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Wednesday, July 22, 2015

Milkweed Magic

Driving around in the lower elevations of Tejon Ranch in the hot days of mid summer feels a bit like being a flea on the flank of a lion--forever surrounded by broad rolling contours of golden brown. While most lower-elevation herbaceous plants have senesced at this time of year, there is a notable exception-- milkweeds! Poking through the gold in contrasting verdant green, one may commonly observe clusters of these extravagantly thick, broad-leaved herbaceous plants.

Milkweeds are one of our few species flowering in the hot dry of summer and thus are a fascinating powerhouse of faunal activity. Lingering by a cluster of flowering plants for a few moments reveals a buzzing maelstrom of invertebrate species coming in to feed from the dense clusters of blossoms. Many of these species have evolved to rely specifically on milkweed for part of their life cycles.

Tarantula hawks (Pepsis sp.) left, and unknown thread-waisted wasp (suspected Podalonia sp.) right, feeding on narrow leaf milkweed (Asclepias fascicularis). Observed in lower elevation grasslands in the San Joaquin Valley, Tejon Ranch.

Milkweeds are so named for their potent milky sap, easily observed when a leaf or stem is broken. The chemical structure of the sap contains steroids called cardenolides which are highly toxic to most living things. There are some species, however, who have developed resistance to the toxic effects of cardenolides, and are brilliantly adapted to sequester these compounds in their own tissues as a personal defense against predation. In ecology, this adaptive trait is called aposematism, and is commonly advertised by bright coloration that warns predators of a distasteful or sick-inducing would-be meal. Here on Tejon Ranch, we have observed several milkweed associate species that exhibit aposematism. Below are a few examples:

Large milkweed bug nymphs (Oncopeltus fasciatus). Observed west of Los Alamos Canyon in the Antelope Valley, Tejon Ranch, feeding on narrow leaf milkweed (Asclepias fascicularis). Both nymphs and adults feed on various species of milkweeds.

Milkweed aphids (Aphis nerii) feeding on desert milkweed (Asclepias erosa). These aphids were apparently introduced from Europe. Observed in various locations in the San Joaquin Valley, Tejon Ranch. All individuals are female and they reproduce by giving birth to live young that are clones.

Milkweed longhorn beetle (suspected Tetraopes basalis) feeding on desert milkweed (Asclepias erosa). Observed in Tunis Canyon in the San Joaquin Valley, Tejon Ranch. Adults and larvae feed on milkweed vegetation. In order to avoid being inundated by the copious toxic and sticky sap, this beetle will cut one or more incisions in the mid-vein of the leaf and forage on the vegetation after the cut(s) have slowed the flow of thick sap. Pretty smart!

Desert milkweed (Asclepias erosa)
 In California we have about 15 native species in the milkweed genus Asclepias (family Apocynaceae). Four of these can be found on Tejon Ranch and are generally distinguished by their erect stature, opposite or sometimes whorled leaves, and unique, highly specialized flower structure. 

California milkweed (Asclepias californica)

Queen butterfly (Danaus gilippus). Larval form require milkweeds. 
This individual was observed in the eastern Mojave Desert on
 Mojave milkweed (Asclepias nyctaginifolia), a CNPS list 2B.1
 species (rare, threatened, or endangered in California).
A renowned aposematic species with a well-documented obligate relationship to milkweeds are monarch butterflies (Danaus plexippus). The life story of the monarch rivals the most riveting and inspiring of dramas—a journey involving several generations traversing international borders for thousands of miles in a perfectly timed and geographically specific cycle relying on countless impossible ecological pieces. Perhaps the most critical piece to their survivorship involves milkweeds--monarch adults lay their eggs on milkweeds and it is the larvae that ingest the plant material, ensuring that the vivid and vulnerable adult travelers are protected from predators on their long journey. They are one of several butterfly species who depend on milkweeds for their reproductive success. Declining milkweed populations throughout the Americas due to the loss of habitat from development and cropland conversion has been implicated in severely decreasing monarch populations—estimated at a more than 80% reduction in their population since the 1990s. In fact, monarchs are currently under review by the US Fish and Wildlife Service to determine if their declining populations warrant protection under the Endangered Species Act. There are many efforts being made to educate the public about the importance of preserving milkweed populations and to encourage the planting of native milkweed species. 

So next time you find yourself near a patch of these magic plants, take a moment to observe the extraordinary diversity of life that they support and appreciate the complicated stories they are central to. Consider doing your part to support these important plants--for the myriad species that have spent millennia evolving unique characteristics with milkweeds, as well as those that flourish foraging from the high summer flowers. Here are some ways to learn more and to help out:  

An informative guide to select California milkweeds:

 A coalition of agencies and organizations focused on monarch conservation:

 A non-profit organization dedicated to conserving invertebrates and their habitat:

Munroe, Lynne and Gene. 2013. Desert Insects & Kin of Southern California. Lyons, Colorado: Merryleaf Press.

Morhardt, Sia and Emil. 2004. California Desert Flowers: An Introduction to Families, Genera, and Species. Berkeley and Los Angeles, California: University of California Press.

Project Milkweed. The Xerces Society for Invertebrate Conservation. Portland, Oregon. Accessed 20 July 2015. <>