A number of people have contacted us over the summer to ask how the monarch population is doing. Some of them mention that they’ve come across very few monarchs up to now. Others have noted that the adults seem to have arrived later than normal. Are we dealing with a smaller population of monarchs this year, or have the monarchs simply been delayed in getting to our latitudes? Before reaching certain conclusions, here are a few points to be aware of with regard to the monarch’s spring migration.
The monarch’s different strategies in springtime
The study and understanding of the monarch migration phenomenon in North America are both relatively recent. Although the monarch butterfly was first described by the taxonomist Carl von Linné in 1758, it wasn’t until 1976 that the scientific community documented the butterfly’s wintering grounds in central Mexico for the first time (Urquhart 1976). And it was only after that “discovery” that it was possible to get a better grasp of how the monarch’s annual “recolonization” in North America ‒ its spring migration ‒ unfolds.
Thanks to a study published by Miller and his colleagues in 2012, it’s now agreed by the scientific community that monarchs use different strategies during that migration. The strategy used by most of the monarchs overwintering in Mexico (90%) consists in migrating to the southern United States, laying eggs on the way and dying, in the process leaving their offspring to recolonize the summer breeding grounds located further north. That strategy involves "successive broods" migrating across the North American region during the spring. However, a small portion of monarchs that wintered in Mexico (10%) will migrate as far as the Great Lakes, deposit their eggs along the way, and thus carry out a “single sweep” from their breeding grounds, without making a stop in the more central U.S. like other monarchs.
Other factors that may influence the monarch’s northern migration
Beyond the different strategies that the monarch may use in its spring migration, other factors may influence the itinerary, the timing and the duration of that migration.
On the one hand, monarchs touch down and momentarily pause their journey north if wind conditions are not conducive to migration. Northward progress may therefore vary slightly from one region to another.
On the other hand, the milkweed plant emergence rate and plant quality may vary across the territory. Thus it’s possible for females to decide not to stop to lay eggs if they reckon that the plants are not “good quality.” That may explain why adults are sometimes seen in certain regions, but not eggs or caterpillars.
Variations in the size of the overwintering population will also influence the scale of this migration phenomenon: the number of monarchs found in Mexico in early spring will determine the number of monarchs we might see later in spring. On that subject, we know that the size of the population wintering in Mexico unfortunately decreased by 53% as compared to last year’s.
As far as the moment when migration begins and how long it lasts, important variations have been noted from one year to the next. It seems that certain environmental factors significantly influence the migration phenomenon; however, there are still no specific studies on that topic.
How can the monarch migration be documented?
To this point, the principal studies on the monarch’s spring migration have been of short duration and undertaken at specific places along migration corridors in the U.S. and in the Great Lakes region. For a better understanding of the migration phenomenon we need large-scale programs that compile data over the long term. That’s where citizen science comes in! Already, programs like Journey North have made a better understanding of the springtime phenomenon possible. Mission Monarch also has a role to play, because that program allows us to document the monarch’s arrival in its northernmost breeding range.
To sum up, it’s not always possible to give concise answers to the highly interesting questions we receive. Nevertheless, one thing’s for certain: actively participating in Mission Monarch will help the scientific community study this magnificent migration phenomenon ‒ and arrive at a better understanding of it.