By Laura Lovett, Gardening with Natives Committee Chair
Population Declines
Monarchs overwintering – Photo by J Ting
As recently as the 1980s, millions of monarch butterflies over-wintered at sites along the California coast, including in Marin. In recent years, citizen scientists have documented a plummeting population, now less than 3% of its historic size. Monarchs have been stressed by habitat degradation, loss of larval host plants, and the use of pesticides. All these factors have also contributed to potential loss of many different beneficial insects important to the food chain, but the Western Monarch, with its bright orange and black coloring, is particularly noticeable in its absence.
Monarch butterflies go through four life stages: the egg, larva (caterpillar), pupa (or chrysalis), and imago or the adult stage—the butterfly. In the pupal stage, the larva undergoes a magical transformation from a caterpillar into a butterfly. At each stage, specific needs must be met for the insect to progress toward the final adult form and begin the cycle anew.
Phase Out Tropical Milkweed
Milkweeds are the only plants that monarch caterpillars will feed on, therefore, the only plants that a female monarch will lay her eggs on. Without these plants, the cycle does not start. Milkweeds contain toxic chemical compounds called cardiac glycosides, which monarch caterpillars can tolerate. As monarch caterpillars consume milkweed plants, they sequester these toxic compounds within their bodies.
Asclepias fascicularis by Vernon Smith
Of the native milkweeds available here, both showy milkweed (Asclepias speciosa) and narrow-leaf milkweed (Asclepias fascicularis) grow well in Marin and are sold through local nurseries as well as Marin Chapter CNPS plant sales. California milkweed (Asclepias californica) will also grow here, but plants are hard to find. There are wild populations in the East Bay, but not in Marin.
Up until now, there has been another alternative available: tropical or Mexican milkweed (Asclepias curassavica). Unlike native milkweed species, tropical milkweed does not die back in winter, which can confuse monarchs into breeding when they should be migrating. Tropical milkweed is also a host plant to a protozoan parasite of monarchs called Ophryocystis elektroscirrha (OE). OE Spores are carried on adult butterflies as they migrate, and the dormant OE spores are deposited on the tropical milkweed leaves. As caterpillars eat the plant, they ingest the OE spores and become infected. Such infections have been linked to lower migration success as well as reduction in body mass, lifespan, mating success, and flight ability.
As of August 24, Marin County Agricultural Commissioner Stefan Parnay has prohibited the sale of tropical milkweed (Asclepias curassavica) by Marin’s retail nurseries. This follows on the heels of the California Department of Food and Agriculture designating tropical milkweed as a B-rated noxious weed.
Asclepias currasavica (mexican milkweed)
Although some researchers feel that a ban on tropical milkweed will not have much effect on monarch populations, butterfly-loving gardeners may want to take some action. Instead of tearing out existing tropical milkweed, prune the stalks to about 6 inches high no later than the end of October. This encourages monarchs to resume their migration and helps to eliminate OE spores on the plants. Over time, consider replacing your tropical milkweed plants with native milkweed species.
Food Sources for Over-Wintering Adults
Monarch butterflies over-wintering on the coast are in reproductive diapause—a period during which development is suspended. The presence of larval host plants close to an over-wintering site—anywhere within about three miles of Marin’s ocean coast—can interrupt the butterfly’s natural migratory cycle and encourage attempts at winter breeding that yield poor survival rates. Please DO NOT plant any milkweed species if your garden is within three miles of Marin’s ocean coastline.
Now that we have taken care of the caterpillars’ food needs, what do the adult butterflies need? Nectar! Specifically, lots of native flowers, especially ones with daisy-like flower heads that make great butterfly resting places. Once the butterflies start to stir themselves in spring, the first thing they must do is recharge their energy for the migration ahead. Please DO plant fall, winter, and early spring blooming species that provide the nectar the adult butterflies need. Among the choices to plant for early spring bloom are Ceanothus and Arctostaphylos species and cultivars, native currants and gooseberries, black sage and other Salvia species, seaside daisy, and annual wildflowers. Blooming in the fall, asters, buckwheats, California fuchsia, coyote brush, goldenrod, gumplant, and yarrow provide resources that support our valuable pollinator populations while filling our gardens with late summer color. If all of us work hard to create gardens with complex and long-blooming habitat that includes both milkweed and floral resources, this endangered species and other pollinators should flourish once again.