Monarch butterflies, milkweed, and migration: The law of unintended consequences

I described the magic of migration by monarch butterflies in Chapter 1 of Tools for Critical Thinking in Biology. These tiny animals fly from breeding areas in the northern US and Canada to overwintering sites in high-elevation forests in central Mexico each fall. After a period of semi-dormancy until spring, the adult monarchs mate, return to the southern US to lay eggs on milkweed plants, and then die. Larvae hatch from these eggs to feed on milkweed foliage, then pupate on milkweed (or nearby lawn furniture) until adult butterflies emerge. Some of these adults fly further north to reproduce, and these successive journeys north continue for 3 or 4 generations until monarchs have reached southern Canada again. In fall, the process repeats itself, with Canadian monarchs flying 2,500 miles to Mexican overwintering sites that they have never seen.

Tagged monarch butterfly

Monarch butterfly wearing an ID (photo by Jim Gagnon)

This story of migration by monarch butterflies is magical because it took 40 years and the help of thousands of volunteers for two Canadian scientists, Fred and Norah Urquhart, to discover the wintering area in the mountains of central Mexico. The Urquharts developed a method of tagging butterflies; volunteers throughout North America reported their captures of tagged butterflies to the Urquharts and did tagging of their own to gradually extend the known range of movement of the animals. The story is also magical because scientists have used observations and experiments to learn what triggers migration in the fall, how monarchs from as far north as Canada find their way to Mexico, and why migration may benefit monarchs. I describe some of this research in my book, but the research continues and important new discoveries have extended the story since the book was published in April 2015. One of these stories links migration and milkweeds, with potential unintended consequences for conservation of monarchs, so I need to give a little background about milkweeds and conservation before telling this story.

Why is milkweed important for monarch butterflies?

Adult monarchs eat nectar from a large variety of flowers, helping to pollinate the flowers in the process. Females lay eggs only on milkweed, however, and larvae eat only milkweed as they grow and develop. Milkweeds contain compounds called cardenolides that are toxic to many vertebrates; these compounds ingested by monarch larvae remain in the tissues of adults that develop from the larvae. Cardenolides make adult monarchs poisonous to predators such as blue jays, which can learn to avoid monarchs from trying to eat a single individual. This is another fascinating story that you can learn about here.

What is the conservation status of monarch butterflies?

Monarch butterflies face many risks, both during their breeding season in the United Sates and southern Canada and in their overwintering sites in central Mexico (and along the Pacific coast of California where monarchs breeding west of the Rocky Mountains spend the winter). Breeding habitat is lost to growth of cities, shopping malls, roads, and agriculture. The milkweed that monarchs depend on grows in and adjacent to agricultural fields; farmers now plant seeds of corn and other crops that are genetically engineered to be resistant to a herbicide called Roundup. This enables farmers to use Roundup to control weeds without killing their crops. These weeds include milkweed, so this process is detrimental to monarchs.

Some overwintering sites along the Pacific coast of California are in towns and cities, making them vulnerable to further development and loss of habitat, although local governments have set aside parks to protect monarchs that winter there. Most monarchs overwinter in the highlands of central Mexico, which have a history of disturbance by logging and cattle grazing. Monarchs in Mexico occupy a handful of sites with a total area of 1 to 18 hectares (2.5 to 45 acres; an acre is about the size of an American football field without the end zones). The Mexican overwintering population fluctuates from year-to-year depending on how successful breeding was the previous summer, how much mortality occurred during migration, and weather conditions in the overwintering sites, but the general trend has been downward since 1995. The lowest estimate of the Mexican overwintering population was 25 million in 2014, although the population rebounded to 150 million in 2016. These may seem like very large numbers, but the Mexican overwintering population has decreased by more than 25 million monarchs eight times since 1995. If this had happened between 2014 and 2015, monarch butterflies east of the Rocky Mountains would be extinct.

Monarch Butterfly Overwintering Populations.

Estimates of number of monarch butterflies overwintering in Mexico, 1995-2016. From Center for Biological Diversity.

What can citizens do to help protect this charismatic insect? One strategy is to plant milkweed in our yards to compensate for the loss of milkweed in agricultural areas. There are about 100 species of milkweeds native to North America, most of which are used by monarch butterflies. However, the most common milkweed available to gardeners in plant nurseries is an exotic species called tropical milkweed. Unlike North American milkweeds, tropical milkweed remains green and continues to flower in fall and winter in places with mild climates, like the Gulf coast states. Because of this newly available food supply, some monarchs migrating from farther north remain through the winter in areas along the Gulf coast rather than continuing to Mexico. These monarchs breed through the winter and have established non-migratory populations that remain along the Gulf coast all year.

Common milkweed, native to North America, and tropical milkweed, often sold in plant nurseries and planted by gardeners in North America, to the possible detriment of monarch butterflies.

Does this sound like a problem? Probably not, until I give you one more piece of information. As I described above, monarchs have an effective chemical defense against many vertebrate predators. They are also attacked by several parasites, the most common of which is a protozoan called Ophryocystis elektroscirrha (OE). There are populations of monarchs that are year-round residents in Hawaii and Florida, and OE is much more prevalent in these populations than in migrating populations, perhaps because migration culls infected butterflies, who can’t fly as well as healthy ones, or because migration enables butterflies to escape areas where they can become infected. In a recent study, Dara Satterfield and colleagues found that the new, non-migratory populations along the Gulf coast have higher infection rates than migratory monarchs. Satterfield’s group suggests that these sedentary monarchs along the Gulf coast could act as a reservoir for OE, causing infection of migrants as they pass through this area on their way to or from the wintering sites in Mexico. In trying to help monarchs by planting exotic tropical milkweed in our gardens, we may instead be putting them at greater risk. If you’re interested in contributing to the conservation of monarch butterflies, why not share this story with your local nurseryman, and ask him to sell native milkweed instead of the exotic tropical species that contributes to the unintended consequence described here?

Monarch migration

Migration of monarch butterflies in eastern North America. Green diamonds show the main locations of overwintering monarchs in the highlands of central Mexico. Red squares show alternative sites where small numbers of migrating monarchs spend the winter. Blue dots show locations of new sedentary (non-migratory) populations along the Gulf Coast. From “Loss of migratory behaviour increases infection risk for a butterfly host”, by D. A. Satterfield et al., 2015.

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