by Karen Oberhauser, Wisconsin Monarch Collaborative
Figure 1. Photo and report submitted to Journey North by observers in Escanaba Michigan. For full details, click here
Monarchs in the Eastern Migratory Population that emerge as adults on or after about August 15 in the Upper Midwest will be in reproductive diapause and will spend their first few months of “adulthood” flying to their overwintering sites in Mexico. Instead of being ready to mate and lay eggs three to five days after emerging, these individuals will delay reproductive maturity until spring 2026. This change was triggered by hormonal changes which were themselves the results of exposure to environmental cues: decreasing daylength, cooler nights, and senescing host plants during development. We’re seeing decreasing numbers of eggs and larvae, and more adults focused on directional flight that will take them toward their winter grounds.
Interestingly, some monarchs that emerge later in the summer but before August 15, while not in reproductive diapause, also head south. Instead of laying all of their eggs where they developed, and where they would be exposed to cooler conditions and thus slower development, these monarchs move southward as they lay eggs. This “pre-migration migration” has been documented by Monarch Larva Monitoring Project (MLMP) volunteers in the middle and south of the eastern half of the U.S., who see increased egg numbers at the end of the summer.
Journey North volunteers in the north are beginning to document migratory roosts; as of this writing (August 21) there are only three roost reports, in Minnesota, the Upper Peninsula of Michigan, and Southern Ontario, but these reports will pick up over the next few weeks. For an up-to-date picture, visit the Monarch Fall Roost page here. I recommend clicking on the dots with white squares for amazing photos! See an example in Figure 1.
How did Wisconsin monarchs fare in summer 2025?
Monarch numbers continued to be relatively high, at least compared to the last several years, through August. MLMP volunteers have documented strong egg and larva densities, and many people are reporting that they’re seeing many adults. Our promising July “density hump” continued into August, with fairly good numbers even past mid-August. All of the eggs and larvae we see after about the end of July will become migratory adults (if they survive), so we should be sending off good numbers to the south. Figure 2 compares 2025 and 2024 egg and larva densities in Wisconsin. For more detailed comparisons across years see Graphs of egg and larva densities per milkweed plant on the MLMP website.
Figure 2. Wisconsin MLMP data from 2025 and 2024. Note the large increase in per plant density of monarch eggs and larvae from June to July in 2025 (left graph, downloaded August 21). The 2025 dips during the weeks of June 8 and July 20 reflect rainy conditions, when monarchs weren’t laying many eggs. Note the bigger increase from the first to the second “humps” in 2025, reflecting more population growth.
Of course, these migratory adults still need to make it all the way to Mexico, but drought conditions in Texas have lessened (albeit in dramatic and sometimes destructive ways) and monarchs should be able to find the nectar needed to sustain both the migratory flight and their overwintering period. Figure 3, from the U.S. Drought Monitor team, shows the current 2025 drought map. For the first time in many years, monarchs have experienced consistently benign conditions throughout their eastern breeding range, and this good luck might continue through the migration. This map bodes well for the population!
Figure 3. NOAA drought map. Downloaded from https://droughtmonitor.unl.edu/on August 21, 2025. Note the lessening of drought conditions throughout the range of the Eastern Migratory Monarch Population. To compare other years, visit the Drought Monitor website.
Thanks to Journey North and Monarch Larva Monitoring Project volunteers for providing the above data, and the U.S. Drought Monitor team (the National Drought Mitigation Center at the University of Nebraska-Lincoln, the National Oceanic and Atmospheric Administration, and the USDA) who provide important statistics on drought conditions throughout the U.S.
