As monarch butterflies make their way through Texas this spring, the migratory population heading north from winter roosts in the Mexican mountains is about 50 percent smaller than last year, the World Wildlife Fund Mexico announced.

Conservationists calculate the monarch butterfly population by measuring the amount of forest occupied by the overwintering insects. Mexican officials said 11 colonies were located this season, covering 2.83 hectares, or about 7 acres. The tally represents a 53 percent decrease from last year’s count of 6.05 hectares, or about 15 acres.

Chip Taylor, founder of Monarch Watch, a citizen science organization devoted to the insects based at the University of Kansas at Lawrence, said in a recent blog post that despite a robust breeding season throughout the summer, monarchs experienced drought conditions in Texas and northeastern Mexico last fall. He speculated that the dry conditions and a late migration took a toll on the insects. He also noted that late-season monarch butterflies sampled last fall in Kansas “were both smaller than average and substantially below average in mass and thus ill-prepared to reach Mexico.”

Andy Davis, a migration studies expert at the Odum School of Ecology at the University of Georgia, believes “the migration itself is the weakest link” and a primary cause of declining monarch butterfly numbers. “[That] the migration is such a critical phase of the life cycle is something that I think a lot of folks, including scientists, have traditionally overlooked,” said Davis.

Texas has always been critically important to the monarch butterfly migration because of its location between the roosting grounds and the milkweed beds and nectar prairies that serve as host and fuel sources for the famous insects. Millions of monarchs pass through what’s known as the “Texas funnel” each spring and fall as they make their multi-generation migration from Mexico to Canada and back. Spring in Texas is a critical time for the monarchs as they seek out milkweed plants – the only plant on which they will lay eggs and reproduce.

Monarch butterflies at the Piedra Herrada sanctuary in Mexico in February 2020. Credit: Courtesy of Veronica Prida

Cathy Downs, Monarch Watch’s conservation specialist based in the Texas Hill County town of Comfort, attributed the “disappointing” downturn to a late migration and drought last year.

“Double whammy, apparently, for overwintering mortality,” she said, adding that warmer weather late in the season causes the insects to burn through fats they’ve built up to get through the winter.

“It’s very disappointing to see such a significant decrease in their population size,” said Lee Marlowe, president of the San Antonio chapter of the Native Plant Society of Texas and ecological restorationist for the San Antonio River Authority. “This is a wake-up call for us to do what we can to conserve, protect, connect, and enhance monarch habitat throughout their migratory pathway.”

The head of the World Wildlife Fund in Mexico, Jorge Rickards, played down the reduced number as “not alarming,” categorizing the large numbers last year as “atypical.”

“The norm has been for the butterflies to cover an average of about three hectares,” he said. “The last season, 2018-19, was very good, with 6.05 hectares of forest cover, but it was certainly atypical, thanks to the fact that the first generation of butterflies in the spring of 2018 encountered favorable weather conditions to reproduce.”

The Lady Bird Johnson Wildflower Center recently predicted an overall good season for wildflowers, so butterflies arriving in San Antonio and the Hill Country should find plenty. A survey of local observers suggests milkweed is just starting to emerge from the soil.

Downs reported dozens of seedlings of two different milkweed species peeking from the earth in Comfort. “I’ve seen three worn and two fresh monarchs so far flying through and nectaring here – no eggs yet,” she said. 

Monarchs have also been spotted along the San Antonio River and in neighborhood pollinator gardens.

“Monarchs in the yard today,” said Drake White, of the Nectar Bar, a local native landscaping company. “Worn ones, so the migration has started.” 

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...