When Heat Breaks an Instinct

New research from University of California, Davis, published in the journal Oecologia, offers a grounded look at the continued decline of the monarch butterfly, a trend that has persisted since the 1990s. Rather than offering simple answers, the study highlights how warming conditions are changing survival outcomes in ways scientists are still working to understand.

The research finds that monarch caterpillars adjust how they respond to threats depending on temperature, but that those adjustments can break down under extreme heat. The experiments focused on caterpillars feeding on milkweed plants. When threatened, caterpillars often drop from the plant to escape. At cooler temperatures, that response was common and relatively safe. As ground temperatures rose, dropping became less frequent. Then, unexpectedly, once temperatures reached about 27 degrees Celsius, or 80 degrees Fahrenheit, dropping increased again, even though survival rates were low.

“The idea is that the temperature is messing with their biology,” said Prabhjot Singh, lead author of the study.

Louie Yang, a UC Davis professor, said caterpillars may escape danger only to land on dangerously hot ground. Monarchs are important pollinators and indicators of ecosystem health, making the findings relevant for agriculture. To learn more, visit https://caes.ucdavis.edu/news/how-hotter-days-increase-risks-monarch-caterpillars