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  • Andy Davis

Findings from new study call into question those from previous studies of monarch habitat selection

Hello everyone,

There was a very interesting new study just published on monarchs and their habitat - as you can see from my blog title this was a study that has some serious significance to all prior work regarding monarch habitat selection. I'll be telling you about this study today. So strap in and get ready to hear about some monarch science! (warning - there are no pretty pictures of butterflies here)

OK, so this paper just came out in the insect journal, Environmental Entomology, and it was authored by Andrew Myers, Christie A Bahlai, and Douglas A Landis. I don't know these folks but from a basic google search it looks like Myers is a phd student in the lab of Dr. Landis at Michigan State University. Here is a link to the study.

Let me say at the start that I was impressed by the amount of work that went into this study. It seems that this project was the result of two years of field work, which included some in-depth and labor-intensive experiments. Kudos to the authors for putting this all together. To be fair, I also have a few critiques, as usual, but nothing major. I'll get to those later.

The main goal of this study was to determine if monarchs show a preference for specific habitats when they lay their eggs. In addition, the authors wanted to know if the level of predation differed across habitats. The habitats included prairie fields, corn fields, soybean fields, and a bare ground "habitat". This last one was a kind of control. Given these habitats, it looks like they were testing if monarchs had a preference for "agricultural" areas or natural areas. This is indeed a great question. There has been a lot of talk and arguing as of late about the role of agriculture in the long-term declines of the overwintering colonies. At the heart of this argument is this idea that the monarchs have lost their "preferred" habitat in the Midwest, which was milkweed growing in the middle of corn and soy fields. Years ago, researchers (I believe, John Pleasants was the main one) had apparently found evidence that milkweeds growing in between row crops appeared to have greater egg and larval densities than in natural areas. Everyone took this to mean that monarchs "preferred" these agricultural habitats for some reason. But the authors of this new paper pointed out that it might be that egg and larval predation is simply lower in the row crop habitats, which would give the impression that egg densities are higher. This was the rationale in the current study for testing the predation levels across these different habitats. Again, kudos to these researchers - this is something that has not been done before.

The authors conducted some clever experiments to test both egg-laying preference and predation, at different time points. Instead of wandering around looking for naturally-growing milkweeds in these different habitats, they placed their own potted milkweed plants (common milkweed) in the 4 habitats and waited for natural monarchs to come lay eggs on them. This approach had a couple of advantages - first, it ensured that all plants were equal in terms of height, nutrition, etc., because they grew these plants in their greenhouse. And second, it allowed the researchers to arrange them the same way across all habitats - i.e. in clusters that resembled a typical patch of milkweed. So in short, the milkweed plants were completely identical across all habitats. This was great, because it eliminated this variable completely so that any differences in oviposition seen could then be attributed to the habitat alone. Apparently, there was some idea that the "preference" for agricultural milkweeds could have been due to the fact that those milkweeds were growing in soil that had been fertilized...

It looks like a total of 144 common milkweed plants were used in the experiments. After the plants were established, the authors (or Myers, at least) monitored each one on a daily basis to see how many eggs were laid. And when he did see an egg he counted it, but then removed it from the plant - this was so that the plant remained fresh (no larval damage), and to avoid double-counting eggs. This was smart from a logistical standpoint, although I'm not sure what he did with all those eggs after that!

Importantly, it also looks like the authors recorded if they saw any adult monarchs at their "milkweed patches" too. This will come up later.

This part of the experiment was run 3 different times during the summer of 2016, and then they repeated the whole thing the next summer. So in all, there was very good replication in this experiment (that means we should believe the results!). Below is a screenshot of a table from the paper that reports how many eggs (and adults) they saw over each iteration of the experiment.

Note from this table that the number of eggs laid over each season differed, as did the pattern of egg-laying over the season. This difference is really important and will come up next.

Now let's get to the habitat results. They used the egg density in each site as their index of monarch "habitat selection", and so they calculated the average egg density in each of the four habitats. Below are graphs showing what they found. Look carefully and especially compare the results from 2016 to 2017.

Do you see what I see in these graphs? I know, this is crazy. According to these data (which are totally believable), it seems that wild, naturally-occurring, female monarchs showed a clear preference (for oviposition) for milkweed in cornfields in one year, but not the next. In the second year, they had a preference for milkweed in prairie habitat! What were the monarchs thinking? Or were they thinking at all? Hmm...

One other thing of note from this was the authors reported that the number of adult monarchs seen visiting their sites was not at all correlated with the density of eggs at those sites. In other words, the number of adult monarchs you see in any given habitat is different than the number of eggs you see. One cannot be used in lieu of the other to gauge monarch abundance.

OK so that's the important bits from the egg-laying experiments. Next are the experiments that were done to try to understand how predation varies across habitats. Following each iteration of the oviposition experiment, the authors conducted a predation trial the very next week. Here, they took monarch eggs that had been laid on plants in their lab, and glued them onto the milkweeds at each of their field "milkweed clusters". Then they returned at specified times and assessed if the egg was still there. I guess they must have had a way to keep track of exactly where on the plants they glued the eggs!

Whoops, I almost forgot - there is actually a youtube video describing this project that was produced by the university - I'll show the video below. It's worth a look because it gives one a visual image of what the sites looked like. I wished they had included some images like these in the actual paper!

Done watching? OK, back to the predation trials. Now here's a crazy part - this student apparently wanted to know when exactly the eggs were predated, and he actually camped out at the milkweed sites so that he could check the plants throughout the night! I know this because it was written in a nifty little presser that went with the video above - Kudos to this student for taking this extra step to track the timing of predation. This is something that most researchers don't do - normally they just come back a few days later and see if the eggs are gone. It looks like the authors monitored the eggs for 72 hours after they were glued, and they knew exactly when the eggs went missing within those 72 hours.

It also looks like they ran three iterations of this egg survival experiment each summer (2016 and 2017), similar to the oviposition trials. So, recall that they were specifically looking to see if predation levels differed across the four habitat types. Below is a graph from their study (I modified it a bit to make it easier to read) that shows the survival of eggs across the four habitats. Each line is a different habitat, and they show the proportion of eggs surviving, averaged across all milkweeds in these habitats.

Once again, there were some crazy differences between the two years! The highest survival in the first year was on plants on bare ground! The prairie milkweeds had abysmal survival that year. But then the very next year this was not the case! In 2017, the eggs in soybean fields took a beating. One consistency was that survival appeared high in cornfield habitats in both years. The authors made note of that specifically.

The other thing to note from these graphs is the fact that apparently, most egg predation seems to happen within 24 hours! In fact, the authors noted that a lot seemed to happen at night.

As an aside - I don't want people to get up in arms about the fact that these eggs were predated. Predation happens. Monarchs are part of the food chain of a healthy ecosystem. Get over it. And don't squish every predator you see on your milkweeds. They have just as much right to be there as monarchs do.

OK, so I think I've covered the meat of this study at this point, and now I'll discuss some of the implications of this work, based on my impressions when reading it.

I think one of the most important findings of both the oviposition trials and the survival experiments, was that they showed different results each year. This is huge, and unfortunately, really really bad for prior work done in this area. Think about it for a second - if monarchs appear to show a preference for one habitat in one year, but then not the next, what can we conclude about their habitat "preference" Do they even have one, or is it completely whimsical? I noticed the authors didn't really dwell on this much in the paper, and they appeared to make some offhand comments about how the weather could have been different each year (I think it was), and that may have altered the habitat "preference". I think this is a stretch. If the weather really does affect habitat preference (or survival) that much, then we shouldn't even bother running these experiments, and we should just be checking the weather forecasts!

I would argue that these results suggest that monarchs don't really have a preferred habitat for egg-laying. If their preference changes from year to year, even when everything about the habitat or milkweed is the same, then how can they have a real, honest-to-goodness, overall preference? In fact, I might even say that the title of this paper is misleading - "Habitat Type Influences Danaus plexippus (Lepidoptera: Nymphalidae) Oviposition and Egg Survival on Asclepias syriaca." To be more acurrate, it should read, "Habitat Type Inconsistently Influences Danaus plexippus (Lepidoptera: Nymphalidae) Oviposition and Egg Survival on Asclepias syriaca."

But what really strikes me is that this study has some serious implications for any prior (or future) studies like it. If for example, there was a prior study that appeared to show that monarchs prefer to lay eggs in cornfields (there was such a study), can we believe those results? Would the monarchs show the same preference the very next year? What about the next year? how many years of data do we need? In fact, if someone wanted to do a future study testing if female monarchs show habitat preferences, it seems that there would need to be many, many years of data collection, since there is such natural variation between years!

Another thing from this work that has some serious implications for prior research is the bit about the timing of egg predation. The fact that it occurs so quickly in nature means that most routine (weekly) monitoring of milkweeds, such as is done in the Monarch Larva Monitoring Project, would probably miss a lot of eggs. In other words, if you only check your milkweeds once a week, then in theory a female could lay eggs and they could vanish in between your checks. you would end up thinking there had been no activity when in fact there was and you missed it!

Boy, this project has done some damage to the monarch research world!

OK, I guess that about covers the paper, in terms of the findings and their implications. Thanks for reading, and feel free to share this post!


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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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