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  • Andy Davis

Is swan milkweed (hairy balls) a better alternative to tropical? Here is the answer

Hello blog readers,

Thanks for tuning in yet again to this blog site and for opening your brain up to some science, facts, and evidence around the critter that everyone loves - monarchs. I hope this site has been useful to you in your own personal journey around monarchs. If I haven't mentioned it in prior posts, I should point out that I don't get paid to write these blogs. I only do this to try to connect everyday people to the world of science, and in so doing, to (hopefully) help this critter by allowing people to make smart conservation decisions. All I ask in return is that you listen with an open mind.

Ironically, today's topic is one that actually has no science around it, because the milkweed in question is rather new, at least to North America. This species, called either swan plant, swan milkweed, or sometimes "hairy balls" or "balloonplant" milkweed (Gomphocarpus physocarpus), is yet another non-native plant (this time from Africa) that has been brought over to feed the insatiable appetite of the monarch-loving consumers in the United States (and also a little in Canada). However as you'll see below, in this case everything we need to find the answer to the original question is already available to us. All we need to do (and what I'm going to do today) is go to the right place to find it.

To back up a bit, let me set the stage. Swan milkweed is becoming more and more popular in nurseries and in people's yards, (I think) because it doesn't have the "stigma" that tropical milkweed does. What I mean is, there is a lot of research on the non-native tropical milkweed and what it does to monarchs in North America, and it is not good. I've covered just about every study on tropical milkweed in this blog, and it all points to one conclusion - it harms the long-distance annual migration to Mexico (or California). It does so partly by influencing the physiology or morphology of the larvae that eat it (making them less suited for migration), or by enhancing the spread of the OE parasite, which itself harms the migration. Because of all of this, tropical milkweed has a very bad reputation (and rightly so), and then people who have tropical milkweed in their yard tend to get shamed (and rightly so). Now, in comes swan milkweed - the next wonder plant that has no reputation at all because there is no research. People are buying it and guess what - there is no shaming! The average consumer must think this is a great thing. But... (you knew there was a but coming), in my book it is ALWAYS dangerous to make conservation decisions without science.

Before getting into the details of the swan milkweed, I need to point out a few things about tropical milkweed, for reasons which will come up later. One of the biggest problems with tropical milkweed is that it lasts longer than native milkweeds do in the course of a growing season. It is not native to North America and so it has an inherently different internal clock. That means the plant does not senesce until late in the season; there are still lush green leaves on the plant all through the summer, and well into the fall. You've probably seen this yourself, and the exact timing depends on where you live. In northern regions, tropical milkweed may last until September, and at mid-latitudes, until October. In the deep south, it may not ever die back. Meanwhile, most native milkweeds have died back at the end of the summer. Some people may have the impression that this is a good thing, but they are wrong. All of this has ramifications for the monarchs' fall migration, which I'll list below.

1. The later senescing can mess with the monarchs' migration timing. The monarch migration in North America has evolved over eons because monarchs need to go somewhere to avoid the harsh winters (Mexico or California). Their bodies and senses are sensitive to certain environmental cues that they use to tell them when it is time to head south at the end of the summer. There are a number of these cues, and we are still learning about how each works, but, we know that one cue is the milkweed age itself. When monarchs see their milkweed die back at the end of the summer, they know it is time to move on. We have (pretty good) evidence on this point. So, if there is still lots of green tropical milkweed around in the fall, the monarchs don't get their cues until too late in the fall.

2. Related to the point above, since tropical milkweed is still in leaf so late in the season, it allows monarchs to continue breeding (ovipositing) well into the fall too. This leads to late-season larvae that then become adults that eclose way too late to have any chance of making it to Mexico. We have very good evidence on this point based on tagging data - late-migrating monarchs have a very poor migration success rate.

3. The prolonged green period of tropical milkweed provides more opportunities for OE parasite transmission. This parasite spreads when infected adults drop OE spores onto milkweed leaves. The longer there are leaves around, the more these spores can build up on the leaves, and the more likely the parasite will be spread. This is NOT just a southern thing! If you live in the north and have tropical milkweed, this is happening with your plants too. And, recall the bombshell paper from early last year which showed how the national prevalence of OE has increased dramatically, and, that this increase leads to less successful migrations, and smaller winter colonies.

OK, so there are three big reasons why tropical milkweed is bad for monarch migration, and they all have to do with how the plant has a prolonged season that lasts well into the migration season for monarchs (Aug-Nov).

Now, I told you all of this so that I can next talk about swan milkweed.

Like I said, I do not know of any specific research on this plant yet, at least in terms of how it affects monarchs here, but, I do know that there are many records of this plant (in North America) now on the online program, I've talked about this wonderful website in the past - it is a place where anyone can upload photos and observations of any species, plant or animal, and then the program collates the observations and creates maps, charts, etc. Specifically, the program can give you a chart showing the "seasonal distribution" of the species in question (i.e. when people report it). So, I just now looked up the swan milkweed (hairy balls) graph and here it is. Keep in mind that this graph is based on records from North America, and mostly the U.S. too.

Notice anything about this graph? Notice how the "peak" number of observations is from August through October. That means this is the time when the plant gets reported to this platform the most. That means that this plant is most noticeable in these months. In other words, the plant is green and in full leaf during these months. And if you haven't put two and two together yet, these are the prime monarch migration months. This is when monarchs are (supposed to be) getting ready for the journey, or, are already traveling south.

For comparison, I'm going to put another graph below of a native milkweed (swamp milkweed). Notice when this plant is no longer sighted (because it has senesced).

That's right, this graph demonstrates that native milkweeds in North America are not in leaf during the fall migration, and this is the way monarchs have evolved - this is the way the MIGRATION has evolved. When the milkweed dies back, the monarchs move on. This is the way it has been for eons. In contrast, swan milkweed in the U.S., is fully in leaf exactly when monarchs should be migrating south (i.e. not laying eggs).

Now, recall all of the problems that tropical milkweed brings us because it is in leaf during the fall migration, including messing with the migration cues, allowing larvae to grow longer, and providing places for OE to build up (just to name a few). Even though there is no specific study that shows us this, it is fairly obvious that these same issues would happen with swan milkweed, simply because of its inherent seasonality. In fact, given that this species is native to southern Africa, it is no wonder that it has a different seasonality than the native milkweeds here do.

So, let's bring this home now, and talk about the implications for your home (see what I did there?).

- Is this plant harmful to monarchs? It is probably not good for the fall migration.

- Should you plant this milkweed in your backyard? No.

- Should you be shamed for having it? I would say yes, especially if you have read this blog and still insist on keeping that plant!

Let me end by reminding everyone of the current status of monarchs in North America, based on the most up-to-date science. Regardless of what you've heard in the news, the North American monarch population is not declining, at least during the breeding season. There has been no wide-scale habitat loss for monarchs. The real problem they face right now is a declining migration. There are many reasons for this decline, but it is very clear that this plant, like tropical milkweed, is only going to exacerbate this.

Please do share this post widely so the message gets heard.



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