Insect migration is a global occurrence, with moths, dragonflies and others taking flight in search of food and breeding grounds. If you grew up in North America, you may be familiar with one species with an especially long journey: the Monarch butterfly. Twice a year, Monarchs take a 3,000-mile journey between overwintering grounds in Mexico and food sources in the United States and southern Canada.
Thousands make the journey successfully, but their route has dangerous roadblocks in the form of climate change and human interference.
The Monarch butterfly (Danaus plexipuss) is a major pollinator and also the only butterfly species to make a twice-yearly migration. There are two populations in North America. Western butterflies, a smaller population, live to the west of the Rocky Mountains, overwintering mostly in California and into Mexico. A much larger population to the east spends the winter roosting on Mexican hillsides. The U.S. Forest Service describes how they return each fall from feeding grounds in north and central U.S. states and southern Canada.
Their journey was first discovered and reported in 1976 in National Geographic by University of Toronto researcher Dr. Frank Urquhart. Prior to this, it was as if the butterflies simply vanished from sight, and no one knew where they disappeared each winter.
Through tagging and one of the earliest citizen science projects, Urquhart traced the disappearing populations. The World Wildlife Fund describes the Monarchs reappearing in Mexico each year around el Diá de los Muertos. The butterflies roost among oyamel and pine trees on the high slopes of Mexico’s Sierra Madre. They cluster together for warmth and safety, forming bright orange, densely packed colonies until triggered to fly north again by the longer days of spring.
Monarchs at Risk
Monarch butterfly numbers are dwindling. They’re not listed as endangered in Canada but are included under the Species at Risk Act. Their status under the Endangered Species Act in the U.S. is under review, along with protection legislation for their habitat.
The downward trend also affects Monarch butterflies west of the Rockies. A report in Frontiers in Ecology and Evolution reveals that western numbers dropped in abundance by 97% between the 1980s and the mid-2010s.
National Geographic notes that, at this rate, models predict an 11% to 57% chance that Monarch numbers will fall so low over the next 20 years that the population will not be able to recover.
Climate Change and Other Impacts
Although the insect migration each year poses a huge threat to individual survival, studies show that deaths along the route are not the primary cause of the population slump. Tagging studies carried out by Iowa State University researchers summarized in Science Daily show that migration mortality due to factors such as car strikes, for example, is not significant, and most butterflies make it to their destination.
Monarch butterflies make their twice-yearly journey for two reasons — food and shelter — and both are under threat from climate change and human activity. Overwintering forests are under threat of logging in Mexico, and food sources are sensitive to herbicides.
Milkweed grows alongside major migratory routes, such as the I-35 running through the central and mid-west U.S. Nectar-rich plants are important energy sources for migrating butterflies, but milkweed supports a vital part of the Monarch’s life cycle. Not only do the butterflies lay their eggs on the underside of milkweed leaves, but the cardiac glycoside toxins in the plant sap help protect the larvae from predators and parasites.
Climate change, through temperature and carbon dioxide levels, is impacting this beneficial habitat. Not only is rising CO2 changing the toxin production in the plants and rendering its protection less effective, but increased temperatures are forcing the butterflies into longer migrations as they journey to breeding grounds further north. And the impacts can be seen already; though Monarchs are built to fly long distances, their wings are now growing even larger to cope with the stress of a longer migration.
Canada, the U.S. and Mexico are all involved in saving Monarch butterfly populations. Efforts mostly involve protecting habitat and food sources. For example, the National Wildlife Federation’s Mayors Monarch Pledge sees municipalities in the three countries taking steps to protect environment. Their actions include protecting and creating habitats for not only the butterflies but all pollinators along the major migration routes.
Captive breeding is also a popular eco initiative. However, a 2019 study in the Proceedings of the National Academy of Science (PNAS) suggests that individuals bred in captivity may not migrate as successfully compared with wild butterflies.
There’s also a Monarch Highway initiative devoted to creating and maintaining pollinator habitats. For example, milkweed benefits from seasonal maintenance. Mowing the plants especially in time for egg laying season ensures a fresh crop of young and tender leaves for the butterfly larvae.
Although actions like legislating environmental protections may be a little beyond what the everyday citizen can accomplish, there are practical steps you can take to protect Monarch butterfly food sources and habitat. Planting a butterfly garden helps to provide energy for the migrating insects and also supports local pollinators. You can also take part in annual Monarch counts and other citizen science observation projects to keep this amazing insect migration in the air.