, leaving monarch butterflies with less fat for their epic migration. The findings highlight an overlooked way climate change threatens pollinators.
Every fall, monarch butterflies embark on one of the most astonishing migrations on Earth, traveling thousands of kilometers from Canadian fields to mountain forests in Mexico. New research from the University of Ottawa suggests that climate change is quietly undermining that journey by weakening the butterflies’ fuel supply.
The problem is not the monarchs themselves, but the flowers they rely on.
In a field experiment in Ottawa, researchers found that a temperature increase of just 0.6 degrees Celsius was enough to reduce both the amount and sugar content of nectar produced by late-season flowers. Monarchs that fed on these slightly warmer plants ended up with about a quarter less body fat than butterflies feeding on plants at normal temperatures.
Those fat reserves are critical. Monarchs do not eat much during their long flight south, so they depend on energy stored from late-summer and fall nectar to power migration and survive the winter.
The study was led by Heather Kharouba, an associate professor in the Department of Biology at the University of Ottawa and University Research Chair in Global Change Ecology. Her team wanted to understand not just how monarchs respond to warming, but how climate change alters the resources they depend on.
The experiment, carried out by researcher Katherine Peel in Kharouba’s lab along with collaborators from Environment and Climate Change Canada and Western University, took place at the Fletcher Wildlife Garden in Ottawa. The scientists carefully warmed only the plants, keeping the butterflies themselves at regular outdoor temperatures.
By separating plant and insect temperatures, the team could pinpoint how heat affects nectar rather than the butterflies directly. As temperatures rose slightly, late-season flowers produced less nectar, and what they did produce had lower sugar levels.
The monarchs tried to compensate by feeding, but it was not enough.
“It’s not that the butterflies are being directly harmed by the heat,” Kharouba said in a news release. “It’s that warming is making the nectar less nutritious. Even though the butterflies could eat as much as they wanted, they couldn’t make up for the lower-quality nectar.”
That finding reveals a subtle but serious threat. Climate change is often discussed in terms of heat waves, storms or direct stress on animals. This work shows that pollinators like monarchs can also be hit indirectly, through changes in the plants they rely on.
“We’re seeing that climate change can hit pollinators indirectly, by degrading the resources they count on,” added Kharouba. “I believe the findings are a wakeup call for anyone working to protect these butterflies and, really, for anyone planting a garden or maintaining a park as the planet heats up.”
Monarch populations have already been declining due to habitat loss, pesticide use and extreme weather. The new study suggests that even modest warming could further weaken the butterflies by cutting into their energy reserves at a crucial time of year.
The research, published in Global Change Biology Communications, adds to a growing body of evidence that climate change is reshaping plant–pollinator relationships.
Nectar is the main energy source for many pollinators, including bees, butterflies and hummingbirds. If warming consistently reduces nectar quantity or quality, it could ripple through entire ecosystems, affecting not only pollinators but also the plants that depend on them for reproduction.
For monarchs, the stakes are especially high. Their multigenerational migration depends on a chain of healthy habitats stretching from Canada through the United States to Mexico. Late-season flowers in northern regions are a key link in that chain, helping monarchs bulk up before they head south.
The University of Ottawa team’s work also sparked an art-science collaboration. Visual artist Valérie Chartrand created an exhibition titled “Flutterings: Monarchs and Climate Change,” using the study as inspiration to explore how people relate to these iconic insects and the changing climate. The project aims to deepen public understanding of complex environmental issues by pairing data with visual storytelling.
While the study focuses on a specific butterfly and a specific stage of its life cycle, its message is broader. Protecting pollinators in a warming world will require more than just planting flowers; it will mean thinking about how climate change alters the quality of those floral resources over time.
For gardeners, land managers and policymakers, that could mean prioritizing diverse, climate-resilient plantings, reducing other stresses like pesticides and habitat loss, and supporting research that tracks how nectar and other key resources change as temperatures rise.
The monarch’s journey has long been a symbol of resilience. This new research suggests that helping these butterflies continue their epic migration will require paying close attention not only to the butterflies themselves, but also to the invisible energy source inside each flower they visit.
Source: University of Ottawa