The new findings indicate that changes to migration habitat are not likely to be a primary cause of population declines and that good migration habitat remains for monarchs, especially in Mexico.

The study drew on data and expertise from both the U.S and Mexico to help complete the picture of habitat change across the full life cycle of the eastern population of migratory monarchs.

“It’s an important example of international scientific collaboration,” said Jay Diffendorfer, USGS research ecologist and lead author.

Monarch butterflies are well-known for their impressive annual migration. Not all monarchs migrate, but for those that do, their journey can be as long as 3,000 miles, a vast distance for an insect that weighs only one-half of a gram. The eastern migratory population flies northward each spring from their wintering habitat in central Mexico, mating and laying eggs on milkweed in Texas before they die. Subsequent generations migrate into the central and eastern U.S. and southern Canada and continue breeding. In the fall, the last generation completes the annual cycle by foregoing breeding and flying south to the wintering grounds in Mexico, passing through Texas on the way.

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Monarch butterfly populations in their wintering habitat in Mexico have sharply declined since the 1990s, with the second-lowest numbers recorded this past winter in an annual survey led by the World Wildlife Fund. 

Diffendorfer was in Mexico when the latest monarch population numbers were published, and his observations in the field reflected the latest numbers. 

“It was shocking to be up there—you're at 10,000 feet in a beautiful fir forest —and there were almost no monarchs,” he said. “It was pretty sobering to say the least.” 

The steep declines in the wintering population have been associated with a loss of milkweed across the midwestern U.S. as a result of the rapid adoption of herbicide-tolerant genetically modified corn and soybeans. The accompanying use of glyphosate herbicide essentially eliminated milkweed in and around agricultural fields in the Midwest.

However, there remains disagreement about the causes of monarch population declines, and some scientists have suggested that fewer monarchs are surviving the long fall migration to Mexico. While the loss of milkweed and other changes to monarch summer breeding habitat have been studied extensively, little research has investigated how land use and land cover change may have affected milkweed and nectar availability in migratory habitat in Mexico and Texas.

The researchers found little evidence of major changes to migratory habitat from 2001 to 2020, estimating only a 2.9% decline in milkweed in Texas, and little to no change in Mexico. Fall and spring nectar resources declined <1% in both regions. Overall, they found that monarch habitat in Mexico and Texas appears relatively more intact than in the midwestern, agricultural landscapes of the U.S.

These findings indicate that habitat loss in the migratory region studied is not likely to be a major cause of monarch butterfly declines, weakening support for the idea that monarchs are not surviving fall migration.

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“If there had been big scale changes like we’ve seen in the Midwest, we would have picked that up,” says Diffendorfer. “The hypothesis that losses are happening during migration could still be correct, but it’s not because of the types of habitat change we measured.”

Despite the low numbers this year, the study highlights how much intact monarch habitat still remains in Mexico and Texas.

“This study provides baseline information to reinforce conservation actions from a Mexican perspective,” said Víctor Sánchez-Cordero, Professor at the Instituto de Biología, Universidad Nacional Autónoma de México and co-author on the study.  “It justifies the idea of expanding conservation efforts throughout the monarch migratory route in Mexico, which has seen little land use change for some decades. For example, a conservation area network can connect decreed protected areas with newly identified priority areas for conserving the migration corridor for monarchs in Mexico during their fall and spring journey.” 

The study was published in Nature Scientific Reports on March 20.

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