MEXICO CITY, March 16 (UPI) -- The number of monarch butterflies that migrated to Mexico for the winter dropped by about 59 percent this year, experts say.
The monarch population in Mexico has seen a drop in population in six of the past seven years and now there are only about 1/15th as many butterflies as there were in 1997, CBC News reported.
"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 World Wildlife Fund and its partner organizations said in a statement.
Omar Vidal, director of the World Wildlife Fund 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.
Lincoln Brower, a leading entomologist at Sweet Briar College in Virginia, said in a statement "the dwindling monarch population 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.
"To blame the low numbers of monarchs solely on what is happening north of Mexico is misleading."
He said use of herbicides on soybean and corn fields is "a serious problem" because it kills milkweed, but that 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.