They have lost vast portions of their habitats to farms and development, drought has killed the milkweed they feed on, and a severe and sudden storm in 2002 killed close to 80 percent of the overwintering monarch population in Mexico.
"That was a very extreme and unusual weather event. It's usually the dry season; there aren't big storms there, but they just had a lot of precipitation. That was followed by cold temperatures, so that juxtaposition of precipitation and cold just killed all the butterflies," said Karen Oberhauser, a professor in the Department of Fisheries, Wildlife and Conservation Biology at the University of Minnesota. "Clearly, that kind of storm is predicted to be more common under climate change scenarios."
The discovery of a cold trigger also raises the possibility of an unseasonal cold spell in the north disrupting the whole migration south.
A different pattern from fall
The temperature trigger is unexpected.
Monarchs take their cue to start migrating in the fall from decreasing hours of sunlight. While studying monarch diapause, a hormonal change that keeps adult butterflies from producing eggs and sperm, Oberhauser found that they showed the greatest response when exposed to changes in both day length and temperature.
"Migratory organisms need to respond to conditions in one place that will predict conditions somewhere else," she said. From an evolutionary perspective, it makes the most sense for them to respond to something that is constant from year to year, she said.
"So it is surprising to me that they are responding to temperature and not day length, which is obviously a fixed cue and is not going to vary from year to year," she said.
Reppert and Guerra tested a group of monarchs by keeping the day length constant from the time they were captured and only decreasing temperature. The butterflies still flew north.
"Subtle changes in day length might still have some influence, but it doesn't seem to be anywhere near as important as temperature," said Reppert. He plans to conduct additional research by using molecular and genetic tools to uncover what proteins and temperature sensory mechanisms are at work.
The monarchs themselves are a hardy lot. As Oberhauser's research shows, they can withstand temperatures up to 40 degrees Celsius and can even weather summer storms by latching onto a plant. What is at stake is their annual cross-country passage.
"It would just be a shame to lose this migration," Taylor said.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500