• Andy Davis

Larvae reared on tropical milkweed become adults with the wrong metabolism for migration

(picture above from the Dallas Morning News - link)

Hello blog readers,

Today I need to once again veer into the contentious territory of tropical milkweed, which is always a topic that stirs emotions among monarch enthusiasts. I'm going to tell you about a brand new study that I just read (I had nothing to do with it and am only the messenger here), that was just published in a high-profile scientific journal. I found the paper to be quite interesting, since it focused on monarch physiology, which is one of my own areas of interest. Anyway, this is a paper that folks should know about, and I need to write down my thoughts on it while it is still fresh in my mind.

The paper was published in the journal, Nature Communications Biology, and I think it is fully open access (link here). The authors were a number of folks from the department of entomology at Penn State, and the lead author was Tori Pocius (pictured below). Some readers may recall that Tori had done a Masters project in the lab of Chip Taylor some years ago. From a simple google search, I see she was a postdoc at the time of this project, working in the lab of Dr. Jared Ali, who is a fairly new professor in their entomology dept. I also see that Dr. Ali (pictured below, holding the laptop) appears to have a lively lab group which conducts research on milkweeds. Perhaps we'll be hearing more from his lab in the future.

OK, lets get to this new study. As any monarch enthusiast knows, there are many different species of milkweeds in North America, and each of these have differences in their plant chemistry. This paper describes an experiment that was designed to test what happens to larvae that eat different milkweed species, and specifically, what is the effect of these milkweeds on the metabolism of adult monarchs. This is a very interesting question, which has not specifically been explored before, but has great importance for the long-distance migration of monarchs. From prior research, we know that monarchs from migratory populations appear to have a lower metabolism than those from non-migratory populations. And within the eastern migratory population, we also know that the migratory generation has a lower metabolism than those from the breeding generation. Combined, these both tell us that having a low metabolism is needed for the long-distance journey, presumably because this saves energy. Think of metabolism as the body's "idling speed", like in a car engine - if the engine is revving too high when you drive, it burns a lot of gas.

Anyway, the authors questioned if different milkweeds can affect metabolism, because of the differences in cardenolide content of the plants. Cardenolides are the substance that makes the milkweeds toxic, and when the monarch larvae eat the plants, they themselves become toxic. But, there is a catch - the cardenolides are hard on the monarchs too, and they can cause some key physiological changes during development, that we are only now beginning to learn. For example, we now know that different milkweeds can affect traits like larval survival, immunity, resistance to parasites, and adult wing morphology. In other words, monarchs that are grown on different species of milkweeds may look similar, but they may have vastly different physiologies.

The researchers examined 8 different species of milkweeds in this experiment, including tropical milkweed. These were chosen because of their difference in cardenolide content, as well as their availability. I'm going to put a screenshot below of a table from the paper that describes the plants used. A lot of these should be familiar to monarch enthusiasts. From reading the methods of the paper, these plants were all grown from seed in temperature controlled plant growth chambers.

You can see above, that tropical milkweed, Asclepias curassavica, has the highest cardenolide content of all of these commonly-found milkweeds. This is one of the reasons why this plant causes so many problems for monarch biology.

It sounds like the monarch eggs were obtained from a breeding colony maintained at Iowa State University, and shipped to the lab at Penn State (more on this later). The researchers grew the eggs until the caterpillars hatched, then placed the caterpillars onto these different milkweeds in their greenhouse (plants were in mesh cages). They allowed the caterpillars to eat and grow on these plants until they metamorphosed into adults, then they conducted a series of measurements on the adults, including their metabolism.

Measuring metabolism in insects is something I do in my lab, so I can help to explain what they did here. The researchers placed an adult monarch in a glass jar (like a large mason jar), which had a tight-fitting lid. The lid had an air hose going from it to a machine that measured the amount of carbon dioxide in the jar. Monarchs, like humans and other animals, breathe, and in so doing, emit carbon dioxide. The amount of carbon dioxide they emit is directly related to their internal metabolism. Therefore, by measuring how much CO2 is being produced in the jar, the researchers can measure the monarch's metabolic rate. In my lab, I have a similar device, but mine measures oxygen, and so I measure metabolic rate of insects (and spiders!) by assessing how much oxygen is used by the critter in question over time.

It sounds like the researchers placed the jar with the monarch in a temperature controlled incubator (with the air hose still connected) and they then measured the "resting" metabolic rate, and the "flight" metabolic rate of the monarch. For the resting state, they simply covered the jar with a black cloth, which more or less kept the monarch calm. They did this for 5 minutes. Then, they removed the cloth, and apparently, the monarch in the jar then flapped around, which more or less simulates "flight." This is an approach I have seen used before. It doesn't really simulate actual migratory flight, but it does simulate "flapping", which is a good proxy for flight. Anyway, the researchers repeated this for all of the monarchs that had been reared on the different milkweed species. They then used the data they gathered on each monarch's CO2 emission to compare across the milkweed species.

Now let's get to what they found.

First, they reported that there were some overall differences on larval survival across the different milkweed species, which I won't get into here (this has been reported many times in other studies), and there were some differences in adult size (also not getting into). Interestingly, the authors also reported the size of the flight muscles of these monarchs - apparently, they had dissected the monarchs after the experiment to determine this. They found that flight muscles of monarchs reared on tropical milkweed were larger than those reared on 6 of the 7 other species. They spent a fair amount of time on this part, though I don't think it is as important as the metabolism parts (next).

I'm pasting another screenshot below, of one of the key figures from the paper, that shows the "flight" metabolism of monarchs, and how the milkweed type affects this trait. They use abbreviations for the different milkweed species, which you can probably figure out.

The key thing to see here, is that monarchs reared on tropical milkweed as larvae (CUR, above), have a very high metabolic rate during flight compared to other milkweeds. There was another graph given that showed the resting metabolic rate, which essentially showed the same thing as this one - monarchs have a higher metabolism when grown on tropical milkweed. Recall that for long-distance migration, we know that lower is better.

So in other words, monarchs reared on tropical milkweed would be burning a lot of energy trying to make the trip to Mexico in the fall.

Next, let me ruminate a bit over this finding. While the implications are pretty clear (in terms of the migration), it is less clear why this was found. The authors did not really have a good explanation for why tropical milkweed produced monarchs with high metabolism, other than some discussion over the cardenolides. In general, I think the high metabolism is probably linked with the aforementioned larger flight muscle mass of these monarchs. The unusually large muscle size probably requires more energy to fuel, and during periods when energy is not critical, this is probably not a problem. But during the migration, this would be a problem. At first glance, one might think that having large flight muscles would be beneficial for migratory monarchs. Interestingly, I don't think there is evidence that migratory monarchs require larger flight muscles than non-migrants, even though it sounds logical they would. Thus, I can't see how this larger flight muscle would benefit the migratory journey, especially if it requires more energy. I guess an over-arching question here is why the high cardenolides cause monarchs to have overly-developed flight muscles in the first place. I can see from the paper that the authors also had these same thoughts.

Now, let me circle back to something that gave me some pause while reading this paper, and if any of the authors are reading this, perhaps they could keep it in mind for the future. I mentioned that the eggs for the project were obtained from a "breeding colony" at Iowa State University. The paper states that the colony had been maintained for 18 generations, but with some wild stock introduced occasionally to prevent inbreeding. There was no indication that these eggs were infected or anything like that, but what concerns me is that the monarchs were all on the small side, which is one consequence of a long-term breeding colony, especially one that has been ongoing for 18 generations. In other words, without the annual impact of the migration to remove any small-winged monarchs, you eventually end up with lots of small-winged monarchs in your breeding stock. I'll paste below another table from the supplemental file of the paper, which shows the morphological measurements of these monarchs. Look specifically at the forewing lengths (FWL), which are all in the mid-40s. Typically, migratory monarchs in the east have lengths over 50mm.

This table tells me that the monarchs used in this experiment may or may not be truly representative of the "migratory" monarch generation. In fact, these wing sizes are similar to those from non-migratory monarchs in Florida, Costa Rica, etc. So does this mean the experiment is flawed? Not at all. By my read, the experiment was done quite well. This is really just me being picky I guess. And really, the issue of small-winged monarchs does not negate the findings regarding tropical milkweed, since all of the monarchs in the study were small. But I think if I had been a reviewer of this manuscript, I might have asked the authors to discuss this limitation.

So, I guess these are my thoughts on this interesting new paper. For those who have followed this blog for a while, you may recall that in a prior post I had put together a list of all of the research surrounding tropical milkweed, which all show how it negatively affects migration in various ways. I guess I'll have to add this one to that list: the paper that shows how tropical milkweed produces monarchs with the wrong metabolism for migration!

That's all for now.


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