The number of Monarch butterflies making it to their winter refuge in Mexico dropped 59% this year, falling to the lowest level since comparable record-keeping began 20 years ago. It was the third straight year of declines for the butterflies that migrate from the US and Canada to spend the winter sheltering in mountaintop fir forests in central Mexico. Six of the last seven years have shown drops, and there are now only one-fifteenth as many butterflies as there were in 1997.
"Ominous," says a leading entomologist at Sweet Briar College in Virginia. "This is not just the lowest population recorded in the 20 years for which we have records. It is the continuation of a statistically significant decrease in the Monarch population that began at least a decade ago." The World Wildlife Fund, one of the groups that sponsored the butterfly census, blamed the drop in part on agricultural practices, especially the use of herbicides that kill off the Monarchs' main food source, milkweed. The entomologist says other factors, such as small-scale logging and the diversion of water resources, are probably at play in all three countries. (Read more monarch butterflies stories.)