Researchers at an environmental group have called the number of Western monarch butterflies migrating along the California coast "disturbingly low."
A recent Xerces Society count has registered less than 30,000 butterflies, a decline of 86% since 2017.
By comparison, the group in 1981 had more than 1 million Western monarchs wintered in California, the San Francisco Chronicle reported.
The Xerces Society conducts the annual Thanksgiving and New Year counts and was not sure what caused the numbers to fall. He said there is no substantial evidence of a delayed migration and butterflies are not being reported in other parts of the country.
A 2017 study by researchers at Washington State University found that the species is likely to be extinct in the coming decades if nothing is done to save it.
Scientists say butterflies are threatened by pesticides, herbicides and destruction along their migratory route. They also noted the impacts of climate change.
Researchers at the University of Michigan and Stanford University found that the car's carbon dioxide and factory exhaust reduced the natural toxin from the syringes that caterpillars use to fight parasites.
Western monarch butterflies are typically seen from November through March in forest woods along the California coast.