Many bee species are changing the timing of their seasonal activity, with most studied species exhibiting mean advances of up to 1 day per decade. Past studies, however, have focused almost exclusively on spring-active species, which may not be representative of all wild bees. Additionally, analysis of mean phenological change may not reflect the degree of change occurring at the beginning or end of a phenological event (e.g., a bee species’ flight season). As a result, species with similar mean rates of phenological change could still experience different changes in the overall duration of activity. In this study, we describe changes in mean timing and duration of activity from 1852–2019 for 50 species of solitary bees native to the northeastern United States. We compiled 18,377 unique museum specimen records based on collection date and location. We assessed differences in patterns of phenological change among three life history traits: flight season (spring vs. fall), diet specialization (generalist vs. specialist), and nesting location (above vs. below-ground). We used generalized linear mixed models to analyze changes in mean date and quantile regression to analyze changes in flight period duration.
Results/Conclusions:
Mean phenological change differed significantly between taxa with different flight seasons, with those active in spring advancing, on average, 0.39 days per decade and fall species delaying their phenology, on average, 0.64 days per decade (X2 = 6.243, df = 1, p = 0.012). When assessing differences in diet specialization, we found no differences in the rate of phenological change between spring-active pollen generalists (–0.35 days/decade) and spring-active pollen specialists (–0.46 days/decade) (X2 = 0.112, df = 1, p = 0.738). Mean date of timing for above-ground nesting species was changing faster (–0.87 days/decade) compared to below-ground nesters (–0.36 days/decade), although not significantly so (X2 = 3.730, df = 1, p = 0.053). No significant change in flight season duration was found in any trait group (average increase in duration of 0.09 days/decade), suggesting that early emerging individuals are changing their phenology at the same rate as late emerging individuals, regardless of life history. Taken together, these findings reveal diverse phenological responses among wild bees and prompt further study into the mechanisms behind these changes.