A polylectic species, with a preference for Asteraceae and Fabaceae pollen. Praz (2017) reports that Megachile rotundata is probably polylectic with a preference for Fabaceae
Mueller & Bansac (2004) state that M. rotundata is polylectic harvesting pollen from at least six different plant families including Asteraceae but not Cardueae (Westrich,1989; Rust et al., 2003).
Banaszak & Romasenko (1989), copied by Stanisavljević (2000), list Cirsium, Helianthus, Onopordon, Carduus, Centaurea (Asteraceae); Medicago, Melilotus, Sophora, Onobrychis, Lotus, Genista, Ononis (Fabaceae); and Thymus (Lamiaceae) as forage sources.
In the US it shows a preference for pollen from Medicago sativa (Fabaceae) flowers suggest preferential oligolectic behaviour (Stephen & Torchio 1961). Pollen from native and introduced flowers including Asclepias (Asclepiadaceae); Cosmos (Asteraceae); Lotus sp. (Fabaceae) and Veronica (Scrophulariaceae)(Krombein et al. 1970). Uses alfalfa for nest building and foraging. Also feeds on nectar (Richards 1984).