New study finds a big clue to the mystery of migratory dropout

Hi folks,

Thanks for tuning in to another post about our favorite butterfly and the science around it. Today's post is all about the monarch fall migration, and what's wrong with it! That is, what is leading to what I call, "migratory dropout." This is a big issue of conservation concern right now, and so any new piece of evidence is great to have. So it looks like this new study does just that. I just read it, and I'm going to make a quick post about it before I forget anything! This was a study that (surprisingly) I had not seen in review, nor was familiar with beforehand - so I guess that means I came upon it with a blank slate perspective. However, given that this study was essentially a follow-up to one that I recently spearheaded in 2024, I'm of course quite interested in the findings. You should be too, if you care about preserving the world-famous fall migration of this charismatic bug.

First, here is a link to the new paper in question. And, the paper is freely-available.

Before going further, let me remind people of the study my team did last year, that pretty much sets the stage for this new one. Recall that my team discovered how there is a growing problem happening with the monarch fall migration in North America - that for some reason, the monarchs are increasingly failing to reach their winter destination, and the fall migration is essentially petering out by the time it reaches Texas. While our study was not meant to find the cause of this problem, we could at least say with confidence that the reason is not related to a lack of migration habitat. That means it has something to do with the monarchs themselves, which are simply "disappearing" during the migration.

Anyway, this new study, led by a research team from the University of Central Florida, seems to suggest that rising temperatures during the fall migration are a big reason (or at least, one reason) for the dropout.

Let me briefly go over how they came to this conclusion. Be sure to read the study itself to get into the details. Basically, the authors had conducted a lab-based study, but using about 500 wild-caught fall migrants from Ohio. The adult monarchs were kept in mesh cages that were themselves in temperature-controlled conditions, and for a number of weeks. The authors purposely raised the temperature on some of the monarchs, but not others, to determine the effect of warming fall temperatures. Also, another set of monarchs were subjected to some gentle "wind" (a fan) to keep them "active" within the cages. This was to simulate the "active" nature of migrating, though the authors acknowledged that this was not an ideal representation.

The researchers monitored the butterflies for survival, weighed them before and after the temperature exposures, measured their wings, and even tested them for OE (more on this later). They also dissected some of the females to see if they had any eggs inside.

After a month of this, the researchers compared the stats for those monarchs that had been exposed to warmer temperatures, versus those that experienced normal fall temperatures. They found that the male monarchs experienced greater mortality, and lost more mass when the fall temperature was warmer. Also, females exposed to warm temperatures developed eggs! The authors report also that some females (which were migrants) began laying eggs on the floor of the mesh cages, despite not having any milkweed in the cages.

So, the implication here is that rising temperatures during the fall leads to poor migration success for males, and to migrant females becoming reproductive. Becoming reproductive is a bad omen for the migration, because being in diapause allows them to not burn energy in making eggs, and, diapause makes them lighter too.

That's not all. A second part of this project involved then taking those same adult monarchs that were used for the "fall" experiments and exposing them to another round of warm "winter" temperatures (4 months), and then monitoring them thereafter. Surprisingly, they found that the monarchs had the lowest winter survival if they had experienced a previously warm migration period! The warm winter temperatures didn't really do as much. This indicates that the warming fall temperatures we're experiencing are not only going to affect the survival of the migration, but there are also going to reduce the ability of monarchs to successfully survive the winter period. Bummer.

Finally, I mentioned before that the authors had also checked all of the 500 monarchs for OE at the beginning of the study, but they included the monarchs in the experiments even if they were infected. Then, they checked to see how the infection affected their survival - as you might guess, it did. But there was one other part of this bit that I noticed that was odd. The authors had included all of their original data in the paper's supplemental section, and I of course, looked at it. While about 8% of the monarchs were heavily infected, the other 92% were carrying small numbers of OE spores! That's right, nearly every single monarch they collected in Ohio had a small number of OE spores. That doesn't seem right. It's very possible that the person doing the collecting was using the same net for all of the butterflies, and the spores from the infected butterflies got into the net (remember they fall like glitter), and then onto ALL other monarchs collected. People who net fall migrants should keep this in mind.

The authors concluded that rising fall temperatures, plus OE infection, are two major factors leading to this growing problem of migratory dropout, and, to the rise of winter-breeding. Both of these things are related. What's worse, is that warming fall temperatures are even a factor affecting overwintering survival!

Regarding the winter breeding, the authors had a lengthy discussion about this, since this is a controversial topic these days. They suggested that rising temperatures alone would be all that is needed for female monarchs to become reproductive during the migration (i.e. develop eggs internally), regardless of whether they are exposed to fall milkweed. That's a pretty big assertion! Though from other work, we know that when migratory females are exposed to green milkweed during the fall, that too leads them to become reproductive. So it is likely a bit of both.

OK, back to the big picture now. All of this is pretty damning news for the monarch migration, since climate change is a big, big, global issue that can't really be stopped. Essentially this new information is saying that no matter what we do, the future migration is doomed because of the warming climate. So, what can the average person do with this new information to support the fall migration?

Here are my thoughts on this question. First, it would still be important to remove any non-native milkweeds from the equation, and to ensure the migrants do not encounter them at all. Even IF the rising temperature alone is leading to increased reproductive activity, the presence of green milkweed in September or October anywhere in the flyway will only hasten that issue, and tell the females that it's time to stop migrating and lay eggs. Second, the OE issue is definitely something that can be managed by us, and ideally, minimized. But it would require people to stop doing things that increases its prevalence in the population, like REARING MONARCHS IN CAPTIVITY and planting non-native milkweed. I've also gone so far as to even recommend that people in Florida remove all of their backyard milkweed, because the OE problem there is so out of control. So yes, the OE issue can be solved, but it's going to take big steps.

Let me end this post by reiterating something - the monarch fall migration is a spectacular feat of adaptation and took eons to evolve, and it is what makes the monarch special. Lincoln Brower always used to say it is as important as the Mona Lisa, and losing either, would be tragic.

The future loss of the migration will be on us.

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Direct link to this blog entry:

https://www.monarchscience.org/single-post/new-study-finds-a-big-clue-to-the-mystery-of-migratory-dropout

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