A male monarch butterfly on the south channel of the San Antonio River in November. Credit: Monika Maeckle / For the San Antonio Report

Unusually wet and cold weather kept monarch butterflies in San Antonio weeks after their usual peak migration season this year. The first appearances of the slow-moving travelers finally occurred at their overwintering sites in the Mexican mountains Nov. 6, later than usual.

For months, scientists and citizen scientists had been predicting the largest migration in a decade, but the insects have taken their time arriving at their winter roosting sites, missing their usual Day of the Dead arrival by almost a week.

Mexico’s Natural Protected Areas Commission (CONANP) said in a statement that the first butterflies of the season were “crossing the sky” at the high-elevation monarch sanctuaries on Nov. 6.

With favorable weather, most butterflies should reach the sanctuaries by Nov. 20, CONANP said.

In Texas, the migratory channel through which all monarch butterflies pass on their way to the wintering grounds, the season has been late, unpredictable, and dogged by stormy weather.

San Antonio experienced its wettest fall in history, with more than 23 inches of rain recorded in September and October. Much of the region’s rainfall came in dramatic and destructive weather events, including a 40-foot rise along the Llano River in the Texas Hill Country, where monarchs often roost in pecan trees along the river bottom. Late-season nectar plants were mowed down by the floods, and some butterflies perished.

When the Llano River flooded in October, some migrating monarch butterflies drowned. Credit: Monika Maeckle / For the San Antonio Report

Record-breaking freezes then descended on the area this week just as monarch stragglers continued their way south.

Monarch butterflies continued to fly. Even caterpillars endured the cold.

“I’m still seeing a few straggling adult monarch butterflies in my habitat,” said Drake White of the Nectar Bar, a native landscaping company on San Antonio’s North Side. “I even have a few fourth and fifth instar [growth stage] caterpillars on giant milkweed.” 

She said the caterpillars survived Tuesday night’s freeze, when 28-degree temperatures broke a 111-year-old record low.

A late-stage monarch butterfly caterpillar on a giant milkweed plant in a pollinator habitat on San Antonio’s North Side following Tuesday night’s freeze. Credit: Courtesy Drake White

Changeable weather seemed to break the monarchs’ migration into disjointed parts in Central and South Texas. A big pulse of monarchs arrived  in mid-October, followed by a break, then another rush around the end of the month. Stragglers have been flying ever since.

“I saw the first scattered monarchs along the river in mid-September before the continuous rain events,” said Lee Marlowe, sustainable landscape ecologist at the San Antonio River Authority. “I didn’t start seeing them again until after the sun came back out and when that happened in late October, they were in full force at our Environmental Center on East Euclid Avenue.”

Will a late, disjointed monarch butterfly migration become the new normal for San Antonio?

Chip Taylor, founder of Monarch Watch, the citizen science tagging program based at the University of Kansas at Lawrence, says times are definitely changing, but it’s too early to declare a trend.

“Yes, this is another late migration relative to the arrival at the overwintering sites,” said Taylor, adding that first-week-of-November arrivals are unusual, but have happened in the past. Rain and cool temperatures presented varying conditions for monarchs moving through Texas, he explained. Similar weather patterns followed in northern Mexico.

Andy Davis, assistant research scientist at the University of Georgia’s Odum School of Ecology and editor of the journal Animal Migration, thinks the trend is pretty clear.

Recent studies indicate the impact of climate change on the monarch migration will result in more seasons like 2018, Davis said. Warmer weather will cause monarchs to breed later in the season and result in delayed or missed cues about when to start migrating.

“Either way, this means the monarchs will experience delays in reaching Mexico, or suffer greater mortality during the trip [because of the longer travel time],” he said via email.

All of the research has been clear on this, he said, although no one expected it to be happening so soon.

“That right there might be the new normal,” he said.

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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...