Six of the past seven years have also seen numbers decline, bringing the population to only one-fifteenth of what it was in 1997.
This winter, the butterflies covered just under 3 acres of forest in Mexico’s Michoacan state, down from just over 7 acres during the previous winter.
Scientists say it is no longer possible to attribute the decline to just yearly or seasonal events that have always affected the species.
The World Wildlife Fund is one of the groups that sponsored the butterfly census. It says climate change and agricultural practices are to blame.
One of the biggest factors is the use of pesticides that kill off the monarch’s main food source of milkweed.
Logging in the butterfly’s wintering home in the forests of western Mexico was once considered the main threat to the species.
But it has been virtually halted in recent years by conservation efforts. At its peak in 2005, the felling of the forests in Michoacan state was devastating more than 1,000 acres of trees each year.
Photo: Casey Overby