Storm in Mexico May Have Significantly Harmed Overwintering Monarchs, Says Expert
Researchers hope to get a better picture of the damage in the coming week
Several days after a severe storm hit the Mexican region where monarch butterflies overwinter, it’s still unclear to scientists how much damage was done to the embattled species.
Loss of habitat from logging, along with use of agricultural pesticides, have been decimating populations of the orange butterfly for years, with its numbers dipping by 90 percent since the early 1990s. This winter was seen as a possible turnaround for the species, with the butterflies blanketing trees over three times as much land as in recent years, across 12 Mexican sanctuaries.
Photo by Pablo Leautaud
The largest butterfly contingent gathers at El Rosario Monarch Sanctuary in Michoacan, Mexico, which this year had more than a fourth of the Eastern population of butterflies, said Elizabeth Howard, director of Journey North, a citizen-science organization. The sanctuary posted photos of the weather damage from the March 10 storm, saying that the butterflies were suffering from the cold, wet conditions.
Howard said it’s known that another sanctuary, Sierra Chincua, also was in the storm’s path. The two sanctuaries hold half the Mexican overwintering population between them. Other sanctuaries might also have been affected.
At first, the expectations were grim. The day after the storm, Dane Elmquist, community program assistant at the Monarch Lab at the University of Minnesota, said it represented the most dangerous kind of weather system for the monarchs. Rain was followed by hail and sub-zero temperatures, he said, which meant the butterflies would be drenched by water that would then turn to ice.
Elmquist said the last time this kind of weather occurred at the sanctuary during overwintering season was in 2002, when some 70 percent to 80 percent of the monarchs there reportedly died.
Yet less than a day after the storm, the Mexican government triumphantly announced that almost all of the butterflies had survived. According to the National Protected Natural Areas Commission, ABC reported, its staff had photographed "branches and trunks covered with butterflies in good condition."
Others doubt that things are quite that good. Some, including Ellen Sharp, who co-owns a bed and breakfast in the area, said in a posting on the Journey North website that “those who benefit from butterfly tourism downplay the damage.”
It might be true that many butterflies still clung to trees following the storm, but the damage could still be significant, said scientists and advocates for the butterflies.
“Although encouraging to see, butterflies hanging in clusters can be dead — and butterflies on the ground can be alive, even if buried in snow,” Howard wrote on the Journey North website.
David James, a monarch expert at Washington State University, agrees.
“Sometimes it takes a few days for the dead butterflies to drop from the trees,” he said. Still, though the number might be much higher than the 2 to 3 percent that Mexican officials rosily predicted, it seems certain that the storm was not as catastrophic to the species as the 2002 disaster.
“Predictions are that it’s possible that about 30 percent of the population remaining at the sites may have perished,” James said Monday. “But we won’t know this for a week or so, when people can get there and do counts of dead butterflies on the ground.”
One important factor, he said, will be how many butterflies had already left the sanctuaries. March is when they start their migration north, flying as far as southern Canada.
Most monarch butterflies in North American never actually make the trip. Those that emerge — or eclose — from their chrysalises in spring and summer live for just a few weeks, long enough to mate and lay eggs on milkweed, the only plant they will use for the purpose. The caterpillars then eat the milkweed — and only the milkweed — before themselves entering the chrysalis stage.
But Eastern butterflies that eclose in autumn — the so-called “fifth generation” — make the migration of up to 2,000 miles to the warmer climate of Mexico. (Western monarch populations are less likely to make the full migration, James said, often overwintering at smaller sites in California and other areas that are relatively warm all year.)
One of the reasons behind the declining numbers of monarchs is a dearth of milkweed throughout the vast agricultural expanses of the Midwest, partly from development but in good part because of the use of herbicides to control weeds.
A 2012 study by researchers at the University of Minnesota and Iowa State University raised questions about whether the popularity of genetically engineered crops worsened the situation. It found that the rise in those crops in the Midwest was accompanied by an 81 percent drop in the region’s monarch population. Though the study was only correlational, scientists theorized that because herbicide-resistant corn and soy allowed farmers to spray more heavily and thoroughly without harming their crops, they were killing off most of the milkweed.
In the coming weeks, researchers will gain a fuller picture of the impact of the harsh winter storm on the beleaguered monarch butterflies.