“Monarch Butterflies Drop Stuns Scientists” was the SF Chronicle front-page headline on January 17, 2019. The story covers the results of the winter 2018 monarch count in 213 forested groves in California this last winter. The results are alarming: the count showed an 86% drop from a year ago, a 99.4% drop since the 1980s, and “...an all-time low for the Pacific Coast, where an estimated 10 million monarchs once blanketed trees from Marin County to the Baja California peninsula…”.
Eastern Monarchs overwinter in Mexico and migrate north through the eastern United States. They had a much better year. The number of Monarchs (scientific name Danaus plexippus plexippus) that overwintered in Mexico was more than twice as large as the prior year. Something to celebrate, certainly, but the long-term trend is not as rosy. The total population is still only about ⅓ of what it was two decades ago.
The Monarch is beloved because it’s big and showy (have you ever seen a eucalyptus grove full of monarch butterflies? If not, take a look at this video. But it’s not just Monarchs; many other butterfly species have seen steep population declines or even gone extinct in the recent past. Dr. Arthur Shapiro, Professor of Evolution and Ecology at UC Davis, is one of the world’s leading butterfly experts. He’s been monitoring butterfly populations in California since 1972. In this interview, he notes that what is happening to Monarchs is also happening to many species in our state, due to the “...large loss of habitat and habitat connectivity, and the [landscape] is increasingly becoming more butterfly-sterile.”
Among the Lepidoptera species that have seen steep population declines in the past 10-20 years are the beautiful blue-and-black pipevine swallowtail (Battus philenor), the callippe silverspot (Speyeria callippe callippe), the mission blue (Icaricia icarioides missionensis), the San Bruno elfin blue (Incisalia mossii bayensis) and the Bay checkerspot (Euphydryas editha bayensis). Battus philenor, for example, has nearly disappeared due to loss of habitat.
The Xerces Society for Invertebrate Conservation, an international nonprofit organization that protects the natural world through the conservation of invertebrates and their habitats, tracks monarch populations. They offer a Western Monarch Call to Action plan to help the monarch population bounce back:
Visit their web site to read more about this plan and learn how you can get involved if you want to.
Mariposa partners with several organizations that do important work to preserve and create pollinator habitat in the Bay Area, and they all offer resources, education, and events that you can use to learn more. They include:
The Xerces Society recommends some steps you can take in your own garden to help Monarchs survive:
If you do plant milkweed, be aware that not all milkweed is alike. Native California species such as narrow-leaf and showy milkweed go dormant in the winter. This encourages the butterflies to migrate as they normally would. Tropical milkweed does not go dormant, blooming through the winter. This can encourage Monarchs to stick around (hey, plenty of food here, why take that dangerous journey?)
When Monarchs don’t migrate it can cause build up of a bacterium that weakens the developing insect as it transforms from caterpillar to butterfly. OE (Ophryocystis elektroscirrha) is a protozoan parasite that infects butterflies in the Danaus species group, those that host on milkweed. In the US, OE affects Monarch (Danaus plexippus), Queen (Danaus gilippus), and Soldier (Danaus eresimus) butterflies. When butterflies don’t migrate, OE within the population can become quite high. As a result many caterpillars are infected and the adult butterflies are weakened or even killed.
To make a long story short: it’s preferable to plant native milkweeds. If you do plant tropical milkweed in your garden, be sure to cut it back in September. Keep it cut back until the spring, to encourage Monarchs to migrate south during the winter months.
Find sources of milkweed
How to grow milkweed