Every fall, millions of monarch butterflies travel up to 3,000 miles from the U.S. and Canada to one forest in Mexico, where they’ll stay for the winter. There in the Monarch Butterfly Biosphere Reserve, located on the border between the Mexican states of Michoacán and Mexico, they almost completely cover the trees and comingle until the spring, when they migrate back north, as their great-great-grandparents had done the year before.
Yet the number of monarchs that make this remarkable journey has been declining. Eastern monarchs, which are those that live east of the Rocky Mountains and make this annual trip, are typically measured by the area of the forest reserve they cover in the winter, which in the 1990s peaked at around 21 hectares. Last December, the butterflies only covered 2.1 hectares of forest, a 26 percent decrease from the same month in 2019 – and a far cry from the minimum of 6 hectares of coverage needed to ensure the species’ survival.
In total, the eastern monarch population is thought to have decreased by 80 percent. And the western monarchs, a separate population that lives west of the Rocky Mountains, has gone down by a whopping 99 percent within the last two decades.
But what’s been the cause of this beloved species’ decline?
The WWF has reported a four-fold increase in illegal logging in the Monarch Butterfly Biosphere Reserve between 2019 and 2020 as compared to the previous year, and this deforestation has certainly affected the monarch. Additionally, the butterflies are very sensitive to changes in weather, which cue them on when to reproduce, migrate and hibernate – and climate change is only seeing weather patterns become increasingly unstable.
But a main cause of their path to extinction is the decline of a native plant called milkweed (of the genus Asclepias) to complete their life cycle, and the loss of this plant through pesticides and changes in the landscape is a primary reason for the drop in numbers. Monarch caterpillars feed solely on the leaves of milkweed, which is commonly regarded as a weed by homeowners and farmers. Estimates vary, but it is thought that at least 1.3 billion new milkweed stems are needed to restore monarch butterfly numbers.
Agricultural lands make up 77 percent of potential monarch habitat. In the past, milkweed grew in much of the corn and soybean fields grown in the “monarch central flyway” – a major path that runs through the Midwest and southwestern U.S. that is used by eastern monarchs to migrate south – and monarchs would travel across fields to lay their eggs across multiple milkweeds. Milkweed stems in farm fields support on average 3.9 more monarch eggs than those in non-agricultural areas.
“The general concept is ‘don’t put all your eggs in one basket.’ [The monarchs] know there are predators out there. There are things that can happen to the milkweed, so they’re forced to keep moving on and find that next patch,” says John Pleasants, an Iowa State University professor who has researched how farming practices affect monarch butterfly populations.
This was before the extensive use of glyphosates, one of the world’s most widely used herbicides, on corn and soy fields. As intended, such herbicides have likely made crop fields become milkweed deserts, and nearly 40 percent of milkweed stems are estimated to have disappeared since 1999.
Given their effectiveness, herbicides like glyphosate are likely going to stay on farms in the near future. However, there might be ways to reintroduce milkweed onto farms without reducing crop output, such as by taking advantage of the Conservation Reserve Program (CRP). The CRP pays farmers to take a portion of their cropland out of agricultural use to grow vegetation that is good for the environment, and almost 8.5 million hectares of farmland were active CRP lands as of January 2021. Farmers entering new CRP contracts can opt for the CP42, a CRP type specifically devoted to protecting pollinators, and at least nine species of pollinator-friendly wildflowers, legumes or shrubs must be planted to qualify.
Although milkweed loss on farmland has been linked to the decline of monarch butterfly numbers, Pleasants emphasizes the need to help farmers benefit from protecting the environment “[Farmers] are not really to blame. It’s not like they knew that monarchs would disappear if they used these farming practices that people told them they should use.”
“You have to think about the bottom line and what’s in the self-interests of farmers,” he says. “They have needs, and times have been tough for agriculture lately, so they need some sort of either financial or personal incentive to do various things.”
Fortunately, conscious growth of more native species has been gaining traction in more densely populated parts of the U.S. too.
A group of researchers from the Field Museum of Natural History in Chicago and the U.S. Fish and Wildlife Service used LIDAR data to examine the possibility for four cities in the monarch central flyway to provide milkweed for monarch butterflies. The team, led by conservation ecologist Abigail Derby Lewis of the Field Museum, used a suite of tools developed to help city planners calculate estimates of current and potential milkweed stem numbers across any region according to land use type.
When the researchers applied the milkweed-calculating tools for the Chicago region, they were not surprised to find that residential areas presented the greatest opportunity for increasing milkweed stems. But when this was extrapolated to all U.S. cities east of the Rocky Mountains, what the team found was shocking: that urban areas could provide for nearly a third of the 1.8 billion more milkweed stems they regarded as needed for monarch survival.
Although urban residential areas might have the greatest potential to increase milkweed stems, other parts of cities should also come into play more, most notably corporate campuses and rights-of-way, which will entail collaboration with businesses, utility companies, the Department of Transportation in different states and railway providers. Under the Mayor’s Monarch Pledge, city, local and tribal governments commit to creating habitat for monarch butterflies and other pollinators.
However, some have been concerned that cities might serve as ecological traps. For example, it has been suggested that roadsides might be less suitable for monarch habitats because the butterflies may be hit by cars, or because the heavy metal pollutants emitted by vehicles could harm them. Derby Lewis also notes concerns over milkweed planting’s potential to lure butterflies into places that undergo mosquito abatement spraying or that simply don’t have enough milkweed to support them.
She notes, however, that resolving this dilemma will come down to community science, such as a current monitoring project also run by the Field Museum: “People are just knocking down the door to be able to be a part of this research project. They go out every single week, and they count their milkweed, and they count the number of [monarch] eggs. They count the [developmental] stages of the caterpillars, and they count the number of flowering plants they have. They record that and submit it.”
“And that’s really going to help us understand how productive these small gardens and small spaces can really be. We need empirical data to really answer [the ecological trap] question, but from what we know qualitatively, these are not ecological traps.”
There are also steps that just about anyone can take to add some of the wild to their homes to help monarch butterflies survive, and a number of organizations, such as Monarch Watch and Monarch Joint Venture, have called for individuals to take action, such as by planting pollinator gardens.
“People are bananas for monarchs. And it is one of the most successful species that I’ve ever seen at convening people through public attention and interest,” says Derby Lewis. “And that’s great, because when you do things to help monarchs, you’re helping all of these other species too.”
Many organizations recommend that people plant native milkweed plants in their yards (or balcony gardens), as well as native nectar plants that provide adult monarchs with the energy needed for their epic journey south. Harmful chemicals should be avoided in pollinator gardens, and milkweed should be organic and free of neonicotinoids, a widely used insecticide that has been proven to harm pollinators such as bees.
However, Derby Lewis recognizes that there are some barriers. One is the public perception of what a garden and lawn should look like, which still entails the frequent mowing of grass-covered turf and the use of chemicals to keep unwanted “weeds” away, which is often reflected in legislation as well.
“[Municipalities] will often have a height restriction as part of weed ordinances. So, for instance, anything over 10 inches would be considered ‘a weed’ and therefore inappropriate to have on the landscape,” says Derby Lewis. “All of our native plants are typically 10 inches or higher.”
In both urban and rural areas, it remains to be seen if these efforts will be enough to save the monarch butterfly. But despite the enormity of the task at hand, there is no doubt that these butterflies can serve as an entry point for many into the conservation world, and for self-empowerment.
“So many things in the world right now feel out of our control, or feel like an individual action just cannot make a difference,” says Derby Lewis. “And so to be able to say, ‘What we plant matters. What you do counts. If you put this milkweed into the ground, they will come,’ it’s something that is exciting for people.
“It’s gratifying and satisfying.”
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