Since early November last fall, most monarchs in the Eastern North American Migratory Population have been overwintering in Central Mexico, waiting for the environmental cues that will signal that it’s time to move back north. Even before the monarchs leave, mating will begin, and they’ll continue to mate and the females will lay eggs as they head north, through northern Mexico and into Texas before spreading into approximately the southeastern quarter of the U.S. This should begin in a little under a month.

Photo by Karen Oberhauser

A month ago, the leading edge of the migration had moved well into Mexico, and I noted that the very first monarchs should begin arriving at their winter homes right on time, around the Day of the Dead celebration on November 2, with larger numbers a week or so behind. Just like clockwork, this is what happened. Estela Romero sent a report to Journey North of monarchs in the skies above the small town of Angangueo Michocan on November 4. The sanctuaries of El Rosario and Sierra Chincua in the Monarch Butterfly Biosphere Reserve officially opened to the public on November 22.

Photo by Brenna Wiesner

A month ago, the southernmost roost sites reported by Journey North volunteers were in Oklahoma. As of this writing (October 29), the leading edge of the migration has moved well into Mexico (figure 1), with one sighting of 16 monarchs in the State of Guanajuato, the state just north of Michoacan where many overwintering sites are found. However, the main part of the leading edge is near Saltillo in the State of Coahuila. Based on these locations, the very first monarchs should begin arriving at their winter homes within a few days, right on time (they usually begin arriving around the Mexican Day of the Dead holiday), while larger numbers will be a week or so behind.

A month ago, Journey North volunteers were just beginning to see monarch roosts, and only in the northern reaches of their summer breeding range: Minnesota, Michigan and Southern Ontario. But since then, the moving front of monarch migration has moved steadily southward, with monarch roosts spotted as far south as Oklahoma.

Photo by Mike Budd/USFWS

Monarchs in the Eastern Migratory Population that emerge as adults on or after about August 15 in the Upper Midwest will be in reproductive diapause and will spend their first few months of “adulthood” flying to their overwintering sites in Mexico. Instead of being ready to mate and lay eggs three to five days after emerging, these individuals will delay reproductive maturity until spring 2026.

Photo by Mike Budd/USFWS

The adult monarchs that we’re seeing in Wisconsin now are mostly offspring of the generation that started as eggs in the spring of this year. These “Gen1” individuals came up from the southern US, and were themselves the offspring of the monarchs that overwintered in Mexico. So we’re seeing “Gen2”, with a few “Gen3s” emerging (the Gen3s we see at the end of July, are the first offspring of the first offspring from Gen1). Journey North maps below show little northward movement after Gen1 recolonizes the summer breeding range.

Photo by Courtney Celley/USFWS