People are seeing fewer monarchs across the country, and lepidopterists (people who study moths and butterflies) believe the population decline is because of two primary factors: destruction of their winter habitat and destruction of their primary food source, milkweed. In Maine, monarch butterflies have been slowly disappearing from the landscape over the past decade.
There’s not much the state can do about it.
“Maine plays a minor role in the fate of the monarch butterfly in North America,” said state biologist Phillip deMaynadier.
These well-known butterflies are affected by both of these factors long before they fly north to Maine each summer.
“Monarchs probably have been monitored more carefully than almost any other butterfly species for the longest period of time,” said deMaynadier.
All of the monarch butterflies living in eastern North America congregate in a specific forest in Mexico each winter. There, the trees are cloaked in vibrant orange as millions of butterflies cling branches and slip into a stupor that lasts for months. This phenomenon has allowed scientists to easily monitor the monarch population since the mid-1990s.
“These fir forests create a microclimate that protects the monarchs from freezing while roosting,” said deMaynadier. “It’s a slightly warmer and humid climate under the trees.”
Instead of counting the individual butterflies, which would be impossible, biologists quantify the monarch population each year by determining the number of acres they occupy while roosting. At the highpoint of their population, during the winter of 1996-97, monarchs occupied trees of about 50 acres. Last winter, they inhabited just 1.5 acres — the smallest population ever documented.
Unfortunately for the monarchs, these forests are being illegally logged, deMaynadier said, which opens up the canopy and destroys the microclimate the butterflies need to survive the winter. In addition, storms (heavy rain, hail, high winds and freezing temperatures) have taken its toll on the overwintering populations.
Those monarchs that do find a suitable roosting area and survive the winter storms begin to stir from their stupor in March, when they will face another dilemma.
In the spring, monarchs leave Mexico and fly northeast in search for milkweed, the host plant for their eggs. Some will stop in Texas, while others will continue to southeastern states such as Florida and Georgia. But because of an increased use of herbicides in agriculture, milkweed is being culled from the land, leaving the monarchs nowhere to breed.
“What’s really fascinating about butterflies as a group is this whole concept of a requirement of a host plant,” deMaynadier said. “The whole life history of butterflies is tied to the location of very particular plants.”
Butterflies evolved about 250 million years ago and have since been in a sort of “co-evolutionary arms race” with flowering plants, deMaynadier said. Over time, flowering plants evolve chemicals to prevent animals from eating them, and in response, different species of butterflies have evolved enzymes and metabolites to break down those chemicals. As a result, some species of butterflies have become so specialized that they can only feed on one type of plant.
“That strategy works well up to a point,” deMaynadier said.
Since milkweed is toxic to most animals, monarchs have little competition for food. On the other hand, their fate is closely tied to the fate of a plant, a reality that has proved problematic.
Monarchs that manage to locate milkweed in the south will breed, lay eggs and die. Their offspring, in the form of caterpillars, will feast on the milkweed, form cocoons and transform into butterflies, which will then continue the migration north to Maine and other northeastern states.
Maine residents typically spot monarchs in fields from July through September, though these sightings are becoming increasingly rare. Again, the monarchs are in search of milkweed, of which Maine has plenty, where they’ll lay eggs (and afterward, die). Their offspring also may reproduce (then die). It is then their offspring that fly 2,500-some-odd miles back to Mexico for the winter.
“What’s amazing is that the monarchs we have in Maine are several generations removed from their great grandparents that made the migration to central Mexico,” deMaynadier said. “And somehow — and this is something scientists have no consensus on — the butterflies all fly across America to that roosting spot in Mexico. How [do] they know where to go?”
Monarchs aren’t the only butterflies that are in trouble. Of the 118 species of butterflies native to Maine, one in five are listed as either rare, threatened, endangered or expatriated. And for the most part, the reason behind the decline is the same — loss of habitat and host plants.
Since 2007, volunteers have been mapping the state’s butterflies for the Maine Butterfly Survey, a citizen science atlasing project led by the Maine Department of Inland Fisheries and Wildlife, Colby College and University of Maine Farmington.
“Butterflies act as an ambassador for the whole invertebrate population,” deMaynadier said. “People like them. They’re colorful, large and easy to work with.”
So far, the survey has trained more than 250 volunteers, half of which have submitted data to the atlas, which consists of about 20,000 records of butterfly species spotted throughout the state. Of those records, 200 are new county records (the first time a species was mapped in a county), 10 are new state records, and one is a U.S. record.
“These surveys really do expedite our ability to get information quickly,” deMaynadier said.
Next year will be the last field season of the butterfly survey, he said, then the partners plan to compile the information in a book. Next, the MDIF&W plans to enlist volunteers to survey bumblebees.
“We’re going to try to launch that next year,” deMaynadier said. “There’s a lot of concern about pollinators in general and the ecological services they provide.”
Anyone interested in taking part in the Maine Butterfly Survey’s final field season can learn more at mbs.umf.maine.edu, where information about Maine butterfly species is available to the public. To learn more about the monarch population study, visit monarchwatch.org.