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  • Andy Davis

Which milkweed plant produces the best monarchs? Reviewing the evidence

Happy new year everyone,

I'm going to start the new year off with a question that I'm sure a lot of folks have thought about - of all the different types of milkweed there are in North America, which species produces the best monarchs? Today I'm going to give an overview of the science on this topic, including some of the most recent research. So hopefully, this post will give you the most scientifically-accurate answer possible!

Before I begin, let me first say that I'm not talking about which milkweed do monarchs like the most, or which one gets the most eggs in a garden. That's a whole other can of worms. I'm talking about the next stage, when larvae are consuming milkweed and growing into adults - which milkweed leads to the biggest, most robust, and most hardy adult monarchs. In other words, which milkweed produces the most migration-capable monarchs. This is a much more important question than the former is, because if you've been following the recent research, you know that the biggest problem monarchs are facing right now is that they are not reaching the overwintering sites like they used to, leading to a decline in overwintering colonies over time. So, to help with this issue, what we (meaning you the homeowners) need to be doing is to ensure that our gardens are yielding the highest-quality adult monarchs. See the difference here? It's not a numbers game. It's not just about growing lots and lots of monarchs. It's about growing monarchs that are most likely to be successful during the migration. That's what we all need to be doing.

So what are the characteristics of a successful migrant monarch? Well we know for sure that large wings are probably the biggest factor - we have many, many studies pointing to this, and you can see the blogs about this here. Next to that, we also think that the shape of the forewings matters - having elongated wings seems to be important, although we are less sure of this. A number of years ago, I coauthored a study (link here) that showed migratory monarch populations tend to have more elongated forewings than the non-migratory populations do, although the evidence in that study was not strong. But, we do know from a lot of studies of birds, bats and other insects that migratory individuals tend to have elongated wings. Basically, more elongated wings are better aerodynamically. Next, migrants need to have strong, sturdy wings that are damage-resistant, for obvious reasons. And finally, we also know that (in monarchs anyway) the color of their wings is somehow linked with migratory success - the redder the better. So when you put all of this together, we can say with confidence that the ideal monarch, or the most migration-capable monarch, should have large wings, and/or very elongated wings, its wings should be very red in color, and its wings should be thick and sturdy.

OK, so lets get to the science on milkweeds. Over the last ten years or so, there have actually been a number of studies done where monarch larvae were grown using different types of milkweeds. These studies have had various goals and their methods have generally varied, but for the most part, in each study the larvae were grown under standardized conditions, meaning that the lighting, temperature, humidity and everything else was the same in the experiment. But (big but here), because the experiments were done by different researchers, in different labs or greenhouses, etc., that means the conditions probably differed between studies. I'm telling you this because in some cases below, you'll see that one study may have found a different result than another did, even though they used the same milkweed types. This happens frequently in the cases below. This is a bit of a problem when you're trying to draw conclusions about something - normally, we like our experimental results to match up. Even so, what we CAN do is look for patterns across studies, and use a weight-of-evidence approach. You'll see what I mean later.

One of the reasons this blog piece is timely is because a brand new scientific study has just been released. A researcher named Leslie Decker and colleagues has a paper forthcoming in the journal, Functional Ecology, which seems to bear on this issue. Here is a link to an early version, but I don't think this is freely available. The paper described an experiment where the researchers tested how larvae grown on different milkweed types affected the wing morphology of the adult monarchs, and under current and rising CO2 conditions.

This was a very meaty paper, and I'm not going to get into the whole thing here. I'll just focus on the key bits where the milkweeds come in, and I'll just relay what they found as bullet points, for the sake of efficiency:

- They grew monarchs on 4 species of milkweed - Asclepias syriaca (common milkweed), A. incarnata (swamp milkweed), A. speciosa (showy milkweed?), and A. curassavica (tropical milkweed).

- After measuring the adults that came from these milkweeds, they found no statistical difference in wing size across all milkweed types

- Adult monarchs did show differences in wing shape - monarchs reared on tropical milkweed had the least elongated wings, while those reared on common milkweed had the most elongated wings.

- Wing thickness also varied - monarchs reared on tropical milkweed had the thinnest wings (would be most likely to be damaged).

- There were some other interesting findings concerning the effects of elevated CO2, but I'll skip those.

Now, let's move on to another study where different milkweeds were examined for their effects on wing morphology, one that I co-authored. I mentioned this one in the previous blog - in the recent collection of monarch studies that came out in Animal Migration, my coauthor, Jaap de Roode and I had a paper that described an experiment where we measured wing morphology on monarchs that had been reared on different milkweeds under the same conditions. We looked at 7 different milkweeds - the 4 above, plus Asclepias fascicularis (Narrow leaf milkweed), A. physocarpa (Balloon plant milkweed), and A. sullivantii (Prairie milkweed).

Here is what we found:

- We found very little difference in monarch wing size across the milkweed types, except that those reared on tropical milkweed tended to be larger than monarchs reared on all of the others.

- We examined the shade of orange-red color on the monarchs too, and again found little effect of milkweed type, except that monarchs reared on tropical milkweed tended to be the least red (meaning they have low migratory propensity).

- We found no effect of milkweed type on the thickness of monarch wings - all milkweeds produced monarchs with equally-thick wings.

Next, there was a study also published in this same collection in Animal Migration, that was authored by Micah Freedman and Hugh Dingle. I also mentioned this one in the previous blog. Recall that these researchers examined museum specimens of monarchs across North America, specifically looking at wing morphology variation across regions and through time! But they also conducted a side experiment where they reared monarchs on different milkweed types and then examined their wing morphology. They used common milkweed, A. fascicularis, tropical milkweed and two other non-natives, Gomphocarpus physocarpus, and G. fruticosus. I believe these last two can be found in gardens here in the US now.

Here is what they found in this study:

- Monarchs reared on common milkweed were significantly larger than those reared on the other three

- They found no differences in wing shape (elongation) across all milkweed types

Then there was a study done by Victoria Pocius and colleagues, which was published in 2017 in Environmental Entomology (here is a link). They had examined characteristics of monarchs that had been reared on 9 different milkweeds native to the midwest. They had mostly focused on larval survival, but they had also performed some measurements on the adults too, which I'll relay here.

I'll paste a table from that study below, which shows a list of the different milkweeds, plus all of the relevant morphological data from the adult monarchs. They had measured the adult wet mass and the dry mass (which is how much the specimen weighs after it is dried in an oven), both the forewing and hindwing length, and also how much lipids were in the specimens.

They reported that there was no statistical difference in the average wet mass of adults, although looking at this table you can see that one plant appeared to produce very heavy monarchs - A. exultata. However, I also note that those adults did not have exceptionally large wing sizes. For reference, wing lengths of 50mm or higher are typical for the average migrant monarch. Usually, wing sizes above 51 are needed to get to Mexico - see a previous blog about this. Given this, it looks like monarchs raised on A. syriaca, A. tuberosa, and A. speciosa had wing sizes in that range.

The lipid assay in this study were interesting, although this is a little harder to interpret, since we don't really know how much lipids monarchs are supposed to have when they first eclose (emerge from metamorphosis). It may not even matter how much they have when they eclose, since they seem to accumulate lipids during the southward migration. But I guess, if you're a migrant monarch, you'd want to have as much lipids as possible from the start. I note that the authors did report that there was a statistically significant difference in lipids. Looking at the table, it seems that monarchs reared on A. syriaca, A. tuberosa, A. exultata and A. incarnata tended to have higher lipid content.

Finally, there is one study that I know of that is not even published yet, but will soon be. Dr. Anurag Agrawal is a coauthor on it and was kind enough to send me the relevant results of it. It is a study that seems to be fairly relevant to this discussion too. It is titled, " Beyond preference and performance: host plant selection by monarch butterflies, Danaus plexippus and consequences for growth and defence sequestration." It is due to be published in the journal, Oikos. This was another experiment where monarch larvae were reared under similar conditions but using different milkweed species. They used the following milkweeds - Asclepias perennis (Aquatic milkweed), A. curassavica (tropical milkweed) and two varieties of swamp milkweed (A. incarnata). In the study, the researchers found that monarch larvae grew at different rates on these milkweed, but by the time they pupated, there was no difference in the mass of pupae across the groups.

- If we assume that pupal size is an indicator of adult size, then their results indicated there were no differences in overall size of adults across the milkweed types.

OK, so are you thoroughly confused yet? Let me try to sort this out next.

By my count, there have been 5 studies where monarchs were reared on different milkweed types. The most common conclusion from these studies was that there was little to no difference in monarch size when reared on different milkweeds. But, if you look closely at these studies across the board there was one milkweed that stood out on more than one case - Asclepias syriaca (common milkweed). These cases are summarized below.

- One study (only one study) found that common milkweed produces the largest monarchs.

- One other study (only one study) found this plant produces monarchs with the most elongated wings.

- Monarchs reared on this plant have large lipid reserves when they eclose, relative to other plants

So let's answer the original question - which milkweed produces the best monarchs, or those that are most migration-capable? While most studies indicate there is minimal difference between milkweeds, there is the most evidence pointing to common milkweed. Monarchs reared on common milkweed appear to be the most migration-capable.

Incidentally, if you wanted to know what milkweed produces the worst monarchs (least migration-ready), it would probably be tropical milkweed. Below are the cases for this.

- One study showed monarchs reared on tropical had pale wing color (not migration-capable)

- One study showed monarchs tended to have thin wings when reared on tropical

- One study showed monarchs had the least elongated wings when reared on tropical

Well, that about wraps up this post. Please feel free to distribute this far and wide to those people who are committed to helping the monarch migration!

Go forth and plant common milkweed!


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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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