Monarch butterfly populations are facing new threats due to rising migratory temperatures, according to a study led by Ken Fedorka, a professor in the University of Central Florida’s Department of Biology. The research, published in Royal Society Open Science, examines how warmer weather during migration affects monarch physiology and behavior, raising concerns for long-term population stability.
Fedorka explains that both eastern and western monarchs migrate thousands of miles to overwintering sites in Mexico and California. During this journey, they enter reproductive diapause—a dormant state that helps them conserve energy and survive until spring. However, the study found that increased temperatures are causing monarchs to end diapause prematurely. “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,” says Fedorka.
Since the 1990s, eastern monarch populations overwintering in Mexico have declined by 70%. Previous studies have linked this drop to factors such as habitat loss, deforestation, tropical milkweed use, and the OE parasite. The current research adds warming migratory temperatures as another significant risk.
The team simulated warm and cold migratory conditions with monarchs to measure survival rates, body condition, reproductive development, and parasite load. Fedorka notes: “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. Both sexes also mated more under warm conditions.”
This change disrupts the timing between reproduction and milkweed availability. Monarch caterpillars depend exclusively on milkweed leaves for food; if eggs are laid before native milkweed emerges in spring, larvae may not survive. “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,” says Fedorka.
The role of tropical milkweed—an ornamental plant often sold in U.S. gardens—remains controversial among conservationists. Some suggest it could help monarchs breed during warmer winters; others warn it can attract butterflies away from migration routes or harbor high levels of parasites like OE. “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.”
Fedorka cautions against widespread planting but acknowledges managed use might aid conservation efforts: “To date, planting native milkweed and minimizing tropical milkweed seems reasonable,” he says. He points out that record heatwaves have already triggered changes; during California’s 2020 population crash fewer than 2,000 migrant butterflies were recorded at overwintering sites compared with millions previously seen.
Despite low numbers at these sites one winter saw unusually high counts of breeding adults elsewhere—suggesting some butterflies may survive outside traditional overwintering areas if conditions allow: “Our data show that monarchs failing to reach overwintering sites may not be dying… Instead they may become reproductive—and if successful—their offspring could re-migrate northward to sustain strong summer populations,” Fedorka states.
Looking ahead he recommends adapting strategies as climate patterns shift: “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,” he says.
Fedorka emphasizes ongoing data collection will be essential for effective conservation: “We need to be cautious with our conservation efforts and focus on gathering more data about our changing environment… This will be crucial moving forward to conserve this iconic species.”
He also highlights citizen scientists’ contributions: “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… These passionate citizen scientists are crucial to monarch conservation.”
