ARS | Publication request: Specialist and generalist bee-visitors of an endemic beardtongue (Penstemon caryi: Plantaginaceae) of the Big Horn Mountains, Wyoming

Submitted to: Western North American Naturalist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 18, 2011
Publication Date: December 31, 2011
Citation: Tepedino, V.J., Griswold, T.L., Freilich, J.E., Shephard, P. 2011. Specialist and generalist bee-visitors of an endemic beardtongue (Penstemon caryi: Plantaginaceae) of the Big Horn Mountains, Wyoming. Western North American Naturalist. 71: 523-528.

Interpretive Summary: The beardtongues, genus Penstemon, comprise the largest genus of endemic plants in North America. Many of the species in this genus, including Caryi's Penstemon (Penstemon caryi), the subject of this report, are rare and have been highlighted for special conservation treatment by the federal government and/or the states in which they occur. Conservation management depends upon accurate biological information. This study was undertaken to discover the pollination mechanism and pollinators of this rare species. Pollinators may be especially important to rare plants because they are usually instrumental to rare plant reproduction. We found that Cary's Penstemon does depend on native bees to move pollen from flower-to-flower, thereby effecting fruit and seed production, that those bees are mostly large, non-social species, and that many are specialists that collect pollen only from Penstemon flowers. Thus, P. caryi and the bees that service it are tied into a mutually dependent association. However, one of the dominant flower-visitors was a species that visits many other types of plants and does not specialize on Penstemon. This species may be a particularly important pollinator when specialists are absent.

Technical Abstract: Insect exclusion from the flowers of Penstemon caryi showed that fruits are not produced unless pollen is moved between flowers by pollinators. We recorded over 30 species of bees visiting the flowers, about a third of which were common. Flower-visitors were primarily pollen-collecting female bees, most of which also carried pollen on areas of the head that strongly implicated them as pollinators. While larger bees were generally more common on the flowers than smaller bees, there appeared to be no difference between size groups in 1) the percent of females collecting Penstemon pollen; 2) the percent of collected pollen that was Penstemon pollen, or 3) the percent of individuals that carried pollen in areas likely to effect pollination. Two long-tongued species, Anthophora ursina (Apidae) and Osmia brevis (Megachilidae), dominated the collections. These two species have very different host association with Penstemon flowers: O. brevis is a Penstemon oligolege which visits many species of beardtongue in the western U.S., while the polylege A. ursina utilizes a wide spectrum of plant species for pollen and seems to express fidelity to Penstemon flowers only when those flowers are abundant. There was no evidence that oligoleges are superior to polyleges as pollinators of P. caryi.