LOS ANGELES, CA (Catholic Online) - The butterflies typically migrate from the United States and Canada to spend the winter sheltering in the mountains of central Mexico. Six of the last seven years have shown butterfly population drops. There is now only one-fifteenth as many butterflies as there were in 1997.

Experts say that the decline in the monarch population 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.

One of the culprits, illegal logging in the reserve established in the monarch wintering grounds was believed to be one of the chief reasons. Such logging has been vastly reduced by increased protection, enforcement and alternative development programs in Mexico.

The World Wildlife Fund blames climate conditions and agricultural practices, especially the use of pesticides that kill off the monarchs' main food source, milkweed. The Monarchs 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 organizations said in a statement.

"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," Omar Vidal, the World Wildlife Fund director in Mexico says.

"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. But a 2012 aerial survey showed very little detectable logging, in the first times that logging had not been found in detectable amounts since the mountaintop forests were declared a nature reserve in 2000.

Milkweed depletion 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.

Unusually hot or dry weather can also 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.

"This is not just the lowest population recorded in the 20 years for which we have records," Lincoln Brower, a leading entomologist at Sweet Briar College in Virginia, says. "The report of the dwindling monarch butterfly winter residence in Mexico is ominous.
"It is the continuation of a statistically significant decrease in the monarch population that began at least a decade ago."

However, Brower differed on whether small-scale logging, the diversion of water resources and other disruptive activity in the reserves in Mexico are playing a role in the decline.

"To blame the low numbers of monarchs solely on what is happening north of Mexico is misleading," Brower said. "Herbiciding of soybean and corn fields that kills milkweed is a serious problem, but the historical decline over the past 19 years has multiple causes.

"All three countries need to face up to the fact that it is our collective activities that are killing the migratory phenomenon of the monarch butterfly," he said.

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