Monarch butterflies are heading to San Antonio in a year that could claim the largest migration in a decade. 

Based on robust activity in the monarchs’ primary Midwestern breeding grounds, Monarch Watch founder and expert Chip Taylor predicted “the migration should be the strongest since 2008.” Taylor shared the promising outlook in a recent population update on the monarch butterfly conservation organization’s blog

On Sept. 27, Journey North, a citizen science organization devoted to tracking wildlife migrations, reported a cold front pushing the leading edge of the migration south into Kansas and Oklahoma. The cool weather resulted in roosts in the Midwest that numbered up to 2,000 butterflies, with some vanguard monarchs already gathering in Texas.

A roost outside Lubbock on Sept. 30. Credit: Courtesy / Jerry Hatfield

Jerry Hatfield, 400 miles north of San Antonio in Lubbock, reported a roost of “100 or more” nectaring on Gayfeather and hanging in clumps in the understories of trees Sept. 30.

All migrating monarch butterflies pass through the Texas Funnel in the fall. The migratory corridor plows through the state and channels the insects from Northern states into Mexico, where they roost in the mountains west of Mexico City. The following spring, they rouse from their diapause, reproduce, lay eggs, and die. The next generation starts the cycle anew, starting the migration through North America over multiple generations.

San Antonio lies in the heart of that migratory path and often serves as the first stop on the migration north in the spring and as the last stop in the fall.

Recent rains in the San Antonio area bolster optimistic predictions at a time when the U.S. Department of the Interior is considering the monarch butterfly for “threatened” status under the Endangered Species Act. The listing is scheduled for decision by June.

Local monarch butterfly fans are already seeing the early arrivals of this year’s migration.

Cathy Downs, of Comfort, said she’s “not seeing anything here yet, but I do have abundant nectar now in the area.” Downs, who serves as a Monarch Watch conservation specialist, said she’s confident the larger roosts of about 1,000-1,500 being reported in the Midwest will be able to develop sufficient lipid mass in Texas to “see their way clear to Mexico with nice, fat abdomens.”

Monarch butterflies migrate through the Texas Funnel en route to and from their roosting grounds in Mexico in the spring and fall.
Monarch butterflies migrate through the Texas Funnel en route to and from their roosting grounds in Mexico in the spring and fall. Credit: Courtesy / Nicolas Rivard

Migrating monarchs broke their diapause Sunday and left Drake White of the Nectar Bar on San Antonio’s North Side “lots of eggs,” White said. And monarch butterflies have been spotted regularly in ones and twos along the South Channel of the San Antonio River in the City’s recently installed pollinator gardens.

Reports of Texas monarch sightings on iNaturalist, a citizen science website and app that allows ordinary folks to share observations of wildlife in real time, have been increasing steadily in recent days.

Peak migration season in San Antonio typically occurs during the last two weeks of  October.

As monarchs move through the San Antonio area, their primary need is nectar, so they can fuel up and build their fat reserves. Late season blooms such as mistflowers, purple coneflower, duranta, lantana, and salvias serve this purpose well.

While a large migrating population is a hopeful sign, it doesn’t necessarily spell continued success for the monarch butterfly migration, scientists caution.

For decades, scientists have determined the health of the migrating monarch butterfly population by calculating the hectares the butterflies occupy at their overwintering sites in the mountains of Mexico. From November through March, millions of butterflies roost in the sanctuaries. The more forest area they occupy, the larger and healthier the estimated population.

That methodology, developed in the late 1990s, is being challenged as recent studies suggest that focusing on the migration’s end-game results is a misleading metric for determining overall monarch butterfly migration health.

The monarch population has been estimated for decades by measuring the amount of forest occupied by the overwintering insects in Mexico.
The monarch population has been estimated for decades by measuring the amount of forest occupied by the overwintering insects in Mexico. Credit: Courtesy / Monarch Watch

Climate change, disease, and the physical difficulties of migrating across a continent are presented as the monarch butterfly migration’s greatest challenges in a recent paper in the journal Science. In it, Anurag Agrawal, chemical ecologist and author of Milkweed and Monarchs at Cornell University, uses the 2017 season as an example of how a stellar migration season doesn’t predict a spectacular overwintering population or subsequent robust numbers of spring migrants.

“This has been one of the biggest years for summer monarchs in the Northeast and Midwest that most folks can remember over the past 20 [years],” Agrawal told the Rivard Report via email. “There was enough milkweed this summer. A huge number of butterflies – and the migration has begun. If the size of the overwintering population in Mexico is not much larger than the last couple of years, it would seem that something is wrong with the migration itself.”

“They are doing well up here, but beyond that I am unwilling to predict the future,” said monarch butterfly expert Karen Oberhauser, founder of Monarch Joint Venture and director of the Arboretum in Madison, Wisconsin. “Too much can happen between now and December.” (Oberhauser will participate in a forum at the forthcoming Monarch Butterfly and Pollinator Festival on Oct. 19.)

Until monarchs arrive en masse in the area, the ongoing snout butterfly invasion that started in August continues, to the delight and annoyance of locals. Some folks find the creatures fascinating; others detest the mess on their car grilles and windshields.

According to Larry Gilbert, a professor of  integrated biology at the University of Texas at Austin, the two butterfly species have nothing to do with each other.

“Monarchs are independent of snouts,” he said. “Not much overlap there.”

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Monika Maeckle

San Antonio Report co-founder Monika Maeckle writes about pollinators, native plants, and the ecosystems that sustain them at the Texas Butterfly Ranch website. She is also the founder and director of...