Monarch butterflies in Northern California are exhibiting a surprising adaptation to climate change: winter breeding. This behavior might be essential for their survival, as outlined in a recent study published in the Journal of the Lepidopterists’ Society.
Research Overview
David James, an associate professor of entomology at Washington State University, led a study that uncovered a 50% survival rate for monarch pupae that developed during the winter of 2021/2022 at an urban site in the San Francisco Bay Area. This builds on earlier research documenting winter breeding by monarchs in the same region during the winter of 2020/2021, likely driven by rising temperatures.
Monarchs and Climate Adaptation
“Monarchs are evolving to meet new environmental challenges,” said James. “Their responses offer valuable insights that can help shape our conservation strategies to support their success.”
Fluctuating Populations
The past five years have seen dramatic shifts in the overwintering populations of monarchs on the California coast. The population dropped from 192,624 in 2017 to a historic low of 1,899 in 2020, before recovering to 247,246 in 2021 and 335,479 in 2022. Alongside this, there has been a notable rise in winter-breeding monarchs feeding on non-native milkweeds in the urban Bay Area.
Study Methodology and Key Findings
Citizen scientist Maria Schaefer, collaborating with James since 2020, observed over 100 wild monarch pupae at the Googleplex campus near Palo Alto during the winter of 2021/2022. Despite some pupae being lost to landscaping activities, the study reported a 49.4% success rate in pupal eclosion, the process where a butterfly emerges from its chrysalis. The researchers also found that winter pupae could survive and produce adults for up to seven weeks, compared to just 10 days during the summer.
Encouraging Results in Challenging Conditions
“Winter is not ideal for breeding, so these findings are encouraging,” James noted.
Parasite Presence
The study also used exuviae, the shed skin of the pupae, to detect the parasite Ophryocystis elektroscirrha. The analysis indicated that about 70% of the winter-breeding population might be infected with the parasite, highlighting the need for further investigation into how parasites and other stressors affect western monarch populations.
Adjusting Conservation Practices
The emergence of winter-breeding monarchs suggests a potential adaptation to warmer winters, providing an alternative to the traditional overwintering sites in places like Pacific Grove, Santa Cruz, and Pismo Beach. James recommended altering conservation practices to support this new behavior, such as changing the timing of pruning non-native milkweed, a crucial food source for monarchs. Instead of early winter, pruning could be done in late summer to foster healthier winter-breeding populations.
Revising Guidelines for Conservation
“Current guidelines advise cutting down ornamental milkweeds during winter to reduce parasite infection,” James explained. “This approach is detrimental to winter-breeding monarchs. We suggest modifying these guidelines to cut back milkweed in late summer rather than late fall. This would still minimize parasite infection while ensuring plants are available for winter breeding.”
Future Research Directions
Looking forward, James and Schaefer will continue their monitoring of winter-breeding monarch populations in the South Bay Area to gather more data. Their aim is that these monarchs will significantly contribute to the sustainability of the western monarch population in the United States.
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