ROBERT SIEGEL, HOST:
Monarch butterflies make an annual migration from North America to the Sierra Madre Mountains in Mexico. It's a journey of thousands of miles. The distinctive orange and black butterflies will rest and mate in Mexico through the end of this month. Their numbers are up this year, but no one is celebrating. More than 90 percent of the monarch population has vanished in the last 25 years. NPR's Carrie Kahn went to see some monarchs before they head back north.
CARRIE KAHN, BYLINE: It's about an hour-long trek straight up the mountain from El Rosario, the closest village to the butterflies' Mexican wintering stop in the southern state of Michoacan. Finally, at the top at the edge of a towering stand of fir trees, Vicki Gray (ph) a retired probation officer from Washington, D.C., gets a glimpse. Tens of thousands of butterflies hang from the branches in bulging clusters. Rays of sun peak through the canopy. And, taking advantage of the warmth, hundreds break from their wintry sleep and hover over her head.
ISABEL GRAY: It exceeds my expectations - absolutely magical. I've always wanted to do this. I never thought I would.
KAHN: Tens of millions of butterflies returned to the forest this season, according to Mexico's annual census. That counts acres inhabited by butterflies, not individuals. The insects are resting in 70 percent more forest. And that's good news since the 2013 census was the lowest on record. Forest biologist Luis Enrique Salcedo Soto (ph) says it's unclear why the butterflies rebounded this year - probably better weather conditions. He says so much about the monarchs' migration is a mystery.
LUIS ENRIQUE SALCEDO: (Foreign language spoken).
KAHN: "Like why do they always come back to the same fir forests," he asks, "especially since the returning butterflies have never been here before?" Salcedo says these individuals are actually the fourth offspring, some even fifth or sixth, from the ones that left last year. While their multigenerational passage may be perplexing, one fact is very clear. Their numbers are declining at an alarming rate. Just 25 years ago, the population stood at nearly a billion. Some scientists say as few as 33 million are left.
EDUARDO RENDON SALINAS: (Foreign language spoken).
KAHN: "This is a staggering decline," says Eduardo Rendon Salinas of Mexico's World Wildlife Fund. He says the causes are many, including global warming and illegal logging in the Mexican forest. But he says the government has made great strides in recent years to crack down on loggers.
Rendon says the threats are now greater in the U.S. The monarch's habitat and main food source, the milkweed plant, has been decimated in recent years. Sylvia Fallon of the Natural Resources Defense Council blames pesticide use and industrial agricultural practices. She says the increasing use of genetically altered crops, modified to be resistant to herbicides, has allowed farmers to spray their crops indiscriminately.
SYLVIA FALLON: What it has done is very effectively killed the milkweed. And so we've seen a huge decrease in the amount of milkweed that's available across the United States and particularly in the major kind of corn belt area, which is critically important for the monarch population.
KAHN: The NRDC announced last week it is suing the government over the use of herbicides, especially the commercially popular Roundup. Fallon of the NRDC says there hasn't been a review of its use since the 1990s, and agricultural practices have changed substantially since then. The Environmental Protection Agency says it will have a response for the NRDC by the spring or summer and is studying what it says are the multiple causes of the decline in the monarch population. While the U.S. decides how to combat the butterflies' decline, tourists continue to flock to Mexico's reserve and catch a glimpse of the amazing insects while they can. Shandalin Zeldin (ph) came from Canada to the reserve. She actually lives near Point Pelee National Park, where many monarchs summer. She says now when she sees the monarch in her garden, it will be a new experience.
SHANDALIN ZELDIN: I can say that I saw your grandmother, your abuela, here.
KAHN: She just hopes her grandchildren will get to see the monarchs, too. Carrie Kahn, NPR News, at the El Rosario Butterfly Reserve in Michoacan. Transcript provided by NPR, Copyright NPR.