Partners Work to Help Monarch Butterflies by Expanding a Locally Scarce Milkweed Species

By Hillary Sardiñas, Biologist, Alameda County Resource Conservation District

The Western population of monarch butterflies is in a precarious position. Over the past 20 years, its population has crashed 99.5% and is currently at the brink of extinction. While there are numerous reasons for the butterfly’s decline, host-plant availability has been indicated as a key factor. Monarch butterflies only lay their eggs on milkweed, which contains the cardenolides – a type of toxin – that makes the caterpillars poisonous to would-be predators.

California milkweed in bloom. Photo by Hillary Sardiñas.

Monarch butterflies produce four to five generations a year, and the overwintering generation is the longest lived. The butterflies cluster together in tree groves along the California coast during the wet, cold months. They then fly east through the coast range searching for milkweed (Asclepias sp.) to lay eggs on. Milkweed grows in a variety of habitats but is most frequently found in grasslands. The adult butterflies oviposit on the milkweed, then die. When the next generation hatches, the caterpillars consume the milkweed, metamorphose, then journey east to find more milkweed. This cycle continues until fall, when the fourth generation returns to the coast to overwinter once again.

However, monarchs are now leaving their overwintering sites earlier in the year, possibly because of climate change or tree maturity and death within their overwintering sites that can alter their preferred microhabitat conditions. Because of this behavioral change, some monarch researchers are concerned that there is currently a lack of early-season milkweed to support the breeding butterflies.

California milkweed seed pods splitting open and spilling seeds. Photo by Hillary Sardiñas.

There are three milkweed species present in Alameda County: narrow-leaf milkweed (Asclepias fascicularis), showy milkweed (A. speciosa) and California milkweed (A. californica). The first two are late-blooming, while the latter is early-blooming. California milkweed is less widespread;in fact it is in the EBCNPS Rare, Unusual, and Significant Plants of Alameda and Contra Costa Counties Database Watch List B. It has likely declined the most – though there is little historic data on its presence. It is also the only species that is not commercially available from local native plant nurseries.

Thanks to a small grant from the Alameda County Fish and Game Commission, the Alameda County Resource Conservation District and the Natural Resources Conservation Service (NRCS) are partnering with the East Bay Regional Park District, The Watershed Nursery (TWN), and local private ranchers to help restore populations of California milkweed to the East Bay. The partners used Calflora historic sightings, recent iNaturalist posts, and local field knowledge from EBCNPS to identify seed collection locations in the area. Chapter member Ling He (who is also a Rangeland Management Specialist with NRCS) coordinated with fellow EBCNPS members Judy Schwartz, Janet Gawthrop, Dianne Lake, and Gregg Weber in the effort. In 2020, TWN will cultivate the milkweed seed collected, which will then be outplanted on five private ranches in the Tri-Valley area.

Ranchers have historically been cautious about milkweed due to concerns over toxicity to livestock. Luckily, California milkweed contains less cardenolides in its plant tissues than the late-blooming species, which means it is potentially less toxic to livestock and therefore more attractive for planting in rangelands. In addition, the partners are educating ranchers about the compatibility of milkweed and cattle- highlighting the fact that there are no known cases of mortality in cattle due to milkweed consumption. This information makes it easier for ranchers to become willing partners in efforts to revitalize the Western monarch population to preserve the iconic butterfly for future generations.

If you see California milkweed – or any milkweed – while out hiking in the East Bay, please post it to iNaturalist. The partners will be able to add your sightings to their milkweed project data as they continue working to preserve the genetics of existing stands and expand milkweed species to new locations. For more information on monarchs and what you can do to help, check out the Xerces Society for Invertebrate Conservation’s Monarch Butterfly Conservation website.

– December 2019