by Karen Oberhauser, Wisconsin Monarch Collaborative
The adult monarchs that we’re seeing in Wisconsin now are mostly offspring of the generation that started as eggs in the spring of this year. These “Gen1” individuals came up from the southern US, and were themselves the offspring of the monarchs that overwintered in Mexico. So we’re seeing “Gen2”, with a few “Gen3s” emerging (the Gen3s we see at the end of July, are the first offspring of the first offspring from Gen1). Journey North maps below show little northward movement after Gen1 recolonizes the summer breeding range.
Figure 1. Note monarchs’ range at the end of June (left, downloaded June 28) and July (right, downloaded July 27). If you look closely, you can see a few more northern sightings in July, but there is little northward range expansion by the second generation of the year.
Perhaps you’ve noticed lots of monarchs lately; monarch numbers have taken off since our last update! We always hope for more monarchs in Generations 2 and 3 than we saw in Generation 1; this increase is a sign of a combination of high fecundity (number of eggs laid by females) and high egg and larval survival. Monarch Larva Monitoring Project (MLMP) volunteers in Wisconsin are documenting such an increase in 2025, with a July peak almost three times as high as the June peak. So far, peak monarch density this year is just under 0.30 monarchs per plant, a number we haven’t seen for four years (see Figure 2). Of course, monarchs still need to finish off the summer strongly, since the butterflies that will migrate to Mexico haven’t emerged yet; those that emerge after August 15 will head south. For more detailed comparisons across years see Graphs of egg and larva densities per milkweed plant on the MLMP website.
Figure 2. Note the large increase in per plant density of monarch eggs and larvae from June to July in 2025 (top graph). The dip during the week of June 8 reflects cool rainy conditions that week, when monarchs weren’t laying many eggs. The peak during the week of 7/13 hasn’t been this high since 2021 (bottom graph). 2025 graph downloaded July 27.
Volunteers submitting data to Wisconsinbutterflies.org are also reporting lots of monarchs, with some seeing so many that they stopped being able to count them!
Thanks to Journey North, Wisconsinbutterflies.org, and Monarch Larva Monitoring Project volunteers for providing the above data. If you haven’t already, please join these great projects!
