Petals and leaves: quantifying the use of nest building materials by the world’s most valuable solitary bee

The alfalfa leafcutter bee (“ALCB”, Megachile rotundata) is a solitary, cavity-nesting bee native to Eurasia, managed as an agricultural pollinator of alfalfa and several other crops in North America since the 1940s (Pitts-Singer and Cane 2011). The most intensively managed and economically valuable solitary bee worldwide, it serves as a model organism for studying solitary bee biology (Pitts-Singer and Cane 2011). Understanding this species’ nesting behavior is critical to its propagation for commercial pollination services. As its common name suggests, female ALCBs construct their nests in above-ground holes using carefully clipped pieces of leaves (Pitts-Singer and Cane 2011).

Methods
At University of California, Riverside (33.965 °N, 117.341 °W), we erected 16 mesh field cages (5.8 m3), each of which contained a fixed ratio of five plant species in 2-L pots (Appendix S1: Table S1), totaling 30 pots per cage: alfalfa (Medicago sativa) and the California-native ornamentals Baileya multiradiata (desert marigold; Fig. 1A), Sphaeralcea ambigua (desert mallow; Fig. 1B), Erigeron glaucus (seaside daisy), and Phacelia tanacetifolia (tansy phacelia). Foraging ALCBs visited all five plant species for pollen and/or nectar. In each cage, we supplied a nesting block (Fig. 1C; face 23 cm x 15 cm; height: 1 m) filled with 60 paper straws (diameter 5 mm, length 12.7 cm), facing southeast. In mid-June 2019 and 2020, we introduced 20 female and 50 male ALCBs to each cage. After six weeks, all straws were collected, stored at 5 °C, then dissected the following spring. We recorded all cells constructed in each nest (straw), the contents of each cell (presence of brood), and the material(s) used to construct each cell. Leaves and petals from all five plant species were identifiable by color and trichome morphology.