iNaturalist records reveal shifts in phenology of caterpillar (Lepidoptera) activity in response to warmer spring temperatures.

Climate change is causing shifts in the phenology of taxa across the globe, with phenological shifts detected in species, including but not limited to, plants, insects, and birds. However, changes in the phenology of caterpillar activity have been relatively understudied, despite the importance of caterpillar activity for conservation, plant and wildlife ecology, forest management practices, and agriculture. To improve understanding of the potential impacts of climate change on caterpillar phenology, we modeled shifts in the onset date of observable activity, termination date of observable activity, and duration of observable caterpillar activity in 147 lepidopteran species using citizen science records collected across the United States and Canada. We found that in response to increased spring temperatures, caterpillars are advancing the onset and termination of activity by an average of 2.5 and 1.1 days (d) per °C, respectively, and lengthening their activity periods by an average of 1.5 d per °C. These shifts in the phenology of caterpillar activity may lead to changes in the exposure of caterpillars to adverse environmental conditions or predation, as well as changes in the seasonal timing of herbivory pressure for caterpillar host plants, prey availability for insectivorous wildlife, and the optimal timing of pest management practices.