Monarchs And Native Milkweeds

Southern California provides important habitat and overwintering sites for western monarch butterfly populations, which have declined by 95% since the 1980's.  This is in part because milkweed, the plant upon which all monarchs depend during the early stages of their lifecycle, is also declining.  

In response, a great number of very well-meaning people throughout San Diego County and beyond began growing tropical milkweed.  This nonnative species of milkweed was readily available at nurseries, had showy flowers, remained green year-round and very successfully attracted monarch butterflies.  Many gardeners were thrilled to have a plant that attracted monarchs to their yard all year round. But at what cost?

It was quickly discovered that planting tropical milkweed was not the panacea it seemed. Indeed, we understand now that planting this nonnative species of milkweed is actually detrimental to the recovery of western monarch butterfly populations.

This is because:

• While milkweeds native to southern California such as narrowleaf milkweed (Asclepias fascicularis), die back in the winter, tropical milkweed provides a year-round food source for monarchs. This induces monarchs to breed in winter and disrupts their natural migration patterns by causing them to remain at an artificially available food source instead of migrating (Satterfield et al. 2015).

• If they do not migrate, prolonged feeding on tropical milkweed results in an accumulation of a debilitating protozoan parasite (Ophryocystis elektroscirrha, aka OE) in their bodies.  At high densities, these parasites decrease the survival rate of caterpillars and shorten the life spans of adult butterflies (Altizer and Oberhauser 1999).

• Pesticides, which are often used on milkweed purchased from nurseries that supply garden and box stores are widely believed to contribute to the overall decline in monarch butterflies (Halsch et al. 2020).

• It has also been found that warmer temperatures increase the concentration of a toxin found in tropical milkweed called cardenolides, to levels that are largely intolerable to monarch butterflies.  This, in combination with the additional stressor of warming temperatures predicted with climate change will likely result in a dramatic decline in their overall survival.  Scientists therefore recommend that native milkweed species be preferentially planted over tropical milkweed in anticipation of warming temperatures (Faldyn et al. 2018). 

What can you do?

• Plant native narrowleaf milkweed, and not tropical milkweed in your yard.  

• If you have tropical milkweed, cut it back to within a few inches of the ground October-Februrary, to better imitate the life cycle of native narrowleaf milkweed.

• Consider replacing your tropical milkweed with milkweed species such as narrowleaf milkweed (Asclepias fascicularis) or woollypod milkweed (Asclepias eriocarpa).

• Check back with Moosa Creek and Butterfly Farms, and also Earth Discovery for updates in the coming months on the availability of narrowleaf milkweed seeds and plants.

• EDI is always looking for weekday volunteers to work on the milkweed farm and support restoration projects on protected lands.  You can also support our work donating, or joing EDI as a member.

Learn about native narrowleaf milkweed in San Diego County:

Additional resources and reading:

• Why Not to Grow Tropical Milkweed: https://xerces.org/blog/tropical-milkweed-a-no-grow
• Protecting Pollinators from Pesticides: https://xerces.org/pesticides/pesticides-your-garden
• Making Your Yard Pollinator Safe: https://xerces.org/publications/fact-sheets/smarter-pest-management-protecting-pollinators-at-home
• Why Captive Rearing Isn’t the Way to Help Monarchs: https://xerces.org/blog/keep-monarchs-wild
• Milkweed FAQs: https://www.xerces.org/milkweed-faq
• Saving Monarchs from Extinction: https://assets.speakcdn.com/assets/2332/program_plan_safe_north_american_monarch_2020-2023.pdf