By Dr. Karen Oberhauser
A month ago, Journey North volunteers were just beginning to see monarch roosts, and only in the northern reaches of their summer breeding range: Minnesota, Michigan, and Southern Ontario. Since then, the moving front of monarch migration has moved steadily southward, with monarch roosts spotted as far south as Oklahoma as of September 25.
In that time, monarchs have moved from 48 degrees north latitude to 36 degrees north. A degree of latitude is about 69 miles, so the distance between the farthest north roost and the farthest south roost (which are conveniently located at about the same longitude) is over 800 miles.
Given that monarchs rarely fly in a straight line, their flight distance is quite a lot farther. Their destination in Mexico is at about 20 degrees south, so the leading edge still has another 1,104 miles to go. That’s a long distance for an insect that weighs about as much as a paper clip.
How did the breeding season end?
Monarch Larva Monitoring Project volunteers documented strong egg and larva densities throughout July and August. Even September egg and larva densities were higher than last year.
These late-season observations document the size of the population at the end of the summer, but Monarch Watch tagging data show that monarchs leaving the north later are less likely to migrate successfully, probably because they are facing cooler temperatures and senescing nectar plants during their migration. For more detailed comparisons across years, see Graphs of egg and larva densities per milkweed plant on the MLMP website.
What’s ahead for 2025 monarchs?
The success of migration is strongly dependent on nectar availability during the migration. The current drought map for the U.S. shows a slightly drier outlook than a month ago, especially through Illinois, Missouri, Arkansas, and southern Texas. Wet conditions along rivers and low areas will be important sources of the nectar needed to fuel the migratory flight. Once monarchs move into Mexico, conditions should improve.
