New study shows captive-reared and commercially-reared monarchs (and even their descendants!) are poor migrants

June 24, 2019


Well folks, get ready...I'm about to blow your mind with this blog entry.


Today I'm going to tell you about a new study that was just published and that is sure to set the internet on fire. And let me also point out that I am just the messenger here, so direct your ire elsewhere if you have any...


Let me quickly set the stage here - eastern North American monarch butterflies are experiencing problems with their migration. This much is certain. Their overwintering colonies have been getting smaller and smaller over the last 2 decades, and we're not sure why. We know they face a barrage of obstacles on their way south each fall, including car strikes, losses of nectar, and droughts, but now it seems there is another threat to the migration - people.


This study was authored by a group of researchers from the University of Chicago. It has this ominous-sounding title: "Contemporary loss of migration in monarch butterflies." The authors' names are Ayse Tenger-Trolander, Wei Lu, Michelle Noyes and Marcus Kronforst. I don't know the first few folks but I do know Marcus, and from what I know of him, his research is simply stellar. You can see for yourself if you take a look at his lab website. His lab studies butterflies mostly. For the last few years his lab has been quietly studying monarchs too, specifically looking at the physiological and genetic traits that are linked with the migration. If you recall, his lab was instrumental in the study that came out a few years ago that showed how the eastern monarch population was the ancestral population of all global monarch populations, based on genetic analyses. I recall reading that paper and being like, oh, wow. Oh, and I should also mention that this new study was published in one of the most prestigious scientific journals in the world, the Proceedings of the National Academy of Sciences. Here is a link, but I'm not sure if it is freely-available. So keep all of this in mind going forward - this was not some lame scientist and a hairball experiment that slipped through the peer-review process. This was a top-notch insect lab with an impeccable track record, and their study was published in a world-class journal.


From my read of this new paper, it looks like in the past few years Marcus' crew has been zeroing in on the growing trend of home-rearing, and commercially-reared monarchs. And rightly so. Conservation groups have all pointed out how dangerous this practice is if people go overboard and rear massive number of monarchs - recall the online arguments that took place last fall when Xerces and Monarch Joint Venture released statements about the risks of mass-rearing and commercial operations (summary of those arguments is here). Scientists believe these practices are risky because we don't really have much science on this issue. That all changes today. This new study specifically examined the genetics and migratory ability of captive-reared monarchs and compared them to wild migrants to see if they are equivalent. Now we have a definitive answer to the question of whether captive-rearing monarchs helps the population. Spoiler alert - it doesn't, and it looks like it even is harmful to the migration.


So this was a pretty hefty paper, with lots of different experiments and data, plus a dizzying array of appendices and methods. I certainly don't have time to go into all of these, so I'll just go over some keys bits and gloss over the rest. 


Let me start with one of the main tools that they used, because most of the results of this study hinge on this, and so it is important that readers know what it is. Marcus' group studies a key migration behavior in monarchs - their orientation ability - and to do this they use an apparatus that sort of looks like a cross between an old washing machine and a rain barrel. There is a drawing of it in the paper, which I'll add here.



I know this thing looks crazy but it is actually one of the best ways to experimentally tell which direction a monarch "wants" to fly in. Basically, you tether a monarch on a stick that hangs in the middle of this barrel, and the monarch can pull the stick, or "fly" in any direction it wants in the barrel. The stick is wired to a computer which logs which direction the monarch wanted to fly, and for how long. I'm a little familiar with this thing because we have one in our lab! Here is our version, which is an exact copy of Marcus' one.



That's our former PhD student, Ania, who worked with this gizmo a couple years ago. And I know what you're thinking, because everyone has this same question - how do you attach the monarch to the stick? It's fairly easy, actually; you just use a type of glue, and it doesn't impede their wings from flapping. We glue monarchs to rods all the time with our flight mill apparatus. So when a monarch is "flown" on this device, the computer tells you what the average direction of their flight was. When you do this for multiple monarchs, you can then determine what the average of all of them was, and then see if that average is statistically different then the average of another group. Very neat!


OK, so that's the flight gizmo. Now let's get to the meat - one of the main goals of this study was to determine if captive-raised monarchs are capable of orienting properly during the fall migration. For the fall migration, monarchs should be orienting south, so anything that deviates from that is not normal. (As an aside, let me say that testing the orienting ability of captive monarchs is very clever - no one has done this before!)


To start with, the researchers obtained a collection of monarchs from a commercial breeder in July of 2016, and they also collected some wild monarchs from the local area (Chicago). They housed them all in outdoor "insectiaries", which I assume means mesh cages. Then, they bred monarchs in each group to produce offspring the next month, which were also raised in these outdoor cages, each group side by side. This is very important - the resulting butterflies had been reared completely outdoors. One group was the progeny of wild monarchs and one was progeny of commercial monarchs - all reared under identical conditions by the researchers. They then compared the orientation ability of these offspring using the device above. This first round was done during the summer of that year (not during migration), so for this test they didn't expect there to be any directionality in the flights, and lo and behold, there wasn't. Neither group displayed any clear direction.


Now here's the key part. They then allowed those offspring to mate and produce a second generation that emerged in the fall, when it is time to migrate. These were also reared outside, under identical conditions. So these groups were composed of the grandsons and granddaughters of the original wild monarchs and commercial monarchs. The researchers then tested these two groups on the flight gizmo. Now, there was a clear difference - the wild offspring showed an overall average orientation direction towards south, as expected. But, the grand-offspring of commercial monarchs showed no clear direction (statistically). You can see for yourself in the diagram below, which is copied from the paper.



Let me walk you through this diagram, because it's probably not something you've seen before. First, "North America" refers to the progeny descended from wild monarchs. The "Commerical" group are the ones descended from monarchs they purchased. The four circles each represent a compass, and the black lines each represent a monarch flight direction (it's overall average). The length of the black line represents the strength of that directionality for that monarch. The red arrow represents the overall average direction for the group, and the longer the red arrow, the stronger that was.


Now that you know what you're looking at, you can see that during the summer (both lefthand compasses), there was no clear flight direction preference in either group, as expected. As I mentioned, there was no reason to expect any directionality during summer. But note the right compasses for the autumn groups. The descendants of wild monarchs showed a clear preference for southward flight, but the descendants from commercial monarchs did not, or at least not very much. Note the length of the red arrow, which shows the overall average. Now to be fair, if you look hard at that compass, you can see that a few of the commerical monarchs did fly in a southward direction. It is unclear if these few simply happened to randomly choose this direction, or if they clearly did know which direction they wanted to go.


Now think about this for a second - all of those monarchs (both groups) had been reared outside and exposed to every possible natural environmental cue there is that tells them that it's time to migrate. But despite this, the descendants of the commercial monarchs, even two generations removed, showed an inability to properly orient. From this result, the only logical conclusion to draw is that the genetic codes for migration are lost in the commercially-reared monarchs, and this "loss of migration ability" is passed down to each successive generation through their genes. You should probably re-read what I just said. If I were holding a mic it would be dropped right now.


Speaking of genetics, the next crazy part of this study was their genetic analyses of the commercially-reared monarchs and their descendants. Marcus' crew compared the genetics of the commercial line to those from wild monarchs. I won't get into the procedures used for this, because they're over my head! But I'll paste a diagram from the paper below that should suffice.



Think of this diagram as like a family tree of sorts. It's a visual depiction of the genetic relatedness of monarchs around the world. And remember, Marcus is the foremost expert on this. Global monarch populations essentially can be grouped into four groups, based on their genetics - North America, Pacific, Atlantic, and Central & South American. Marcus threw in some Danaus erippus (the "southern monarch") in for good measure here. Here's the crazy part - those commercial monarchs didn't match any known genetic group! They were so distinctly separated from the North American monarchs that they branched off into their own group. Imagine here that I picked up that mic and just dropped it again.


They didn't specify in the paper who the breeder was where the original commercial monarchs were purchased. But whoever it is, they appear to be creating genetic franken-monarchs in their operation. I doubt they even know they are doing this. In fact I have no idea how this is even possible. Nor do we know what else about these monarchs is abnormal. The scary thing here is that these commercial monarchs are being shipped everywhere, every year, by the tens of thousands (or hundreds of thousands?), and released into the population...Think of the genetic damage being wrought on the entire population when those monarchs mate with natural ones! This part should make everyone livid! I know I am.


One other thing to note here about this genetic part, is that their analyses also showed that the commercial monarchs were not from any non-migratory populations, like Florida. Nor were there any Florida genes that had sneaked into the stock.


OK, so by now you're probably thinking, well, all of this bad stuff only applies to commercial breeding operations, surely, my little kitchen table rearing is ok... sorry, but it looks like the answer is no.


There were a couple of other experiments conducted that bear on this. One was from the fall of 2018, where the team reared wild monarchs in their outdoor insectiaries and they had all eclosed by mid-September. Meanwhile they also reared some wild monarchs indoors in incubators, which are basically like special fridges with lights inside. These incubators can be set to any temperature desired, plus any light duration desired. We use these all the time in our work. Marcus' team set their incubators to closely match the fall temperature and lighting conditions. When they tested both groups they found that the outdoor-reared monarchs oriented properly (south), but those monarchs that were reared inside in artificial conditions did not! Thus, even though the incubators closely matched the environmental conditions of the outside, the fact that it was still an artificial environment was enough to abolish the navigational ability of the monarchs. As a control of their procedures, the team also caught migrating monarchs during that fall and tested them on the rain-barrel-thingy too - they oriented perfectly.


Amazingly, at one point during their fall experiments, the researchers had taken a subset of the WILD monarch pupae from the outdoor cages, and brought them inside to finish their remaining development in their lab. They were placed in the incubators, which were set to fall-like conditions. It sounds like these pupae were really only in the incubators for a few days before they eclosed. When those pupae became adults, they were tested on the orientation apparatus. Amazingly, most of these monarchs failed to properly orient! Meanwhile the brothers and sisters of those wild monarchs that were reared completely outside did orient properly! Think about this for a second - even though these monarchs were wild, and had spent most of their lives outdoors, the fact that they were brought inside in an unnatural environment for the final few days of development was enough to abolish their orientation ability.


I can believe these last results because of something that happened here a few years ago. I mentioned that our former student Ania had done some work with this same orientation apparatus. What I didn't mention was that she could never get this thing to work - or so we thought. She had wanted to test the flight direction of a bunch of monarchs for something and so she had this thing built exactly to the specifications of the original. But, none of the monarchs she used ever showed any evidence of directionality, even those that had been reared (in an incubator) under fall-like conditions. She eventually gave up and we all thought that we were doing something wrong, or that this gizmo didn't work. Now that I read this new paper it looks like our monarchs didn't want to fly properly because they were reared inside in artificial conditions!


So the take-home message of this part is that even taking wild monarch larvae from your backyard milkweeds and rearing them in artificial conditions (i.e. indoors) can disrupt their ability to properly navigate (another mic drop). And, recall that this inability to navigate is passed down to each successive generation, meaning that even if you do your rearing early in the summer, well before the fall, it can still cause problems, because your reared monarchs will then have passed down their inability to their offspring, and even their grand-offspring which will emerge in the fall.


I should point out that the researchers did not know what specific element of the artificial conditions caused the loss of orientation ability, but if you think about it, there are many possibilities. The incubators usually have halogen lamps for lighting, and the wavelength (or color) of these could be a factor. The temperatures can be controlled, but it looks like Marcus had them set to a constant, cool temperature all day, so maybe the monarchs need to experience the lower temperatures early in the day, and then feel it get warmer during the day. I know also that these incubators can be noisy inside, because of all the machinery, and I know that loud noise can stress monarch larvae. Or maybe it is the stress from human handling that comes with rearing the monarchs indoors, since their containers need to be cleaned daily, new plant stems added, etc. Or maybe developing monarch larvae need to experience the singing of birds! Who knows? The point is, since we don't know what it is about the indoor rearing that causes the adult navigational problems then there is no simple change that people can make to "fix" their rearing practices.


I know, this is all crazy.


Still don't believe all of this? Think about this then - while it was not in the paper itself, I can tell you that these results are very consistent with other information we have about captive rearing. There have been at least two papers published in the past 5 years that have looked at monarch tagging rates during the fall migration, one from Pennsylvania and one from Arizona. Both studies compared (overwintering) tag recovery rates between wild monarchs and those that were reared in captivity. Both studies found that captive reared monarchs have a much lower recovery rate than wild monarchs, which implies their migration ability is poorer. Secondly, recall that I had done an informal analysis of monarch wing color a few years back, which I blogged about last fall. Remember that wing color is a predictor of migration and flying ability, and in my study I had found that captive-reared monarchs tend to be paler than wild monarchs, which implies poorer migration ability. So the fact that Marcus' crew discovered that captive-reared monarchs cannot navigate very well actually explains a lot. And, the consistency between all of these studies means that there is something to this.


OK, so there was a lot that I didn't cover in that study, and I would encourage folks to try to get a copy to see what else was done. I really only covered about half of the paper here. For now though we should step back and think about what's happening with the eastern monarch population. As I mentioned at the beginning, we have good evidence showing that eastern monarchs face a range of threats during their critical fall migration. The last thing we need is to be making this harder for them by watering down the population with monarchs that can't properly navigate.


Before I sign off, given the controversial nature of these findings, I feel like I should end this blog entry with some discussion of what people should do now.


I'm sure there are a lot of folks reading this who have baby monarchs on their kitchen tables right now. After reading this you will probably do some hard thinking. You may share this blog post with your favorite monarch group, to solicit opinions from others. You may try to track down the original study and read it directly (I hope you do this one!). But really, there are just one or two questions you should be asking yourself right about now - given all of this new information, should I keep rearing dozens (or hundreds!) of monarchs in my kitchen, garage, or whatever? Am I really helping the population? Similarly, do commercially-reared monarchs really help the population like the butterfly breeders say they do? After all, if a few captive-reared monarchs actually do make it to Mexico (a few do), doesn't that prove that the entire practice is safe? Let me answer that question with one of my own - how many monarchs has your home rearing operation (or a commercial one) prevented from reaching Mexico?


Let me explain this last thought with some simple math: say you've released 1000 reared monarchs into the world in the last couple of years (some people really do!), and let's say that 1 one these was recovered in Mexico, which sounds great. But, about 500 of the other monarchs went on to mate with wild monarchs throughout your local region, giving rise to offspring (and grand-offspring!) that lack the proper navigational ability. In this case then, your reared monarchs have literally prevented those offspring from successfully migrating by passing on their poor-navigating genes. After all, we now know that this happens.


So what do I think people should do? I'm glad you asked! There is certainly something to be gained from the experience of raising a few monarchs, in terms of education and fostering a connection with nature. There is also the benefit of contributing to citizen science programs like MLMP and MonarchHealth when you rear a few and then contribute your observations (or samples). And it sure is fun for kids! So my advice is to limit your rearing to 10 monarchs per year, and, do the entire rearing outside. That's enough to do all of those things but still not adversely affect the entire population.


I should probably leave it here. There's lot's to ponder over this new study. I'm sure we'll all be discussing this at length in the coming months, or even years!



Direct link to this blog entry:


Please reload

The science of monarch butterflies

A blog about monarchs, written by a monarch scientist, for people who love monarchs