(above picture from University of Minnesota)
Hi blog readers,
Today I'm diving back into the controversy around roadside habitat for monarchs and other insects. This is where people and organizations work to enhance or create insect habitats, flowering plants, and/or milkweed patches on the vacant strips of land right next to roadsides, thinking that this will help those critters by providing habitat. There has been a lot of research on this issue in the past 10 years. This is also something that I've covered extensively in this blog in the past too, and I've even been a part of some of this research (see here for a complete list). So, I'm well-versed in the controversy for sure, and I also know that there are lots of differing opinions. I'm diving back in today because a new study has just been published by some researchers from the University of Minnesota that opens up the worm can again. So in today's post I briefly go over this project based on my read of the paper. To be completely upfront, I was less-than impressed by some parts of this project, which I'll get to as well.
Let's start with the basics - it was published in the journal, Insect Conservation and Diversity, and here is a link. I believe it is open-access. The lead author was Alison Banks Cariveau, who used to work for Monarch Joint Venture, and the other notable authors include Emilie Snell-Rood (who is a professor at Univ. of MN and has also done some research on roadside habitats for monarchs), and also Karen Oberhauser (former professor at UMN), plus a number of junior researchers and technicians, I think. From the acknowledgements section, they write that this project was a part of a broader project spearheaded by Monarch Joint Venture, and funded by the "Transportation Research Board" of the National Academy of Sciences. The other bit of background I'll give here is that from what I can tell, Monarch Joint Venture has been at the forefront of efforts to create or enhance roadside habitats for monarchs in recent years. In fact, whenever I talk with someone who either manages or creates roadside habitats in various states, they always point to MJV as the group that backed their efforts, or provided them with guidance.
The title of the paper - "Host plants and landscape predict use of roadside habitat by breeding monarchs" gives you the gist of the study. From my read, the project appeared to be a survey of roadsides in Minnesota, whereby the researchers looked for milkweeds and also eggs and larvae of monarchs on those plants that were next to roads. They surveyed about 250 different sites, and they compared milkweed densities, and monarch densities, at roadside sites that were next to agricultural fields versus other landscapes like grasslands. I think the idea here was to see which roadside landscapes had more monarchs or milkweeds, which they said would be good information to give to the roadside management teams. I'm simplifying this a lot for brevity - the paper itself does contain a lot of data and results. There was also a nifty comparison of mowed versus unmowed roadsides, and the effect on milkweeds and monarchs. I know this is a question that comes up a lot.
In the end, they had compiled a list of all of the milkweed species they found (the vast majority was common milkweed), plus the numbers of eggs and larvae found on each plant. I'll paste a screenshot of one of the main tables below. You can see that not only was common milkweed the most common milkweed found (ha!), but monarchs were also more likely to use that species too.
As for the mowing thing, the results were quire clear - there tended to be more milkweeds, and more monarchs, in roadsides that were mowed. A screenshot of their graph is below. Don't worry about the units on the y-axis, as those are log-transformed numbers.
The above pattern may seem counterintuitive, but this pattern is actually consistent with a number of other studies looking at the effect of mowing, and not just along roadsides. Basically, cutting the milkweed down stimulates the milkweed plants to grow more shoots, which will have more younger, fresher, leaves, which is what female monarchs like to lay their eggs on. Some researchers argue that this should be done mid-summer, to stimulate a larger late-summer population.
In the results and discussion of the paper, the authors focused a lot of their attention on the fact that they found eggs and larvae on some of these roadside milkweeds. According to the authors - "We found that 57% of sites with milkweed had monarch eggs or larvae. Monarch eggs and larvae were more likely to be present at sites with high milkweed density..." This appeared to be the result they were looking for. In the conclusions section, the authors state - "our results support the general promise of roadsides as habitat for monarchs, especially in the upper Midwest. We documented high densities of milkweed host plants in roadside habitat, especially in areas with adjacent agriculture and grassland."
So basically, the authors are arguing that because they found lots of milkweeds and monarchs next to roadsides, then that means these habitat patches are "good" for monarchs, and, that we should plant more of them next to roadsides. Sounds logical right?
Wrong. And let me explain why below. And to be fair, this is an issue that lots of people, even scientists, can't seem to wrap their heads around.
Here's the thing - the mere presence of milkweed or even monarch larvae next to a roadside does not signify that it is "good" monarch habitat, nor does it do so even if there are lots and lots of eggs or larvae. Consider for instance, that roadside milkweeds tend to have higher levels of pollutants and chemicals in them, which then gets into the monarch larvae. That's not really a good thing for those larvae, and this evidence comes directly from other research from the Snell-Rood lab too. Then there is the less-than-obvious effect of road traffic noise on the larvae, which is something I have studied in my own lab. Roadsides are really, really loud, especially if we're talking about interstates, and in my lab, I've found that this noise can be stressful to monarch larvae. And finally, perhaps the biggest problem with attracting monarchs (or other insects) next to roadsides is that it leads to their eventual death by car strikes. This has been studied in monarchs, plus other insects, and the results are always dire. In fact Doug Tallamy (yes, that Doug Tallamy) even found that insect mortality is highest in places where there is insect habitat next to roads, and lowest when there is no habitat (see the blog on that here).
So given all of this, how can it be a good thing if there are milkweeds next to roads, which are basically attracting adult monarchs to fly next to the roads, and enticing them to lay eggs, so that their larvae can develop, only to become stressed, polluted, or crushed later? None of these issues were examined in the above study, nor were they mentioned even in the overall discussion of the results. The authors largely ignored this body of work except for some cursory mentions early in the paper. They even ignored a highly-relevant review paper that was published earlier this year (link here), which was written by a separate lab, and which pointed out this very conundrum with roadside habitat research - i.e. that researchers often conflate insect abundance with benefit, but fail to consider the added mortality. In other words, the review paper points out (rightly so) that there has been no research at all that has determined if roadside habitats PRODUCE more insects than they KILL.
For monarchs in particular, which have a unique, long-distance migration each fall, this issue of road mortality is heightened even further, since this 3000-mile trip forces them to cross hundreds and hundreds of roads over the span of two months. Even without roadside habitats, these crossing attempts lead to the deaths of millions upon millions of monarchs each fall (I've done the math). So why in the world should we entice migrating (or breeding) monarchs to fly even closer to roads by placing flowers, or milkweeds, next to them?
If it is not clear already, my stance on this issue is pretty firm - roadsides are no place for monarch or insect habitat, period. My stance on this comes from studying not only the roadside issue myself, but also from my more recent efforts to document the massive DECLINE in monarch migration success over time in N. America (see the prior blog). And, despite what the researchers here have concluded, this new study does not prove that roadsides are good for monarchs. If anything, it shows how dangerous they are, because these habitats essentially act as lures to monarchs - i.e. luring them to hang out in a very dangerous place.
Finally, I'll also add here that I did reach out to the lead author and Dr. Snell-Rood, and told them of my concerns with their interpretation of their own data. They did politely respond and told me that they stand by their study, and that they were indeed aware of the controversial nature of this issue. They also provided me with a copy of their data, which I did look through (it was an impressive dataset). They also indicated that the study was not designed to test if roadsides were good habitats or not. I told them that I disagreed with that - their own conclusion from the paper is that roadsides are "effective habitats for monarchs" (their words).
I worry that this paper will be used to further justify this dangerous practice.
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