Taisha is again the author:
Today’s photographs (image 1 | image 2) are of Asclepias asperula. The first image is that of a scanned slide uploaded to the Botany Photo of the Day Flickr Pool by Michael Huft@Flickr. Michael photographed this milkweed in Woodward County, Oklahoma back in 1980! The second photograph (also uploaded via the Flickr pool) is courtesy of sonnia hill@Flickr, taken in Texas’s Ellis County in April 2003. Thanks Michael and sonnia!
Asclepias asperula is a member of the Apocynaceae or dogbane family. This family is distributed over much of the world, excluding only Antarctica and the northern North Hemisphere. This species, though, is only found in a narrow part of the entire family’s distribution. It occurs in northern Mexico and central & southwest USA, where it is typically found in sandy soils along roadsides, pastures and open hillsides. Asclepias asperula has many common names including antelope-horns, green-flowered milkweed, spider milkweed, and spider antelope-horns.
A perennial clump-forming species, Asclepias asperula has a densely hairy stem and narrow leaves that are slightly folded lengthwise. In the summer, umbels of greenish-yellow flowers appear. Later, large follicles are produced. These curve to resemble an antelope horn, hence one of its common names. Seeds are arranged spirally around a central axis within the fruit. Long white hairs are attached to the seeds, aiding in wind-dispersal when the follicle dehisces.
Monarch butterfly (Danaus plexippus) larvae are obligate herbivores requiring members of the Apocynaceae to feed upon. Typically, these are members of the genus Asclepias, as mentioned in earlier posts on Botany Photo of the Day: Asclepias tuberosa and Liatris ligulistylis.
As many readers are likely aware, the monarch butterfly is in decline. In a survey conducted by the WWF-Tecel Alliance and CONAP, the percentage of Mexican forest occupied by monarch butterflies is used as an indicator of the number of butterflies that reach the country for hibernation. This year the numbers are at the lowest level in the past 20 years. The decrease in the butterfly population size is attributed to several factors, including a decrease in milkweed. There is an international effort to conserve monarch populations, with one method being to conserve and restore milkweed species in suitable habitats within their native range (see: Luna, T., and Dumroese, RK. 2013. Monarchs (Danaus plexippus) and milkweeds (Asclepias species): The current situation and methods for propagating milkweeds. Native Plants Journal. 14(1):5-15).