The number of monarch butterflies at their overwintering areas in Mexico dropped precipitously this year to the second-lowest level on record, according to an annual survey.
The census, considered a benchmark of the species’s health, found that the butterflies occupied only about 2.2 acres of forest in central Mexico, down 59 percent from the prior year. Only the winter of 2013-14 had fewer butterflies.
Scientists said the decline appeared to be driven by hot, dry conditions in the United States and Canada that reduced the quality of available milkweed, the only plants monarch caterpillars can eat, as well as the availability of nectar from many kinds of flowers, which they feed on as butterflies.
“It’s telling us that we need to intensify conservation and restoration measures,” said Jorge Rickards, the general director of World Wildlife Fund in Mexico, which conducted the survey with the National Commission of Protected Natural Areas and other partners.
Migratory monarchs are listed as vulnerable, or threatened with extinction, by the International Union for Conservation of Nature Red List of Threatened Species, the world’s leading scientific authority on the status of species. They were initially classified in the more imperiled category of endangered, but their status was adjusted in September.
The United States government has placed monarchs on what is essentially a waiting list for protection under the Endangered Species Act; the species qualifies for protection, officials have said, but others take priority.
It’s normal for insect population totals to swing up and down drastically, but drops become dangerous when they have been chronically eroded, as with monarchs, said Karen Oberhauser, professor emerita of entomology at the University of Wisconsin-Madison, who has studied monarchs for decades.
“When we start with a low number, a catastrophic event could send the population into a tailspin that would be hard to recover from,” Dr. Oberhauser said.
A drop this large has never occurred after a number as low as last winter, she added. Dr. Oberhauser also noted that deforestation in the butterflies’ winter habitat in Mexico was low last year, so it doesn’t appear to be a factor in the decline.
Monarch numbers during the summer breeding season were only slightly lower than last year, with some areas like the Northeast showing higher totals. But drought conditions in the south-central United States and northern Mexico probably led to lower nectar availability and a less successful autumn migration, Dr. Oberhauser said.
The survey made public Wednesday measured Eastern Monarchs, which live east of the Rocky Mountains. Western Monarchs, on the other hand, overwinter mainly in California. Their annual count is conducted differently, and this year tallied 233,394 butterflies. That was down from the previous year but far higher than a perilous low of less than 2,000 in 2020.
Still, Western monarchs were just 5 percent of their population in the 1980s, when numbers were often in the low millions, according to Xerces Society, a nonprofit group dedicated to insect conservation that participated in both surveys.