Foliar traits, taxonomic preferences, and foraging effort by <i>Osmia bruneri</i> bees acquiring leaf pulp for nest construction (Hymenoptera: Megachilidae)

Abstract Several genera of osmiine bees (Osmia and Hoplitis) are unique among animals for building nests using leaf pulp (mastic). Only scattered anecdotes indicate their leaf choices. To learn more, a large captive nesting population of cavity-nesting Osmia bruneri was offered foliage from 46 plant species (18 families) differing in scent, pubescence, latex, and cutting resistance. The choice trials presented four species over 12 min, counting approaches by females, landings, foliage bites, and successful pulping bouts. Daily and lifetime efforts to acquire pulp balls were estimated. Leaf pulping by O. bruneri was not random. Foliage of some families was consistently rejected (e.g. Asteraceae); others were regularly pulped (e.g. Malvaceae). Anecdotal usage of some leaf species was confirmed. Bees preferred leaves bearing stellate trichomes, but dense pubescence or abundant long trichomes interfered with pulp ball formation. Scented foliage was also rejected. Leaf thickness and leaf fracture (cutting) force were not correlated with pulping preference among the more flexible leaves tested. Pulp balls were acquired with ∼150 bites in 40–60 s. Over a lifetime, &amp;gt;100 pulp balls were acquired through &amp;gt;15 000 bites; mandibles were only moderately worn. Some preferred leaf species are readily grown in greenhouses or outside for leaf-pulping Osmia used in commercial pollination.