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  • Andy Davis

Are sites with winter-breeding monarchs good or bad for the population? A debatable question

Hello blog readers,

Happy new year to all. I have some interesting news to share with folks today, and that relates to the hot-button issue raised by this blog title. This week, there were two new online articles that were published in the journal, Insects, that describe an interesting academic debate that I have had with Dr. David James, a fellow monarch scientist. Today I'm going to fill you in on this debate and boil down the main points as best as I can. This post should be of interest to anyone who cares for monarchs, and who wants to do the right thing to conserve them. And importantly, if you have monarch caterpillars in your yard right now (in January), then this is something you should definitely care about. Be warned first though, that you will need your thinking cap on to follow this!

Let's start with some background. We all know that monarchs in North America undergo annual migrations each fall to wintering sites in central Mexico (where most of the population goes), and also to scattered sites along the California coastline. The adults spend the winters here in a state of reproductive dormancy, waiting until the spring, when they all re-migrate northward, laying eggs along the way to recolonize their breeding range. This is the narrative that we have all learned, and this is the way the monarchs have probably evolved over thousands of years. But, this narrative has been changing in recent times - a growing number of monarchs are not reaching their Mexico or California wintering colonies. In the eastern U.S., a growing number are wintering in coastal areas of Louisiana, Alabama, Georgia and South Carolina. In fact, scientists are even now trying to get a handle on how many monarchs do this, by asking for people to report winter sightings to Journey North. In these areas, the winter temperatures are becoming milder each year, thanks to climate change, so that there are fewer and fewer killing frosts. Adding to this issue, is the increasing use of non-native milkweeds by homeowners in these areas, and these milkweeds don't die back in the fall or winter like native milkweeds do. Thus the presence of year-round milkweed, plus the warmer temperatures in these areas entices monarchs to breed all throughout the winter. The same thing is happening now in southern California. The mild winters, plus year-round milkweed, is creating a growing "resident" population of monarchs.

The worry is that these areas are actually luring migratory monarchs out of their migration, thereby reducing the total numbers that reach their winter destination. The science on this issue is ongoing but incomplete. There is experimental evidence that tropical milkweed does promote reproductive development in migrant female monarchs, and there is also evidence that monarch larvae that feed on tropical milkweed have reduced propensity for migration. Then there is the problem of OE at these places with year-round milkweed. The prevalence of this pathogen is super-high in these areas. So, there is a range of evidence that exotic milkweed can be harmful to the population. I wrote a blog here describing the science around tropical milkweed.

Here's the problem - it is very hard to convince a typical homeowner of these risks because they can't see it happening. If a homeowner plants tropical milkweed in their yard, they will quickly see monarch larvae on it, and to the homeowner, this seems like a win. And since most homeowners don't test their adult monarchs for OE, they also don't see how these milkweeds are increasing the spread of this disease. To them, they just see pretty butterflies flying around their backyards - even in winter.

OK, so this is the background for the debate around non-native milkweeds and winter-breeding, which are definitely confounded. On the one hand, there is evidence that these milkweeds are not good for the population, but on the other, someone can say that by planting them, they do help to boost the overall population (from a simple mathematical argument).

Now, let me describe a recent scientific study that was the focus of these two new articles. In 2021, a paper was published by Dr. David James, and three citizen-scientists, in the journal Insects. Here is a link to it (freely available). This paper described an investigation into the nature of winter-breeding in the San Francisco Bay area of coastal California. This is an area with a growing resident population of monarchs, and it is very close to some of the traditional migratory colonies. There is a lot of exotic milkweed in this area, which seems to be driving the growth of the year-round resident monarch population.

In the winter of 2021, there was an apparent crash of monarchs arriving at the traditional migrant winter colonies - at the time, only about 2,000 monarchs showed up. At the same time, there was what appeared to be an increase in sightings of monarch larvae and adults flying around in California backyards that winter. From the outside then, it gave the appearance that the migratory monarchs somehow "switched" that year and decided to take up residency at these nearby tropical milkweed sites, instead of going to their winter colonies, or at least, this is what Dr. James contends. His theory, which he has provided many times though social media, is that these sites with abundant tropical milkweed provide monarchs with a choice, or an "alternative winter lifestyle". The idea goes that if the conditions warrant, fall-migrating monarchs can choose to become winter-breeders, and then if the conditions for migration become favorable again, they or their offspring will "switch back" to a migratory lifestyle in the spring. This theory creates a very rosy picture of these sites with exotic milkweed, as it implies that they have "conservation value" to the monarch population.

But, here's the thing - the switch to winter breeding may be a one-way street. Once a monarch develops their reproductive tissue and becomes a breeder, they, and their offspring, may not ever return to a migratory life. We know that monarchs in non-migratory populations retain at least some of the physiological traits from their former life as migrants, based on some prior research by Dr. Micah Freedman (that I blogged about), but thus far we don't have solid evidence one way or another about this. And, this is the linchpin of the entire argument Dr. James is making. In other words, the idea that winter-breeding sites have "conservation value" hinges on whether or not the winter-breeding monarchs "switch back" to their migratory ways. If they do, it implies that these sites can indeed bolster the overall population. But if they cannot (i.e. if it is indeed a one-way street), then these sites do nothing but lure migrants out of the population, and they are technically "population sinks".

Here's where Dr. James and his colleagues came in last year. Their 2021 study was designed to learn more about these winter-breeding monarchs in the Bay area, and to see if the winter larvae in these areas eventually become adults that show any evidence of migration in the spring. Their study involved having citizens in the Bay area rear some of the larvae in their backyards into adults (in captivity), and then they measured their wings, tested them for OE, and tagged them. They attempted to track the movements of these adults by following reports by other homeowners of the tagged individuals. Then, they tried to piece together these movements by mapping the locations, and importantly, the directions of travel.

In a nutshell, they reared 500 monarchs, and found that about 20% of them were later found (recovered) by other homeowners in the Bay area in Jan-Feb. But, during the spring months (Mar-Apr), a lower percentage (12%) were recovered. These few recovered monarchs appeared to have a northward trajectory, based on where they were found. Mostly from these two findings, Dr. James concluded that a lot of the monarchs must have become migratory during the spring, and therefore that the winter-breeding monarchs can indeed switch back to a migratory life. This makes the case that the winter-breeding sites are a boon to the overall population.

When I read this paper, or heard Dr. James speak of it in news articles, I was skeptical of this claim, not only because there were little to no rigorous statistical tests done to prove it, but for another reason that was overlooked by the authors. So, I then wrote a "rebuttal" paper that described my arguments, and submitted it to this same journal. It was just published and here is a link to this article. Please read it!

As you can see, my paper focuses on the wing sizes of the monarchs reared by James et al. In their paper, they reported that they measured the wing sizes of the 500 reared monarchs, and the average length of the forewings was 47mm. That is quite small for a migratory monarch. As someone who has spent a lifetime looking at and measuring monarch wings, I can attest to this. In fact, there is a lot of prior research showing what the average forewing length of monarchs are, and in various places around the world. And, in all of the places where monarchs have a non-migratory lifestyle, their wings are always small - around 45-47mm. In the traditional migratory population in western and eastern North America, their wing lengths are closer to 51-52mm. See also a prior blog where I talked about wing sizes and migration.

In my rebuttal paper, I even made this chart showing the reported wing lengths of migratory and non-migratory monarchs from around the world, for comparison to the California monarchs raised by James et al. (which are the red bar).

From this comparison, it seems pretty reasonable to conclude that the California larvae reared by James et al in the winter were not large enough to be real migrants. If these were the offspring of formerly-migratory monarchs (that had switched to a winter-breeding lifestyle), then they should have become large-winged adults, like their parents. Since they actually became small-winged adults, this implies that they were the offspring of the small-winged, permanently-resident monarchs from the local area. This fact then implies that these monarchs did not rejoin the migration, and therefore, that these monarchs did not contribute to the overall migratory population. In other words, the year-round milkweed sites produced resident monarchs, not migratory monarchs. This means that these sites did not enhance the migratory population.

In addition to this argument about wing sizes, I also pointed out in my paper that the 500 monarchs raised by James et al. were heavily infected with OE (based on their own data), which is something that I don't think James takes seriously enough. This high prevalence is typical of monarchs in areas with year-round milkweed, and it is NOT a good thing for the monarch population. I've blogged about the dangers of this in the past (like here).

OK, so these are the main arguments I raised in my rebuttal paper. As is customary in a rebuttal situation, the original authors are allowed a chance to respond, and James et al. did so. Their "counter-rebuttal" article was also just published in the same journal and is linked here. Please read this as well! I read through it and see that they do have some interesting responses. Not sure if I agree with them though. Their main response seems to be that the winter-breeding monarchs don't have to be large! Interesting thought...I'll let readers be the judge - please do read that article too.

If you've reached the end of this blog you may be left wondering what to believe, or more importantly, who to believe? In this case, there are two scientists, each an expert, but each saying something different. Hopefully, you will do your reading and decide for yourself how best to help the monarchs, or even if they need our help at all! Regarding winter-breeding and exotic milkweed, I think the thing that this debate highlights is how little we actually know about the full impact of exotic milkweeds to the behavior and lifecycle of monarchs in North America. I always recommend the cautionary approach in situations like this.

I want to end this blog post by commending the authors for at least making the effort to generate some useable data on this evolving situation in California. It is really the interpretation of those data that is at question here. And, getting this interpretation right is critical for effective monarch conservation efforts.

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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