Increasing agricultural habitat reduces solitary bee offspring number and weight in apple orchards through reduced floral diet diversity and increased fungicide risk

1. Threats to bee pollinators such as land use change, high pesticide risk, and reduced floral diet diversity are usually assessed independently, even though they often co-occur to impact bees in agroecosystems.

2. We established populations of the non-native mason bee O. cornifrons at 17 NY apple orchards varying in proportion of surrounding agriculture and measured floral diet diversity and pesticide risk levels in the pollen provisions they produced. We used path analysis to test the direct and indirect effects of different habitats, diet diversity, and pesticide risk on emergent female offspring number and weight.

3. Our results showed that high proportions of agricultural habitat surrounding bee nests indirectly reduced the number of female offspring produced, by reducing floral diet diversity in pollen.

4. When proportion agriculture surrounding bee nests was high, bees collected increased proportions of Rosaceae in their pollen provisions, which marginally (0.05<p<0.1) increased fungicide risk levels in pollen, which, in turn, marginally reduced female offspring weight. In contrast, female offspring weight increased as proportion surrounding open habitat (wildflowers, grassland, pasture) increased, but this effect was not influenced by proportion Rosaceae or fungicide risk levels in pollen.

5. Synthesis and Applications: To promote healthy O. cornifrons populations in apple, we must maintain floral resource diversity and open habitats, while reducing fungicide risk levels and agricultural habitats. More broadly, our results show that land use change, in the form of increasing agricultural habitat, can negatively impact bee populations in agroecosystems indirectly through multiple, simultaneous threats. We must strive to understand these complex interactions between simultaneous threats to maintain healthy bee populations in agroecosystems, where we rely on them for pollination.

Methods
Please refer to the "Materials and Methods" section of the associated article, as well as Appendix S1 and S2 of the Supporting Information, where you will find all information related to methodogy and data processing.