A report released on July 8 by the Xerces Society shows a sharp two-decade decline in the number of monarchs which overwinter along the California coast. The report, State of the Monarch Butterfly Overwintering Sites in California, provides an analysis of western monarch population trends. A comparison of the average number of butterflies overwintering in California during the periods 1997–2001 and 2010–2014 shows a decline of 74 percent, a figure that is comparable to declines documented at monarch overwintering sites in Mexico.

The report presents a list of the fifty California overwintering sites most in need of protection and management, based on the severity of butterfly declines at each site and their importance to the remaining population. Listed sites span the monarch’s overwintering range along the Pacific coast and the East Bay of San Francisco, including Pismo Beach and Morro Bay Golf Course in San Luis Obispo County, Ellwood Main in Santa Barbara County, Pacific Grove Sanctuary in Monterey County, Lighthouse Field State Beach and Moran Lake in Santa Cruz County, and San Leandro Golf Course in Alameda County.

The loss of breeding habitat, especially milkweed, due to the increased use of genetically modified herbicide-resistant crops, is an important factor influencing the decline of monarchs in the eastern United States. Evidence for breeding habitat loss is less clear in the West, and the continent-wide decline in monarch numbers may be compounded by other stressors such as disease, increased frequency of drought driven by climate change and the widespread use of neonicotinoid insecticides, as well as destruction of their overwintering habitat.

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