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  • Andy Davis

Evidence that tropical milkweed entices fall-migrating monarchs to lay eggs (which all die!)

Hi everyone,

Today's post is a bit overdue. There was a small study that was published a few years ago, and which largely went unnoticed, but which has important implications for the world of monarch conservation. So with this in mind, today I'll take you through that study and show you what the authors found. Hint - it's basically what the blog title indicates...

The study in question was published in a rather obscure plant journal, The Oklahoma Native Plant Record, in December 2020. The paper is fully online, and here is a link, in case you don't want to listen to my blathering about it. The project appears to have been conducted by people from the University of Oklahoma, who are knowledgeable about native and non-native plants. From my read of the paper, they also are well-aware of the controversy around this plant, and what it can do to monarchs.

This project actually addresses something that has not yet been examined scientifically, but yet is something that I'm sure everyone has either seen firsthand, or heard from a friend. That is, the degree to which tropical milkweed "attracts" fall-migrating females, and entices them to become reproductive. This plant is well-known to remain in full leaf for much longer than milkweeds native to North America, and I'm sure everyone has seen this. While most milkweeds have senesced by late-August or September, tropical milkweed still has lush green leave well into October, or later, depending on what latitude you are at. In Texas, Florida and the Gulf region, it may remain in leaf year-round! This fact has rather large implications for the spread of OE, which is another story altogether.

OK, so from my read of this paper it looks like the authors went about this experiment in a very straightforward way. During the summer of 2019 they established a series of outdoor milkweed plots on raised beds, like those shown in the picture below. Some of the plots had tropical milkweed (Asclepias curassavica) and some had native milkweed. They also had some nectar plants too. It sounds like they started with Asclepias viridis and A. speciosa for the native plants. They started with 48 plants of each type, although they indicated later that some of the natives did not survive to the end of the summer. But some of them did (20 or so), or enough to continue the experiment.

In the fall, the authors began recording the presence of monarch eggs and larvae on each. They did this twice a week from 12 August onward throughout the fall. Sounds pretty simple right? They also kept track of the plant quality as they went. They quickly found (as expected) that the native milkweeds mostly died back in late-August. Meanwhile, the tropical milkweed remained in leaf through October.

The results from the egg surveys were also what you might expect. The authors report: "From 12 August to 15 October, we observed 145 eggs and 39 caterpillars on 40 of the 48 A. curassavica plants and 1 egg on an A. viridis plant." They also report that of these eggs, "The majority of the eggs were laid between 10 September and 26 September." "The final eggs we found were on 1 October." Finally, they report that on October 12, there was a hard freeze, which killed all of the tropical milkweed plants, and presumably all of the monarch eggs or larvae. The authors state they could find no living larvae thereafter.

That basically summarizes the data and results of the study. I told you is was very straightforward. The authors indicated that they did not conduct any statistical analyses on these data, though it doesn't seem like any are needed, since the results are quite clear - tropical milkweed remained in leaf throughout the fall and continued attracting oviposition during September. And keep in mind the location - Oklahoma is directly in the fall migration flyway.

The discussion of the paper describes how these findings dovetail with other research, such as the study by former UGA grad student Ania Majewska, who showed that the presence of tropical milkweed entices migrant female monarchs to become reproductive. So really, these results are not at all unexpected, and they reinforce the idea that this plant makes migrating monarchs want to breed.

My take on this study is that it shows what I think everyone who has this plant has seen for themselves - that it remains in leaf well into the fall, and continues to entice monarchs to lay eggs into the fall too. Some people seem to think this is a great thing - "Hey, look at how many eggs I got this September!" These people are sadly misinformed. These eggs are not supposed to be there. Monarchs in North America are not supposed to be laying eggs in the fall. They have evolved for eons with the hundred or so native milkweeds in North America, which all naturally die back at the end of the summer. This senescing is a signal to the monarchs that it is time to migrate. So the presence of lush green milkweed in the fall is likely screwing with the migratory urges of the monarchs.

The other problem is that these fall eggs will eventually become adult monarchs (if they make it that far) that are VERY late to the migration party. Of the readers here, who has seen this first-hand, where someone on Facebook reports having "late" monarchs, and is wondering what to do with them? Some people even try to ship these late monarchs to Texas in a ridiculous attempt to save them. Keep in mind that with native milkweeds, none of this happens. But even IF these late monarchs were released "so they can join the migration" as people claim, they would likely perish during the trip. The fall migration is already treacherous, and it gets even more so in October because of the weather. And, there is plenty of evidence from other studies showing how late-migrating monarchs rarely make it to Mexico. So really, promoting late monarchs in any way is quite unethical.

Finally, let's talk briefly about how this "new" study will be received by the monarch masses. I will warn everyone now, that if you are in a Facebook group that is run by people who sell tropical milkweed, then you will probably hear a lot of talk about how the authors didn't consider this or that, or, how the scientists are all biased, or some bullshit like that (yes, I said it). By now, with ALL of the accumulated science around this plant, the evidence is quite clear - it is bad for the monarch migration. Anyone who says otherwise is an idiot, and you should not be listening to them.

OK so that wraps up my summary of this "new" paper. I'll add this to the growing list I keep of all of the science on it, which is in another post (link to that is here). I'll also add a separate post about this study in The Thoughtful Monarch Facebook group, where real science around monarchs is discussed. Join us if you want to talk about the facts!

Thanks for reading.


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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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