Soapweed (Yucca glauca) COSEWIC assessment and status report 2013: chapter 6
COSEWIC Status Report on the Soapweed Yucca glauca in Canada - 2013
Scientific Name: Yucca glauca Nuttall
Synonyms: Yucca angustifolia Pursh, Yucca glauca var. gurneyi McKelvey, Yucca glauca var. stricta (J. Sims) Trelease, Yucca stricta J. Sims
Common Name: Soapweed, Yucca, Yucca glauque (French), Eksisoke or aiksisooki (Blackfoot), hupestola (Lakota), Small Soapweed Yucca, Great Plains Yucca, Spanish Bayonet, Beargrass, Narrow-leafed Yucca
Family: Agavaceae; sometimes included in the Liliaceae or Asparagaceae
Yucca glauca Nutt. is the only species of the genus native to Canada. It hybridizes with Palmella (Y. elata) in eastern Colorado, central New Mexico, and Arizona and is thought to hybridize with Y. constricta, Y. baileyi and Y. angustissima (Webber 1953; Groen 2005) in parts of the species range. None of these Yucca species occurs naturally in Canada, although they may be present in horticultural settings.
Soapweed is a long-lived, polycarpic perennial that grows as a single rosette or cluster of rosettes of long, narrow, sharply-pointed leaves (Figure 1). Clusters of rosettes are part of the same genetic individual (genet) and are referred to as clones following established terminology for this species. Leaves reach 25 to 40 cm long in mature plants (Kingsolver 1984; Csotonyi and Hurlburt 2000). At flowering, each rosette can produce a single inflorescence containing 15 to 75 nodding, perfect flowers and reaching 30 to 85 cm in height (Csotonyi and Hurlburt 2000). Most inflorescences in Alberta have one or two flowers at each axil; however, they can also be branched with up to 5 flowers at each axil. Flowers are large and white to greenish-white or pinkish-white (Csotonyi and Hurlburt 2000). Fruits are woody once mature, kiwi-sized and dehiscent. The fruits of Soapweed plants at the Onefour Research Substation (henceforth Onefour), contain an average of 146 (± 94) flat, black seeds arranged in six locules or rows (Hurlburt 2004; Figure 2). Soapweed clones have 1 or 2 large diameter rhizomes from which smaller rhizomes protrude (Groen 2005), forming a mat as deep as the hardpan layer below the soil surface in Alberta (Hurlburt, D., pers. obs.).
Figure 1. Soapweed clone at Pinhorn Grazing Reserve, AB in late June 2002.
Photo: D. Hurlburt.
Description of Figure 1
Photo of a Soapweed rosette and inflorescence (foreground). The leaves are long, narrow, and sharply pointed. The flowers are greenish-white.
Figure 2. A locule of a Soapweed fruit, showing viable (black) and non-viable (white) seeds.
The Yucca Moth larva has destroyed a portion of the viable seed through feeding. Photo: D. Hurlburt.
Description of Figure 2
Photo of a section of a Soapwood fruit showing a locule or row of flat seeds. Some of the seeds are black (viable) and some are white (non-viable). The exterior of the fruit is green. A pinkish Yucca Moth larva is attached to some of the black seeds.
No subspecies of Soapweed are recognized, all the plants occur in a single COSEWIC National Ecological Area (Prairies), and there are no distinctions that warrant consideration of multiple designatable units. This report is based on a single designatable unit.
Soapweed engages in an obligate mutualistic relationship with its pollinator, the Yucca Moth (Tegeticula yuccasella), in which both species depend on their interactions to complete their life cycle. Few interspecific relationships of this nature are documented worldwide and this may be the only example of an obligate mutualism among free-living species in Canada.
Soapweed is also the sole host to three other species of Lepidoptera, in addition to the Yucca Moth. The Non-pollinating Yucca Moth (T. corruptrix) and the Five-spotted Bogus Yucca Moth (Prodoxus quinquepunctellus) use Soapweed fruits and inflorescences respectively, as oviposition sites. These two species and the Yucca Moth are listed as 'Endangered' under Canada's Species at Risk Act (SARA), Schedule 1. Strecker's Giant Skipper (Megathymus streckeri) relies on the leaves, stems and roots of Soapweed for rearing larvae (Anweiler 2005). This species is on COSEWIC’s Candidate list, but it has not yet been assessed. This web of 5 highly interdependent, specialized organisms, plus their complex interactions with ants and aphids (Perry 2001; Perry et al. 2004, Snell 2008a; 2008b), illustrates the interdependence of species, in which a decline in one member of the set can negatively alter the fate of all others.
Soapweed and Yucca Moth have unique characteristics in Canada that allow the species and their mutualism to persist despite highly variable biological and environmental conditions at the northern edge of their ranges. Soapweed in Alberta have the longest flowering seasons documented in any Yucca spp., which allows some pollination and seed set to occur even when moth emergence is delayed. The plants can selectively abscise fruits with fewer moth eggs, thus apparently allocating resources for fruit development in a manner that tends to increase the reproductive success of the moths. This process has not been described elsewhere (Hurlburt 2004). Unlike most yuccas, Soapweed in Alberta can readily retain self-pollinated flowers with no apparent negative effect of inbreeding on progeny (Hurlburt 2004, although Yucca Moths are still required for pollination of selfed flowers). The unique attributes of these peripheral populations make them especially significant targets for conservation.
In addition, these peripheral populations of Soapweed and Yucca Moths are expected to be on the leading edge of range expansion and may be adapted to different environmental conditions than populations at the centre of the species’ ranges. These populations may be more resilient to anthropogenic disturbance or climate change than others (Lesica and Allendorf 1995, Lomolina and Channell 1998).
Soapweed is of commercial value as horticultural stock and is commonly available in nurseries throughout Canada (Hurlburt 2001). Yucca roots contain steroidal saponigens and have been used as the soapy agent in shampoos and soaps. Yucca extracts are also popular components of herbal remedies and drugs, and have been promoted as cure-alls (Hurlburt 2001). Experimental efforts are ongoing to determine if the addition of Yucca extracts to animal feed can reduce ammonia compounds in manure or reduce methane production (in cattle) without compromising digestion (e.g. Holtshausen et al. 2009).
Soapweed is known to be of significance to Aboriginal groups in the United States for food and fibre and it is expected that ATK, including knowledge of the interactions between the Soapweed and Yucca Moth, may exist in those areas where Soapweed is more common. No ATK was found to be available in Canada, nor are any Aboriginal artifacts containing yucca fibre known to occur.
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