Monarch butterflies — best known for their color pattern of black, orange and white — are one of the most beloved species of North American butterflies (or even insects for that matter). They are notable for their role in pollinating flowers and for their annual migration across the United States. In late summer and autumn, the western North American monarch population (those found west of the Rocky Mountains) migrates to southern California, while the eastern North American monarchs (those from the northern and central United States east of the Rocky Mountains) migrate to Mexico. The monarch butterflies we spot in Louisiana are of the latter variety, making their way to and from Mexico in a migration that is quite simply an astounding act of nature. During the fall migration, monarchs cover thousands of miles with a corresponding multi-generational return north.
According to Beau Gregory, state zoologist at the Louisiana Department of Wildlife & Fisheries, there are no distinguishing physical characteristics between the eastern and western monarchs.
“Although they winter in different locations, genetically they are one population and should still resemble each other for the most part,” Gregory says.
“Why monarchs migrate is a splendid question,” says Zack Lemann, curator of animal collections at the Audubon Butterfly Garden Insectarium. “Most insects that live in places that get cold — let’s say under 50 degrees for a month or more at a time — have adapted means of surviving that period of the year. Monarchs, instead of having a diapause when they are simply inactive, migrate instead. We know that the ancestors of monarchs were tropical, so even though time has enabled this species to expand its range northward, the physical ability of monarchs to remain in cold places did not follow along. It’s a neat mystery.”
What is definite though, is that monarchs migrate as an evolutionary strategy triggered by changes in day length and temperature.
“These changes produce hormonal changes in the adult insects that changes their behavior to migratory mode,” says Dr. Chris Carlton, LSU Department of Entomology taxonomist/systematist and director of the Arthropod Museum.
While the monarch migration is in response to temperature changes, their migratory route is mostly tied to dietary requirements for their young and necessary environmental conditions.
“Like many children, monarch babies (the caterpillar phase) are very picky eaters,” Gregory says. “In fact, they only have a taste for one group of plants: the milkweeds. In order to reproduce successfully, monarchs need milkweed plants to lay their eggs on and thus they spend the warm time of year in areas where milkweeds grow. The only problem with that is that they also require very specific environmental conditions in order to survive the winter. There is one small place monarchs have found in the mountains of Mexico that most consistently provides those conditions, and that is why the majority migrates there for the winter.”
Because milkweed is so important to the monarch caterpillars, the locations of plants can vary their migratory route.
“The vast majority of the North American population spends the winter in one small area in Mexico or a few places in California,” Gregory says. “These few sites consistently provide the proper conditions for winter survival. When they migrate north, they spread out to any areas that contain adequate flowers for food (nectar) and milkweed plants for their caterpillars to eat.”
While monarch populations follow similar patterns during their migrations, the details are not well understood. One generation of monarchs flies south, but it takes two to three generations for the monarch population to return from Mexico to the northern United States. According to Lemann, the southbound monarchs generally will not lay eggs during their trek to Mexico. Rather, it is the northbound monarchs that over-wintered in Mexico and start to lay eggs as soon as they hit the shores of the Gulf Coast.
“During migration, monarchs generally follow the same route both directions,” Gregory says. “However, in the spring, they typically start spreading out or dispersing as soon as they reach the United States. In the fall, a larger proportion of the population comes together in the Midwest and passes directly through Texas.”
Here in Louisiana, it’s easy to spot a monarch almost any time of year. That is because both migratory and non-migratory populations are here. According to Carlton, estimating populations of resident and migrating insects is difficult and the necessary resources are not available.
“Monarch migrations occur from September to December, with some adults lingering into mid-winter,” he says. “Migratory and non-migratory populations seem to be genetically distinct from each other.”
For those looking to spot migrating monarchs, a good time to be on the lookout is during the fall. During this time, the southbound migration happens in a smaller time-frame than the trickling-in effect of the northbound monarchs that happens in March and April.
“If you were to drive over Lake Pontchartrain between Oct. 15 and Nov. 15 on any given year, I would bet that you’d spot monarchs — sometimes two or three every minute — flying in a generally southwesterly direction as they cross the lake,” Lemann says. “There may be aggregation spots of which I’m unaware, but the I-10 and Causeway crossing points are pretty good bets for seeing flight.”
How long a monarch lives depends on temperature and larval food quality. The life cycle from egg to adult is six to eight weeks; when it’s warm, adult monarchs live only two to four weeks on average.
“The generation that flies to Mexico arrives in high elevation forests where the cool weather slows down their metabolism,” Lemann says. “The result is that less energy is used, and, even though the butterflies may be hanging on trees doing nothing, they end up living for several months. After it warms up and these particular butterflies begin flying again, they do not live more than a couple of weeks.”
According to Carlton, the number of individual butterflies is unknown, but during 1990, an estimated 1 billion monarchs migrated south. Unfortunately, there is no way of knowing the percentage of migrating butterflies that survive the trek each year. Immature butterflies experience high mortality rates, so only a small percentage reach adulthood and reproduce.
“They compensate by producing far more progeny than required for simple replacement,” Carlton says.
While the Endangered Species Act does not list monarchs as threatened or endangered, they are considered to be an “at-risk species” — they have been petitioned for listing as threatened or endangered, but a decision has not been reached by the U.S. Fish and Wildlife Service.
“They are labeled as a Species of Greatest Conservation Need by the Louisiana Department of Wildlife and Fisheries,” Gregory says. “Causes for the current decline in monarchs have primarily been attributed to deforestation in their wintering grounds and a widespread reduction of milkweed plants in their breeding range. Effects due to climate change have not surfaced at this time, but significant changes to average summer and winter temperatures could have negative impacts in the future.”
Additionally, the mass overwintering populations in Mexico may be endangered and could be eliminated through forest mismanagement and possibly climate change. “Complete elimination of the Mexican winter migrants would have a significant impact on the species as a whole, and would be a biological tragedy,” Carlton says.
This beautiful creature can be supported by creating a butterfly garden that incorporates native milkweed plants. However, be sure to stay away from tropical milkweed. It is not indigenous to Louisiana and can actually be harmful to monarchs.
“On a larger scale, it is important to promote the conservation and proper management of native habitats, especially open habitats — such as savannas and prairies — that support monarch host plants and adult nectar sources,” Carlton says. “Monarchs are great, charismatic insects that are great poster species for conservation and insect awareness, but they are only a single species among many thousands of smaller, less charismatic, native insects that require conservation initiatives and environmental activism to insure their survival in the best interest of global stewardship.”
A group of butterflies is called a swarm, a flutter, a rabble or a kaleidoscope.
The toxin that monarch caterpillars eat is called a cardenolide. Even though the insect goes through an incredible metamorphosis in which its entire body changes shape, these poisons stay in the butterfly so that it is defended from predators as an adult.
Monarchs are not confined to North America. In fact, the same species occurs in places as distant as Australia and New Zealand, where they are referred to as “the wanderer.” Other common names (depending on region) include milkweed, common tiger and black-veined brown.
Butterflies have a long, curled tongue that they use like a straw to feed on nectar and water.
It’s a mystery how monarchs know how to find the exact same wintering grounds year after year.
There are several fun citizen science projects where members of the public can collect important data that scientists will use to determine the best way to stabilize monarch populations. Find a partial list of available projects at monarchjointventure.org. For more information on monarchs, visit monarchwatch.org and journeynorth.org.