The Marvel of the Monarch!

Monarch on a prairie plant
© T. Kemmerer

It is one of our most common butterflies, yet it has one incredible story. The monarch butterfly (Danaus plexippus) is on Conservancy preserves and in our backyards. Even the most inexperienced gardener can usually distinguish the monarch by it's orange, black, and white markings.

There are two populations of monarchs in the United States. The western population exists west of the Rockies and winters on the California coast. The eastern population stretches from southern Canada to Mexico. Every fall, the eastern population of this butterfly, that weighs between .27 and .75 grams, flies up to 3,000 miles from its northern habitat to Mexico!

The publication date of this newsletter (third week of September) coincides with the peak time of the monarch migration in central and southern Ohio, though you can still see stragglers through October. If you are reading this newsletter on your porch, you may be lucky enough to watch one monarch after another fly by at a leisurely pace (about 12 miles per hour) going southwest on their long journey. Their destination is a small area in the mountains just west of Mexico City, with an estimated arrival time of November 18^th.

Ohio Chapter volunteers play a big part in the monarchs ability to make this journey and to breed because of their work to restore native plants such as milkweed, which these butterflies are dependent on for survival. Our volunteers not only protect milkweed by eradicating non-natives that threaten this plant, but we also have project days when volunteers plant natives, such as butterfly and tall green milkweed, at Kitty Todd. Milkweed species in Ohio include sand, tall green, swamp, purple, common, prairie, butterfly, whorled, spider, and green flowered.

Why is milkweed so important to the monarch butterfly?

The milkweed plant contains a poison that is not harmful to monarchs and actually protects them from predators by making the butterflies taste unpleasant. Thus, predators associate the bad taste with the bright colors of the butterfly (the viceroy butterfly has evolved to liken its colors to the monarch, even though it isn't poisonous, as a self-defense mechanism). When a female monarch lays her eggs, it only looks for the milkweed plant. The female can lay many eggs in one day, but only one at a time. She lays them on the underside of the leaves, and depending on temperature, three to twelve days later a caterpillar (larva) is hatched. The caterpillar eats the leaves of the plant for a few weeks for nourishment while growing and then forms a shiny green and gold speckled chrysalis (pupa). The pupa hangs from the stem or leaf of the milkweed. The pupa slowly becomes clear, and after 14 days, the adult monarch butterfly emerges from the chrysalis. The adults use nectar from blooms of milkweed and other native plants for nourishment.

Monarchs have four generations of adults per year. The first three generations of adults live about 2 to 6 weeks. It is the fourth generation that makes the flight down to Mexico, and if they are able to avoid all the hazards of the journey, could live up to 9 months.

Getting from there to here and back

Like other insects, the monarch is unable to survive through a long cold winter. To have the species survive, Eastern Monarchs migrate only to the Transvolcanic Mountains in Mexico, where there are only eleven to fourteen known sites each year. Each site is a few hectares in size. Two to five hundred million dormant monarchs cluster together in these mountains to try and stay warm. Towards the middle of March, they begin to migrate north. They need to head north as milkweed does not grow at their wintering site and they have to travel where it is abundant.

They arrive on the United States Gulf of Mexico coast a few weeks later, mate, lay eggs on southern milkweed, and die. These offspring go through the life cycle and continue the migration north to repeat this process once they reach the Great Lakes region in early June. They then lay eggs producing the second generation and die. Two more generations of the butterfly will be produced.

The last generation emerges from their chrysalis in late summer and early fall. They are different biologically and behaviorally from those emerging in the summer. They will not mate or lay eggs until the following spring on the Gulf of Mexico coast. Instead, they invest energies into getting ready for their long journey south. In fact, as they stop for nectar on their flight, they actually weigh more at the end of the trip than when they started! The fat they build up is key not only for the journey to Mexico, but is also critical for them to survive the winter and have enough energy the following spring for the flight back north.

If they complete the trip, they will go to the same tree in the Mexican forest that their ancestor left back in March!

Mystery of the Monarch

It has been a mystery to scientists how the monarch knows when to migrate and how they get to the exact tree it's ancestors were previously. And how do they make such a long journey when the organism is so small? [The Aztec believed the adult monarch butterflies to be the incarnation of their fallen warriors, wearing the colors of battle!]

A recent study whose findings were released in May shed some "light" on why they migrate. It has been known that monarchs are very adept at sensing ultraviolet light, the wavelength of light that the human eye cannot see and is responsible for causing skin cancer. Their ability to see ultraviolet light is so precise, they can even detect the sun's angle on a cloudy day, allowing them to always head south.

In a study led by Steven Reppert of University of Massachusetts Medical School, he found that they actually rely on polarized ultraviolet light. Polarized light is filtered to vibrate on one plane instead of all directions. The study also found that ultraviolet photoreceptors dominate the part of the monarch eye that specializes in polarized light detection. When they blocked ultraviolet light in flight tests, monarchs lost their sense of direction.

They also pinpointed the location of the monarchs circadian clock, a processor in the brain that governs 24-hour activity and metabolic cycles of animals, from humans to insects. Key genes of the clock are found in the dorsolateral protocerebrum section of the brain, where tiny neural fibers connect the clock to polarization photoreceptors in the eye. This is a critical link for the butterfly to compensate for the time of day in using their "sun compass". Says Reppert, "This pathway has not been described in any other insect, and it may be a hallmark feature of butterflies that use a time-compensated sun compass."

While this may explain how the monarch finds its way south, it still does not explain how it finds the same tree as it's ancestor.

And how can they make such a long flight when they are so small? Many scientists believe that monarchs conserve their "fuel" in flight by gliding on air currents. They also use thermal air rising from the ground to help with their lift.

Monarch getting nourishment
© T. Kemmerer

Threats to the monarch

While there are hundreds of millions of monarchs, it is hard to think that they may be a threatened species. However, human activities and global climate change are having effects on the monarch.

In their northern habitat, monarchs face habitat destruction caused by new roads, housing developments, and agricultural expansion. Milkweed, which is considered a noxious weed by many, is often destroyed by landscapers, farmers, gardeners, and others. In some areas, milkweed is being affected by ozone. Monarchs are also being killed outright by many pesticides.

In the Transvolcanic Mountains in Mexico, monarchs high concentration in only a few small sites make any possible habitat destruction in the area very serious. The trees they roost in, such as the oyamel tree, are valuable economic resources to the local people. Another serious issue is that non-roosting trees taken out open the canopy in the forest, which let in rain and snow that makes the roosting monarchs more susceptible to freezing.

Global climate changes are also a threat to the roosting monarchs. Winters in the mountains are cool, but as long as it is not too wet, the butterflies can survive. Experts say that there have been major die-offs three out of the past five years due to bitter cold, severe rain storms, and drought, which killed up to 75% of the dormant monarchs in 2002 and 2004. They were able to recover in 2002, but in 2004 Chip Taylor, founder of Monarch Watch and professor at the University of Kansas said, ". . . there was not a good recovery because of the low spring population, the summer being extremely cold, and the loss of habitat in the upper Midwest."

A study led by Karen Oberhauser of the University of Minnesota looked at the effects global climate changes may have on monarchs. This study found that all current climate models predict more storms in places where monarchs spend the winter, and this increased moisture will result in more frequent storms. According to Oberhauser, "The conditions that monarchs need to survive the winter are not predicted to exist anywhere near the present overwintering sites." She explains that, "The temperatures won't change much, but the combination of coolness and increased rain will hurt."

What can you do to help the monarch butterflies?

There are many ways that you can help the monarchs while they are in their northern habitat. Volunteer on Conservancy preserves to help eradicate non-native plants that crowd out natives like milkweed. Plant a butterfly garden in your backyard, and plant plenty of milkweed to support the monarch lifecycle. Educate others about the monarch and share with them the marvel of their story. Perhaps they, too, will plant milkweed in a newly planted butterfly garden and volunteer on Conservancy preserves across Ohio to restore native habitat.

For more information on the monarch, www.monarchwatch.org is a good resource.

Resources

Adams, L.J. (2003). After huge losses last year, Monarch butterflies come back strong. Pittsburgh, PA: Pittsburgh Post-Gazette. Retrieved August 16, 2005, from http://www.post-gazette.com/healthscience/20030223butterfliesenviron2p2.asp

Butterfly's navigation secret revealed: Flight simulator shows sensors hard-wired to circadian clocks. (2005). New York, NY: MSNBC.com. Retrieved August 17, 2005, from http://www.msnbc.msn.com/id/7736111/

Conservation. (1999). Lawrence, KS: Monarch Watch. Retrieved August 15, 2005, from http://www.monarchwatch.com/conserve/index.htm

Gokal, S. (2005). Experts: Monarch butterfly population in jeopardy. Atlanta, GA: CNN. Retrieved August 15, 2005, from http://www.cnn.com/2005/TECH/science/04/20/monarch.butterfly/

How monarch butterflies are wired for navigation. (2005). Physorg.com. Retrieved August 17, 2005, from http://www.physorg.com/news3978.html

Malcolm, S. (1997). Monarch Migrations . . . with clarifications. Kalamazoo, MI: Butterflywebsite.com. Retrieved August 15, 2005, from http://butterflywebsite.com/articles/monarch.htm

Migration and Tagging. (2004). Lawrence, KS: Monarch Watch. Retrieved August 15, 2005, from http://www.monarchwatch.org/tagmig/

Monarch Butterfly Lifecycle. (2000). Palisades, NY: The Wild Ones Animal Index. Retrieved August 15, 2005 from http://www.thewildones.org/Animals/monarch.html

Monarch Butterfly Page. (2005). Monarch Butterfly Website. Retrieved August 16, 2005, from http://www.monarch-butterfly.com/

Spice, B. (2003). Tagging program tracks fall migration of gorgeous Monarch butterfly. Pittsburgh, PA: Pittsburgh Post-Gazette. Retrieved August 16, 2005, from http://www.post-gazette.com/pg/03251/219200.stm

The Monarch Butterfly. (2005). Kidzone. Retrieved August 17, 2005, from http://www.kidzone.ws/animals/monarch_butterfly.htm