Winter-breeding monarchs in California - good or bad? A new study adds more confusion
- Andy Davis
- 1 day ago
- 7 min read
Hi blog readers,
Today I'm going to provide an objective overview of a new study that was just published that was focused on the contentious issue of winter breeding, and homeowners who encourage it by providing non-native milkweeds. This is a practice that has grown in the last decade, especially in places in the U.S. where it remains warm in the winter, like Florida, the Gulf Coast, and importantly, in coastal California, where this study took place. There, homeowners have been increasingly planting milkweeds to "help the monarchs", and in many cases, they have been planting non-native milkweeds like tropical milkweed or hairy balls milkweeds, which are often available at local nurseries. I have extensively covered the issues associated with these plants elsewhere in this blogsite (hint - they are all negative issues).
Winter breeding in California is a unique situation since the western wintering sites are in the same area, meaning that each fall, the migratory monarchs from the Pacific Northwest travel into the coastal California region to spend the winter. The big worry here is that the presence of year-round milkweed nearby will encourage those migratory monarchs to abandon their migratory behavior and to take up residency in those areas with abundant milkweed. If this happens year after year, this would lead to an overall reduction in migratory monarchs and fewer monarchs at the wintering colonies. This has been a bit of an ongoing debate among scientists as to whether these winter breeding sites (i.e. places with non-native milkweed) are helping or hurting the western monarchs. This new study appears to come to an interesting conclusion about this debate - that they neither help, nor hurt!
So, the paper was published and is fully available online in the journal, Ecosphere (link here), and was led by the labs of Elizabeth Crone and Sheryl Schultz, plus a post-doc named Emily Erickson. Surprisingly, I was not asked to review this paper by the journal, and so I am only reading this for the first time myself. However, I did give the paper a thorough read, and I'll provide the basics below.
First, the study was essentially an observational one - the lead author performed walking surveys through neighborhoods in the San Francisco Bay area (the study location) each month for 2 years. During these surveys she recorded the presence of milkweed (native or non), the presence of eggs or larvae on those milkweeds, and the presence of adult monarchs in the area. She also captured some of the adult monarchs to measure their wing size, amount of wing wear (as measure of age), and importantly, she sampled them for the OE parasite. This last bit is a very important part of the debate here, because prior research showed how these areas with non-native milkweeds and lots of winter-breeding are loaded with OE-infected monarchs. I'll come back to this later. I also note that the authors appeared to have the necessary permits to conduct the sampling.
From these data, the authors looked at the monthly changes in each variable to see if they could spot any trends that would indicate if the Bay region was receiving annual influxes of migratory monarchs each fall, which is the big worry. Specifically, they looked to see if there was a noticeable increase in the numbers of monarchs in the fall, or, an increase in wing size in the fall (because migratory monarchs tend to be bigger), or, if there was a noticeable drop in OE prevalence in the fall. This last bit seems to be the linchpin of their conclusions, and they put a lot of emphasis on it in the paper. The idea here is that if the migratory monarchs were indeed entering these winter-breeding sizes in the fall, then this would equate to an influx of a lot of OE-free monarchs into the mix, which would lower the overall prevalence. This is something that has been found at similar sites in the east.
So, here's what they found.
First, there was no real noticeable increase in adult monarchs in the fall. Nor was there an increase in wing size in the fall. And, there was no overall drop in OE prevalence in the fall. From all of these bits of evidence, the authors concluded that there was no large influx of migratory monarchs into these winter-breeding sites. I think that is some good news here.
Here is where it gets even more interesting - the authors also addressed the idea that these winter-breeding sites are a beneficial "source" for the western subpopulation, that is, they are providing a boost of monarchs, or otherwise "helping" the population. It turns out, they are not. The authors looked to see if there was a noticeable exodus of monarchs in the spring, which would indicate that some of these breeding monarchs were leaving the area and re-entering the migratory cohort. There was not.
So, from this evidence, the authors concluded that these winter-breeding sites in California are neither helping nor hurting the western (migratory) subpopulation. Moreover, they also included some text that suggested even that removing non-native milkweeds would have little conservation value, or impact. I think they are basically arguing that these winter-breeding sites in California are essentially operating independently!
OK, so now I have some thoughts here, which you can take or leave.
First, I think the authors were being a bit too cavalier with this paper, both with their conclusions and their suggestions for conservation. I don't question the actual data, as I know the researchers here to be very competent, but, I think these data have some limitations as to what we can conclude from them. For one, the study was based on only two years of sampling. In some ways, this was a lot of work and a lot of sampling, but on the other hand, a lot of things change from year to year, especially in the west. The authors did point this out too. They also seemed to be swayed by some human emotional arguments - they wrote that their own conversations with homeowners convinced them that the non-native milkweeds help to teach people to appreciate nature, and this then leads people to plant more flowers or other insect-friendly landscapes.
Also, the entire study was essentially an observational one, and the authors were drawing conclusions about migratory and non-migratory monarchs without actually knowing which was which. In other words, they didn't test the captured monarchs to tell where they came from. This can be done using lab analyses that examines their wing tissues, and it lets you know if the monarchs really are migrants or are residents. That would have been a really helpful thing to do with this study. The authors did acknowledge this too.
But...the big kicker for me is the OE data, which the authors seemed to poo poo a bit in the paper. Across both years of the study, the overall prevalence of OE at these sites was 50%, and in some months it was over 75%. That means at least every other monarch flying around in these neighborhoods is dropping OE spores on all milkweeds it contacts. In other words, these neighborhoods are OE hotspots. This is something that the project monarchhealth data also shows. So, even if the migratory monarchs don't really overlap that much with these neighborhoods in California, this high OE level is still a problem - for the neighborhoods themselves.
The thing about a high OE prevalence that never gets talked about (and the authors didn't either) is the massive level of death and deformity it leads to within these sites. Those pretty butterflies you see flying around in those neighborhoods are really just the lucky few monarchs that were able to survive to the adult stage while being infected. The rest of the monarchs didn't, and they died a slow death hidden away under a bush, because their wings were too deformed to fly. Some infected monarchs simply die trying to break free of their pupal case. And because of the high OE prevalence at these sites, it doesn't matter if a homeowner uses native or non-native milkweed, because they all become contaminated with OE spores, and the same thing happens.
I recall talking with some monarch-savvy students a number of years ago who had visited neighborhoods like the ones here in California, and a direct quote from them was "it is a horror show." When they actually looked closely at the milkweeds and surrounding plants, they could see exactly what I mentioned - deformities everywhere and monarchs dying slow deaths. For illustrative purposes, I'm going to post a picture taken by someone in Florida, who experienced this death and deformity first hand. When you really look closely in the bushes and milkweeds at these places, this is what you see.
As someone who has been part of this ongoing debate, I can see some value in this new paper, in that it at least tells me that these winter-breeding sites in California are probably not population sinks for the western (migratory) subpopulation. From the data gathered, I can concur with what the authors found. But I don't think I would conclude that they are not harmful. They are harming monarchs.
I also worry that this new paper would add yet more confusion to the mix, and especially to homeowners in California. In fact, at one point in the paper the authors even pointed out how homeowners there seemed to be very confused about this very thing, and there were different opinions and stories about native and non-native milkweeds, and their impacts.
Maybe at this point in the crazy times we're living in it doesn't matter anyway, because so many people are simply choosing to ignore science and evidence. This is happening within the world of monarchs, and also outside of it.
That's all for now.
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