Phenology in adult and larval Lepidoptera from structured and unstructured surveys across eastern North America

Caterpillars (larval Lepidoptera) are an essential link in trophic networks of forest ecosystems, as they serve as herbivores of vegetation and a food source for many organisms. Phenological mismatches between caterpillars, host plants, or predators may have negative effects across multiple trophic levels. Seasonal timing of caterpillar emergence and peak occurrence may be impacted by climate change, however, studying caterpillar phenology at broad spatial scales is challenging due to lack of data availability. Here, we examine two sources of caterpillar observations, opportunistic records from iNaturalist and structured surveys of forest caterpillars, and compare whether phenology patterns in these datasets are consistent across larval datasets and with more numerous records of adult butterflies. Despite substantial taxonomic differences between these three datasets, we found concurrence in patterns of early and late years in spring onset between datasets. However, the datasets do differ in how well they capture phenological responses to warmer spring temperatures. More data-rich iNaturalist caterpillar and adult butterfly records may provide a reasonable proxy of interannual deviations in forest caterpillars, however, expansions in structured survey efforts are needed to capture changing patterns in other ecologically important measures such as abundance and biomass.