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Groundplum milk-vetch (Astragalus crassicarpus Nutt.) is a native legume found on gravelly to sandy prairie uplands throughout central North America. The large and fleshy fruits, or pods, of this species were consumed by indigenous people and European settlers, and are cached by rodents. Our previous research on native legumes of the northern Great Plains indicated seed predation by insects, notably bruchid beetles belonging to the genus Acanthoscelides Schilsky, was a dominant factor determining viable seed production in natural and artificial plant assemblages. However, although we have studied the reproductive biology of several species of Astragalus L., none had fleshy pods. Therefore, the objective of this study was to quantify the factors influencing viable seed production in natural populations of GPMV in South Dakota. Mature pods were collected from two natural populations on the South Dakota State University Oak Lake Field Station during July 2000 and from one natural population on the Grand River National Grassland near Lodgepole, SD, in July 2003. Pods were dissected and number of ovules that: 1) produced viable seed, 2) were unfertilized, 3) produced aborted seed, and 4) produced seeds that were predated by larvae of the bruchid beetle Acanthoscelides fraterculus (Horn) (Coleoptera: Chrysomelidae: Bruchinae) were recorded for each of the two valves of individual pods. Our results indicated average seed set, i.e., percent of ovules that produced viable seed not predated, was about 55%, averaged across populations. Unfertilized ovules were 42% of the total. Predation by the bruchid beetle occurred in about 2% of the developed seeds, with aborted seed making up the rest of the ovule total. About 35% of pods infested contained larvae, or adults that were immured within the pod while attempting to exit, presumably due to the thick and leathery layers of the ovary wall. Predispersal seed predation was a relatively minor factor in the reduction of viable seed set in GPMV in South Dakota.

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Proceedings of the South Dakota Academy of Science



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South Dakota Academy of Science


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