California Monarchs

September 23, 2021

Question: Are there two populations of monarch butterflies in California?

Answer: The bottom line is that it’s unclear, but we continue to track the research, as we are responsible for their conservation and successful migration.

In addition to the declining population of migratory western monarch butterflies, scientists are seeing an increase in resident monarchs that breed year-round. Resident monarchs have been reported in higher numbers in coastal areas from San Diego to the San Francisco Bay Area. Historically, the migratory monarch population overwintered in coastal groves from October to March. During the rest of the year, monarchs migrated and bred throughout states west of the Rocky Mountains. In the past, winter breeding may have occurred at a low level. However, over the past few years it has expanded concurrent with the decline of the migratory population. A 2021 scientific study(opens in new tab) estimated that there were approximately 12,000 resident monarchs — more than six times the remaining migratory population.

Scientists are currently looking into which factors are influencing the transition to year-round breeding. One hypothesis is that the expansion of nonnative tropical milkweed (Asclepias curassavica) in home gardens may induce winter breeding. Climate change could also play a role as warmer winter weather exposes monarchs to temperatures that can cause them to break reproductive diapause early.

It is unclear whether resident monarchs represent a separate population from the migratory population or if there is intermixing. If they are distinct populations, questions remain over whether the resident and migratory populations can persist side by side. Finally, scientists are still trying to determine if the transition to year-round breeding represents a persistent trend or is a short-term adaptation to local conditions.

As a trustee agency, the California Department of Fish and Wildlife (CDFW) is tasked with protecting California’s flora and fauna, including terrestrial invertebrates like monarch butterflies for their ecological value and enjoyment by the public. California Fish and Game Code (FGC) section 1021 directs CDFW to “take feasible actions to conserve monarch butterflies and the unique habitats they depend upon for successful migration.”