The orange-and-black insects are a popular site along the coast each winter.
The non-profit conservation group the Xerces Society says the western monarch population has fallen 74 percent in just the last two decades.
Biologist Emma Pelton says the decline is even deeper than expected. "The monarch butterfly is especially concerning because it used to be one of the most common, and most widely distributed butterfly species," Pelton told KCBS. "So if that one is declining, it's a question of what other insect species are declining."
Pelton says there are likely several factors behind the decline including more development in monarch habitats.
"Insecticides and herbicides aren't just agricultural issues, they're also widely used in gardens and homes, and often those are at really high rates – higher rates than agriculture," Pelton said.
Environmental groups are pushing for monarchs to be classified as endangered but a decision from the federal government is not likely until 2019.
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