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  • Andy Davis

Summary and comments on the new study of tropical milkweed and climate change

Hello everyone,

This month there was a small bomb that went off in the monarch world, and it had everyone talking. A new study was published that looked at the effects of climate warming on monarchs reared on tropical milkweed. Today, I'll summarize this study and provide some commentary.

Before I get started, let me first say that I am only the messenger here - I had nothing to do with this project, so please direct your ire elsewhere, if you're one of those people who think that scientists are all conspiring to vilify this plant. More on this later.

The paper was just published online in the journal, Ecology, and was authored by two persons from Louisiana State University (Matthew Faldyn and Bret Elderd), and Mark Hunter, from the University of Michigan. It was titled, "Climate change and an invasive, tropical milkweed: an ecological trap for monarch butterflies." Here is a link, but I believe the paper is not accessible to all. If this link doesn't work, the authors have made a free copy available to everyone here. I don't really know these gentlemen personally, but I know Hunter is an expert on all things plant chemistry, including milkweed cardenolides - that's the toxic chemical in the milkweeds that the monarchs ingest, and that turns the monarchs toxic themselves. Remember this for later.

So there was a lot of online discussion over this paper, because of the ominous title of the paper, plus some equally-splashy articles in the press about it, like this one - "Climate Change and Invasive Milkweed Could Make Toxic Cocktail for Monarchs, Study Finds". I noticed that the university had their own press release about the paper, and I saw that all of the articles were shared widely on social media. There is nothing like the tropical milkweed issue that brings out the comments, and ire of people. I know this from experience! Interestingly, from my reading of the comments on social media, it looks like most people did not read the actual study, but rather just the press release, or maybe just the abstract of the paper. Perhaps my summary of it here will help.

As the title indicated, the study was about climate change, monarchs and milkweed. We all know what climate change is - the earth has been steadily warming over the last 100 years and with each new year we keep breaking records for the warmest year on record. This is not in dispute, despite what you hear from Fox News. With this in mind, the authors of this study wanted to examine what will happen to monarch larvae in the future - i.e. when it gets warmer. In the lab, we know that monarch larvae have a thermal upper limit beyond which they tend to die, or develop slow, etc. In other words, monarch larvae don't like it too hot! What we don't know is what happens to the milkweeds when it gets too hot, and this is where the experiment comes in.

Here is my summary of the experiment.

The experiment was conducted using two species of milkweed - tropical (Asclepias currasavica) and swamp (Asclepias incarnata) milkweed. They chose these two because each of them differs greatly in how much cardenolides they have - tropical milkweed is high in cardenolides, and swamp milkweed has low amounts.

The authors grew their milkweed in a greenhouse so that they were all the same age when the experiment began. Since they wanted to know what would happen to the plants (and the monarch larvae) during warming conditions, they next created a very nifty field setup, which is pictured below. They placed single plants in mesh bags out in a field, and placed 1st-instar monarch larvae (grown in the lab) on each plant. Then they placed these crafty little mini-greenhouses over some of the plants, which are pictured below. These are just glass panes surrounding the plants, which raised the daily temperature within each greenhouse. Other plants were left alone, with no greenhouse. See where this is going?

They also put little temperature sensors on each plant, which confirmed that the greenhouses raised the ambient temperature around their plants. From the paper: "During the daytime, temperatures in the greenhouse enclosures were raised by 3°C, maintaining an average temperature around 35°C, compared to ambient plots with an average temperature of 32°C." Then they state, "In daytime hours, monarchs in the greenhouse plants experienced brief peaks in temperature up to a maximum of 46°C, and in open, ambient plots monarchs experienced temperature peaks of up to 38°C." Andy here - this part of the experiment was very important, because what they did here was to effectively mimic the natural fluctuations in daily temperatures that plants would experience, and at the same time, they raised the temperature in half of their plants by 3°C. This may not seem like much to you or I (that's about 5°F), but this is what is predicted to happen in 2080. The strength of this approach is that it mimics nature - the plants get rained on, they are exposed to natural temperature fluctuations, etc. It's very hard to do this justice in the lab.

After the plants had been set up for a few days, they put early-stage monarch larvae on each plant (I think 2 per plant?). Then they tracked the growth and survival of the monarchs on the warmed plants and the control plants until they became pupae, at which time they brought them back to their lab to eclose. When they became adults, they measured them - the weight and wing length of the butterflies. This part here is important, because it really wasn't discussed much in the news articles, or in the online discussions of the study.

Here are their results:

Remember they were looking at two factors - the effect of warming, plus the difference in the host plants. First, they found that the survival of larvae was affected by warming temperatures, but only on the tropical milkweed. Below is a screenshot of their graph on this result. The circles represent the average survival (proportion) across the study.

This graph shows the number of monarch larvae that survived on swamp milkweed (incarnata - the low-cardenolide one) did not change when the temperature was warmed. On curassavica (tropical milkweed - high-cardenolides), there was a big difference in survival. In fact, hardly any larvae survived when the temp was raised. This is probably where the authors got their idea to use the words, "ecological trap," because if everyone keeps planting tropical milkweed (to "save the monarchs"), at some point this plant will not be suitable in the future. And remember the authors had put mesh bags around the plants so that there were no predators or anything that could eat the monarchs. So this low survival is not from being eaten, or from simply dropping off the plant. The take-home message of this is that warming temperatures result in low larval survival on tropical milkweed.

OK, so this is the part of the story that I saw the most kerfuffle about in the online arguments. Based on the news articles, or wherever people got their information, there was a lot of talk about the actual temperature limit for milkweeds, and specifically, for tropical milkweed. People thought the article was saying that the milkweed, or the monarchs, would not "do well" above a certain temperature (which is not what it said). Then everyone chimed in with their own anecdotes from their gardens, like "well, here in (fill in the blank) it gets above 100F, and MY milkweed is fine," A lot of folks then wrote off this paper as "bogus" (mostly the tropical milkweed planting folks), because they didn't, or don't, believe high temperatures affect the plant, or the monarchs.

Here's the part of the story that was overlooked - it probably wasn't the warmed temperatures that caused the low survival, but more likely something about the plants that changed in response to the temperatures. The authors specifically looked at how much cardenolides were in the plants before and after the experiment, and they found that the concentrations increased in tropical milkweed under warming conditions, but not in swamp milkweed. This is more likely what caused the dramatically low survival on tropical plants. From what I understand (and Mark Hunter is the expert), cardenolides help to make monarchs toxic to predators, but they are difficult for the monarchs to process, so it's a bit of a double-edged sword. And keep in mind that tropical milkweed already has very high concentrations of cardenolides - this study showed that global warming (even 3 degrees C) will increase this concentration even more. So is this a bad thing for monarchs? Don't know.

Recall the authors also measured wing and body size in the adult monarchs. It turns out that the plant species didn't have much effect on the wing and body size. However, warming temperatures overall resulted in monarchs with smaller wings, and this happened on both plant species. Below is a graph showing this.

This graph shows that average forewing lengths (the circles) were smaller for monarchs reared on warmed plants, by about 2-4mm, and this happened for both plant species. Doing the math here, this translates to about a 4-6% decrease in wing length. Again, while this doesn't seem like that much to you and me, to a monarch this is huge. I've blogged about wing size in monarchs a couple of times (see list of blogs here), and from my research on this topic I can tell you that a 5% reduction in wing length would be disastrous for monarch migration. Small-winged monarchs have a much harder time migrating. While it is possible for them to be successful, the vast majority of monarchs that make it to Mexico are the large-winged ones. This is why over time, the migratory populations of monarchs become large, and vice-versa - non-migratory populations tend to have smaller wings, because they don't have this annual fly-or-die scenario.

What does this mean for the future of the monarch migration? You can probably guess - if warming temperatures lead to small wing sizes in monarchs, this will result in lower migratory success. And recall that the actual amount of warming that we're talking about here is an average increase of 3°C by 2080.

OK, so now let me talk about the craziness that surrounds tropical milkweed. This paper does not paint a good picture of tropical milkweed, which certainly adds fuel to a fire that has already been burning for a while now. As most monarch folks know, there are a number of other studies that have also provided evidence that this plant is harmful to the monarch population (though not to individual monarchs). I've blogged about all of these (link here for a list). I've also blogged recently about how there is no debate among monarchs scientists over this plant - they all recommend that people don't plant it. The "controversy" seems to come because there is a small sect of monarch folks out there who are convinced that their use of tropical milkweed is not harmful, and they are very vocal about it. They further think that this plant is being "vilified" by scientists! This is a little conspiracy-esque, if you ask me. And, these folks also engage in heated debates with everyone who dares point to the evidence in these studies, because in their mind, these studies are all flawed in some way. Here, I can add my two cents. By my estimation, this study in question certainly had some limitations, like the fact that they only used two species of milkweed. Five or six species would have been better. But this wasn't a "flaw". There's a difference. A flaw is something that is clearly wrong, either with their data-collection, or statistical analyses, or even their interpretation of the results. This was a limitation. Their data were not wrong. Their data clearly showed that it is better to have a low-cardenolide milkweed in your garden in the future.

I can also say to those folks who insist on finding "flaws" with all studies on tropical milkweed - all studies, even those conducted by the most brilliant and well-respected scientists in the world, have limitations. And, if one tries hard, one can ALWAYS find these limitations in any study. And from my view, the tropical milkweed folks seem to be trying very hard.

OK, enough of my soap-box. I think that about covers it for this paper. Thanks for reading, both this blog, and the paper itself (because I know you did - wink).


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The science of monarch butterflies

A blog about monarchs, written by a monarch scientist, for people who love monarchs

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