The number of Monarch butterflies hibernating in Mexican forests decreased by 22% last year, and the number of trees lost from their favored wintering grounds tripled, according to an annual report from the National Commission of Protected Natural Areas and the WWF-Fundación TELMEX Telcel Alliance.

Frost and "extreme temperatures" in the United States may have played a role in the butterfly's decline during the most recent winter season, said Humberto Peña, director of Mexico's nature reserves.

Due to a myriad of factors, monarch numbers have dropped in recent years. Experts say drought, severe weather and loss of habitat — especially of the milkweed where the monarchs lay their eggs — as well as pesticide and herbicide use and climate change all pose threats to the species' migration.

Monarchs, which migrate from Mexico and California in the winter to summer breeding grounds in the United States and Canada, have seen their population decrease between 22% and 72% over the past decade, the International Union for Conservation of Nature said last year. The Western population had dropped from 10 million butterflies in the 1980s to just 1,914 monarch butterflies in 2021, IUCN said. Since then, California has seen a rebound with 330,000 monarch butterflies recorded in 2023.

The annual butterfly count doesn't calculate the individual number of butterflies, but rather the number of acres they cover when they clump together on tree boughs.

Monarchs east of the Rocky Mountains in the United States and Canada overwinter in the fir forests of the western state of Michoacan, west of Mexico City. The total area they occupied this past winter dropped to 5.4 acres (2.21 hectares), from 7 acres (2.84 hectares) last year.

Gloria Tavera, conservation director of Mexico's Commission for National Protected Areas, said the area of forest cover appropriate for the butterflies that was lost rose to 145 acres (58.7 hectares), from 46.2 acres (18.8 hectares) last year.

Illegal logging has been a major threat to the pine and fir forests where the butterflies gather in clumps to keep warm. But experts said that this year, more than half the tree loss was due to removal of dead or sick trees affected by fires, storms or pests. Tavera said a lack of rain had plunged trees into hydric stress, making them more vulnerable to diseases, pests and fires.

Jorge Rickards, WWF Mexico's general director, blamed climate change,

"The monarch butterfly is an indicator of these changes," Rickards said.

Critics say that in the past, removal of diseased trees has been used as a pretext for felling healthy trees for timber.

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