Modeling long-term occupancy trends in Indiana bumble bees (Hymenoptera: Apidae)

Bumble bees (Hymenoptera: Apidae: Bombus spp.) are crucial pollinators of many natural and agricultural systems, but some species are undergoing declines in response to diverse stressors, including climate change and habitat loss. For example, some species (eg Bombus affinis) have undergone significant range contractions, while others have remained stable or even expanded their range, and the factors driving this disparity remain unclear. We used records of preserved specimens from over 35 unique sources to study occupancy trends in 9 common Indiana bumble bee species from the early 1900s until 2022. We created 2 occupancy models to estimate the effects of crop richness, crop extent, and temperature on county-level bumble bee occupancy. We found that the probability of occupancy increased over time for 4 species (Bombus bimaculatus, Bombus citrinus, Bombus fervidus, and Bombus griseocollis). Responses to temperature were highly variable among species, with Bombus fervidus and Bombus vagans having a significantly lower likelihood of occupancy with higher temperatures. Crop richness and crop extent did not have a significant effect on occupancy. With future climate warming, some bumble bee species are more likely to persist than others, which could lead to significant restructuring in pollinator communities in Indiana. Our dataset al.o reveals that only 47 out of 92 Indiana counties have specimen records post-1990, highlighting the need for expanded sampling throughout the state. With declines already documented in the Midwest and beyond, it is crucial to continue monitoring our pollinators to understand how to best conserve their populations.