Over the past 50 years, common butterfly species have been appearing in the Central Valley earlier in the spring.  

Over the winter, many butterfly species become dormant, seeking shelter in places like cracks in tree barks and empty logs. Most butterflies hibernate as eggs, catterpillars, or pupae, while a few hibernate as adults. This dormancy helps them conserve energy and withstand harsh weather conditions when food supply is scarce. As the temperatures rise in the spring, butterflies resume activity once again. Shifts in the timing of butterfly emergence showcase how some butterflies are responding to changing climatic and environmental conditions, which may have far-reaching effects on the ecosystem. For more information, download the Spring Flight of Central Valley Butterflies chapter. 

This graph tracks the “date of first flight,” when butterflies are first seen in the field each spring. The value for each year represents observations across 23 butterfly species (the gray area represents the spread in each year’s value). A negative value means that the butterfly species altogether appear earlier than the historical average, while a positive value indicates they appear later than the average.  

Source: Forister and Shapiro (2003), updated 2021

• Over the past 50 years, butterfly species in the Central Valley (Yolo, Sacramento, and Solano Counties) have been appearing earlier in the spring.  
• Out of the 23 butterfly species examined, 18 have shown earlier dates of first flight. This includes nine species with significant trends, advancing about six days each decade.  

• Butterfly populations are declining across the western United States. By tracking butterfly emergence, researchers can better understand how climate change contributes to their decline.  
• Butterflies that emerge too early, before the flowers bloom, may not find enough nectar. This timing mismatch impacts not only the butterflies themselves, but also the plants that rely on them for pollination.  

• Hotter and drier conditions in the Central Valley, especially in the winter, can affect when butterflies become active.  
• In addition to climate, factors like habitat loss and degradation, pollution, and pesticide use influence when butterflies appear in the Central Valley.  

Butterflies that have been appearing earlier in the Central Valley include, from left to right: Pieris rapae (European Cabbage Butterfly), Vanessa atalanta (Red Admiral), and Papilio rutulus (Western Tiger Swallowtail).

Credit (left to right): John Heyse, Henk Wallays, and T. W. Davies, California Academy of Sciences. Art Shapiro’s Butterfly site 

• UC Davis, Art Shapiro’s Butterfly Site 
• Matt Forister and Art Shapiro, Monitoring Western Butterflies