Karen Oberhauser, Monarch Butterfly Fund Board of Directors and Professor Emerita of UW-Madison Department of Entomology
On March 6 the World Wildlife Fund-Telmex Telcel Foundation Alliance (WWF) and the National Commission of Protected Natural Areas in Mexico (CONANP) released data from the winter 2024–25 monarch butterfly population counts. In December 2024, monarchs occupied 1.79 hectares, compared to 0.9 hectares at the same time in 2023. This represents a 99% increase, the biggest jump we’ve seen since the monarch overwintering area increased from 2.48 hectares in December 2017 to 6.05 hectares in December 2018, a 144% jump. While any increase is great news, this number is still below the average for the past decade, 2.81 hectares, which is itself far less than the area when researchers first started measuring colony size in the early 1990s (see graph). The decline in their numbers led to the December 2024 recommendation by U.S. Fish and Wildlife Service that monarchs be designated as threatened and thus receive the protection of the Endangered Species Act.
At the wintering sites in central Mexico, where most monarchs from east of the Rocky Mountains overwinter, monarch population size is compared from year to year by the number of hectares (one hectare = 2.5 acres) occupied by trees with clusters of monarchs. WWF and CONANP have been monitoring this area since 2004, with similar data from 1993-2003 collected by the Monarch Butterfly Biosphere Reserve (MBBR). While the number of monarchs in a hectare varies from year to year and is difficult to estimate, our best estimate is that it is about 21 million.
Monarch numbers are driven by factors that affect recruitment (the number of eggs that are laid and that survive to adulthood) and adult survival and successful navigation of the annual cycle of breeding, migrating and overwintering. Recruitment and survival are affected by the availability of quality habitat, weather conditions, and the presence of lethal agents (natural enemies such as predators, diseases, and parasites; environmental toxins and pesticides; and other agents of mortality that are often difficult to quantify, such as vehicles). Thus, land conversion, climate change, and pesticide use can all affect monarch numbers.
The amount of breeding habitat available for monarchs sets an upper limit, or ceiling, for the size of the monarch population. Numbers vary under and up to that ceiling depending on weather conditions. We probably got close to the ceiling in the summer of 2018, when the colonies were the largest that they had been for over a decade.
Over the past century, many factors have affected the amount of habitat available to monarchs, and the relative importance of these factors is probably changing. In the 1800s and early 1900s, forested area in the eastern U.S. was cleared for farmland and cities, but much of that farmland has since been taken out of cultivation, and forests are recovering.
Early North American settlers in the Midwestern U.S. converted vast expanses of prairie, good monarch habitat, into farmland, but these farms tended to have small fields with milkweed and nectar plants growing on the edges, and chemical pesticides were not available. Even with the growing predominance of large, cultivated fields after the middle of the 20th century, weed control methods (usually disking) did not kill milkweed; on the contrary, most milkweed in the Upper Midwestern U.S. in the 1980s and 1990s was probably found in corn and soybean fields. Thus, early farming practices in the U.S. probably benefited monarchs; in the east, farmland replaced forests that were not good monarch habitat, and the land disturbance that accompanied agriculture promoted the growth of what became eastern monarchs’ most important food source, common milkweed (Asclepias syriaca).
In the late 1990s, the development of genetically-modified herbicide-tolerant crops (first Roundup® Ready and then a series of crops resistant to other herbicides) allowed farmers to spray their fields with herbicides after the crop plants emerged, and milkweed quickly disappeared from these fields. Insecticide use to control agricultural, nuisance pests, and insects that can vector human pathogens has also increased in the past few decades. Climate change is another factor altering the conditions for population growth as drought reduces the quality of milkweed nectar plants and availability of nectar for migrating monarchs. Additionally, damaging storms increase in intensity.
The best way to support monarchs is to raise the population ceiling by creating more habitat.
That means restoring habitat in our yards, places of work, schools, and churches; along roadsides, utility rights-of-ways, and railroads; and in areas currently used for crops that aren’t very productive. But we also need to do what we can to mitigate climate change. The conditions predicted by climate models will be bad for monarchs, but they’ll also be harmful to us and most other organisms on Earth.
For background on the factors affecting monarch numbers, see this Monarch Joint Venture webinar on Climate, Weather, and Monarchs.
