Western Great Lakes Bird and Bat Observatory

Citizen Science

The Miraculous Monarch

Monarchs by KR MonCat by KR

Monarchs by Kate Redmond  

By Kate Redmond

Ecologically speaking, putting almost all of your eggs in one basket is a really bad idea. 

Quick review: Monarch butterflies (Danaus plexippus) feed on a variety of flowers but lay their eggs only on plants in the milkweed family. The bright, aposematic (warning) colors of both the butterfly and its caterpillar alert potential predators to back off. (Poisonous milkweed sap renders the caterpillar toxic, and it carries its toxicity into adulthood.) 

Monarchs migrate. Let’s unpack that simple, two-word sentence, because the biology behind it is complicated (and miraculous), and whole books have been written about it.

There is a Pacific population of Monarchs that migrate only as far as southern California in winter (though some free spirits have wandered as far as Hawaii and beyond), and there are year-round breeding populations in the Florida peninsula and along the Rio Grande. Historically, all of the migratory Monarchs east of the Rockies overwintered on a dozen or so smallish sites in the oyamel fir forests of the Central Highlands northwest of Mexico City. 

Monarchs usually return to Wisconsin by mid-May. Researchers don’t know whether the late migrations recorded in the past few years are random events or are due to climate change. Once they get here, there are several short-lived generations that serve to produce more Monarchs, but in mid-August, a generation is produced that uses its energy to migrate rather than to procreate. This final brood, called the Super-generation or Gen 5 (the fifth generation removed from Mexico), is exceptionally long-lived and is the only generation that is tagged for migration studies. Experiments suggest that “host plant quality” (Monarch caterpillars prefer tender, young leaves, not the leathery plants of late August), decreasing day length, cooler overnight temperatures, and the changing tilt of the earth combine to flip a switch in the Monarch’s brain (which is about the size of the head of a pin). Wherever Monarchs are, they are cued to start moving south when the sun recedes to around 57 degrees above their southern horizon. (They migrate along the shore of Lake Michigan.)    They navigate using an internal “sun compass” that allows them to adjust their orientation in relation to the sun’s movements across the sky, and they probably also sense magnetic fields with their antennae. This allows Monarchs in Michigan, Maine, and Montana to set correct (but different) flight plans for central Mexico. 

The butterflies move south in the fall, spending the winter some 1500 miles south of Wisconsin. Monarchs eat little on their wintering grounds, depending on fat reserves built up during their leisurely migration. (A newly emerged butterfly has about 20 mg of fat in its body, but a Monarch newly arrived in Mexico carries 125 mg of fat.) In spring, signaled by warming temperatures and the availability of milkweed, Monarchs become reproductively ready and head back north, being careful not to outpace the emerging milkweed plants. They lay eggs in the southern tier of states, and their offspring continue the journey.

The spectacle of Monarch butterfly migration is one that inspires international awe (and, not surprisingly, sometimes-invasive ecotourism), and the drastic decline in Monarch numbers over the past decade has resulted in quite an outcry. Some numbers: In the mid-1990s, an estimated one billion Monarchs migrated to central Mexico, with another million counted in California. Today, the entire population is estimated at 56.6 million—a decline of more than 80%. (A 2013 World Wildlife Fund count was 33 million.) During the winter of 1996-97, wintering butterflies covered 21 hectares (one hectare is just under 2.5 acres). In 2013-14, they occupied only two-thirds of one hectare of forest, and 80% of the butterflies were found on just a few sites. Predictions for this year, if all the ducks stay in a row, are more optimistic, but far from rosy.

What’s happening? In a nutshell:  


Monarch survival depends on favorable weather in their summer range, during migration, and on the wintering grounds. The extreme weather events associated with climate change will affect Monarchs negatively—cold, wet winters; a warmer climate in Mexico that may negatively impact the oyamel fir trees; large storms or stubborn frontal systems at the start of the spring migration; and prolonged heat and drought that shorten the blooming periods of flowers (and therefore the availability of nectar), especially in plants that the butterflies depend on during migration. (Remember 2012, when even the prairie plants looked wilted?) 

It’s cold up in the mountains, but wintering Monarchs can tolerate cold temperatures if they’re dry, and they burn fewer calories idling in cool weather. Rain or snow that wets the butterflies, followed by freezing temperatures, are a one-two punch, especially in January and February, when butterfly thoughts are turning northward and their fat supplies are waning. In the winter of 1995, snowstorms on the wintering grounds caused greater than 60% mortality. As the butterflies headed north in the spring of 1996, el Nino delivered heavy rains in the southeastern United States, and more died. In 2002, 2004, and 2010, huge Pacific weather systems brought heavy rain, hail, and frigid air into the wintering areas, decimating the Monarch population. The 2002 storms resulted in a 75% casualty rate.  


Although the Mexican government has banned logging in the area, illegal logging continues to nibble away at the oyamel forests, which affects the microclimate created by the trees’ canopy and allows greater temperature fluctuations within the forests. The trees are also threatened by bark beetles. North of the Rio Grande, commercial-urban-suburban development is replacing Monarch habitat at a rate, according to one source, of 2.2 million acres per year (6,000 acres a day). The biofuel movement accounts for some of this loss, with land being taken out of the Conservation Reserve Program to produce corn to make ethanol. The tricky part, of course, is balancing the needs of the Monarchs and the needs of the people who live in the area.  


The Corn Belt produces more than half of the Monarchs that migrate to Mexico. Common milkweed (Asclepias syriaca) is their food of choice, but common milkweed is disappearing, largely due to today’s agricultural practices. Crops like corn and soybeans are genetically modified so that they can survive exposure to herbicides like Roundup, allowing farmers to spray more often. (Crops are in the works that will have multiple resistances.) Sprayed chemicals often drift past the borders of agricultural fields, and roadside flowers become collateral damage. Neonicotinoid insecticides kill agricultural pests, but innocent bystanders are also exposed to this neurotoxin when they nectar on adjacent wildflowers. 

The clean farming trend that advocates removing fence rows and non-agricultural vegetation between cultivated fields leads to the loss of native plants and wildlife, and roadside mowing takes a toll, too. Milkweed is slow to regenerate, and one study suggests that 60% of milkweed has been eliminated from the grassland ecosystem in the Midwest (http://www.mlmp.org/results/findings/pleasants_and_oberhauser_2012_milkweed_loss_in_ag_fields.pdf). The tricky part, of course, is balancing the needs of the Monarchs against our desire for inexpensive food.


Ophryocystis elektroscirrha (OE) (http://monarchparasites.uga.edu/whatisOE/) is a protozoan parasite that infects only Monarchs and the related Queen butterflies. One theory is that Monarchs pick up the protozoan from the leaves of tropical milkweeds that don’t die back in the winter and so have a longer period to harbor the parasite and its spores. Female Monarchs pass it on to their eggs and to the plants they land on, and caterpillars ingest it as they feed. Most of the damage is done during pupation, and emerging adults are covered with spores. Infected adults may simply be less fit (the long migration south is Darwinian natural selection in action), but a heavily infested Monarch may have difficulty emerging from its chrysalis and may have deformed wings. OE was first noted in South Florida and has been around for almost 50 years, and although it’s been found in all of the populations of Monarchs, the highest rates of infection are found in non-migratory groups (reaching 70% in South Florida). The infection rate in eastern migratory Monarchs is between 7% and 8%.  


There were three hurricanes and two tropical storms at the start of the 2017 fall migration period https://monarchwatch.org/blog/2017/09/15/forgotten-victims-of-harvey-the-pollinators/.. The fall migration of 2017 along the Atlantic Coast was late, with some butterflies lingering into late October and even early November, lulled by unseasonably warm weather, the late migrants left susceptible to storms and freezes.  

Chip Taylor at Monarch Watch (https://monarchwatch.org/) issues frequent updates about Monarch populations. As of March 18, World Wildlife Fund Mexico in collaboration with CONANP and the Monarch Butterfly Biosphere Reserve, announced the total forest area occupied by overwintering Monarch colonies. Nine colonies were located, with a total area of 2.48 hectares, a 14.77% decrease from the previous season. 

He also says, “we need a comprehensive plan on how to manage the fragmented edges and marginal areas created by development and agriculture since it is these edges that support Monarchs, many of our pollinators, and the many forms of wildlife that are sustained by the seeds, fruits, nuts, berries, and foliage that result from pollination.”  


The plight of the Monarch has energized people and organizations across the country. The Monarch Butterfly Biosphere Reserve, which contains most of the overwintering sites, was created in 1980 and declared a World Heritage Site in 2008. The International Union for Conservation of Nature declared Monarch migration as a Threatened Phenomenon, and the US Fish and Wildlife Service is considering adding Monarchs to the Endangered Species list. Monarchs are expected to benefit from the 1,500-mile-long “Butterfly Highway” proposed by President Obama as part of his efforts to help native pollinators (http://www.csmonitor.com/Environment/2015/0522/How-Obama-s-butterfly-highway-paves-way-to-save-embattled-monarchs). 

The Monarch’s population plunge has been publicized by (among others) the National Wildlife Federation, the Defenders of Wildlife, the Xerces Society, World Wildlife Foundation, USDA Natural Resources Conservation Service, the Natural Resources Defense Council (suing the EPA over pesticide regulations), and a whole bunch of newspapers and periodicals. The US Fish and Wildlife Service is committing $20 million for habitat restoration over five years. The Monarch Joint Venture (http://www.monarchjointventure.org/) is a partnership of federal and state agencies, non-governmental organizations, and academic programs working together to support and coordinate efforts to protect the Monarch migration across the lower 48 United States. And there are many grassroots organizations. 

WHAT YOU CAN DO (among other things):

* Plant native milkweed. (You’ll be able to choose from three native species at our plant sale during our World Migratory Bird Day celebration.)

* Plant flowers for butterflies to nectar on. (At our plant sale, we’ll also be selling native nectar plants that adult Monarchs rely on.)

* Join the growing number of people who collect Monarch eggs from vulnerable locations (like the paths of mowers) and raise them indoors for release (instructions on-line).

* Refrain from celebrating a birth or death by releasing commercially raised butterflies.

* Be ultra-cautious in your use of pesticides.

  * And be a citizen scientist—that is, add your observations of eggs and adults to the data at Monarch Watch (http://www.monarchwatch.org/) and/or the Monarch Larva Monitoring Project (http://www.mlmp.org/).

Such a fragile balance. "The Monarch butterfly unites the three countries of North America in peace. It is an ambassador of peace which requires protected areas and ecosystems that are preserved through sustainable agricultural and forestry practices. Let us continue to work together to maintain a healthy and balanced ecosystem for all North America."—President Jimmy Carter