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  • Andy Davis

I asked 12 monarch experts for their opinion on tropical milkweed - here is what they said

Happy new year all,

To start the blogging year off with a bang, I'm going to wade once again into the fray that is created by tropical milkweed. You know this plant - it's the one that has been the center of a growing amount of "debate" among folks who have or promote butterfly gardens. Note that I put the word debate in quotations for a reason here, which I'll explain later. For this blog entry, I'm not specifically covering any one research project, as I normally do, but instead I'm going to present the results of a brief survey I recently conducted within the community of scientists who study monarchs for a living - in other words, the experts.

To get everyone up to speed on the controversy, let me quickly go over the basics. Tropical milkweed (Ascepias currasavica) is a non-native milkweed to the US, which is commonly available for purchase at nurseries and other big-box stores. Apparently, it is more commonly-sold than the native milkweeds (maybe because it looks pretty?). Anyway, homeowners really want to help monarchs by planting milkweed so they go out and buy this plant. Thus, it is quickly becoming ubiquitous in home gardens around the country, especially in the southern states and in the western states, where it seems to prefer the mild climate. Since it is native to a more tropical climate, it's growing period is much longer than the natives, so it tends to stay in leaf for longer during the year than natives. And, in the south and west (California), it will remain in leaf almost year-round. Ever since this plant has come onto the market, there have been questions about what this would do to the monarch biology. In the past two years there have been a number of scientific studies done to begin to examine the effects of this plant, and these studies have all shown it can have negative effects to the monarch population. This is where the controversy comes in. For those interested, I have some previous blog entries that explain some of this research - link. You can also see some essays by other bloggers here and there, like this one.

In the past year or so, I've noticed a growing amount of chatter on social media around this plant, which I think, mirrors the controversy. Essentially the "debate" (again in quotes) boils down to two sides - on one side are those who follow and agree with the science around this plant, and on the other are those who do not agree with the science. For those that do not agree with it, I note that most of the people seem to be home gardeners or people involved in community gardens, etc (not scientists). The other thing that has added to this controversy is the fact that one notable person, Jeff Glassberg, the former president of the North American Butterfly Association, has been a strong opponent of the science around this plant. And, in 2015 he wrote an essay in his magazine (American Butterflies) that made an argument for why this plant does not harm the monarch population. I've noticed that this one essay tends to get brought up by the naysayers whenever there is discussion over this plant. Some have even gone so far as to say that this essay "debunks" all of the research. I could go into a long monologue here about why this is ridiculous, but perhaps I'll just politely point out that this was an essay in a magazine, and I'll leave it at that. I'll be getting trolled enough over this posting by the anti-science facebookers as it is.

Given all of this controversy, my survey of expert opinion was an attempt to understand just how much debate there actually is among the monarch butterfly scientists over this plant. I know that we've (the scientists) have all casually discussed this issue at conferences, and in one-on-one communications, but I don't think we've ever tried to consolidate the opinions like this, so this exercise was actually useful. Thus, over the holidays I created an online survey with just five basic questions, and sent this to every scientist who currently studies or has studied monarchs in North America. This was a list of about 20 or so names. Of those, there were 12 people who completed the survey. I'm purposely not including their names here (so they don't get trolled), but suffice it to say that the people taking this survey were among the leading experts on monarch butterfly biology in the world. Below is the summary of the questions and their answers.

The survey was designed with 5 questions, and the respondents could only answer yes, no or not sure. According to my survey account, most respondents finished the survey within 2 minutes. So I guess they didn't take long to answer these questions. This is important because it means their opinions on these topics are already formed, and they didn't have to think about them for very long.

Question 1 - Are you familiar with the research on tropical milkweed? This was just an introductory question to gauge whether I was asking the right people (which I'm pretty sure I was). All 12 (100%) said yes.

Question 2 - Does tropical milkweed contribute to the spread of OE in monarchs? For those readers unfamiliar, OE is the monarch-parasite that causes wing deformities, reduced fitness, and high levels of mortality. On this question 66.7% said yes, 33.3% said not sure

Question 3 - Does tropical milkweed contribute to winter-breeding? On this question, again 66.7% said yes, 33.3% said not sure

Question 4 - Does tropical milkweed interfere with spring and fall migration? This is a rather big, complex issue, and I purposely tried to boil it down into a very simple, yes or no answer. I'm not sure it worked, because the answers here seemed to reflect a high level of uncertainty. Here, 42% said yes, and 58% said not sure.

Finally, question 5 was the big one and has the most relevance to the average home-gardener:

Question 5 - Do you recommend planting tropical milkweed to homeowners? Here, 92% said no, 8% (1 person) said not sure. I note that no scientist actually said yes to this question.

That was it!

In my opinion (ha! see what I did there?), the answers to these questions were very interesting, and somewhat surprising, especially for question 4. Here, it looks like there is the most uncertainty among the experts - on whether tropical milkweed actually interferes with the migration. As I said, this is a very complex issue with no simple answers. Plus the word "interfere" could mean a lot of things, like interfering by promoting infections, which limit flight potential. Or it could mean causing monarchs to break diapause and begin breeding, because the plant is still in leaf well into the fall migration season. There are probably a variety of ways in which this plant could be affecting the migration. What also doesn't help is that there is not a lot of research on this aspect - the migration thing, anyway. And most scientists, in true scientist form, are wary of going out on a limb without having any evidence to back up their conclusions. I believe there is some coming though, and I'll be sure to blog about it when it comes out.

Speaking of upcoming evidence, I'm familiar with most of the work being done on this plant in the various research labs around the country, and I know there is a fair amount of work that has yet to be published on it. When this does come out, I expect that this ongoing conversation will surely intensify - all of the unpublished work I'm aware of shows negative effects of tropical milkweed. Stay tuned.

Getting back to this idea that there is a "debate" going on about tropical milkweed - I believe the answers in this survey are telling. According to this, there is no real debate over whether this plant should be planted - that is, among monarch butterfly scientists. 100% of monarch scientists said they would not recommend planting it. If there is any debate at all, is in the details of how this plant is screwing with monarch biology - of that we don't have all the answers yet. In fact, after this survey was completed, all of us scientists proceeded immediately to have a very scientific email debate over just how this plant causes, or does not affect diapause, OE spread, etc. This also speaks to the uncertainty we still have, and how much more work needs to be done to research the effects of the plant on monarchs.

So the take-aways of this survey I think are this - the people who are the world experts on monarch butterflies are of the opinion that tropical milkweed has some negative effects on the monarch population, though there is some uncertainty as to how much, or at what stage. Importantly though, no scientist that I surveyed recommended planting tropical milkweed. I don't want to put words in their mouths, but I believe their collective response on this question reflects the degree of uncertainly and potential risks that are associated with this non-native plant.

That's all for now.


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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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