An American nonprofit group says the number of monarch butterflies spending the winter in the western United States has dropped to its second lowest mark in nearly thirty years.
The Xerces Society for Invertebrate Conservation, based in Oregon, said the causes for this include the use of chemicals to kill insects, known as pesticides.
The group also blames reductions in habitat and climate change.
Monarch butterflies are known for their clear orange and black markings.
They live across North America.
Monarchs in the eastern United States spend the winter in Mexico.
The World Wildlife Fund, a nonprofit based in Switzerland, counts them there but has not yet released data for this year.
Monarchs west of the Rocky Mountains usually spend the winter along the California coast.
The Xerces Society for Invertebrate Conservation has been counting western overwinter populations in California and Arizona for the last twenty-eight years.
The highest number recorded was one point two million in nineteen ninety-seven.
The organization announced Friday that it counted just nine thousand one hundred nineteen monarchs in twenty twenty-four, a decrease of ninety-six percent from two hundred thirty-three thousand three hundred ninety-four in twenty twenty-three.
The total was the second lowest since the survey began in nineteen ninety-seven.
The record low count was one thousand nine hundred one monarchs in twenty twenty.
The survey noted that a place in Santa Barbara that saw thirty-three thousand two hundred monarchs last winter had only one hundred ninety-eight butterflies this year.
Monarchs across North America face increasing threats.
One is the supply of a plant, milkweed, which they feed on as caterpillars.
Monarch Joint Venture is a nonprofit group based in Minnesota that aims to protect monarchs.
The group said milkweed has been disappearing because of dry weather, wildfires, agriculture, and urban development.
The Xerces Society said pesticides might be on the remaining plants.
Emma Pelton is a biologist with the Xerces Society.
She said it is unclear what caused such a sharp decrease in the western population in just one year.
"The monarch population is already small," she said, "and heat above thirty-seven point seven degrees Celsius in the western states last year might have slowed breeding."
Monarchs suffer when the temperature gets up to thirty-seven point seven degrees Celsius, and any temperatures above forty-two point two degrees Celsius will kill the insects, Pelton said.
The western states saw a heat wave in July with temperatures in some areas well past thirty-seven point seven degrees.
Palm Springs, for example, reportedly reached a record fifty-one point one degrees Celsius on July fifth.
Another heat wave hit Northern California in early October, with several cities breaking heat records.
Pelton said that it is too early to tell what long-term effect the sharp drop might have on the overall western monarch population.
"Insects can reproduce very quickly," Pelton said.
After a low point of one thousand nine hundred one butterflies in twenty twenty, the population recovered to two hundred forty-seven thousand two hundred forty-six insects the following year, an increase of nearly thirteen thousand percent.
The year after that, the survey recorded three hundred thirty-five thousand four hundred seventy-nine monarchs.
"This is bad news," Pelton said of the twenty twenty-four population drop, "but we have seen incredible recovery.
This doesn't mean we're not going to have western monarchs."
The U.S. Fish and Wildlife Service announced in December twenty twenty-four that it wanted to list monarchs as threatened.
Such a move would ban people from killing or transporting them.
The proposed change would also ban property owners from making changes to their property that would make it unusable to the butterflies.
For example, removing all milkweed from a property would be banned by law.
Earthjustice is an environmental nonprofit legal group based in San Francisco.
It asked the Environmental Protection Agency in December twenty twenty-four to require testing of pesticides' effects on insects such as bees, moths, and butterflies.
I'm Jill Robbins.