Global Plant Protection News

USDA

Milkweeds for Monarch Butterfly Habitat in the Southwestern U.S.

Milkweeds are an iconic pollinator plant, with seventy-three native species that can be found in most habitats across the United States. These plants are in the genus Asclepias and are mostly composed of herbaceous perennials with milky sap and showy flowers. The plants are prolific nectar producers, providing nourishment for scores of native bees, flies, butterflies, hummingbirds, and many other animals. Milkweeds are perhaps most famous for their association with the life cycle of the monarch butterfly (Danaus plexippus). Milkweeds are vital to the monarch’s survival as the larvae feed exclusively on milkweed plants. Monarch butterflies are divided into two migratory populations, the eastern population overwinters in Mexico while the western population overwinters on the California coast.

Unfortunately, milkweed populations nationwide have been in significant decline due to habitat loss because of invasive weeds, urbanization, and agriculture. In addition to these stresses, milkweeds are often viewed as a weed, and are actively eradicated from roadsides, ditches, and agricultural areas. The reduction in milkweeds has been identified as a major contributing factor to the dramatic decline in monarch populations over the past few decades. The Western Monarch Thanksgiving Count      is an annual population census that has counted over-wintering monarchs in California since the 1980’s. In 1997, they counted over 1.2 million individuals. In 2020, they counted a record low of 1,914 monarchs: a decline of over 99.9% since then.

The Southwestern Plant Materials Centers, located in Lockeford, California (CAPMC); Fallon, Nevada (NVPMC); Tucson, Arizona (AZPMC), and Los Lunas, New Mexico (NMPMC), have conducted studies and developed publications to enhance milkweed conservation in their respective areas. Much of this work has been in collaboration with the Xerces Society, Universities, and other partners.  The CAPMC evaluated the propagation and establishment of Asclepias speciosa, A. eriocarpa, and A. fascicularis by seed, rhizomes and transplants detailed in a series of three reports titled Milkweed Establishment in California’s Central Valley.  The NVPMC published a technical note on milkweed pollination biology and a report on the fourteen milkweed species that occur in the Great Basin. The AZPMC published a technical note on the forty-one milkweed species found in the Mojave, Sonoran, and Chihuahaun deserts. Milkweed conservation work conducted at the NMPMC included seed production trials of A. speciosa, A. latifolia, and A. asperula.

Additional resources and assistance programs to support monarch butterfly conservation are available on the Plant Materials Monarch Conservation and the NRCS Monarch Butterfly webpages.

Technical information and guidance on the use of conservation plants to address resource concerns is available on the Plant Materials Program website or contact the nearest Plant Materials Center or plant materials specialist. For additional information on species of plants mentioned, please see the USDA PLANTS database.