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MEXICO CITY – The number of monarch butterflies making it to their winter refuge in Mexico dropped 59 percent this year, falling to the lowest level since comparable record-keeping began 20 years ago, scientists reported Wednesday.

It was the third straight year of declines for the orange-and-black butterflies that migrate from the United States 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.

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, the experts said.

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 organizations 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.