The southern journey of the monarch butterfly is probably one of the longest routes taken for any insect migration. Yet, it is crucial for their survival. Their behavior and reactions to environmental signals also tell researchers a lot about other things happening in the environment.
You probably notice their presence among other butterflies in your backyard every fall. It’s hard to ignore the bright orange and black pattern on their wings and there is something about monarch butterflies that makes them stand out among other butterflies and insects. In fact, there many reasons we are attracted to these remarkable insects.
It can be because of their beauty. It can be because they are pollinators of wildflowers. It can also be because they inspire you to learn more about nature, especially when you begin to uncover their great migration story.
Monarch butterflies can freeze and die in the cold and wet weather of winter. Thus, they have to migrate to a warmer place to survive the long winter months. Among the monarch populations, butterflies from eastern North America and Canada journey the farthest and may fly southward up to 3,000 miles during migration.
Most of these eastern North American monarch butterflies spend the winter season in the mountainous regions of Central Mexico located within the Oyamel Fir Forests. The Oyamel Fir plants protect the clustering monarchs from the cold weather. Remarkably, only one generation of monarchs successfully completes this annual southern journey.
Figure 1. Illustration of fall, spring and summer monarch migration paths.
hen the spring comes, the monarch butterflies leave their overwintering site. They begin flying north to the U.S. and Canada in search of milkweed to lay their eggs. The hatching monarch larvae eat milkweed exclusively. They store the toxic plant compounds in their body making them inedible for predators.
After the first generation of butterflies emerges, it takes at least three or more generations to complete the journey from Mexico to North America and Southern Canada.
Unfortunately, over the last 20 years, the number of overwintering monarch butterflies in Oyamel Fir Forests has been declining. Tracking monarch population and migration patterns is important because monarch butterflies are indicator species.
Indicator species tell us about the health of our environment. When the quality of the environment declines, the population or the health of this species decreases.
For an example, nematodes living in the soil are sensitive to soil environmental stressors. In the ecosystem, most nematodes help to decompose organic matter into minerals and nutrients. Therefore, the decrease of nematode population due to chemical pollution can further impact the diversity of plant community.
Figure 2. Nematode under a microscope
Monarchs are sensitive to environmental changes and signals. They can also tell us about the changes in biodiversity. The decline in population of monarchs may be caused by a combination of multiple environmental stressors including climate change, agricultural practices (such as: pesticide treatment), and habitat loss.
If their milkweed and wildflower habitats disappear, monarchs may lose their breeding and food sources along their migration path. This habitat loss can also impact many other important pollinators (for example: bees and other butterflies) and should be considered when establishing conservation practices within the U.S. and Canada. After all, we still depend on these beneficial insects to pollinate many agricultural crops, such as: pumpkin, cocoa, apple, pear, almond, and avocado.
The monarch migration journey is long and perilous but important, and there are many ways for us to help monarch butterflies along their way. One approach is to plant native milkweeds, flowering plants, and wildflowers in our yards. The nectar from these flowers will become food and energy sources for monarch butterflies during their long migration.
Figure 3. Monarch on a wildflower (likely native Joe Pye weed)
Some people have begun to collect monarch eggs, rear larvae on milkweed plants, and release the butterflies back into the wild. However, our help and interference without natural phenomena can sometimes impact nature with a negative cost. A recent study by Tenger-Trolander et al. found that monarch butterflies sold by one commercial breeder and those reared indoor lost their essential migration ability to orient properly.
This study comes as a reminder that monarch butterflies do need exposure to their natural environment conditions if they are to maintain their successful migratory behavior. Thus, according to this study the best approach to supplement monarch populations in the summer and fall may be to collect eggs or larvae from the wild and rear them outdoors in their natural environmental settings.
To better promote conservation efforts and aid monarchs, biologists have also encouraged people to prevent deforestation and protect the monarch’s natural overwintering sites. In addition to that, researchers have continued to investigate the environment and genetic factors influencing monarch migration so that we may one day be better suited to assist in the conservation of this remarkable species.
Every year, eastern North American monarch butterflies travel far to the south to a place that they have never visited before. Doing so means monarchs require both external environmental stimuli and internal factors to lead their way. Some external signals include cold temperatures and shorter day lengths.
Monarchs use their eyes to watch the sun’s position in the sky for a direction. They also use an internal circadian clock and stimulus receptors in their antennae to orient and know about the appropriate timing to migrate.
Like monarchs, we also use an internal circadian clock to tell our body about changes to rhythmic cycles and how to adapt to external environmental stimuli. One common stimulus for us is outside darkness which signals to us that it’s time to sleep (day-night cycle).
In insects, the internal circadian clocks are controlled by the activity of the clock genes encoded naturally in their genomes.
Researchers are currently investigating whether the monarch circadian clock can turn their clock genes on or off to start the migratory state. By using gene editing tools such as CRISPR/Cas9, researchers have successfully developed mutants of eastern monarch butterflies encoding a disruption or deletion of two clock genes.
The CRISPR/Cas9 method is a technology used to edit DNA sequences by employing a specific Cas9 enzyme and a piece of RNA (RNA guide) that bind to and alter a specific target DNA sequence. What makes this technology exciting for research in monarch migration is that researchers can pick a candidate gene and study its specific role in the migratory switch of monarchs.
The ultimate goal of studying the biology and genetics of monarch migration is to better understand this unique adaptation and enable more informed strategies to assist in monarch preservation.
Watch a video of a breathtaking monarch butterfly swarm below:
References:
Agrawal, A. A. (2019). Advances in understanding the long-term population decline of monarch butterflies. PNAS, 116(17), 8093-8095.
Markert, M. J., Zhang, Y., Enuameh, M. S., Reppert, S. M., Wolfe, S. A., & Merlin, C. (2016). Genomic Access to Monarch Migration Using TALEN and CRISPR/Cas9-Mediated Targeted Mutagenesis. GENES, GENOMES, GENETICS, 6(4), 905-915.
Monarch butterfly populations are on the rise. (2019, January 30). Retrieved December 2019, from https://www.worldwildlife.org/stories/monarch-butterfly-populations-are-on-the-rise.
Neher, D. A. (2001). Role of Nematodes in Soil Health and Their Use as Indicators. Journal of Nematology, 33(4), 161-168.
Pollination Handbook: (n.d.). Retrieved December 2019, from https://www.ars.usda.gov/pacific-west-area/tucson-az/honey-bee-research/docs/online-books.
Reppert, S.M. & J. C. de Roode (2018). Demystifying Monarch Butterfly Migration. Current Biology, 28(17), R1009-R1022.
Tenger-Trolander, A., Lu, W., Noyes, M., & Kronforst, M. R. ( 2019). Contemporary loss of migration in monarch butterflies. PNAS, 116(29), 14671-14676.
U.S. Forest Service. (n.d.). Retrieved December 2019, from https://www.fs.fed.us/wildflowers/pollinators/Monarch_Butterfly/migration.
