Host-switching by a bee where its usual pollen host is not present: Diadasia diminuta (Cresson, 1878) (Apidae: Eucerinae: Emphorini) uses the rare mallow, Iliamna latibracteata Wiggins (Malvaceae), as its pollen host in northwestern California and southwestern Oregon

Abstract. Diadasia diminuta (Cresson, 1878) (Apidae: Eucerinae: Emphorini) uses Sphaeralcea St. Hil. (Malvaceae) as its pollen host throughout much of its range in dry habitats of the western U.S. and Central America. Here we document a regional shift in pollen host and habitat. Based on observations of foraging behavior spanning 1–11 years at 17 study sites and analyses of scopal pollen loads collected at nest entrances, we show that D. diminuta relies almost exclusively on a rare, fire-following mallow, Iliamna latibracteata Wiggins (Malvaceae), as its pollen host in relatively mesic forest habitats of northwestern California and southwestern Oregon. The generalized flowers of I. latibracteata attract a wide array of visitors, but D. diminuta is likely the most important pollinator; it was present at 16 of 17 sites and accounted for at least 70% of visits in more than half of 52 site-year visitor surveys. Although D. diminuta is widespread and abundant across most of its range, we argue that the rare disjunct populations studied here may be genetically distinct and perhaps represent a case of incipient speciation, making them worthy of conservation concern.