Western monarch butterflies head south from the Pacific Northwest to California each winter, returning to the same places and even the same trees, where they flock to keep warm.
Monarchs generally arrive in California in early November and are spread across the country once warmer weather arrives in March.
On the eastern side of the Rocky Mountains, another population of monarchs travel from southern Canada and the northeastern United States for thousands of miles to winter in central Mexico.
Scientists estimate that the monarch population in the eastern US has fallen about 80 percent since the mid-1990s, but the decline in the western US has been even more pronounced.
The Xerces Society, a nonprofit environmental organization that focuses on invertebrate conservation, recorded about 29,000 butterflies in its annual survey last winter.
That was not much different from the count last winter, when a record low of 27,000 monarchs was posted.
But this year’s tally is lousy. At the iconic monarch wintering sites in the city of Pacific Grove, volunteers didn’t see a single butterfly this winter.
Other well-known locations, such as the Pismo State Beach Monarch Butterfly Grove and Natural Bridges State Park, were only home to a few hundred butterflies, the researchers said.
“These sites are typically home to thousands of butterflies, and their absence this year was heartbreaking for volunteers and visitors who flocked to these sites hoping to catch a glimpse of the impressive groups of monarch butterflies,” said Sarina Jepsen, director of endangered species in the Xerces Society.
Scientists say butterflies are at critically low levels in western states due to the destruction of their milkweed habitat along their migration route as homes expand into their territory and pesticide and herbicide use increases. .
Researchers have also noted the effect of climate change.
Along with agriculture, climate change is one of the main drivers of the monarch’s threat of extinction, disrupting an annual migration of 4,828 kilometers (3,000 miles) synchronized with spring and the blooming of wild flowers.
Massive wildfires across the western US last year may have influenced their reproduction and migration, the researchers said.
A 2017 study by researchers at Washington State University predicted that if the monarch population fell below 30,000, the species would likely become extinct in the next few decades if nothing is done to save them.
Monarch butterflies lack state and federal legal protection to prevent their habitat from being destroyed or degraded.
In December, federal officials declared the monarch a “candidate” for threatened or endangered status, but said no action would be taken for several years due to the many other species awaiting such designation.
The Xerces Society said it will continue to seek protection for the monarch and will work with a wide variety of partners “to implement science-based conservation actions that are urgently needed to aid the iconic and beloved migration of the western monarch butterfly.”
People can help the colorful insects by planting early-blooming flowers and milkweed to feed migratory monarchs on their way to other states, the Xerces Society said.