December 2012 Number 1 - Utah Native Plant Society

Utah Native Plant Society
Figure 7. A Mason bee (Osmia ribifloris ribifloris) pollinating
Sentry milkvetch.
Figure 6. A Painted lady butterfly visits Sentry milkvetch.
USFWS 2006. Sentry Milk-vetch (Astragalus
cremnophylax Barneby var. cremnophylax Barneby)
Recovery Plan. U.S. Fish and Wildlife Service, Albuquerque,
New Mexico, i-vii + 44pp.
ADDENDUM
This paper reported on the earliest efforts of Grand
Canyon National Park to begin implementation of the
2006 USFWS recovery plan for Sentry milkvetch.
Since my presentation at the University of Utah in
2009, four years of intensive work to recover this species
has been completed. Some of this work is documented
in Falk and others (2011) and Busco and others
(2011).
Preliminary pollination studies in spring 2010 established
the identity of three pollinators - two mason
bee (Osmia ribifloris and O. ribifloris ribifloris, Figure
7) and a hoverfly (Syrphidae) (Busco et al. 2011).
We have confirmed the presence of these generalist
pollinators throughout Sentry milkvetch populations
in repeat studies in 2011 and 2012.
At the time of my presentation in 2009, only 725
individuals of Sentry milkvetch were known. Today
there are an estimated 3552 known naturally-occurring
individuals in wild populations, and 425 plants in
reintroduction areas. The increase in numbers within
wild populations is largely the result of the discovery
of new groups of Sentry milkvetch plants on limestone
fingers above the rim and on lower limestone
levels below the rim during revisits to these populations
in 2010-2012 (Figure 8), as well as the result of
continued protection of the Maricopa Point population.
While two of the three populations are apparently
stable or increasing in number, the third small
population is increasingly threatened. The area below
the rim of this population has crumbled away and
fallen into the canyon; the few remaining individuals on
a solitary boulder above may likely follow.
Plant reintroductions at Maricopa Point began in July
2010 and continue to this date (Figure 9). The first
small planting trial was completed in July 2010 – 5 Sentry
milkvetch plants that were planted from greenhousegrown
plants at that time are all alive today. Seeds were
less successful in that reintroduction - 10 groups of three
seeds each were sown in April 2011 and today one seedling
from this cohort is alive and has reached reproductive
maturity. Of eighty greenhouse-grown plants and
Figure 8. Newly discovered Sentry milkvetch population
in Grand Canyon National Park.
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