The last time we visited the magnificent monarch butterfly was August 27, 2018. That particular August turned out to be a rather good season for monarchs at my home in Columbia, MD. Monarch season this year got off to a pretty slow start with the first monarch arriving in the last week of July. Since then, on a daily basis, monarchs have been gliding about the flower beds, sipping nectar from the zinnias and cup plants during daylight hours. Their journey to grace my garden began months ago in early spring. In the Eastern US, these remarkable vagabonds depart their winter refuge in Mexico and continue their journey north for months, reaching milkweed patches in northern states and Southern Canada. In the Western US, a similar migration occurs as monarchs depart overwintering grounds in coastal California and head north to exploit patches of tender milkweeds as they become available in spring and summer. As summer wains days grow shorter and temperatures cool, and these travelers return to their hibernal redoubts. Eastern monarchs embark on an epic journey thousands of miles south to Oyamel fir forests in the mountains of Mexico. Western monarchs return to eucalypt and pine forests of coastal California from summering grounds in Washington, Oregon, Idaho, and several other western states. While 2018 was a rather good year for monarchs in my garden in Maryland, the outlook for migratory monarchs in Eastern and Western North America isn’t so rosy.
In a previous episode, we communed with thousands of western monarchs in Pacific Grove, CA, also known as “Butterfly Town, USA.” During the brief six years since our last visit to Pacific Grove in 2014, overwintering populations of monarchs in California nose-dived from tens of thousands to a shocking 1,914 counted in the 2020 winter census, a decline of more than 99.9% from historic levels. Eastern monarchs have fared better but still their numbers have declined dramatically to around 20% of historic high levels. Many believe it may already be too late to save the Western migratory monarchs and Eastern monarchs may not be far behind. Attempts to have monarchs declared an endangered species by the US Fish and Wildlife Service failed in December of 2020 as resources were needed to focus on “higher-priority listing actions” according to agency officials. The case for endangered species status will be revisited in upcoming years.
Scientists studying monarch butterflies believe multiple factors conspire to threaten populations of monarchs. Illegal logging of trees in the mountains of Mexico has reduced the critical overwintering habitat for monarchs. Without this refuge monarchs cannot survive winter. Scientists suggest that more severe weather events associated with climate change may also threaten monarchs. Freakish winter weather in the mountains of Mexico in 2002 killed an estimated 75% of overwintering monarchs. In the winter of 2015–2016, a late winter storm killed more than 7% of overwintering monarchs. These events translated into tens of millions fewer monarchs making their way north for the annual migration.
In the US Midwest and Pacific Northwest, record summer heat in 2012 and 2015 killed untold numbers of monarch caterpillars. Throughout the US, urban sprawl and the use of herbicides in agricultural production greatly reduce populations of milkweed plants that are vital for the survival of monarch caterpillars. Increasing levels of atmospheric carbon dioxide may change the chemical composition of milkweeds, altering levels of pharmacological chemicals that help monarch caterpillars thwart disease-causing microbes. There is also concern that planting the exotic tropical milkweed, Asclepias curassavica, may interrupt the primal migration behavior of monarchs and cause them to take up residence in areas where tropical milkweeds are planted, derailing their age-old migrations. Another study suggests that when tropical milkweed grows at high temperatures, it becomes more toxic to monarch caterpillars. As the world warms and the growing season get longer, monarchs may expand their range further northward into Canada in search of high-quality milkweeds. This may increase risks of starvation or predation as monarchs attempt an already very long and perilous journey to overwintering grounds in Mexico.
What can be done to help save these unique and charismatic creatures? Globally, mitigating climate change, reducing unnecessary pesticide use, and conserving resources and habitats for wildlife will help. Locally, providing milkweeds for monarch caterpillars and nectar plants for adults will facilitate reproduction and survival. Regional references for milkweed plants can be found at this link https://xerces.org/milkweed and references for monarch nectar plants can be found at this link https://xerces.org/monarchs/monarch-nectar-plant-guides. Be sure to consult a reference to learn what milkweeds work well in your geographic region. Here in Maryland, species including common milkweed, Asclepias syriaca, swamp milkweed, Asclepias incarnata, and butterfly weed, Asclepias tuberosa, are good choices.
After imbibing nectar, the monarch finds just the right leaf on which to place an egg. Under the watchful gaze of a milkweed leaf beetle, a tiny monarch caterpillar consumes a leaf but soon it will grow into a behemoth capable of consuming milkweed seedpods. Within a dazzling chrysalis, a caterpillar becomes a butterfly ready to feed, fly, and spawn the next generation.
In the waning weeks of summer, go to the meadow and enjoy these beauties. Next spring plan to include milkweed and monarch nectar plants in your perennial gardens. We have a role to play in conserving these remarkable wanderers.
The excellent references, “Decline of monarch butterflies overwintering in Mexico: is the migratory phenomenon at risk?” by Lincoln Brower and colleagues, and “Quantifying impacts of climate change on species interactions while fostering undergraduate research experiences using the Monarch (Danaus plexippus)- Milkweed (Asclepias Sp.) system” by Matthew J. Faldyn, “Monarch butterflies denied endangered species listing despite shocking decline” by Farah Eltohamy, and “ We’re losing monarchs fast—here’s why. It’s not too late to save them, but it’s a question of whether we make the effort” by Carrie Arnold, were consulted for this episode. To learn more about monarchs, their migrations and perils, and how to conserve them, please visit the following websites: