The beloved Monarch butterfly (Danaus plexippus) is experiencing a rapid population decline. In California alone, there were nearly 4.5 million Monarchs just 30 years ago. During the most recent Monarch count this past January the western population of Monarchs was estimated to be 28,429 individuals—an 86% decrease from a count done in 2017.
As part of a national call to action to help the western monarch population bounce back from its extremely low size, the Salmon Protection and Watershed Network, or SPAWN, is providing Marin County residents with guidelines on how planting native nectar plants can improve habitat for Monarchs.
“There is no fast and easy way to restore the population of Monarchs, but there are ways to help this iconic species survive,” said Native Plant Nursery Manager Audrey Fusco. “If you live in Marin, the best way to help is to restore habitat by adding native nectar plants that support the adult Monarchs that arrive in the fall and overwinter on the coast near Bolinas and Muir Beach.”
Monarch butterflies are famous for their long annual migration. Adult butterflies arrive at the coast in the fall and stay at overwintering sites on the coast where temperatures are warmer through winter. During this time the butterflies need to conserve their energy and are in a period of reproductive diapause. They use environmental cues, such as increased daylight time and warmer temperatures, to know when it is time to leave their overwintering grounds.
The Marin population of monarchs typically mate and disperse in February. The females fly off in search of milkweed, the only host plant on which Monarchs can lay their eggs. Successive generations continue to travel east throughout the spring and summer. The butterflies that return to the coast in the fall are the third or fourth generation descendants of the prior year’s overwintering population.
To best support overwintering Monarchs, Marin County residents that live in an inland area—more than five miles from the coast—it’s helpful to add the locally native species of milkweed, narrowleaf milkweed (Asclepias fascicularis), for adults that may be in Marin during their reproductive time. Please do not plant tropical milkweed (Asclepias curassavica) as this species commonly hosts a protozoan parasite that infects Monarchs. Narrowleaf milkweed grows best in wet areas such as swales and ditches. Milkweed is toxic to livestock.
If you live in coastal areas, or within five miles of the coast, plant native nectar plants, particularly those that bloom in late fall, winter, and early spring. Recent research indicates that it is not helpful to add milkweed within five miles of the coast, as having milkweed near overwintering grounds disrupts the period of reproductive diapause.
Please refer to SPAWN’s General List of Nectar Plants for Monarchs to determine which plants you should use based on the plant community you live in.
These same tips can be applied to places outside of Marin by using plants native to your area.
Many factors have contributed to the decline of western Monarchs, including the use of pesticides and herbicides, increased wildfires, loss of habitat and habitat fragmentation, and loss of milkweed, which is necessary to Monarchs for reproduction.
For additional information about habitat restoration in Marin, please contact Audrey Fusco, SPAWN’s native plant nurs-ery manager, at firstname.lastname@example.org. For more ways to help save Western Monarchs visit the Xerces website.