Reproductive Ecology and Bee Associates of Packard’s Milkvetch (Astragalus cusickii var. packardiae), A Rare Plant Endemic to Southwestern Idaho

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Abstract:

Conservation of rare plants is dependent on identifying mechanisms that limit their population or persistence. Successful pollination of flowers is one life history trait that is often limiting in plants occurring in fragmented landscapes. We studied pollinator associates and seed production in Astragalus cusickii var. packardiae (Packard’s milkvetch), which is imperiled as a result of non-native grasses, increased wildfire, recreational OHV (off-highway vehicle) use, and livestock grazing. In May and June 2012, we used pollinator exclusion bags to examine the breeding system of Packard’s milkvetch. We collected and videotaped floral visitors to determine the makeup of the associates and the visitation rate to flowers. Our study demonstrates that Packard’s milkvetch is highly dependent on pollinators for reproductive success. Plants that were excluded from pollinator visitation produced a mean of 0.11 ± 0.09 SE seeds/fruit, while plants open to pollinator visitation produced 2.73 ± 0.26 SE seeds/fruit--a highly significant difference (Wilcoxon signed-rank test; n = 23, p < 0.001). We collected a total of 203 floral visitors of A. cusickii var. packardiae. The order Hymenoptera (bees and wasps) made up 86% of the visitors, of which wasps were merely 1% of the collected insects. Lepidoptera (butterflies), Coleoptera (beetles), and Diptera (flies) accounted for the remaining 14% of visitors. The bee associates that we found consisted of solitary bees and were markedly composed of Osmia bee species. Visitation rates of Packard’s milkvetch were 3.8 visits/flower/hour, which is within the range of other studied Astragalus species. We also observed a high rate of predispersal seed predation. Packard’s milkvetch floral visitors are susceptible to disturbance from non-native grasses, wildfire, OHV activity, and possibly livestock use. Since seed production in Packard’s milkvetch is limited by native bees, future management plans for Packard’s milkvetch should incorporate the conservation of native bee habitat.

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Reference Code: U13FUL01IDUS <br>

Full Citation: Fulkerson, J.R., and C.L. Kinter. 2013. Reproductive Ecology and Bee Associates of Packard’s Milkvetch (Astragalus cusickii var. packardiae), A Rare Plant Endemic to Southwestern Idaho. Idaho Natural Heritage Program, Idaho Department of Fish and Game, Boise. 33 pp. plus appendices.. <br>

Location: ELECTRONIC FILE - BOTANY: PLANT SPECIES: { Astragalus cusickii var. packardiae } <br>

Keywords: Astragalus cusickii var. packardiae, pollinators, OHV use, rare plant conservation, Osmia, predispersal seed predation <br>