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  • Andy Davis

Is tropical milkweed really medicinal? (answer: yes, and that's really really bad for your garde

Greetings readers,

This week at MonarchScience I'm going to once again tackle that controversial topic that all monarch people know and love, tropical milkweed. If you are one of those people who still insists this plant is not harmful to monarchs, don't bother reading this, because there is nothing in here that will help your cause. In this blog post, I'll discuss some of the research coming from the lab of Dr. Jaap de Roode, at Emory University, where they study the monarch parasite, OE, and how this parasite is affected by tropical milkweed.

First, a few words about Jaap (pronounced "Yaap" - he's Dutch). He was once a postdoc in Sonia's lab back in 2004 to 2006 I think, and that's where he got his start in the monarch business. Although I should say he is really in the OE business, since a lot of his and his lab's research revolves around that parasite, as well as other parasites, like malaria. Jaap famously once said that he never cared much for monarchs before he learned they carry a cool parasite! So he is really a very exceptional parasitologist, who happens to study a monarch parasite. And his lab is very prolific - they have tonnes of scientific publications on OE, although a lot of these are very technical and not an easy read for the average monarch enthusiast. I'll put a link to his lab website, where you can see for yourself how busy he and his people are - link here.

Back to the story for today - one of the most notable projects that Jaap has worked on and published was one where he described how female monarchs use tropical milkweed in a "medicinal" way. You may have heard news reports about this - here's one from an online science news website, ScienceDaily. So let me explain this story, because I think a lot of people only read the headlines of these stories, and then they get a false impression about the whole thing.

Tropical milkweed happens to have very high concentrations of cardenolides, the chemical compound that monarchs sequester internally and which is what makes monarchs toxic to predators. All milkweeds have this in varying concentrations. Tropical milkweed has the highest levels, or close to it. Let me come back to this point because it comes up later.

Let me also give some basic info on OE, in case there are readers who aren't familiar with it - though if you are a monarch enthusiast, shame on you if you aren't. This is a parasite that infects monarchs and related Danaid butterflies, and is is spread from female to offspring. Infected females have millions of OE spores on their bodies and when they lay eggs on milkweeds, some of these spores fall off onto the plant. The catrpillars then consume the spores when they eat the milkweed leaves, and then the infected caterpillars turn into adult monarchs, once again covered with OE spores. See my wife's page about OE for more info on this parasite -

So Jaap discovered that when an OE-infected female monarch has a choice between laying her eggs on a low-cardenolide milkweed (like swamp milkweed) and a high cardenolide one (tropical), she tends to lay most eggs on tropical. Then, he also discovered that when those eggs on the tropical milkweed turn into adult monarchs, their OE spore load ends up being less than it would if the larvae grew up on swamp milkweed. So Jaap reasoned that the infected females somehow know they are infected, and then they choose to lay their eggs on the milkweed that would most benefit their offspring. In case you are wondering, Jaap found uninfected females have no preference for milkweed type, so it only works if they are infected. So Jaap called this "trans-generational medicine" (or something along those lines), because the female (who cannot be cured herself) appears to be using the milkweed as "medicine" for her offspring.

So here's the thing that a lot of people get wrong when they read about this project - the tropical milkweed does not cure the offspring, or even prevent them from getting infected. All it does is reduce the OE spore load on the offspring (i.e. the number of OE spores on the abdomen), and that kind of reduces their symptoms from the infection. Some of Yaap's other research shows a very clear relationship between the spore load of infected monarchs and their lifespan - the more spores you have, the shorter your life. The idea here is that more OE spores means more damage is caused to the host tissues, thus reducing host lifespan. So if tropical milkweed reduces OE spore loads, that allows the infected monarchs to live longer. So make sure you got all of this because it's important for the next part. Remember, the gist here is that infected females prefer to lay eggs on tropical milkweed because it allows their infected offspring to live longer.

Here's the reason why this is really, really bad - the last thing you want in your garden or habitat patch is for OE-infected monarchs to live longer! The longer they live the more opportunities they have to spread their infections around, thereby raising the overall infection prevalence in the local population. So while the tropical milkweed actually is medicinal to the individual monarch, it ultimately leads to higher infection prevalence in the population! As if we needed another reason not to plant tropical milkweed, right?

Why does tropical milkweed reduce spore loads of infected monarchs? Remember the cardenolide thing? Apparently, the high levels in tropical milkweed somehow interfere with OE replication. In fact, some of the latest work from Jaap's lab (not sure if it's even published yet) shows that infected monarch larvae that are raised on high-cardenolide plants end up with smaller OE spores. This is further evidence of how cardenolides reduce OE development internally.

OK, so that's the medicinal milkweed thing in a nutshell. Perhaps in a future blog I'll visit some of Jaap's other work - he's done some really nice research on the genetics of monarch populations - just fascinating stuff.

Hope everyone learned something new today. Remember, if you like reading this blog, share it!


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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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