Late-Emerging Monarchs: Migration Success and Transport Considerations

[George DeWolf]

Hi Everyone,

I'm making another attempt today to head out and tag some Monarchs, after having no luck just a few days ago. Recently, someone who is rearing a few Monarchs in captivity asked me about the likelihood that a couple of her caterpillars—one in its 3rd or 4th instar and one preparing to pupate—would successfully make it to Mexico. She was also considering giving these captive-reared Monarchs a “boost” by transporting them to Massachusetts and wanted my thoughts on this.

If you’ve ever raised Monarchs, you understand the unique joy of nurturing these delicate creatures, watching them grow from caterpillar to adult butterfly, and you can surely appreciate the heartfelt concern behind these questions.

I don’t claim to be an expert on the subject, but I’ve done a fair amount of research. Perhaps some of you will enjoy the following response. Please feel free to share your thoughts or correct anything you think may be inaccurate. Happy Labor Day!

My response: 

Regarding your question about whether a caterpillar barely an inch long (probably 3rd or 4th instar) or one just entering the chrysalis stage ("hanging J") here in NH in late August or early September has enough time to survive the journey to Mexico:

To start, Monarchs in central NH begin their southward journey at different times; it’s not a coordinated migration and can take place over more than a month. Some Monarchs may begin moving as early as mid-August—perhaps even sooner. At our latitude (Quincy Bog in Rumney, 43.791344°), peak migration probably falls around September 8–11, but you can still see large migrating congregations refueling on goldenrod before and after those dates. For example, I was at Thomson Tree Farm in Orford on August 30, 2022, and observed about 500 Monarchs settling down for the evening on goldenrod in a small meadow at about 2,000 feet in elevation (see Monarchs Everywhere! video). Quite a sight!

The odds are stacked against any Monarch from our region making it all the way to Mexico—regardless of when they start the journey. Obstacles include weather, food sources, predation, and disease. Studies show that northeastern Monarchs are much less successful than their midwestern counterparts: the average recovery rate for tagged Monarchs from midwestern populations is 3.93 times higher than that from northeastern populations (source: Evaluating the Migration Mortality Hypothesis Using Monarch Tagging Data). Roughly 1 in 100 Monarchs tagged in eastern North America are recovered in Mexico (source: Journey North). Eastern Monarchs face greater challenges: longer distances, stronger headwinds, and less availability of nectar sources for refueling.

Given these odds, a Monarch 3rd or 4th instar caterpillar—or even a 5th instar that's in the process of pupating (in late August/early September)—faces an even slimmer chance of completing the journey. Cooler nights mean the pupal stage will likely last longer, perhaps two weeks or more, so emergence as an adult butterfly would occur in mid-September. While favorable winds, temperatures, and nectar availability could help, the probability of completing the full journey is low. I've observed Monarchs here in my yard in Wentworth as late as October 5th, and sightings occur statewide into early November (source: iNaturalist observations). By that time, they are much more likely to encounter freezing temperatures and scarce nectar at some point in their journey. Unlike some species, such as Mourning Cloaks or Question Marks, Monarchs do not tolerate freezing temperatures.

Now, regarding the idea of giving late-emerging Monarchs a head start by transporting them to Massachusetts. I so applaud this sentiment, and we do indeed need more people around who care to the degree you do, but I would caution against it for the following reasons:

• Monarchs rely on sun position, magnetic cues, and other mechanisms to navigate south. I can't know for sure, but moving Monarchs artificially in this way could disrupt their orientation.

• Handling and transporting could stress the butterfly, reducing its chances of survival.

• Human intervention can interfere with natural selection. Helping a butterfly with potentially weaker genetics survive may unintentionally pass on traits that would otherwise not persist.

I personally believe in letting nature be nature while working to reverse the damage humans have caused—habitat loss, climate change, herbicide and pesticide use—and in promoting conditions that support healthy natural selection to ensure the survival of the Monarch.

Keep fostering your enthusiasm and asking thoughtful questions!

Best, 

George