A complete summary of the previous and new science on tropical milkweed and monarchs
UPDATE - July 2023 - New review paper added showing how tropical milkweed influences winter-breeding
UPDATE - April 2023 - Study from Oklahoma added showing how tropical milkweed entices oviposition in the fall
UPDATE - May 2022 - New research regarding the effects of tropical milkweed on adult flight metabolism was just added to this list.
As the blog title implied, today I'm going to cover, in depth, the topic of tropical milkweed, and the science around it. This non-native milkweed is being increasingly sold at nurseries around the US and Canada and people are incorporating it into their gardens, or raising monarchs on them. In the past, I've covered single studies where researchers examined this plant and how it affects monarchs. Some people may be familiar with a few of those well-publicized studies, although there have actually been a number of new publications over the past 5 years which have not really gotten much exposure. Today then, I'm going to try my best to summarize ALL of them. That's right, everything. I'll list each study where tropical milkweed was used to understand how it affects monarchs, and I'll provide an explanation of the main results from each experiment. Then at the end, I'll try to tie everything together. So in the end this blog will be the ultimate guide to the science on this plant, and what we know so far about what its effects are on individual monarchs and the monarch population. Sound good? Here we go...
Before getting too far into this, let me first give a brief overview of the monarch parasite, Ophryocystis elektroscirrha, or OE for short. You need to know this, since this parasite will factor into the results of some of these studies. This is a single-celled organism that infects monarchs and close relatives (and maybe even a few other Lepidopteran species - see a prior blog!). Infected adult monarchs have these on their bodies as spores (see picture below), which fall off when the adult lays eggs. The larvae then eat the spores on their milkweed, and then the spore replicates within the larvae, thus completing the cycle. For more information on this disease, go to Dr. Sonia Altizer's (my wife) citizen science project that focuses on this disease - www.monarchparasites.org.
This picture shows the OE spore, as it appears under a microscope - cool picture, right? I made this for a manuscript once, and now I see it getting reproduced everywhere.
OK, let's get to it. I'm going to list each study next, and these are listed in more or less the order in which they were published...
Satterfield, D. A., J. C. Maerz, and S. Altizer. 2015. Loss of migratory behaviour increases infection risk for a butterfly host. Proceedings of the Royal Society B-Biological Sciences 282: 9.
This study looked at the prevalence of OE in the eastern monarch population throughout their breeding range and compared that to the prevalence of monarchs that skip migration and end up in winter-breeding locations along the Gulf and Atlantic coasts. The important piece here is that each of the winter-breeding locations had tropical milkweed, according to the volunteer reports. From a research perspective, since these non-migratory clusters are becoming more and more frequent, the authors wanted to know how this loss of migration affects the OE levels. They did this through a combination of citizen science reporting as well as from their own sampling of adult monarchs. Below is a map from the study showing the sites that were examined for OE. In each case, the authors reported how many adult monarchs were infected. They wanted to see if there were differences in infection prevalence between the three different life stages. This sampling was done between 2011 and 2013.
Bottom line from this project - the prevalence of OE at winter-breeding sites with tropical milkweed was between 50-60% (at the time this study was done). In comparison, the prevalence within monarchs at summer-breeding sites was about 15%, and at the Mexican colonies it was about 10%.
Satterfield, D. A., F. X. Villablanca, J. C. Maerz, and S. Altizer. 2016. Migratory monarchs wintering in California experience low infection risk compared to monarchs breeding year-round on non-native milkweed. Integrative and Comparative Biology 56: 343-352.
This study was essentially a follow-up to the one above. The take-home message of the paper is given in the title of the paper too. The authors wanted to follow up the previous project, and see if the same result could be found in a different region, where monarchs, again, appear to be foregoing the migration, and forming permanent-resident breeding sites in southern California. These sites all have tropical milkweed, just like the coastal sites in the eastern range. So, they conducted another effort to obtain samples of adult monarchs to look at OE levels, and they did this from areas along the California coast (map below). These were collected between 2013 and 2016.
The authors looked at monarchs at overwintering sites, at places where monarchs breed year-round, and they also looked at sites where monarchs only breed in the summer (seasonal breeding). These seasonal sites only had native milkweed, including Asclepius eriocarpa, A. fascicularis, and A. speciosa. They compared the OE levels of monarchs at each of these groups.
Bottom line for this study - average prevalence of OE at year-round breeding sites with tropical milkweed was between 70 and 80%. This is compared to 10-20% at overwintering sites, and 35% at summer-breeding sites. And, there was one other point of interest in the study that a lot of people have forgotten. There were three winter-breeding sites in which the garden owners reported that they regularly cut back their tropical milkweed. The OE prevalence at those sites was 73%.
Satterfield, D. A., J. C. Maerz, M. D. Hunter, D. T. T. Flockhart, K. A. Hobson, D. R. Norris, H. Streit, J. C. de Roode, and S. Altizer. 2018. Migratory monarchs that encounter resident monarchs show life-history differences and higher rates of parasite infection. Ecology Letters 21: 1670-1680.
This study was another follow-up to the prior two, and it examined the impact of tropical milkweed at sites along the fall migration corridor in Texas. The idea here is that there are some places in Texas that now have tropical milkweed, which is creating year-round breeding spots for monarchs, just like it does in coastal California and the Gulf states. If the local OE prevalence is high at these Texas sites, what happens when fall-migrating monarchs then pass through these sites? Do they interact with those highly-infected resident monarchs and become infected too? The other important question they asked, was what happens when migratory monarchs (which aren't in a breeding state) encounter the lush, green tropical milkweed at these sites - do they then become reproductive? This is a very important question right now. We really need to know if tropical milkweed is causing monarchs to want to come out of their migratory state and attempt to breed in the fall. Once they become breeders, it is likely they would not complete the fall migration.
In this project, Satterfield herself went to specific sites in Texas during one fall migration season, and she sampled monarchs that she caught at different sites. Some sites had monarchs breeding year-round (on tropical milkweed), and some had no milkweed, and the monarchs were all migratory. They actually know that what the migratory status was for each monarch because they tested their tissues using stable isotopes! For each monarch, she checked for infection, and also checked how many eggs the females had, and then put the males in cages with females to see if they wanted to mate. Both of these were to check for reproductive activity.
The monarchs she caught at year-round breeding sites were heavily infected - 95%. Only 9% of those caught at migration stopover sites were infected.
Here is the interesting part about reproductive activity. The MIGRATORY monarchs captured at year-round breeding sites were more about 3x more likely to be reproductive. See the chart below from the paper.
Remember, these are all known migrants here (based on isotope analysis), and the group on the left were caught at places with no year-round breeding and no tropical milkweed (i.e. stopover sites). The group on the right are migrating monarchs that were caught at tropical milkweed sites where there were year-round breeding monarchs.
This evidence implies that the migrant monarchs that encounter the lush tropical milkweed are less likely to be in the migratory state of reproductive diapause (since they were more reproductively active). This certainly is in line with what is suspected - that tropical milkweed is pulling monarchs out of reproductive diapause. However, it is also possible that these monarchs in the right side of the graph had ALREADY come out of reproductive diapause when they encountered the tropical milkweed. But, this in turn implies that the tropical milkweed is acting as a lure to those monarchs who do want to breed.
Majewska, A. A., and S. Altizer. 2019. Exposure to non-native tropical milkweed promotes reproductive development in migratory monarch butterflies. Insects 10: 17.
This study is good follow-up to the last one, since it examines how tropical milkweed is affecting reproductive activity during migration. This was an experimental study where the lead author collected migrating monarchs in the fall from various places in the eastern US, and then placed them in outdoor flight cages with milkweed. There were multiple cages - some had tropical milkweed, and some had native swamp milkweed, and some had no milkweed (as a control). The monarchs were kept in the cages for a week, and then they were euthanized to determine how many eggs the females had (as a measure of reproductive level). Below is a graph showing this result. For each female, the authors counted the number and size of the eggs within females to come up with a development score. This graph shows the average score for all females in each treatment.
All female monarchs had a little egg development, even in the control group. But, the big result here was that migrant female monarchs that are exposed to tropical milkweed began developing eggs, way more than what is normal. Those exposed to native milkweed did not.
Next, the authors also examined how rearing monarch larvae on tropical milkweed affects their migratory state. Here, they reared monarch larvae on tropical milkweed, or native milkweed, and this was done in an environmental chamber that created fall-like conditions - that is, a cooler temperature and declining daylength. These conditions are known to trigger larval monarchs to develop into adults that are in reproductive diapause (migratory state).
The larvae were grown as such into adults, at which point they were dissected to determine their reproductive status (i.e. examining their reproductive tissue). I'm going to paste a graph from this part below, and also paste the legend from the graph, which explains the treatment groups.
This graph shows that monarchs that are reared on tropical milkweed are more likely to develop into adults which are reproductive, than those grown on native swamp milkweed. Remember, being reproductive is not conducive to migration. Monarchs need to be in reproductive diapause for successful fall migration to Mexico. When they start to develop their reproductive tissue, this is extra weight that they must then carry.
Bottom line from this study - monarchs reared on tropical milkweed are more likely to develop into reproductive adults, and, migrating adults that encounter tropical milkweed have a greater tendency to become reproductive.
Tropical milkweed, the "medicinal" milkweed
I'm going to put in a catch-all summary here for the work from the de Roode lab at Emory University, which has made this phrase popular. I'll add some links here to some of their papers, but really, there are a number of them that tell a similar story.
de Roode, J. C., A. B. Pedersen, M. D. Hunter, and S. Altizer. 2008. Host plant species affects virulence in monarch butterfly parasites. Journal of Animal Ecology 77: 120-126.
Tan, W. H., L. L. Tao, K. M. Hoang, M. D. Hunter, and J. C. de Roode. 2018. The effects of milkweed induced defense on parasite resistance in monarch butterflies, Danaus plexippus. Journal of Chemical Ecology 44: 1040-1044.
de Roode, J. C., and M. D. Hunter. 2019. Self-medication in insects: when altered behaviors of infected insects are a defense instead of a parasite manipulation. Current Opinion in Insect Science 33: 1-6.
For brevity's sake, I'll put a link to the de Roode lab here.
So, the gist of a lot of this work has been that tropical milkweed appears to have some "medicinal" properties to it for monarchs, and that monarchs seem to know this, and so they "self-medicate." These same catch-phrases and statements have appeared in a lot of press stories around this work, but a lot of people have misinterpreted what this means. This does not mean that tropical milkweed is good for combatting OE in your garden - it is actually worse for your garden, and I'll explain. This collective research has shown that the naturally high cardenolide content of tropical milkweed seems to reduce the number of OE spores on infected monarchs. In other words, if a monarch larva becomes infected while eating tropical milkweed, it will become an adult with fewer spores, and therefore it will live slightly longer. And, the other interesting thing is that female monarchs seem to know this. The deRoode group has tested if female monarchs preferentially choose where to lay their eggs. They have found that when the females are themselves uninfected, they don't have a preference for any milkweed type, but when they are infected, they prefer to lay on tropical milkweed. So, they seem to know that their (infected) offspring will survive longer if they develop on that species - this is where the idea came from that the females are "self-medicating." They are themselves infected, but they are attempting to medicate their offspring. It's also been called "cross-generational medication."
But, here's the thing - infected larvae growing on tropical milkweed still become infected adults, only now, because of their lower spore load, they live longer, and spread their infection longer! So, yes, the tropical milkweed is "medicinal," but it actually makes things worse in your local garden.
Davis, A. K., and J. C. De Roode. 2018. Effects of the parasite, Ophryocystis elektroscirrha, on wing characteristics important for migration in the monarch butterfly. Animal Migration 5: 84-93.
This one is from the lab of yours truly. This was a study conducted not necessarily on the effects of tropical milkweed, but, there was an important milkweed component to the study. Jaap deRoode and I were asking the question, how does OE infection affect the wing properties of monarchs. We know that OE tends to reduce the overall vigor of monarchs during flight, and that OE makes monarchs less successful during migration, but, we don't really know how infection affects tissue development during metamorphosis. After all, for a monarch, the most important thing they need is well-developed wings. So in this study we did a deep dive into the physical properties, and strength, of wings.
For this study, we examined an archived collection of monarchs (i.e. frozen) that were leftover from a prior experiment. Don't worry about the fact that they were frozen. We showed very convincingly that the freezing did not affect the results. Importantly, these monarchs came from a project where larvae were fed one of many different milkweeds, including tropical milkweed. There were some native milkweeds too. So, each of the adults we examined had been reared on a different milkweed.
For this project we had scanned the wings of the adults and collected some detailed measurements on their physical properties, including how orange they were. Prior research had shown how the shade of orange actually can tell you how well the monarch can fly! The picture below shows where we measured the wing "orange-ness". As you can see, we also measured wing area, and wing thickness.
Here's what we found - Monarchs reared on tropical milkweed had slightly larger forewings than those reared on native species. Their wing thickness was not affected. But, monarchs reared on tropical milkweed were much paler in their orange. Recall that the shade of orange is a strong predictor of migration success - monarchs with paler orange have lower flight ability in the lab and are less successful during migration. These conclusions are known from multiple prior studies. In fact, for monarchs, we know that the ones with the best chances of a successful migration are those with a deep, dark orange shade, nearing brick-red. The image above shows a monarch with a fairly pale orange coloration.
Decker, L. E., J. C. de Roode, and M. D. Hunter. 2018. Elevated atmospheric concentrations of carbon dioxide reduce monarch tolerance and increase parasite virulence by altering the medicinal properties of milkweeds. Ecology Letters 21: 1353-1363.
This was one of two studies that were published by this group, apparently these came from a PhD thesis of Leslie Decker, who did some fantastic research. This team examined how monarchs will fare under a future, climate change, scenario. They had a nifty setup where they reared monarchs in "chambers" or mini-greenhouses, and in the chambers they could alter the CO2 levels and/or the temperature. A schematic from the paper is below.
In these studies, they reared monarch larvae on different milkweeds, including tropical. They were specifically interested in the "medicinal" properties of tropical milkweed, and they wanted to know if this medicinal effect would hold up in the future when the climate changes.
In this experiment, they reared monarchs in future conditions (CO2 levels, temperature) and in current conditions, and on 4 different species. They infected half of the larvae with OE, so they could track how well each one survives.
Bottom line from this project - monarchs reared on tropical milkweed in current conditions had low OE levels, as expected, because of the medicinal properties. But, those raised on tropical milkweed under future conditions did not have a reduction - so the medicinal qualities of tropical milkweed were gone. So, the OE strains they used all appeared to become stronger (they made monarchs sicker) under the future climate conditions.
Decker, L. E., A. J. Soule, J. C. de Roode, and M. D. Hunter. 2019. Phytochemical changes in milkweed induced by elevated CO2 alter wing morphology but not toxin sequestration in monarch butterflies. Functional Ecology 33: 411-421.
This study was the second publication by this group, and was also conducted to find out how monarchs will fare in the future, when the earth becomes warmer. This study used the same experimental setup, where the researchers placed milkweed inside tiny plastic greenhouses to create a future climate scenario. They allowed monarch larvae to develop from larvae to adult in these greenhouse things, where some were in a future climate and some were in a current climate. They used 4 different milkweeds in the experiment, including tropical milkweed. They also exposed half of the larvae to OE. This was to find out how the combination of future CO2 levels and infection affect monarch development.
The larvae were reared to adulthood in these conditions. When they became adults, they measured a variety of wing features, including some that were measured in the Davis and de Roode study above.
Here is what they found - monarchs reared on tropical milkweed had less elongated wings than those that were reared on native milkweeds. Long, elongated wings is a known trait of the migratory population, and is thought to be important for successful migration. Thus, this finding of less-elongated wings has implications for migration success. Another interesting finding was that monarchs reared on tropical milkweed in this case did end up having thinner wings (recall this was not found in the Davis and de Roode study above). Thinness in this case means the thickness of the flat wing, like comparing cardboard to paper. Wing thickness is an important part of overall wing toughness, or tear-resistance. It is well-established that wing damage can occur during migration, and it is usually fatal.
Soule, A. J., L. E. Decker, and M. D. Hunter. 2020. Effects of diet and temperature on monarch butterfly wing morphology and flight ability. Journal of Insect Conservation 24: 961-975.
This was yet another project from this same group, and it is one that I recently blogged about (link). In a nutshell, this study examined the effects of milkweed diet on monarch wing morphology and flight ability, using a flightmill apparatus. The authors reared monarch larvae on three types of milkweed, swamp milkweed, common milkweed, and tropical milkweed, and when the adults emerged, they scanned their wings and used image analysis methods to precisely measure the size and shape of the forewings. They also tracked how fast and how far they flew on an automated flightmill.
I'll paste a figure below, which is one that I myself made, based on their own data, which they had supplied in their publication. For some reason, they did not present this very well in the paper, but to me, this seems like the most important finding - how far the monarchs flew, based on what milkweed they developed on.
The other thing they had found, which had been found in their prior study, is that once again, the monarchs reared on tropical milkweed had less elongated wings than those reared on native milkweed. In other words, the tropical milkweed produced monarchs that were less prepared for a long-distance migration.
Pocius et al. 2022. Impacts of larval host plant species on dispersal traits and free-flight energetics of adult butterflies. Nature Communications Biology 5, Article number: 469 (2022)
This study addressed a novel question about the effects of different milkweeds, and asked how do these milkweeds affect metabolism in adult monarchs. It was conducted by a group from Penn State entomology dept., and the researchers grew monarch larvae on 8 different species of milkweeds, including tropical milkweed. When the monarchs turned into adults, the researchers measured their carbon dioxide emission rates during rest, and during flight. Comparing these rates across all milkweed types, they determined that monarchs reared on tropical milkweed had very high metabolism compared to most other plant types. Since lower metabolism is better for migration (more energetically efficient), this implies that tropical milkweed produces monarchs not well-suited (physiologically) to long-distance migration. Click here for a blog summary of the paper.
Below is a figure showing the metabolic rate (CO2 emission) for adult monarchs reared on different milkweed species.
Clement and Crawford. 2020. Fall available tropical milkweed may be a population sink for the monarch butterfly. Oklahoma Native Plant Record. Vol 20. 68-78.
This observational study examined how the presence of tropical milkweed entices oviposition in the fall. The authors set up outdoor beds with native milkweed and tropical milkweed, and then surveyed all plants twice weekly for the presence of eggs and larvae. They found that most native milkweeds had senesced by late-August, but tropical milkweed remained in leaf through October. The documented over 100 eggs and larvae, and all but one were on the tropical plants. In October, there was a freeze, and all plants (and larvae) died. This evidence speaks to how this plant can entice migrating monarchs to want to breed (when they should be migrating).
Steele et al 2023. Extent and impacts of winter breeding in the North American monarch butterfly. Current Opinion in Insect Science.
This was a review paper that looked at all of the prior research and anecdotal observations, reports, etc., on the issue of winter-breeding in the U.S. This is where monarchs forgo the fall migration to Mexico (or coastal California) and instead "choose" to remain in the U.S. for the winter and enter breeding colonies. These colonies and micro-populations are becoming more frequent and numerous in places like Florida, southern California, southern Texas, lots of Gulf states, etc. The review paper demonstrated that all of these places have two things in common - they all remain warm enough in the winter for larvae to exist, and, they all have tropical milkweed. Both are needed for this to happen. The concern is that these winter-breeder colonies are pulling migrants out of the journey to Mexico, and/or that they are helping to exacerbate the national OE problem, since most winter-breeding sites are heavily infected.
OK, I'm winding down now. I think there might even be one or two other studies that I haven't covered here, but I do need to get back to my own research...
Here is a quick summary.
Tropical milkweed leads to extremely high OE levels (10x normal levels) in areas where it persists year-round, and there is evidence that this happens even if the milkweed is cut back periodically.
Infected monarch larvae reared on tropical milkweed tend to live longer as (infected) adults, thereby spreading the infection longer.
Migrating monarchs that encounter tropical milkweed are more likely to be in reproductive condition (eggs and mating, etc.), suggesting the tropical milkweed is luring these migrants out of the journey and turning them into breeders.
Migrating female monarchs that encounter tropical milkweed are more likely to begin developing eggs internally (getting ready for reproduction).
Tropical milkweed has been shown to entice migrants to lay eggs (become reproductive).
Monarch larvae reared on tropical milkweed are less likely to develop into adults in the migratory state of reproductive diapause (they tend to develop into breeders).
Monarchs reared on tropical milkweed have paler orange pigment, which is a known sign of poor migration potential.
Monarchs reared on tropical milkweed have less-elongated forewings, which is not optimal for migration (successful migrants have elongated wings).
Monarchs reared on tropical milkweed have thinner wings, which would be more prone to damage during migration (any damage during migration is fatal).
Monarchs reared on tropical milkweed don't fly as far as those reared on native milkweeds.
Monarchs reared on tropical milkweed have higher metabolic rates, which is not conducive for long-distance migration.
Tropical milkweed enables the formation of winter-breeding colonies (monarchs that forgo the migration).
OK, there is a lot I could say here to sum this all up. Since this is already a fairly lengthy post, let me just say that the collective research on tropical milkweed is all telling us one thing - it's bad for migration. 1) It leads to less-optimal wing development for long-distance flight; 2) it makes monarchs want to become reproductive, even if they are migrants; 3) it increases local and population levels of OE, which we already know hinders migration potential; and 4) it actually reduces their flying ability!
Should you plant it to help the monarchs? You be the judge. If you make decisions based on science, the answer is clear.
That's all for now.
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