Small Decisions, Big Consequences for Monarchs

New research from University of California, Davis, published in the journal Oecologia, offers a realistic look at the continued decline of the monarch butterfly, a trend that has persisted since the 1990s. For farmers and land managers across the Southeast, where hot summers and warming ground temperatures are already a fact of life, the findings raise important questions about pollinator survival in agricultural landscapes.

The study finds that monarch caterpillars change how they respond to threats depending on temperature, but that those adjustments can break down under extreme heat. Researchers 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 increased, 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 research especially relevant for Southeastern agriculture.