The number of Monarch butterflies making it to their winter refuge in Mexico has dropped 59 percent this year, falling to the lowest level since comparable record-keeping began 20 years ago, according to scientists.
It was the third straight year of declines for the orange-and-black 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, experts said on Wednesday.
The decline in the Monarch population now marks a statistical long-term trend and can no longer be seen as a combination of yearly or seasonal events.
But they differed on the possible causes.
Illegal logging in the reserve established in the Monarch wintering grounds was long thought to contribute, but such logging has been vastly reduced by increased protection, enforcement and alternative development programs in Mexico.
The World Wildlife Fund, one of the groups that sponsored the butterfly census, blamed climate conditions and agricultural practices, especially the use of pesticides that kill off the Monarchs' main food source, milkweed.
The butterflies breed and live in the north in the summer, and migrate to Mexico in the winter.
"The decrease of Monarch butterflies ... probably is due to the negative effects of reduction in milkweed and extreme variation in the United States and Canada," the fund and its partner organisations said in a statement.
Omar Vidal, the World Wildlife Fund director in Mexico, said: "The conservation of the Monarch butterfly is a shared responsibility between Mexico, the United States and Canada. By protecting the reserves and having practically eliminated large-scale illegal logging, Mexico has done its part."
"It is now necessary for the United States and Canada to do their part and protect the butterflies' habitat in their territories," Vidal said.
Logging was once considered the main threat to the reserve, located west of Mexico City. At its peak in 2005, logging devastated as many as 461 hectares annually in the reserve, which covers 56,259-hectares.
But a 2012 aerial survey showed almost no detectable logging, the first time that logging had not been found in detectable amounts since the mountaintop forests were declared a nature reserve in 2000.
The loss of milkweed in the Monarchs' summering areas in the north can make it hard for the butterflies to lay eggs, and for the offspring that do hatch to find enough food to grow to maturity. In addition, unusually hot or dry weather can kill eggs, meaning fewer adult butterflies.
For butterflies that reach adulthood, unusual cold, lack of water or tree cover in Mexico can mean they're less likely to survive the winter.
Lincoln Brower, a leading entomologist at Sweet Briar College in Virginia, said in a statement that "the report of the dwindling Monarch butterfly winter residence in Mexico is ominous."
"This is not just the lowest population recorded in the 20 years for which we have records," Brower said. "It is the continuation of a statistically significant decrease in the Monarch population that began at least a decade ago."