MEXICO CITY - The population of monarch butterflies in the Mexican forests where they spend the winter fell to the second-lowest figure on record this season, offering a grim snapshot of the already endangered orange-and-black insects.

The iconic butterflies' presence was only documented in 2.2 acres (0.9 hectares) of forest spanning a couple Mexican states where they traditionally hunker down for the winter, according to the latest annual study published on Wednesday and conducted by Mexico's protected natural areas commission and the Swiss-based World Wildlife Fund (WWF), an environmental group.

In the mid-1990s, monarch butterflies could be found on around 45 acres of the same forests covered largely by pine and fir trees, along the border between Michoacan and Mexico states.

monarch butterfly rests on a leaf at the Sierra Chincua butterfly sanctuary in Angangeo, Michoacan state, Mexico December 3, 2022. REUTERS/Raquel Cunha/File Photo

The 2023-2024 winter figure marks the second-lowest since the population survey began over three decades ago, and is down nearly 60% from just the previous winter.

Biologists pin the blame for the nosedive on higher-than-usual temperatures and drought conditions where the butterfly reproduces, mostly in northwestern U.S. states like Washington, Oregon and California.

The adverse weather, made worse by climate change, translates into less milkweed, the plant where the butterflies mostly lay their eggs and what their caterpillar offspring feed on.

In one of the planet's epic wildlife migrations, the fluttering butterflies travel south as many as 2,800 miles (4,500 km) from spots as far north as Canada to eventually spend the winter in warmer Mexico, where millions cling to trees that protect them from the rain and chill.

Monarch butterflies fly at the Sierra Chincua butterfly sanctuary in Angangeo, Michoacan state, Mexico. December 3, 2022 REUTERS/Raquel Cunha TPX IMAGES OF THE DAY/File Photo

Monarch butterfly populations fluctuate year to year, and as recently as 2021, the same survey showed a 35% uptick to cover around 7 acres.

Officials and activists called for more action to boost the species, including the need to confront threats posed by herbicides that decimate milkweed plus forest conservation.

"We can't lower our guard," Jorge Rickards, head of WWF's Mexico office, told reporters after releasing the latest data.

Monarch butterflies rest on a tree at the Sierra Chincua butterfly sanctuary in Angangeo, Michoacan state, Mexico December 6, 2023. REUTERS/Raquel Cunha/File Photo

Monarch butterflies rest on a tree at the Sierra Chincua butterfly sanctuary in Angangeo, Michoacan state, Mexico December 6, 2023. REUTERS/Raquel Cunha/File Photo