Milkweed and the Monarch by Heather Doherty was originally published in the BFEC Newsletter, Vol. 18/No. 2, Spring 2014.
North America’s most well-known butterfly is quickly becoming it's most threatened due to loss of the plant that it can’t live without.
We Americans are lovers of wildlife, though we are selective. We adore fuzzy mammals like dogs and bunnies, and diminutive songbirds that visit backyard feeders. Insects, however, don’t usually make the list, save for a tiny echelon that by some accident of nature qualify as “cute.”
Lightning bugs, ladybugs (before the invasive Asian form started swarming homes) and perhaps dragonflies are in this group, as is the monarch butterfly.
The monarch is the most well-known butterfly in North America, or in some cases, the only known butterfly. Young visitors to the BFEC wildlife garden often proclaim “it’s a monarch!” anytime a vaguely brown, black or orange-ish butterfly flitters by.
Why the attention? The monarch is abundant, beautiful and rather large by butterfly standards at up to four inches wide. But even more captivating is the amazing feat it pulls off every fall. It is the longest-migrating butterfly in the world, travelling up to 2,500 miles one-way from as far as southern Canada to central Mexico.
The monarch is also unique in that it requires a single kind of plant, milkweed (Asclepias) to survive, though this same plant is toxic to other animals. Its relationship with the plant, which it uses to lay eggs, has certainly made the monarch successful, but it is now becoming a vulnerability. This winter saw the lowest monarch population levels since record-keeping began twenty years ago, due largely to milkweed’s disappearance.
Monarchs usually appear in Ohio in late summer seeking food (flower nectar), mates and one of a dozen or so species of North American milkweed to lay eggs. When the eggs hatch, the young caterpillars feed exclusively on the milkweed.
“Common milkweed” is a species that is indeed a common “weed” in Ohio, where it often grows on roadsides. If you break a leaf, you’ll encounter a white, sticky sap, which gives the plant its name. It contains compounds that may irritate your skin, but gives monarchs a special advantage.
Caterpillars are usually easy targets for hungry birds, but predators will promptly spit out a monarch caterpillar thanks to its milkweed diet, which makes it unpalatable. The plant compound stays with the caterpillar through metamorphosis and continues to protect the flying adult.
Adults typically live six weeks or so, except for the last brood of the year that matures in early fall. As the days become shorter and nights cooler, monarchs cease egg-laying to begin their southern migration.
A small, mountainous region of central Mexico is the destination for monarchs living east of the Rockies into southern Canada and New England. A smaller population occurring west of the Rockies migrates to over-wintering sites on the coast of Southern California. Though the predecessors of these butterflies have been to these over-wintering sites before, the individuals who make this journey have not.
The monarchs congregate in ever larger groups as they head south, forming sizeable colonies. Usually by late October they arrive at a spot high in the mountains, about 100 miles west of Mexico City that has been protected as the Monarch Butterfly Biosphere Reserve.
Their arrival coincides with an important Mexican holiday, Día de los Muertos, or Day of the Dead, in which spirits of deceased relatives are believed to return home, and are honored with feasts and celebrations. According to local legend, spirits may take the form of the returning monarchs.
The butterflies spend the winter clustered tightly together on trees in the reserve. In late winter they mate and fly the first leg of the spring migration north, making it as far as Texas or Oklahoma. At up to eight moths old (ancient in butterfly years) and with perhaps 4,000 miles logged, these butterflies die, but not before laying eggs that will become the butterflies that continue the journey. It is the second, third, and fourth generations that return to Ohio and points north and east in July and August.
In 1994, biologists began recording the size of the monarch populations at their over-wintering grounds in Mexico by measuring the area that the densely-packed butterfly colonies occupy. The population hit a high point in the winter of 96-97 at nearly 21 hectares (or 8.5 acres), which was covered by hundreds of millions of monarchs.
Fast forward to last winter (2012-13) and the population measured just 1.19 hectares, much to the horror of monarch enthusiasts. The population had seen a steady decline over a dozen years, but this was a new low.
During the spring that followed, the monarch migration got off to a slow start, with the number moving north in May also at an all-time low. This trend was observed in Ohio, and at the BFEC, where we saw a half-dozen monarchs all summer and no caterpillars. This may be normal for some species that are less common, but unheard of for monarchs, which can seem ubiquitous.
The slow summer rounded to fall and time for the southward migration. But on the Day of the Dead in Mexico last November, the monarchs did not appear. They did start to straggle in a week later, but the winter population measured in January of this year was beyond bleak. At just .69 hectares, it represents a 94 percent reduction from the recorded high.
As with most problems, the contributing factors are many, and include the weather. Monarchs are cold-blooded animals, and need warm conditions to grow, fly and find food. A cold snap at the wrong time can be fatal to large swaths of the population. Cold rain or freezing temperatures in Mexico during hibernation can cause massive die-offs, which happened most recently in early 2010.
Weather that is too hot and dry can also lead to desiccation of host plants and larvae. Drought that gripped Texas, the central plains states and parts of the Midwest last summer delivered a very poorly-timed blow to the already small population of monarchs moving north.
Deforestation near over-wintering grounds has also been blamed, but more recently, Americans have focused on problems at home with loss of natural habitats and milkweed.
Loss of milkweed is in itself attributed to multiple factors. Urban and suburban growth, for instance, gobbles up massive acreage every year that formerly contained the plant.
Changes in the way that Americans farm has also reduced milkweed availability. The push for biofuels in the U.S. has led to a 10 percent increase in soybean and corn production since 2007. With corn prices high, farmers are incentivized to leave smaller and smaller strips of land along roadsides and fence rows, a niche where milkweed formerly thrived.
But perhaps the biggest issue of all was quantified in a study released in early 2012 by researchers at the University of Minnesota and Iowa State University, who studied the use of all herbicide Roundup on farm fields. In the mid-90s, Monsanto, maker of Roundup, introduced corn and soybean seeds that were genetically modified to withstand roundup applications on farm fields. This allowed for the application of Roundup across huge section of land, leaving crops unharmed while weeds perished.
“Roundup-ready” seeds now account for 94 percent of all soybeans and 70 percent of all corn grown in the U.S. According to the researchers, this use corresponds to a 58 percent decline in milkweed plants growing in the Midwest from 1999-2010.
To add insult to injury, they fount that milkweed growing on the edge of farm fields is more productive for monarch breeding than other patches, leading to a disproportionately high decline in monarch egg production, which fell by 81 percent.
The researchers acknowledged that wide-spread use of herbicides is unlikely to change, which makes conservation and enhancement of non-agricultural habitat more important than ever. Sprawling lawns in front of corporate offices or the miles-upon-miles of mowed grass along interstate highways, for instance, could be put to better use.
Organizations like the Monarch joint Venture, with partners in federal agencies, nonprofits, and academic institutions, have been created to support projects like these, as well as education and research (monarchjointventure.org).
There’s another habitat in the U.S. that could also support milkweed and butterfly-friendly plants: the ubiquitous residential lawn. Have some open, sunny space that you’d like to stop mowing? Consider planting milkweed, along with other food sources for adult butterflies like coneflower, black-eyed susan, bee-balm and goldenrod.
Ready to plant milkweed? GOOD! Here are a few things to know about three varieties that grow readily in central Ohio and are good hosts for monarch eggs. In general, milkweeds feature clusters of small, star-shaped flowers, prefer full sun and are “seldom damaged” by deer.
Common Milkweed (Asclepias syriaca): This is the 3-4 inch milkweed variety that grows commonly on roadsides and ditches. Its flowers are white and pink and have an amazing, lilac-like fragrance. Seed pods release seeds with tiny “parachutes” that float in the breeze. This plant has limited commercial availability, but look for seeds in catalogs.
Butterfly Weed (Asclepias tuberosa): This compact milkweed is great for more formal gardens. It grows 2-3 inches tall, has beautiful, bright orange flowers and prefers medium to dry soil. It’s also the easiest variety to get your hands on — look for it at local nurseries.
Swamp Milkweed (Asclepias incarnate): Also known as red milkweed, this plant grows wild in wet soils that border ponds and creeks. It grows to about 3 inches and is adorned with pink flowers. Visit the BFEC’s favorite native plant nursery online to purchase plants: prairienursery.com. Best in wet soil.