Last month, Govt officials met in Washington, DC, for the first Monarch Butterfly Summit, just as the milkweed at “Monarch Waystations” now ubiquitous on American lawns was beginning to bloom. Like everyone, they were worried about the fate of the iconic insect, after decades of significant population decline in the butterfly’s winter colonies.
There are two distinct (but genetically identical) populations of monarchs in the United States, and both are migratory. Monarchs west of the Rockies spend the winters in southern California, while those east of the range fly thousands of miles from as far north as Ontario to central Mexico, where they await the cold months in the trees of the Ojamela fir. Since the mid-1990s, scientists have found that the number of butterflies reaching Mexico has dropped by about 70 percent. Bad weather, deforestation and car crashes are blamed for the decline.
In 2020 alone, 26 percent fewer eastern monarchs made it to Mexico than the year before, after being stranded by storms and drought. Those who survived the journey found that their already small wintering grounds had been reduced by illegal logging. Researchers concluded in 2019 that the western monarch was “hovering on the brink of quasi-extinction” after a 97 percent decline in that subpopulation since the 1980s.
So it might be surprising—and perhaps controversial—that a recent study was published in a journal Global Change Biology suggests that some monarch butterfly populations are actually on Grow. “There is no monarch butterfly apocalypse,” says Andrew Davis, a professor of ecology at the University of Georgia (UGA) and co-author of the study. “Not in the United States anyway.”
His group’s work is unusual because it focuses on insect breeding sites rather than their migration stations. In other words, the team looked at counts that took place in the summer across the US, not in the winter in Mexico or southern California. Davis and his fellow researchers drew on more than 135,000 monarch observations made on both sides of the Rockies between 1993 and 2018 during the North American Butterfly Association’s (NABA) annual census. These events invite citizen scientists to record any butterflies they see within a 15-mile radius over two days in early July.
While the research team noted a slight decline in some regions of the US, particularly the Midwest and New England, areas such as the Southeast and Pacific Northwest have more monarchs. Taken together, the data suggest a total annual increase of 1.36 percent across the species’ entire summer range, meaning that over a 25-year period the U.S. summer monarch population has increased by about 35 percent.
Davis says his team’s findings show that butterfly reproduction in the summer makes up for the losses the insects experience during the winter. “They are able to recover and repopulate their entire breeding range each year, regardless of how many there are in winter colonies,” he says. “It’s just math. One female can lay 500 eggs. If the conditions are right, the population explodes.”