Milkweed is key for monarchs
The relationship between monarch butterflies (Danaus plexippus) and milkweed (Asclepius spp.) is an example of the ingenious harmony of nature.
The monarch butterflies observed in North America, including Western Pennsylvania, belong to a migration of successive generations that travels back and forth to Mexico over thousands of miles every year.
Each spring, milkweed emerges from dormancy as monarchs are leaving their overwintering sites in Mexico. As each generation of the monarch flies northward, North American species of milkweed along the migration route are blooming in sync with the monarch’s migration.
Once they arrive in western Pennsylvania the butterflies find three species of milkweed in bloom: common milkweed (Asclepias syriaca), swamp milkweed (A. incarnata), and butterfly weed (A. tuberosa).
The arriving monarch butterflies not only feed on the nectar of milkweed but lay their eggs, one at a time, upon milkweed leaves. When caterpillars emerge in four to five days, they consume the leaves as they transition through their four remaining growth stages, called instars.
Milkweed is their only host plant. Monarchs don’t eat or lay eggs on any other plant. In eastern North American the final generation of the season emerges from the chrysalis in late summer, and the adult from that generation is the individual that will migrate back to Mexico for the winter.
The migratory subspecies of monarch butterfly, Danaus plexippus plexippus, has recently been designated by the International Union for Conservation of Nature (IUCN) as endangered due to declines in the population of between 22% and 72% over the past decade.
Many factors are responsible for this dramatic decline, including activities such as urban development, pesticide use, deforestation and climate change. Factors like herbicide use and drought contribute to the loss of milkweed, upon which the monarch life cycle depends throughout their migration.
What can we do to help sustain and promote population growth for the monarch? We can incorporate any or all the three species of milkweed into our landscapes.
Milkweed can be grown from seed. Like many other native plants, the seeds require a period of cold weather to germinate successfully. In nature, the seeds are released from their large gray pods and drift through the air on the wisps of white silky filaments called coma.
To add milkweed to a landscape, it is easiest to mimic nature by scattering seed in late fall and covering them with a thin layer of soil. Plant in full sun.
Milkweed seeds can be germinated indoors for outside planting in late spring but must be cold treated in the refrigerator for several weeks to mimic the natural cold cycle.
Milkweed is a perennial and is becoming more available at nurseries and greenhouses, especially swamp milkweed and butterfly weed. Rifts create a dramatic effect, or plant in combination with other natives that support pollinators, such as bee balm (Monarda didyma), purple coneflower (Echinacea purpurea), and anise hyssop (Agastache foeniculum).
The decline in the migratory monarch population can be positively impacted by using good gardening practices. Besides planting milkweed to support these butterflies and other pollinators through all their life stages, you can remove invasive plant species and reduce pesticide and herbicide use.
Penn State Extension has additional information on milkweed plants that support monarch butterflies at extension.psu.edu/monarchs-and-milkweed; extension.psu.edu/rain-garden-plants-swamp-milkweed; and extension.psu.edu/programs/master-gardener/counties/york/native-plants/fact-sheets/save-the-monarch.
If you have questions about planting milkweed for monarchs and other gardening practices, call the Master Gardeners of Butler County Garden Hotline at 724-287-4761, ext. 7, or email the Master Gardeners at firstname.lastname@example.org.
Dawne Lawton is a Penn State Extension Master Gardener of Butler County.