This column follows the previous column about ways to reduce the threat of extinction of Monarch butterflies. Readers’ responses to that column indicated strong interest in protecting the Monarchs that overwinter in the Monterey Bay area, enhancing our environment. Readers also called for additional details about ways to help the Monarchs.

Growing Nectar-producing Plants

Interested persons can improve the habitats of the Monarchs’ overwintering sites by planting nectar-producing plants. As reported earlier, the Xerces Society’s publication “California Coast: Monarch Nectar Plants” (https://tinyurl.com/ycsy8jp4) lists several good selections. In addition, Pacific Grove’s Nectar Plant Project has tested and recommended the following plants:

• Butterfly Bush (Buddlea x weyriana). This is a hybrid cultivar that blooms in October and produces much nectar.
• Daisy Tree (Montanoa Grandiflora). A native of Mexico, this woody shrub grows to eight feet or more, and produces a cloud of fragrant white flowers that bloom from late October to late November, just when the Monarchs need nectar.
• Mallow (Lavatera assurgentiflora). Another woody shrub, this California native produces red flowers year-round.
• Yellow Daisy (Euryops pectinatatus ‘viridis’). This South African native is a mainstay for the Monarchs, blooming year-round with bright yellow daisy flowers. It’s also not attractive to deer.

Daisy Tree (Montanao Grandiflora)

The Importance of Milkweed

As noted earlier, milkweed is essential to the wellbeing of Monarch caterpillars. This plant contains toxins known as cardenolides. Monarchs are immune to these toxins, and have evolved to store them in their bodies. The toxin makes them poisonous to birds, which avoid eating the Monarchs. Without this protection, birds would be major (but not the only) predators of the Monarchs.  

The adult Monarchs always deposit their eggs on milkweed plants, so that the larvae begin immediately to eat the plants and accumulate the toxin in their bodies.

Where to Grow Milkweed Plants

The Western Monarchs mate around January, lay their eggs at a wide range of sites throughout the American west, and return to the Pacific coast to overwinter during a four-month period. In the Monterey Bay area, the familiar overwintering sites are at Natural Bridges State Beach and Pacific Grove.

(Other Monarchs have a different migration route. It begins in the northern United States and southern Canada, and continues to overwintering sites in Mexico. )

Milkweeds are needed where the Monarchs lay their eggs. Milkweed grows naturally in Monarch breeding areas, but not at the overwintering sites on California’s central or northern coastal areas. The migration cycle moves between the breeding areas to the overwintering sites. Planting milkweed close to overwintering sites could encourage Monarchs to breed and lay eggs during the winter, and thereby disrupt the migration cycle. Given this concern, the Xerces Society and other experts recommend that milkweed should not be planted with 5-to-10 miles of an overwintering site.

Which Milkweed Plants to Grow

The milkweed genus Asclepias includes over 200 species. California native species that grow naturally near the Western Monarchs’ home territory are preferred.

1. Widely Available Species

• Mexican Whorled Milkweed (Asclepias fascicularis) – dry climates and plains
• Showy Milkweed (A. speciosa) – savannahs and prairies
• Butterfly Weed (A. tuberosa) – well-drained soils; a non-native species that has naturalized in California; Perennial Plant Association named it Perennial Plant of the Year for 2018

2. Other California Native Species

• California Milkweed (A. californica) – grassy areas; native to central California
• Desert Milkweed (A. erosa) – desert regions
• Heartleaf Milkweed (A. cordifolia) – rocky slopes; early budding
• Woolly Milkweed (A. vestita) – dry deserts and plains
• Woolly Pod Milkweed (A. eriocarpa) – clay soils and dry areas; early budding

3. Avoid Tropical Milkweed (A. curassavica), which is evergreen, but which allows development of a protozoan parasite (Ophryocystis elektroscirrho) that harms or even kills Monarchs that eat the plant.

Gardeners in the Monterey Bay area who wish to improve the Monarchs’ habitat should emphasize the planting of nectar-producing plants more than milkweed. Those plants are good for bees, as well. This would be a good year to plant annual seeds in the early spring.

Recent news reports have described a dramatic decrease in California’s population of Western Monarch butterflies. Thirty years ago, over 4.5 million of these beautiful flying insects migrated annually from Mexico to overwinter on the Pacific Coast, with huge, fascinating clusters at famous sites, including Pacific Grove and Natural Bridges State Beach. Their numbers have declined over the years: this year the count was under 30,000 individuals, 0.6% of the population’s historic size. Researchers had established 30,000 as the “quasi extinction threshold,” so coming under this threshold suggests that this species (Danaus plexippus plexippus) is approaching extinction.

Another, larger number of monarchs migrate from Mexico to the northern plains of the United States, so extinction is not imminent, but that population also has been shrinking in recent decades.

The causes of the decline in the monarch population begin with the loss of suitable habitat, defined as areas that include both milkweeds (Asclepias sp.) that Monarch caterpillars depend upon, and pollen plants that provide nutrition for the adults. Milkweed plants are too often deleted from the environment by agricultural pesticides, particularly glysophate (RoundUp), which is used to kill unwanted plants among commercial crops.

Other factors include pesticide use, climate change, and logging and development projects that degrade overwintering sites.

Gardeners can help rescue monarch butterflies back from the threat of extinction by growing milkweed plants in their gardens, with preference for locally native species of this plant. Fifteen species of milkweed are native to California. Nearby sources of seeds for native species include Pacific Coast Seed, in Livermore; Seedhunt, in Freedom; and S&S Seeds, in Carpenteria. For information on growing milkweed, download “Native Milkweed in California: Planting and Establishment.”

A related project for gardeners is to include pollen plants for the adult monarch butterflies. Useful advice for such a project is available in “California Coast: Monarch Nectar Plants,” available here.

Both of these publications are free downloads from the Xerces Society for Invertebrate Conservation.

The Xerces Society has also produced a five-step call to action for recovering the western monarch population. This plan emphasizes protection of overwintering and breeding habitats, protection from pesticides, and continuing research on western monarch recovery. In addition to actions in their own gardens, people concerned with threats to the monarch population could support large-scale initiatives that respond to the Society’s call to action.

Protecting the Western Monarch Butterfly from extinction has at least three broad objectives. One is for us all to enjoy the phenomenon of these beautiful creatures flitting about in our gardens. A second objective, oriented to the ecosystem, is to maintain these butterflies as food sources for birds, which we also appreciate and enjoy. Thirdly, Karen Oberhauser, director of The Monarch Butterfly Lab at the University of Minneapolis, advises that monarch butterflies are valuable subjects for ongoing studies of migration, species interaction, insect population dynamics, and insect reproduction.

We can strive to keep monarch butterflies in our environment, but we should also acknowledge the larger picture of the extinction of species. Before humans spread across the globe, species extinctions occurred for various reasons at a very slow rate. Human activity has increased the average rate of extinction by somewhere between 100 to 1,000 times the previous historical rate., and is accelerating

Today, according to Edward O. Wilson, distinguished American biologist, the science community estimates that Planet Earth has about 10 million living species, one of which is human people (Homo sapiens). About 20 percent of these species are known, and 80 percent are undiscovered. If the extinction of species by human activity continues to accelerate, E.O. Wilson says, we could eliminate more than half of all species by the end of this century.

We can slow this trend a little by working as individuals. Consider adding California native milkweed and pollen plants to your garden this year, to sustain the monarch butterflies.