WI Monarch Butterfly Fall Migration Begins
Excerpted from: Monarch Butterflies Head From Midwest, Canada To Mexico For Winter
People across the state of Wisconsin are seeing an increase in monarch butterflies, and the
distinctive orange and black winged insect isn't here by chance.
Wisconsin and other Midwest states are right in the middle of the monarch migration trail or flyway.
During spring and summer, the butterflies travel north toward Canada to repopulate. Now they are
preparing to migrate south for the winter.
What people are seeing when they are looking into their backyards and along the shores of Lake
Michigan, they are seeing a part of a huge wave of monarchs that are moving south right now.
Eastern Monarch Population Status
The yearly count of monarch butterflies that overwintered in Mexico continues
to show imperilment for the migratory butterfly. This year’s count of 7 acres of occupied winter
habitat is up slightly from last year but still below the threshold scientists say is necessary for
the iconic pollinator to be out of the extinction danger zone in North America.
Overall, eastern monarchs have declined by around 85% since the mid-1990s.
The eastern monarch population is made up the butterflies east of the Rocky Mountains and
accounts for roughly 99% of all North American monarchs. They migrate each winter from the
northern United States and southern Canada to Oyamel fir forests on high-elevation mountaintops
in central Mexico.
Scientists estimate the population size by measuring the area of trees turned orange by the clustering
butterflies. Their population has been perilously low since 2008.
Peak migration in Wisconsin is in the first 2-3 weeks of September.
Emerged Monarchs who
migrate south from Wisconsin are part of a generation that will fly over 3,000 miles and have an
eight-nine month lifespan within which to make it back to the Michoacán Mountains.
These butterflies enter reproductive diapause when they emerge, which allows them to conserve energy
for their flight to Mexico. It also means that they don’t reproduce until later.
The Monarchs will stay there until spring, when they leave reproductive diapause, becoming
sexually mature. They will migrate some distance northward, lay eggs, and perish.
Wisconsin is in the core breeding ground for the eastern migratory population of monarchs. The
state's milkweed feeds and produces several generations of the iconic black and orange beauties
each spring and summer before a final wave gorges itself on wildflower nectar and embarks on a
1,700-mile journey to central Mexico.
Monarch butterflies have a four-generation migration strategy. Right now, people in Wisconsin
and the Midwest are seeing the fourth generation of butterflies. The first generation starts in
Mexico, as monarch butterflies stay in the mountains of Mexico all winter. When they begin to
migrate north during the spring they start to reproduce and repopulate, these are the second
and third generations.
The butterflies being seen now are the offspring of the butterflies that migrated from Mexico last
winter. Monarchs spend most of the summer in the northern part of the United States and southern
Through August and October, numbers are high as the butterflies prepare to migrate, but they will
be headed south toward the end of October.
The population of monarch butterflies has been in decline for several years, but numbers are
increasing due to good weather conditions and proper nesting spots. Barbara Agnew with the
Friends of The Monarch
Trail and others calls this the "Goldilocks effect."
"In the past 20 years monarchs have been declining, but in the past two years they have had good
weather," Agew explained. "They have had a little bit easier winters in Mexico, so the Monarch
population has been able to increase."
"Even though it seems like there's just a ton now, they're simply getting back up to where they
should be and we've got a long way to go," Agnew continued.
Monarch butterfly numbers have been dropping precipitously for more than two decades —
partly due to shrinking winter habitat and increased herbicide use in the Midwest that eliminates
their host plants from summer breeding grounds. However, herbicide use and habitat loss have
diminished over the past decade, yet monarchs continue to decline.
These key facts told scientists that a big part of the story was left out, says senior author Elise
Zipkin, an integrative biologist.
“Migratory periods are missing from most research because they are the most difficult periods
to investigate,” she says. “For monarchs, the fall is the least-studied season because the data
are so sparse and are generally opportunistic.”
Unsurprisingly, are important; summer, fall, and winter factors are all connected. In particular,
landscape greenness during the fall migration, in addition to the peak summer population size
and the amount of habitat at local winter colonies, were the key factors influencing the winter
Butterflies and Moths of North America
Southern Wisconsin Butterfly Assn (NABA)
The Butterfly Site