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  • Andy Davis

Review paper concludes: tropical milkweed, plus climate change, equals more winter breeding

Hi everyone,

I just read a brand new peer-reviewed study that was just made available, and I'm going to pass on my (short) summary of it here. It was a "review paper" that was focused on the topic of winter-breeding in monarchs, which is a growing threat to the famous migration in North America. This is where monarchs form continuously-breeding, non-migratory micro-populations in places where it doesn't freeze in the U.S. (i.e. instead of migrating to Mexico). The concern is that with the perilous fall migration already being in trouble, that the growth of "winter-breeder" colonies around the southern U.S. could be pulling migrants out of the journey, thereby further reducing the migratory pool that travels to Mexico. That's certainly my own concern! But I'm getting ahead of myself.

Let's get to the paper. Feel free to read it yourself, since it was a quick read, and it is fully online. It is currently in an early (pre-publication) format at the journal, called "Current Opinion in Insect Science, and a link is here. You'll see that it was written by Christen Steele (a recent PhD graduate from Univ. of New Orleans), Izzy Ragonese (a current PhD student in Sonia's lab at UGA), and Ania Majewska (A former PhD graduate from Sonia's lab). Each of these people has studied the monarch-OE system extensively for their graduate research, and thus are very knowledgeable about winter-breeding and what it does to monarchs. The title of the paper is - "Extent and impacts of winter breeding in the North American monarch butterfly."

From my read, this review paper set out to summarize all of the available evidence and science around the winter-breeding issue, and the authors then drew some conclusions based on what they read. I see that they looked at research going back to the 1960s, all the way up to the present. They brought in findings from laboratory experiments, as well as field observations. It appears that the goal was to identify some common themes or conclusions from all of this work, that could explain the increase in this winter-breeding phenomenon, and what it will do to the migration.

They first point out that there has long been evidence of at least some winter-breeding in the U.S., in selected places like southern Florida, and even in one site in southern California. Now though, there is evidence of winter-breeding in most of Florida, a growing region in southern California, some parts of southern Texas, lots of pockets along the Gulf coast, the Atlantic coast, and even northern Mexico. It really is growing. They point out that all of these places have two things in common. They all have 1) warm temperatures in the winter - i.e. warm enough for monarch larvae to exist, and 2) the presence of non-native milkweed, usually tropical milkweed. Occasionally, there are some native milkweeds that persist in the winter in some places, like Arizona, but by far, it is typically non-native milkweed that is the culprit. They also point out that in all of these places, the prevalence of the OE parasite is really high. That's because the year-round availability of milkweed allows the OE parasite spores to build up on the leaves. Not to mention the fact that tropical milkweed allows infected monarchs to live longer, thereby spreading the infection longer.

I want to pause here and make sure people reading this are clear about one thing - these winter-breeding sites are being formed because of people - people who have planted non-native milkweed. Read this sentence again. In other words, if you live in one of these places and you yourself have planted some non-native milkweed, then you have created the perfect conditions for winter-breeding. Without the milkweed, the monarchs probably would not be forming these winter colonies. According to these authors, warm temperatures alone are not the cause. I added all of this in here for emphasis, because I think a lot of people aren't aware of just how much this phenomenon has increased over the years, mostly because of well-meaning, but uninformed people.

Now, here's where the climate change thing comes in. It seems pretty clear that with a warming planet, these regions of warm winter temperatures around the U.S. will continue to grow. More and more places will become suitable (at least in terms of temperature) for winter-breeding. The authors state "The warming climate in the fall is expected to increase the chances of forming winter breeding populations." This was a recurring theme throughout the paper, about how these regions will grow, and so too will the winter-breeding (and OE), if there is also available milkweed.

There was an odd section in the middle of the paper titled "Barriers to Migration" that I'll touch on here. The authors (rightly) pointed out that the fall migration itself is getting harder each year. There are multiple dangers or stressors to the migration, like the growing prevalence of the OE parasite, which hinders monarch flight, the increasing amount of car strikes along the way, and the increasing storms caused by climate change. All of these are novel stressors placed on the migration (caused by humans) which the monarchs have not really evolved to deal with. But here, the authors suggested that these stressors could be causing monarchs to forgo the migration (because it is too stressful). They stated, "Thus, migratory monarchs experience multiple, relatively novel environmental stressors, which might drive the insects to favor winter breeding as a strategy when given the opportunity." I can see the logic behind this reasoning, though I am a little skeptical that monarchs would "choose" to forgo the migration, as if they thought about it and decided against it because it was getting harder. Since each adult monarch in the fall generation has never been on the journey before, they have no way of "knowing" if it is stressful or not.

The last section of the paper was titled, Implications, and here the authors laid out what this phenomenon (increasing winter-breeding) would do to the overall North American monarch population. The short answer here is that we don't know for sure, mostly because of a lack of research. Here, the authors (rightly) pointed out that there are still many unresolved questions about this behavior/phenomenon, and they even presented a table listing these questions. Probably the biggest unknown here is the extent to which these winter colonies are "pulling" monarchs out of the migration. To be fair, we already know that when fall-migrating monarchs encounter pockets of tropical milkweed in Texas, they are indeed pulled in, and they then mix with the resident monarchs at the site (which are mostly infected with OE). Here is a link to the blog describing that study. However, we don't really have a handle on HOW MUCH this occurs, or how MANY migrants are being pulled out of the migration. Is it 10% of the migratory cohort, 50%? That part we don't know.

Another important question refers to what happens when monarchs enter these winter-breeding colonies - are they capable of reverting back to being a migrant when they want to? From the research I've read, it seems to be a one-way transformation. And if so, those are now monarchs that will never again enter the migratory pool. I had a lengthy debate with Dr. David James about this last year (see the blog here).

OK, so that's basically my summary of this review paper. Now let me make a couple of important points of my own. First, the fact that tropical milkweed is the main cause of this phenomenon should not be surprising. Based on all of the experimental and lab-based research that has been done, we now know that this plant basically stimulates reproductive activity in monarchs, even if they are migrants. And if caterpillars eat this plant, it makes them grow up to be reproductive adults too. See the list of studies I've compiled previously about the research around this plant (link here). They all point to the same thing, really, that tropical milkweed makes monarchs want to not migrate (purposely oversimplified, but you get the point).

Secondly, let me also point out something to the people in the cold, northern areas of the U.S. and Canada, who are reading this now and don't think this is something that they need to care about. You do. Remember, these southern winter colonies are all heavily infected with OE, which means that most milkweed in these areas (native and non-native) are covered with OE spores. If any healthy spring migrants from Mexico pass through these sites and lay their eggs on those plants, then those offspring will become infected, and, THEY WILL BE HEADED YOUR WAY! In other words, these infected, winter-breeding colonies could very well be driving the increasing OE prevalence on a national level. So yes, you do need to care about what happens down south. If nothing else, you can help to sway your southern neighbors into helping to fix this problem.

And finally, to the people who complain that "tropical milkweed is the only milkweed I can find" I say, then don't have milkweed. In places where there is even a chance that monarchs could form a winter-breeding colony, it really would be safer - for them, and for the population - to not have any milkweed at all. That way, there would be absolutely no chance of further exacerbating the national OE problem we have now, and no chance of your milkweed helping to interfere with the monarch migration.

OK that about covers this new paper, plus my two cents. I'll add this one to the growing list of research around tropical milkweed...


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