Turley NE, Biddinger DJ, Joshi NK, López-Uribe MM. 2022. Six years of wild bee monitoring shows changes in biodiversity within and across years and declines in abundance. Ecology and Evolution.   

Wild bees form diverse communities that pollinate plants in both native and agricultural ecosystems making them both ecologically and economically important. The growing evidence of bee declines has sparked increased interest in monitoring bee community and population dynamics using standardized methods. Here, we studied the dynamics of bee biodiversity within and across years by monitoring wild bees adjacent to four apple orchard locations in Southern Pennsylvania, USA. We collected bees using passive Blue Vane traps continuously from April to October for six years (2014-2019) amassing over 26,000 bees representing 144 species. We quantified total abundance, richness, diversity, composition, and phylogenetic structure. There were large seasonal changes in all measures of biodiversity with month explaining an average of 72% of the variation in our models. Changes over time were less dramatic with years explaining an average of 44% of the variation in biodiversity metrics. We found declines in all measures of biodiversity especially in the last 3 years, though additional years of sampling are needed to say if changes over time are part of a larger trend. Analyses of population dynamics over time for the 40 most abundant species indicate that about one third of species showed at least some evidence for declines in abundance. Bee family explained variation in species-level seasonal patterns but we found no consistent family-level patterns in declines, though bumble bees and sweat bees were groups that declined the most. Overall, our results show that season-wide standardized sampling across multiple years can reveal nuanced patterns in bee biodiversity, phenological patterns of bees, and population trends over time of many co-occurring species. These datasets could be used to quantify the relative effects that different aspects of environmental change have on bee communities and to help identify species of conservation concern. 

Study site

Our study took place between 2014 and 2019 at the Pennsylvania State Fruit Research and Extension Center in Adams County, Pennsylvania, USA (39.935226, -77.254530) and nearby apple orchards. This site has an average yearly rainfall of 112 cm, average summer temperature ranging from 16 °C to 28 °C, and average winter temperatures of -5 °C to 5 °C (Biddinger et al., 2018). The landscape is hilly with well-drained soils and the broader area is approximately 56% broadleaf forest fragments, 25% pastureland, 9% developed areas, and 8% commercial orchards (Biddinger et al., 2018). All orchards were managed under growers’ choice conventional pest management programs that use pesticide classes including insect growth regulators, anthranilic diamide, tetramic acid, microbials, and neonicotinoid insecticides (Biddinger et al., 2018). We sampled bees at 8 locations adjacent to 4 different active apple orchards. Sampling locations were within 150 m of orchards and 250 m of a forest fragment (Figure 1), which have diverse plant and pollinator communities (Kammerer et al., 2016). Often orchards rely, in part, on managed honey bee colonies for pollination, which have the potential to negatively impact native bee populations (Mallinger et al., 2017). However, our sampling sites did not have managed honey bee hives within 2 km and growers managing the adjacent orchards had not rented honey bees for at least 15 years.

Our bee monitoring traps were located within perennial wildflower strips approximately 50 m x 10 m in size that were sown between 2-3 years before the beginning of our study. Wildflower sites used in this study were established and managed using the specific planting guidelines developed by the Pennsylvania USDA-NRCS and the Xerces Society for Invertebrate Conservation (NRCS, 2011). They were sown with 21 species of native forbs and grasses sourced from a local native seed supplier (Ernst Conservation Seed, Meadville, PA 16335). All wildflower sites were mowed once a year and received spot sprays of common selective herbicides to control non-native plants as needed.

Bee collections

We trapped bees continuously from April to October from 2014 to 2019 using Blue Vane traps (BanfieldBio Inc., Woodinville WA). A previous study in this region showed that Blue Vane traps collect a higher abundance and total richness of bees than colored bowl traps, also called pan traps (Joshi et al., 2015). Although the overall community composition of bees captured in Blue Vane traps was different from bowl traps, nearly all species were more likely to be captured in Blue Vane traps over bowls, except some Andrena and Lasioglossum species (Joshi et al., 2015). In our study, Blue Vane traps were filled with about 7 cm of 60% ethylene glycol (Supertech® Wal-Mart Stores, Inc., Bentonville, AR), hung from posts about 1.5 m off the ground. At each of our 8 locations, we placed 2 traps 25 m apart. Traps were left outside continuously from April to October every year and traps were replaced each year in case wear over time decreased their attractiveness. Each week, all specimens were removed and the ethylene glycol was replaced. Bee specimens were separated from other insects collected in the traps and stored in 70% alcohol until they were washed, pinned, and labeled. All bees were identified to the species level except 14 individuals that were removed from analyses because of uncertain species-level identification. For bee identification, we used published dichotomous keys (Mitchell, 1960, 1962; Michener et al., 1994, Michener, 2000) and various interactive bee identification guides available at Discover Life (http://www.discoverlife.org). Species identifications were conducted by David Biddinger (Pennsylvania State University), Robert Jean (Senior Entomologist, Environmental Solutions and Innovations, Inc.), Jason Gibbs (University of Manitoba), and Sam Droege (United States Geological Survey). All specimens from this study are stored at the Pennsylvania State Fruit Research and Extension Center, Biglerville, PA, or the Frost Insect Museum at Pennsylvania State University, University Park, PA.