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  • Andy Davis

New study - Monarch larvae don't care about climate change!

Hello everyone,

Thanks for tuning in to another interesting post about the amazing research around our favorite insect. Today, I am going to take a look back to earlier this fall, when a very important scientific study came out, and was rather neglected. I bet most readers here even are unaware of it. As you might guess from my blog title, this paper was all about climate change and monarchs (cue the eerie music...). I know that climate change seems to be the new boogie man that is supposedly wreaking havoc with our North American monarchs, or at least, that's what you might have heard lately. After reading about this new paper, you may want to re-think this idea.

This study was conducted by people from the lab of Dr. Heather Kharouba, who is a professor at the University of Ottawa, Canada. From my own understanding, this is one of several new studies on monarchs from this lab, so perhaps we will see more from them in the future. From a simple search of her lab's website (link here), you can see that the lab's research focus appears to be on butterflies (many species), and how climate change is impacting them. The fact that this lab is physically located in Ottawa, a northern city, may also be the reason why this is their focus (though I'm just speculating). What I mean is, climate change is expected to have the greatest impacts on these northern regions. For monarchs, this is certainly true. A number of years ago, a study was (quietly) published that described how climate change is going to shift the distribution of monarchs northward into Canada, because the summer temperatures in Canada will become more and more conducive for milkweed growth. I had blogged about that paper too (link here). And recall also the 2019 study by Flockhart et al who described how the monarch breeding range in Canada appears to be increasing (blog on that study is here).

Anyway, this new study was published earlier this fall, and the last I checked, it was free to download (link here). From my read of it, the study was designed to answer some questions about how climate change will impact monarchs during the breeding season, i.e. during the caterpillar stage. They specifically wanted to know if the timing of the main summer monarch larval period (in the Ottawa area) matched the timing of when the milkweed was at its best. In this area, common milkweed is the dominant type, and apparently it is very common there (see what I did there?). They reasoned that climate change is likely to lead to changes in WHEN the milkweed peaks each summer, and, perhaps this may lead to a "mismatch" between the needs of the caterpillars and the milkweed growth. They pointed out that we already know that monarch females seem to prefer to lay their eggs on the tender young milkweed, but, we don't know if climate change is going to shift the timing of when the tender plants are available. All of this is indeed a good rationale. And, they pointed out that there is already evidence that the timing of both monarch arrival in the spring, plus milkweed growth, is starting to become wonky.

To conduct this project, they performed some observational field surveys of milkweeds around the Ottawa area, in the summer of 2019. I'll put a screenshot of their map below, which shows this location. They describe how they visited milkweed patches (red dots in the right panel), measured the sizes of the plants, recorded their height and growth stage (if they were young, or in flower, or senescing) and then checked for monarch eggs and larvae. Apparently, they did this for 2800 milkweed plants. What they wanted to find out was when was the peak time of the summer for milkweed, an when does it start to senesce. And, they wanted to know if the timing of the monarch use of milkweed coincides with this milkweed peak.

Another part of the study was a field experiment, which was conducted over two different summers. The authors wanted to know when the milkweed quality peaks, and when it begins to decline throughout the summer season, from the perspective of the monarchs. In other words, they wanted to know if caterpillars actually do better on the tender young milkweed plants, which we know that momma monarch likes to lay her eggs on. For this part, the authors obtained some monarch eggs (by having mated monarchs lay eggs in a cage), and then placed these eggs on milkweeds of different sizes in a large open field. Apparently, this was wild-growing milkweed in this field, and the researchers had manipulated their growth rates earlier using mowers, to create patches of differently-sized plants. Some of the milkweed was small and tender, and some was tall and older. Then they tracked how well the caterpillars grew, how many survived, etc, on these different plants.

I'll get to the results next.

From their milkweed surveys, they reported that there was a modest peak in the timing of when eggs and caterpillars were found on the wild milkweed, which was in the last two weeks of July. They also found that there were more eggs on the smallest, youngest milkweed stalks, as they had expected. Next, they found that the milkweed plants did show an expected progression of growth over the season, BUT they found that there were small plants available throughout the season too. And, the milkweed apparently didn't ever get to be too old and tough before the peak usage by monarch caterpillars. So in other words, even if there will be a seasonal shift in milkweed growth in the future, it doesn't seem like monarch caterpillars will be affected, because there was enough variety of milkweeds present all summer.

The other interesting finding here was from the field experiment. After recording the survival and growth rates of caterpillars on differently-sized milkweed plants, they concluded that "Milkweed height did not have an effect on any of the larval performance estimates in either year." This includes development time, growth rate and survival. I'll put in a screenshot of one of their figures which basically shows this.

So in other words, the monarch caterpillars didn't care what stage the milkweed was at - they did just fine on the small milkweeds and on the large milkweeds. This is also in line with the prior finding, meaning even if there was a seasonal shift in milkweed growth in the future, monarch caterpillars should be fine - apparently, they can do well on milkweed of any size, at least with this common milkweed.

Near the end of the paper, the authors pointed out something else of interest, which I will also highlight. We know from a few prior studies (and most monarch people know this too) that momma monarch likes to lay her eggs on the tender young milkweeds. Their own field surveys showed this too. But, apparently, this is not because the monarch caterpillars will grow better on those plants! This conclusion is reminiscent of a prior study (which I also blogged about) from the Agrawal lab, in which the researchers concluded that momma monarch doesn't really know what she is doing! Recall from that study, that momma monarch will lay just as many eggs on plants that are not as good for her offspring, as she does on the "good" plants.

Maybe momma monarch simply prefers the smallest and youngest plants in general because she knows that this will ensure her offspring will have enough milkweed to last them throughout their development. Who knows?

Anyway, the results from this study make it pretty clear that if climate change is going to cause shifts in when the milkweed becomes "ready" for monarchs in the summer (or even if it already is causing a shift), that monarchs should be fine. Apparently, they won't care!

For the long-time readers here, you'll notice that once again, the take-home message of this study, along with many others that I have been highlighting in this blog, is that monarchs appear to be highly-adaptable creatures, capable of overcoming any changes and challenges we humans are throwing at them. To be sure, climate change is clearly happening, and it likely will affect the monarchs. But, it will probably not spell their doom.

OK, that's about it for now. Thanks for reading.


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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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