Western populations of Monarch butterflies overwinter in groves of trees,
such as eucalyptus, along the Pacific Coast (Photo credit: Marisa Ishimatsu)
With its distinctive bright orange and black wings, the monarch butterfly is one of the most well-studied, charismatic, and culturally significant insects native to North America. These iconic butterflies migrate each fall from summer breeding grounds to groves of trees, where they overwinter in spectacular colonies. West of the Rocky Mountains, monarchs primarily overwinter along the California coastline. Western monarch populations will often return to the same overwintering sites each year, such as groves located in Pismo Beach State Park in San Luis Obispo County and Pacific Grove Sanctuary in Monterey County. Unfortunately, recent counts at overwintering sites suggest that western monarch populations have declined by 99.9% since the mid-1980s. Despite a recent status assessment by the U.S. Fish and Wildlife Service under the Endangered Species Act, monarchs are only a candidate species, and currently receive no protection off federal lands.
Increased media coverage has sparked enthusiasm for designing habitat to promote monarchs. While well-intentioned, many landscape designers, land managers, and gardening enthusiasts make decisions to create monarch-friendly habitat that are uninformed by scientific research. These decisions may ultimately cause more harm than good. This article highlights four common mistakes that are made when attempting to create monarch habitat, and how these mistakes can be rectified.
1. Planting the wrong kind of milkweed
The most popular approach to creating monarch habitat is to plant milkweed (Asclepias spp.), because monarch caterpillars solely feed on milkweed. However, there is currently little scientific evidence that insufficient milkweed plays a role in western monarch population declines. Major identified causes of population declines include larger, harder to address problems such as habitat loss, climate change, and disease. In addition, planting the wrong kind of milkweed can potentially cause problems for western monarchs.
In California, landscapers commonly plant tropical milkweed (A. curassavica) in favor of native milkweed species. Tropical milkweed is more widely commercially available and produces showy flowers and foliage year-round, while native milkweeds can be harder to procure and die back in the fall. Unfortunately, the presence of year-round milkweed foliage can disrupt monarch behavior by encouraging adults to continue to breed in the fall, when they would typically begin their migration. Milkweed that has year-round foliage can also lead to a buildup of the spores of a parasite harmful to monarchs, Ophryocystis elektroscirrha (OE). If you choose to plant tropical milkweed, it is important to aggressively prune plants to the base each winter to prevent the buildup of OE spores.
Planting regionally-appropriate native milkweed is a better alternative to tropical milkweed. There are currently two commercially available species of California native milkweed: narrow leaf milkweed (A. fascicularis) and showy milkweed (A. speciosa). Narrow leaf milkweed naturally occurs throughout most of California, except the Sonoran desert and high elevations in the Sierra Nevada, and establishes well from seed. Showy milkweed has a more limited range, and occurs primarily in northern and central California. Showy milkweed establishes best from transplants.
2. Planting milkweed along the coast
Even California native milkweeds should not be planted in regions where they do not historically occur, especially along the coast where monarchs overwinter. The presence of milkweed in overwintering areas can disrupt normal migratory and overwintering behavior. In coastal regions, even native California milkweed species may grow year-round, which can also lead to the buildup of OE spores. Currently, experts recommend milkweed should not be planted within 5 miles of the California coast (or within 1 mile of the coast south of Santa Barbara).
Narrow leaf milkweed (Asclepias fascicularis) is a commercially-available
milkweed species that naturally occurs throughout much of California.
3. Mis-timing management practices
Disruptive management practices, such as mowing and pesticide application, can harm monarch caterpillars. Therefore, these practices should be avoided during the breeding season in areas where milkweed has been planted. In southern and central California, these practices should be avoided from October 31 to March 15. In northern California, they should be avoided from October 31 to April 1.
4. Overlooking the benefits of nectar plants
Adult monarchs feed on nectar from flowers, rather than by eating milkweed leaves. Planting species that have flowers that provide food for adult monarchs is an excellent way to support monarch populations. Nectar plants provide benefits for adult monarchs wherever they occur, especially in migration and overwintering areas along the coast. Some good nectar plants for monarchs include yarrow (Acchillea millefolium), coyote brush (Baccharis pilularis), mule-fat (B. salicifolia), common buttonbush (Cephalanthus occidentalis), western goldentop (Euthamia occidentalis), sunflowers (Helianthus spp.), black sage (Salvia mellifera), and western vervain (Verbena lasiostachys).
In summary, the following are suggestions to help create quality habitat for monarch butterflies based on the best current scientific evidence:
- Plant the native narrow leaf or showy milkweed that are regionally-appropriate (or aggressively prune tropical milkweed in the winter).
- Avoid planting milkweed within 5 miles of the coast (or 1 mile of the coast south of Santa Barbara).
- Time disruptive management practices for the late fall and winter to avoid impacts to monarch caterpillars.
- Plant nectar plants for adult monarchs both on the coast and in inland areas.
- The Xerces Society for Invertebrate Conservation (https://xerces.org/monarchs)
- The Monarch Joint Venture (https://monarchjointventure.org/)
- Western Monarch Milkweed Mapper (https://www.monarchmilkweedmapper.org/)