Monarch butterflies in Northern California are adapting to a changing climate by embracing an unexpected strategy: breeding in winter. This shift could be key to the iconic insect’s survival, according to a new study published in the Journal of the Lepidopterists’ Society.
The study, led by David James, associate professor of entomology at Washington State University, found that monarch pupae that developed in an urban environment in the San Francisco Bay Area during the winter of 2021/2022 had a survival rate of about 50%. This finding builds on a previous study that documented for the first time winter breeding activities by monarchs in the region during the winter of 2020/2021, likely due to climate warming.
“We are witnessing monarchs developing and adapting to new environmental conditions,” James said. “We can learn a lot from what they do, and this will inform our strategies to provide the resources they need to be successful.”
Over the past five years, wintering monarch populations on California’s coast have seen dramatic fluctuations, from 192,624 in 2017 to an all-time low of just 1,899 in 2020, before recovering to 247,246 and 335,479 butterflies in 2021 and 2022, respectively. There has been a significant increase in the number of winter-breeding monarchs feeding on non-native milkweed in the urban Bay Area. The study by James and his team is the first to estimate the viability of monarch pupae during winter.
For the analysis, Maria Schaefer, a citizen scientist who has been working with James since 2020, monitored more than 100 wild monarch pupae at the Googleplex campus near Palo Alto during the winter of 2021/2022. Despite some losses due to landscaping activities, the study found a 49.4% success rate in pupal budding, the process by which the butterfly emerges as an adult. Additionally, the researchers found that pupae can survive and produce adults for up to seven weeks in winter, compared to just ten days in summer.
“Given that winter is a suboptimal breeding environment, these results are promising,” James said.
The study also explored the use of exuviae, the shed skin of the pupae, to detect the presence of the parasite Ophryocystiselektroscirrha. The analysis suggested that approximately 70% of the winter breeding population was likely infected with the parasite, highlighting the need for further research into its impact and that of other stressors on western monarch populations under natural conditions.
The emergence of winter-breeding monarchs indicates a possible adaptation to warmer winters and provides an alternative to the traditional overwintering of non-breeding butterflies in locations such as Pacific Grove, Santa Cruz and Pismo Beach. To support these new behaviors, conservation practices may need to adapt, James noted. For example, the timing of pruning of the area’s monarch’s main food source, non-native milkweed, which currently occurs in early winter, could be shifted to late summer to support healthier winter breeding populations.
“Currently, conservation guidelines dictate that ornamental milkweed, the one that winter-breeding monarchs primarily use, should be cut down in winter to minimize parasitic infections,” James said. “This is clearly bad for winter breeding. We suggest that these guidelines should be adjusted to reduce milkweed production in late summer rather than late fall, which should still minimize parasitic infection while increasing the availability of plants for monarch winter breeding.”
Looking ahead, James and Schaefer will continue to monitor winter-breeding monarch populations in the South Bay Area to collect more data. Their hope is that these monarchs will play an important role in ensuring the sustainability of the country’s western monarch population.
