Favorable weather boosts Mexican overwintering population but numbers remain relatively low.
PORTLAND, Ore., February 26, 2016 — The latest count of monarch butterflies overwintering in Mexico found that the population which will migrate to the United States rebounded again this year. Monarch numbers increased to 150 million from 42 million last year, according to data collected by the World Wildlife Fund Mexico and announced today. Today’s numbers show a substantial increase from the last two years (the two lowest years on record) but are still far below a number that most scientists consider sustainable. Scientists estimate the population size by counting the number of hectares of trees covered by monarchs, and found that 4.0 hectares were occupied this year. Researchers estimate that there are approximately 37.5 million monarchs per hectare.
The population was expected to be up this winter due to good spring and summer weather conditions in the monarch’s U.S. and Canadian breeding areas. Although the 150-million figure is very good news, the numbers of monarchs are still well below the 22-year average and the 5-year target of 225 million monarchs (6 hectares) set by the U.S. Fish and Wildlife Service.
“This is great news and give us some breathing room as we work to recover monarch numbers,” said Scott Hoffman Black, executive director of the Xerces Society for Invertebrate Conservation, a conservation group leading rescue efforts. “But there is still a long way to go to ensure that my grandchildren will be able to see monarchs every summer.”
Monarch butterflies (Danaus plexippus) of North America are renowned for their long-distance seasonal migration and spectacular winter gatherings in Mexico and California. Most monarchs east of the Rocky Mountains migrate to sites in the oyamel fir forests north of Mexico City to spend the winter.
Many monarchs west of the continental divide overwinter along the California coast, and new research has demonstrated that some western monarchs also go to Mexico to spend the winter. Populations of monarchs at California overwintering sites saw a modest increase this year.
The monarch butterfly population has recently declined to dangerously low levels. In the 1990s, estimates of up to one billion monarchs made the epic flight each fall from the northern plains of the U.S. and Canada to sites in the oyamel fir forests north of Mexico City. By contrast this year’s estimate of 150 million monarchs is still well below the high numbers in the 1990s.
Because the weather conditions in much of the southern and Midwestern U.S. for last two years were very good for monarchs, there is a concern that this population size is the largest that the habitat can currently support. Unfortunately, the loss of over 150 million acres of habitat to corn and soy production may limit the number of monarchs that can be produced on the habitat that remains. It is vital that we work to restore habitat across the breeding range to grow the population to a level that will not be impacted by winter storms. A single winter storm in 2002 killed an estimated 500 million monarchs, more than three times the size of the current population, even with this year’s boost.
Federal and state agencies and many nonprofit organizations are working to protect and restore habitat for monarch butterflies. The Xerces Society has been leading efforts to implement habitat projects in all landscapes including farms, roadsides, wildlands and urban and suburban areas across the US.
Many unresolved challenges exist to fully recovering this species. Much of the monarch’s habitat is now dominated by corn and soybeans that are genetically modified to allow large scale herbicide use which eliminates milkweed the—monarchs only host plant. Additionally, highly toxic, persistent insecticides like neonicotinoids are used everywhere and milkweed is often mown or sprayed because people perceive it as a weed. Logging at overwintering sites in Mexico also threatens monarchs.
“When you consider that in the mid-1990s the population reached nearly 700 million butterflies, this is still a pretty low number,” said Sarina Jepsen, endangered species program director for the Xerces Society. “Monarchs are still far from recovered. We will still need a focused effort to address the many threats that monarchs face—from pesticide use and habitat loss to climate change and disease.”
All people can help monarchs by planting native milkweed and other native flowers and eliminating insecticide use.
For More Information
Read more about Xerces’ Monarch Conservation Campaign, including efforts to conserve overwintering sites in California and restore breeding habitat in key regions of the United States at www.xerces.org/monarchs/
The Xerces Society for Invertebrate Conservation
Protecting the Life that Sustains Us
The Xerces Society has the world’s largest pollinator conservation team, with several staff currently working on monarch conservation. Efforts include conservation and management of critical habitat across central and western U.S., habitat restoration throughout the country, milkweed production best practices, restoration of overwintering sites in California, and engagement of citizens in monarch research and protection. Our staff participates in the Federal Monarch Butterfly High Level Working Group, the USGS Monarch Science Partnership, and co-chair the Monarch Joint Venture. We collaborate with many federal and state agencies or contract and work closely with university researchers and other NGOs to advance the science and practice of monarch conservation.
To learn more about our work, please visit www.xerces.org.