Many bee species restrict pollen collection to a narrow suite of floral hosts. Recognizing which species are pollen specialists and which hosts they require is challenging because even specialists may visit many floral types for nectar. Furthermore, pollen collecting behavior can be cryptic and may reflect temporal local preferences rather than evolved behaviors. Evidence of pollen fidelity at broad geographic and temporal scales is typically used for asserting pollen specialization, but manipulative experiments can also help distinguish opportunistic behaviors from evolved foraging requirements. Osmia texana is a widespread bee species in North America but little studied, especially in the eastern part of its range. It is considered an oligolege of Asteraceae, but in our region is most commonly observed on different types of thistle. We used an emergent population from Capon Bridge West Virginia in a large screenhouse with diverse floral resources to examine host plant visitation by females, pollen composition in brood cells, and mate searching behavior by males. Available floral resources included 5 species of Asteraceae (the thistle Carduus acanthoides, 3 species in the Helianthiae -Heliopsis helianthoides, Helianthus maximiliani and Helianthus annuus- and Erigeron annuus) and an additional 25 plant species.
Results/Conclusions
Females predominately visited Carduus acanthoides, rarely visited Heliopsis helianthoides, and ignored other species, even when pollen became scarce on Carduus. Pollen provisions from 15 brood cells in 8 nests were almost exclusively Carduus acanthoides: 12 were pure Carduus and the other 3 were 98% Carduus. Males mate searched on both Carduus and Heliopsis but did so over twice as frequently on Carduus. We conclude that Osmia texana, at least in our area, has a strong, density-independent preference for Carduus over other Asteraceae tested. Natural field observations make it likely that this preference extends to other types of thistle, including both native and exotic species in the genera Carduus and Cirsium. We sketch out additional manipulations that need to be done in this and other cases to further distinguish strong preferences from evolved dependence on particular hosts.
