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  • Andy Davis

It's official - tropical milkweed interferes with monarch migration

Hello everyone,

This week another bombshell study was published in the monarch world, this time about the effects of tropical milkweed (yes, that again). And like all previous studies on this plant, this one is sure to generate some interest, controversy, flak, and rankling. Today's post is devoted to this new study.

The paper in question recently appeared in the scientific journal, Ecology Letters, and was a collaboration between a whole bunch of well-known monarch researchers, and was spearheaded by Dr. Dara Satterfield, who is now based at the Smithsonian. Here is a link. It is titled, "Migratory monarchs that encounter resident monarchs show life‐history differences and higher rates of parasite infection", and this is basically the gist of the study.

Let me set the stage for you first. As most monarch folks know, tropical milkweed is not native to the United States, but is now being sold at nurseries around the country, where people happily buy it and plant it in their gardens (thinking they are "helping the monarchs"). But because it is native to Central America, it does not die back during the fall like all other milkweeds do, and then this leads to problems, especially in the southern regions where it can actually remain in leaf well into the fall, or even the winter. Dara's earlier work with this issue showed that in the places where this plant grows year round, you tend to see local ("resident") monarch populations with extremely high prevalence of OE - the protozoan parasite that infects monarchs. This is because the OE spores tend to build up on the plants since the plants remain in leaf so long. In native milkweeds, this does not happen because the plants die back at the end of the summer, thus eliminating the OE spores.

The other issue with this plant is that is thought to promote reproductive activity in fall migrants, which is not really good for the migration. Normally, most fall migrants are in "reproductive diapause" (meaning their internal boy and girl parts have not developed), and this is a key feature of the migratory (super) generation. We think it helps them complete the migratory journey, because they don't have to carry around this extra weight when they don't need it. They basically delay becoming reproductive until the following spring. In other words, migrant monarchs are not supposed to be reproductively active. In fact, the die-back of the native milkweeds throughout their breeding range in the late summer is one of the main cues for the last summer generation to enter diapause (other cues include daylength and temperature, I believe). However, it has long been suspected that when migrating monarchs travel southward and then encounter the lush green tropical milkweed along the way, this causes them to "break diapause" and switch to reproduction mode, thereby ending their migratory journey. This was always suspected, but never demonstrated with data - until now.

For a couple of reasons then, these southern places with lots of tropical milkweed and high OE prevalence are the focus of this current study. Because many of these places are located in the fall migration flyway, the question on everyone's mind has been - what happens when fall migrants pass through these sites? And also what happens on the northern journey in the spring? Do spring migrants from Mexico pass through these sites where there are local resident monarchs that are all infected? Do fall migrants that encounter the tropical milkweeds break diapause and become reproductive? These are the questions that Dara and her team set out to answer. I should point out that this was the first study that has actually directly addressed these questions in monarchs, but also for migratory animals in general - i.e. how much mixing is there between migratory and resident individuals, and what happens when they do. As someone who studies migration for a living, I find this to be a fascinating topic.

Dara traveled to a number of sites in Texas to capture monarchs in the fall and spring. She visited sites where there is only native milkweed, and she visited sites where there is tropical milkweed. At each site, she captured monarchs, and she tested them for OE. She then saved the specimens for later analyses in the lab. There, she determined which ones were reproductively active, or in diapause. They did this by dissecting the females to determine if they carried eggs, and then they placed the males in cages with lab females, to see if they were frisky. Next, she (and her colleagues) did some very sophisticated chemical analyses on all of the monarchs' wing tissues to determine where they came from (isotope analyses combined with cardenolide fingerprinting). Essentially, with these tests you can tell if a monarchs came from higher latitudes or lower latitudes, with a fair degree of certainty. This is an oversimplification, but the bottom line is that from these tests, she was able to determine if the monarchs she caught were local residents, or migrants.

Remember they were looking to see how many of the migrant monarchs were infected at sites with tropical milkweed or with native milkweed, and they also wanted to know if migrant monarchs break diapause at tropical milkweed sites. Here is what they found:

First, I'll paste a figure from the study that shows both the collecting locations, plus the numbers of monarchs that were migrants vs resident. The map on the top (a) shows the fall collecting sites, and the one on the bottom (c) shows the spring. At each site is a pie chart showing the proportion of collected monarchs that were migrants (black). The residents are in blue. The graphs on the right are not important to the story right now.

From the top map you can see that there are some sites in the fall where all of the monarchs there were migrants - the authors called these "stopover sites", which means that all of the monarchs present at the time were just passing through. That seems reasonable. And recall there was no tropical milkweed at these sites. At the tropical milkweed sites there was a "mix" of migrants and residents present. That means that there were both resident and migrant monarchs all flying around the same place (all low enough to the ground to be collected with a net). Interestingly, these were clustered near the eastern coastline. I'm not sure if this is an artifact of where she had travelled, or if it reflects something biological. The bottom map shows that in the spring the three sites she went to all had a mix of migrants and residents. This part here is a really big deal - I'll come back to this later.

Now, here are the results for the diapause and OE tests. These graphs below were pretty much the primary results of the entire paper. The first graph shows the proportion of monarchs that were reproductively active (graph a), and the lower one shows the proportion that were infected with OE (graph b), grouped based on the location. Take a second to take these in...

Let's start with the top graph regarding the reproductive-ness of the monarchs (if that's not a word, then I just made it up). The bar on the left shows the proportion of monarchs captured at the stopover sites (all migrants) that were reproductively active - i.e. they had some reproductive tissue and the males were frisky in the cages. About 10% of all migrants were reproductive. This number jibes with others that have been tossed around over the years, so it seems reasonable. Remember these stopover sites did not have tropical milkweed. Now look at the right bars, showing the sites where there was tropical milkweed and year-round breeding. Of the resident monarchs collected there, about half were reproductive. But the real thing to see here is that 35% of the migrants at these sites were also reproductive - what? Recall that normally, only 10% of migrants are reproductive, but at tropical milkweed sites, 35% of them are - that's more than a third of the migrants. And there was no doubt that these were migratory monarchs remember.

For the OE results, the authors reported that less than 5% of the migrants at stopover sites were infected - also reasonable. But at the tropical milkweed sites about 30% were. Again, this is crazy. Not surprisingly, prevalence within the residents was basically 100% - this has been shown before.

OK, so these are crazy results, and these should make everyone who owns a tropical milkweed plant (especially located within the fall flyway) really, really do some soul-searching. Based on these data, it is now very clear that tropical milkweed is messing up the migration. It seems to be causing migratory monarchs to want to breed, much more so than they should be. This cannot be good for the migration. If they arrest their migration southward in order to reproduce, not only will it delay them considerably, but they will also begin adding mass (reproductive tissue), which would make it harder to fly long distances. And this is even assuming they will WANT to continue migrating, after they have broken diapause and started laying eggs, etc.

The OE data also makes the case that the tropical milkweed sites are spreading the infection to migratory monarchs. Either that, or these tropical milkweed sites are somehow attracting infected migrants to them. Both scenarios are possible with these data.

Now, here's the really scary part (I know, right? As if the stuff above wasn't scary enough). The results from the spring sampling demonstrated that the healthy, northward-migrating adults appear to be coming to these sites where there is 100% infection levels among the resident monarchs. This is a terrible situation. That means that female migrants are laying the first-generation eggs right in the middle of ground-zero for OE. All of those eggs they lay will become infected. Once that happens, each of them will spread it to their offspring, and so on. So essentially, these infection hotspots are giving OE a head-start. How many of these first-generation monarchs do not make it and perish? How many monarchs would we have in the summer if the OE had not been given this head start? And remember from previous posts that the most important thing that monarchs need to have to increase their summer numbers is a successful spring migration.

I could go on here about how bad this looks for tropical milkweed, but I don't want to digress too much from the paper here. Bottom line here - it is now clear that tropical milkweed is hindering the monarch migration. And, now think about how the latest research is showing that the declines at the overwintering sites appear to be due more to reductions in migration success than to milkweed losses.

Now, this will cue the slew of comments from people who refuse to believe this and all of the other studies on this plant.


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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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