IRA FLATOW: Every November, volunteers set out through the mountains of California with one goal in mind– to count monarch butterflies as they gather for winter hibernation. And the recent numbers have been bad news. For example, back in the 1990s, the Western population numbered more than a million. But in 2018 and 2019, volunteers only counted 20,000. And in 2020, the count turned up a terrifyingly small 2,000 butterflies.

So you might imagine there was concern about how many butterflies might be found or not found this year. Well, drumroll please– the 2021 Thanksgiving count turned up 250,000. Yes, 250,000 butterflies in winter enclaves throughout California. What a victory!

OK, so how did the population bounce back so dramatically? And is this number a blip on the radar or the start of better times for the beleaguered butterfly? Here to help us unpack the news, Dr. Louie Yang, a professor and ecologist at the Department of Entomology and Nematology, UC Davis. Hey, welcome to Science Friday.

LOUIE YANG: Thank you. I’m glad to be here.

IRA FLATOW: Was it surprising to you to see such a rebound in such a small population?

LOUIE YANG: Absolutely. Yeah, I think it was astounding, actually. The magnitude of this increase is unprecedented.

I say that knowing that populations of butterflies, and monarchs in particular, are extremely variable. They have the capacity to increase tremendously in a given year. A single female monarch can lay hundreds of eggs. And so they do have the capacity to increase dramatically across multiple generations.

But that cuts both ways. We’ve seen this population decline dramatically in ways that were utterly unanticipated and surprising. And then we’ve seen it bounce back in ways that were even more surprising, I think. This more than hundredfold increase in the population in one year is a remarkable event. We still don’t fully understand.

IRA FLATOW: OK, let’s talk about the why. How could– how could they have rebounded so quickly in just one year?

LOUIE YANG: That’s a great question. And it’s a question that I think a lot of monarch researchers are working to understand right now. It seems reasonable to say that an increase of this magnitude would probably require a series of fortunate conditions throughout the breeding season that would sustain population growth across multiple generations.

IRA FLATOW: Such as?

LOUIE YANG: You know, my research has really focused on understanding the seasonal dynamics of milkweed-monarch interactions. How do monarch caterpillars develop on their milkweed host plants? And what are the conditions that allow them to develop well and survive to adulthood? And what are the conditions that cause lower survivorship?

Some of the main factors that likely limit monarch populations are the kind of bread and butter of ecology that we often think about, things like resource limitation or competition, predation, and also things like disease. And there are also some other drivers, like environmental chemicals and changes in land use and other things that are certainly happening. But my research has really focused on understanding, what are the limitations in the early season, and what are the limitations in the late season?

IRA FLATOW: Let’s talk about that. What are the early limitations? What kind of– you say resources. I take that to mean food. Can they get enough? Can they get enough to eat? Would that be correct? And what does your research show you about the populations and their eating habits?

LOUIE YANG: Yeah, so monarch butterflies lay their eggs on milkweed. There are several species of milkweed across North America and a tremendous diversity of milkweed host plants in the Western range. And that’s the main resource that they need to grow and develop. The caterpillars eat milkweed, and they grow at a tremendous rate.

IRA FLATOW: So this year, they could have had more food earlier in the breeding season?

LOUIE YANG: Yeah, we’ve been very interested in this idea of a potential phenological mismatch in the timing of milkweed availability and the timing of monarch demand for that milkweed. And that seems to be a pattern we’re seeing some evidence of, especially in the early season.

So maybe to take a step back, what we’ve seen over several years of experiments and observational studies is that if you experimentally introduce monarch eggs to host plants, milkweeds, at different points in the season, we see evidence for seasonal windows of opportunity. We see evidence for a window of opportunity where they’re developing quite well in the late spring or early summer. And that’s followed by a mid-summer slump where they don’t seem to develop very well. And then in the late summer or maybe early fall, there’s a second window of opportunity.

When we look more closely at the early season and late season windows, we see that they’re different. They’re different in character. A recent observational study that we conducted before the big crash showed that the early season window was characterized by large numbers of eggs on relatively small numbers of milkweed plants, resulting in high densities on those plants. And those eggs– relatively few of them survived to develop as caterpillars and as adults. So low survivorship on relatively small early season milkweed plants.

The late season seemed to be characterized by a different pattern, where we saw higher survivorship. The plants in the late season are much bigger. However, they’ve been busy ramping up their defensive traits across the season. So by the end of the season, those plants are big and robust and well defended with things like toxic latex.

And we think that the second window of opportunity actually coincides with the point in the season where the plant starts to draw back those resources. So during this period, the leaves get a little bit less well defended. And that seems to coincide with this second window of opportunity.

IRA FLATOW: This is Science Friday from WNYC Studios. Let me ask you this about the changes in the population. Could climate and climate change help explain the timing of milkweed and how well the monarchs are doing?

LOUIE YANG: I think it’s certainly related. I think we’ve seen that climate is fundamental to most of the factors that limit monarchs. Even if it’s not the direct driver, it relates to all the drivers that seem to affect monarchs, or many of the drivers that affect monarch development. In particular, the timing of milkweed emergence from the ground and the timing of milkweed senescence at the end of the season both seem to be strongly related to climatic drivers, maybe most strongly precipitation in the previous winter.

There’s also warming trends that seem to have strong effects on monarch development. So warming could influence when the monarchs begin their spring inland migration. And it could also affect the rate at which caterpillars develop on these plants.

And especially in the late season, we’re starting to see some evidence that heat waves might have negative effects on monarch development. We know that these monarch caterpillars experience sublethal thermal stress when the temperatures get too high. And very high temperatures can even be lethal to them. And those temperatures are temperatures we’re starting to see during heat wave periods in California over the summer.

IRA FLATOW: How many years will it take for you to know whether this is– you know, goes up and down now, up and down, up and down? Or is this a blip on the radar screen that’s just a one-year event, coming back in great numbers like this?

LOUIE YANG: It will take time. But we have good hypotheses, and we have good questions to ask. I actually think that the number of motivated folks studying these questions will have some answers for us soon. What we’ve seen is a long-term trajectory of declining populations, even before the recent population variability.

I think one thing to emphasize is that we really didn’t have much data on how a population that is that small responds. The population for the last several years has been around 2,000 butterflies across all of Western North America. And that’s a tremendously small population considering that just a few years earlier, we had 200,000 butterflies, and not that long before that, we were talking about numbers that were projected to be in the millions.

So we don’t have a lot of data to understand how this specific population responds at such low population densities. But it is possible that there was some benefit to being at that low population that allowed them to increase in this one year. But it’s also useful to recognize that this population was at that low density for several years, and it didn’t increase in those previous years.

So we shouldn’t be complacent or assume that populations will always bounce back from low densities. The opposite could have been true. It could still be true in the future. I think a lot of folks are breathing a sigh of relief that the population has increased as much as it has over the past year. And I think we all share that sense of relief and joy that the population has increased. But also, there is that note of caution that there is a lot about the dynamics of this population that we don’t yet understand and we’re still working on.

IRA FLATOW: What a great place to stop, Dr. Yang. Thank you for taking time to be with us today.

LOUIE YANG: Thank you.

IRA FLATOW: Dr. Louie Yang, professor and nematologist at the Department of Entomology and Nematology at the University of California at Davis.

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