Monarch butterflies (Danaus plexippus) have one of the most impressive migrations in the animal kingdom, but rising temperatures are disrupting their physiology, migration success and long-term population stability, raising new questions about conservation strategies.
In a recent study published in Royal Society Open Science, researchers found that warmer migratory temperatures may be disrupting their reproductive development, reducing overall fitness and increasing mortality.
Led by UCF Professor of Biology Ken Fedorka and colleagues, the study warns that if warming weather trends continue, the monarch’s migratory phenotype could be lost in North America.
“Eastern and western monarch populations migrate thousands of miles to overwintering sites in Mexico and California, entering reproductive diapause, or a dormancy stage, to conserve energy and survive the journey and winter,” Fedorka says. “But warming temperatures are activating their reproductive development earlier and causing them to drop out of migration, even in the absence of native milkweed, the crucial plant they rely on for breeding success.”
Since the 1990s, the eastern monarch populations overwintering in Mexico have declined by 70 %, a drop tied to habitat loss, deforestation, tropical milkweed and Ophryocystis elektroscirrha (OE) parasite, with warming temperatures now emerging as newly identified risk in this study.
Each August, a new generation of monarchs emerges in reproductive diapause, storing fat to fuel the roughly two-month southward migration to the mountains of Mexico, just west of Mexico City. There, they cluster in trees and survive on stored fat until early spring, when they begin investing in reproductive tissues and mating.
“Only the south-migrating generation lives up to eight months,” Fedorka says. “Once they start reproducing, they live around four to five weeks, leaving the next generation to continue the cycle. Typically, four to five generations migrate north and south every year.”
To conduct the study, researchers simulated warm and cold migratory temperatures with monarchs to measure survival, body condition, reproductive development and parasite load.
“We found that during the migration phase, warm temperatures increased male mortality and reduced fitness, while females ended diapause early, developed eggs and even laid them in the absence of milkweed,” Fedorka says. “Both sexes also mated more under warm conditions.”
This pattern can create issues for survival of the offspring. During the overwintering phase, Fedorka and his team found that monarchs exposed to warm migration conditions faced higher mortality, with females reproducing early and parasite-heavy butterflies most likely to die. More notably, their study points at the effect of warming temperatures in shifting the delicate cycle of monarch’s reproductive process.
“Normally, monarchs begin reproduction in spring after surviving winter diapause when native milkweed re-emerges,” Fedorka says. “Monarchs rely on milkweed for reproduction, with females laying eggs on the plant and caterpillars feeding exclusively on its leaves. However, if warmer temperatures cause monarchs to drop out of migration earlier and begin mating and laying eggs before milkweed is available, their offspring will not have a food source, leading to reproductive failure.”
Milkweed is central to the conservation debate. Monarchs rely on milkweed for reproduction, but non-native tropical milkweed, a widely sold ornamental flower in the U.S. , may be complicating efforts.
“In the conservation world, tropical milkweed is controversial,” Ferdorka says. “Some argue it could support monarchs as a warming climate force more winter breeding. Others warn it can lure monarchs out of migration and harbor high levels of OE, a parasite that weakens their fitness.”
The study’s authors caution planting tropical milkweed widely but note it could play a role in conservation if carefully managed.
“To date, planting native milkweed and minimizing tropical milkweed seems reasonable,” Fedorka says. “However, my data and David James’ observations during the 2020 California monarch crash, suggest that unusually warm migratory temperatures may present the biggest risk, triggering migratory failure.”
In the fall of 2020, California’s western monarch population collapsed, with fewer than 2,000 migrant butterflies recorded at overwintering sites compared to the millions that once migrated there. Record heat likely disrupted diapause, though that same December, people documented a record high number of breeding adults, eggs and larvae on milkweeds, suggesting a winter breeding switch. The following year, the overwintering population rebounded, with about 240,000 monarchs believed to have been descendants from the winter breeders.
“One of the current puzzles surrounding monarchs is how such low overwintering populations can give rise to such robust summer breeding populations,” Fedorka says. “Our data show that monarchs failing to reach overwintering sites may not be dying, as generally expected. Instead, they may become reproductive, and if successful, their offspring could re-migrate northward to sustain strong summer populations. That’s something we need to consider when developing conservation strategies.”
Fedorka added that if the climate continues to warm, more monarchs may drop out migration and attempt reproduction sooner and if tropical milkweed is banned, they would face reproductive failure.
“If we adapt conservation efforts and plant tropical milkweed along the southern range of the migration route, it could help retain migration dropouts and support the next generation,” Fedorka says.
While this study highlights warming climate as a major driver of the monarch overwintering population decline, Fedorka emphasized the need for more data.
“We need to be cautious with our conservation efforts and focus on gathering more data about our changing environment,” he says. “This will be crucial moving forward to conserve this iconic species.”
Citizen scientists remain a critical part of that effort.
“Thousands of people across the U.S. track monarchs and milkweed from their backyards or travels, providing data no single research team could gather alone,” Fedorka says. “These passionate citizen scientists are crucial to monarch conservation.”
Looking ahead, Fedorka and his team are building on this research to study how temperature alone may drive diapause disruptions.
“I’ve always been fascinated by the quirks of nature and asking questions,” he says. “I see it as a giant puzzle to be solved, and the real joy comes from chipping away at the mystery and monarch migration is one of them.”
