Monarch Winter 2021–22 and Spring 2022 Population Numbers


Published: 05/18/2022

We still haven’t received the official count data from Mexico, but the first 2022 generation of eastern North American monarchs has moved from the southern U.S. into their northern breeding grounds. These monarchs are the offspring of individuals that flew from the north last autumn, spent the winter in Mexico, and then moved back north into the southern parts of the U.S.

For unknown reasons, many monarchs appeared to leave their winter grounds later than normal in spring 2022, and also arrived in the south later. Then, a cool spring in the Midwest slowed the northern movement of the first 2022 generation and reports to Journey North reported them behind where they normally are in early May. But as of May 10, the leading front of monarchs moved into Minnesota, Wisconsin and Michigan, bolstered by strong southerly winds. For an up-to-the-hour report on monarch sightings, check out the Journey North map of first sightings. You can compare sightings from previous years by changing the year shown in the box to the right of the map.

Monarchs from the western migratory population are also moving away from their winter sites to the east and north. The Xerces Society for Invertebrate Conservation released numbers for this population in January (see link here: the western population is measured in numbers of butterflies vs. numbers of hectares of trees with monarchs in them in Mexico). Volunteers counted almost 250,000 butterflies, as opposed to fewer than 2000 during the winter of 2020-2021. According to Iris Howard and Emma Pelton from the Xerces Society, it is not clear why western monarchs were so much more abundant in 2021, but there are probably multiple causes. The population has undergone a sustained and significant decline and we simply don’t have enough experience with migratory monarch populations at the low levels that have been observed recently to make good predictions.

Breeding monarch numbers in any given year are caused by a complex mix of factors—the number that were present the previous year, how well they survived in their overwintering sites, the conditions they encountered during the spring migration, and conditions in their main breeding grounds. Now that we have many years of data we are better able to understand the factors associated with monarch numbers. One thing that we do know is that weather is important. A recent paper published by a group of researchers from the US and Mexico showed that hot and dry or cold and wet springs in the southern U.S. aren’t good for summer numbers. And what we see in the northern breeding range is partly dependent on where we are. In the northern parts of the breeding range, numbers increase with increases in precipitation and temperature. But in the southern part of the breeding range, high temperatures lead to lower monarch numbers. These associations are summarized in a recent paper by several researchers from the US and Canada

Data from Journey North volunteers will help us put together the story of the 2022 breeding season as it unfolds. Please keep reporting your monarch and milkweed observations throughout spring migration and the upcoming summer breeding and fall migration seasons.

—Karen Oberhauser

Dr. Karen Oberhauser is Professor of Entomology and Director of the University of Wisconsin Madison–Arboretum, chair of the Steering Committee of the Monarch Joint Venture, an officer and founding member of the Monarch Butterfly Fund, and a member of state and national organizations focused on pollinator conservation and citizen science.