Toastier fall weather might cause migrating monarch butterflies to wing it and change their flight plans, starting the countdown toward death.
Eastern monarchs captured during their autumn migration and exposed to warm temperatures in the lab came out of their usual reproductive hiatus, evolutionary biologist Ken Fedorka and colleagues report August 12 in Royal Society Open Science. Breaking that hiatus means the butterflies will likely die sooner than they normally would.
“Once you decide to go reproductive, your clock starts ticking,” says Fedorka, of the University of Central Florida in Orlando.
In North America, monarch butterflies (Danaus plexippus) live about a month, but the last generation of the year, the ones that migrate south, can live around eight months. This generation emerges in late summer in breeding grounds across the northern United States and southern Canada. The butterflies are born in reproductive diapause and fly up to thousands of kilometers south to overwintering sites. Come spring, the monarchs begin reproducing and then die, with subsequent generations migrating north again.
Since the 1990s, monarch populations have dropped, particularly at overwintering sites. Populations found east of the Rocky Mountains have declined by around 80 percent. Higher loads of the parasite Ophryocystis elektroscirrha could be contributing to the decline, as could the loss of habitat and milkweed, the flower on which monarchs lay their eggs and on which their larvae exclusively feed.
Researchers also think that higher temperatures caused by climate change could be detrimental. But until now, no studies have specifically looked at temperature’s impact on diapause, says Karen Oberhauser, a conservation biologistat the University of Wisconsin-Madison who wasnot involved with the new work.
Fedorka and colleagues collected nearly 500 butterflies from the wild, just as the monarchs began their fall migration. In the lab, the butterflies were placed in mesh cages within a temperature- and light-controlled incubator. Using 70 years of climate data, the team calculated the typical temperature ranges along the butterflies’ migration path and exposed the butterflies to the higher and lower ends. One group experienced an incubator temperature averaging around 23° Celsius, while the other group was subjected to about 19° C. After 26 days,the surviving butterflies were randomly assigned to either warmer or cooler overwintering conditions and monitored again.
Many of the monarchs exposed to higher temperatures began breeding in the migration phase, leading to females laying eggs despite the lack of milkweed. “These monarchs are ready to drop out of migration on a moment’s notice,” Fedorka says. A statistical analysis found that warm males had an 88 percent increase in the risk of death compared with those in the cold treatment. The warmer males that died also were in worse physical shape, based on body weight divided by wing length.
During overwintering, monarchs of both sexes that experienced a warmer migration phase had a 28 percent higher risk of death, regardless of the overwintering temperatures. The higher risk of death for males during migration — and the lack of significant deaths in females — could be a result of sample size or males’ condition that year, Fedorka notes.
The team says that further research is needed to see how the results apply in the wild. Even so, Oberhauser says, “it’s an important piece of the puzzle in our understanding of how human activities will affect monarchs.”
Insect ecologist Sonia Altizer notes that the butterflies’ propensity for abandoning their migration in warm weather means they could one day swap their fully migratory lifestyles for partial ones. And while monarchs do contribute as pollinators, the loss of their migration would also mean the loss of a fantastic feat of nature.
“It’s an incredibly beautiful, and amazing and rare phenomenon,” says Altizer, of the University of Georgia in Athens. “If we lose these migratory populations, we can’t just bring them back.”