- February 20, 2018
The Guinness World record for the longest known migration of a Lepidopteran species is a male monarch butterfly (Danaus plexippus) recorded in 1989.open_in_new Tagged in September 1988 in Brighton, Ontario and captured again in April 1989 in Austin, TX, this individual is thought to have migrated from Ontario, to Mexico–to overwintering grounds–and captured again on his trip north to breed. It is amazing that a species that weighs less than a gram (0.25-0.75g) can travel up to 2,880 miles (4,635 km) for survival. Explore with us, this small but impressively strong and extremely distinctive butterfly species, the monarch.
First, lets begin with monarch ecology. The life cycle of the butterfly is depicted in the above and below images. Adult butterflies mate and the female lays her eggs on the leaves of a milkweed plant (Ascelpias spp.). One female can lay between 100-300 eggs. After about four days, the eggs hatch into the larval stage of life, commonly known as caterpillars. A caterpillar will spend the next 10-14 days consuming parts of the milkweed plant, growing, molting its skin (repeated four times making a total of five instar stages) before reaching 25-45mm in length and an average weight of 1.5 grams.
At this point, the caterpillar will enter into the pupal stage and form a chrysalis for 10-14 days where the tissues and organs of the caterpillar are broken down and rearranged into an adult butterfly. When the adult emerges from the chrysalis in the spring, they typically live for two to six weeks (with the single goal of reproduction). Adults emerging in the fall will experience delayed reproduction (diapause) and migrate south–living a much longer eight- to nine-month life.
Monarch butterflies migrate south to avoid the harsh winters of their northern breeding grounds. They are one of the only butterflies that winter in the adult stage of their life cycle–possible only because they migrate to warm and moist climates during the colder season. The mechanisms that guide monarchs along this journey south are still not completely understood, but some research suggests that they use a combination of sun compass and circadian clock, also known as a time-compensated sun compass.open_in_new
Monarchs that breed on the eastern side of the Rocky Mountains mainly overwinter in the Oyamel fir forest of central Mexico at an elevation of 2,400-3,600 meters (+10,000 feet). There is a smaller population that overwinters along the Gulf or southern Atlantic Coast. Monarchs breeding west of the Rockies overwinter in California along the coast from the Mexico border north to San Francisco.
Monarch butterflies travel much like humans‒during the day, stopping at night to rest in a safe place. These “monarch motels” are called roosting sites, which are typically located in evergreen tree species like cedar, pine, and fur and can be used year after year. Little is known about how monarchs determine or locate their migration roosts or their final wintering roosting sites in Mexico or California, but it is predicted that there is a “destination beacon,” which is most likely a combination of olfactory and/or magnetic cues.open_in_new
What is even more impressive is, in early spring, the species disperse from the wintering grounds, mate, and begin their first leg migrating north again. Researchers Guerra and Reppert suggests that the cue that switches the sun compass from pointing south in the fall to pointing north in the spring is cold exposure.open_in_new Without experiencing the cold exposure of the wintering grounds, the butterfly’s sun compass continues to point south.
The journey north for the eastern breeding monarch is a long one. The butterflies that overwintered in Mexico, usually make it to Texas and lay their eggs on milkweed plants. At this point the longer-lived generation which overwintered dies, and their offspring laid in Texas emerge and migrate north for two to six weeks, reproduce, and die. This two- to six-week life cycle of migrating north, reproducing, and dying occurs three to four times until the early fall offspring hatch and start the entire migration south again.
Not as populous, but just as impressive is the western breeding monarch. From Southern California, the butterflies disperse in early spring and migrate up California’s Central Valley and the Sierra Nevada foothills.open_in_new Over three to four generations of migrating offspring, the western monarch can reach as far north as British Columbia before the fall migration south begins.
What is the difference between summer and fall butterflies?
One of the larger unanswered questions about the monarch’s life cycle is what makes the spring/summer, shorter-lived, north-traveling generations of monarch different from the longer-lived, south-migrating generation? Scientists continue to investigate this question but initial research is providing some interesting evidence. Here is what we know or don’t know…
- First, It is unknown what is driving the spring/summer generations to migrate north. It could be genetically programmed or the butterflies could be simply following the seasonal timing of the milkweed abundance northward.open_in_new
- Second, research has found fall generations of monarchs do experience an increase in juvenile hormones which delay reproduction and increase longevity. These juvenile hormones, however, do not seem to be responsible for the change in migratory direction, just the delay of breeding behaviors.open_in_new
- Finally, scientists have observed that the change in migratory direction from summer to fall might be associated with a change in a suite of 40 genes which are expressed differently but not connected to the change in juvenile hormones produced.open_in_new
Additional facts about the monarch:
Habitat loss is one of the largest factors influencing the conservation status of the monarch butterfly. The image above is of the Lomas de Aparicio monarch colony wintering grounds. The image on the left is from March 2004 and the one on the right is from February 2008. Monarchs overwinter on 12 mountaintops in central Mexico, originally covering 20 hectares (50 acres).open_in_new Today it is estimated that valuable overwintering grounds have been reduced to four hectares (10 acres). The degradation and deforestation of the monarch overwintering grounds is a major threat to the survival of this migrating population.open_in_new
In addition to diminishing wintering grounds, the butterflies are also experiencing habitat loss along their migratory path. Milkweed populations are being reduced due to urban and roadside development and use of roundup ready crops.open_in_new Researchers have found an 81% declineopen_in_new in milkweeds in Midwestern agriculture in the past decade. It is predicted that the decline in milkweed and nectar sourcesopen_in_new during migration to be a predominant reason for the decline in monarch populations observed.open_in_new
Climate change is also a concern for monarch populations.open_in_new Projected climate change in central Mexico, where the majority of the northern species overwinters, could make the current wintering ground unsuitable. Mortality due to cold exposure would certainly be reduced, however, the increase in precipitation and storm intensity could threaten roosting monarchs.
Finally, pesticides, specifically neonicotinoidsopen_in_new, are a threat to many insects that consume or pollinate treated or contaminated plants. Neonicotinoids are broad-spectrum, persistent insecticides that are related chemically to nicotine which targets the nervous system of insects. It can be applied by spraying or soil soaking but is more commonly applied to the seeds before sowing. What makes neonicotinoids unique are their persistence in the environment and their water solubility which allows the pesticide to infiltrate all parts of the plant, including pollen and nectar, to protect it from all types of predators.
The 2016-2017 Mexico overwintering population of monarchs was estimated at 145 million butterflies, which is down 27% from 2015-16 estimate, but up 600% from the lowest count in 2012-14.open_in_new There have been many efforts to create more milkweed habitat across the three countries this species inhabits (Mexico, U.S., and Canada), as well as to protect the overwintering grounds in Mexico from further development. It seems that these efforts have slowed the downward trend of the eastern population within the past decade and are currently showing signs of stability.open_in_new
The outlook for the western butterfly isn’t looking any better than the eastern population. In the past 35 years, the population has been reduced from an estimated 10 million to 300,000.open_in_new A recent citizen science monitoring study has allowed scientists to estimate the extinction probability of the western population at 72% in 20 years, and 86% in 50 years.open_in_new
Everyone has a role to play when it comes to protecting pollinators. Monarchs are just one example of the many pollinator species also experiencing the same downward trend in population due to habitat loss, parasites, and pesticide use.open_in_new One fairly easy action to benefit pollinators at home is to commit to installing a pollinator garden. Pitching in a patch of your property for flowering native plant species provides important habitat resources to pollinators. Check out our Introduction to Pollinator Gardening and Planting Palette articles for more resources on how to support monarch butterflies and pollinators. And, don’t doubt that many people planting even a small pollinator garden can have a measurable impact.