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.

Peak Migration South

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.

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 Migration Trail

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 Canada.

Through August and October, numbers are high as the butterflies prepare to migrate, but they will be headed south toward the end of October.

Monarchs Are In Decline

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.

Missing Piece

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 population size.

Further Information:

 Butterflies and Moths of North America
 WisconsinButterflies.org
 Southern Wisconsin Butterfly Assn (NABA)
 The Butterfly Site