(Updated Feb 21, 2019)
Hello everyone,
There was a very interesting study published earlier this month, and today I'm going to tell you about it!
As you can see from the title of this blog, the study was about milkweeds and monarchs - two topics that go hand in hand. Specifically, this study examined which milkweeds are chosen by female monarchs to lay their eggs on. I'm sure this is a topic of great interest to a lot of butterfly gardeners. In fact, I know that a lot of people reading this probably already have a good idea which of the milkweeds in their gardens are preferred by female monarchs, or in other words, which milkweeds get the most eggs. Well, prepare to have your mind blown, because the results of this new study says that this isn't really that important!
The paper was just released online in the journal, Oikos, which is a fairly prestigious ecology journal. It was authored by Patricia Jones and Anurag Agrawal, from Cornell University, and it was titled, "Beyond preference and performance: host plant selection by monarch butterflies, Danaus plexippus." Here is a link, but I'm afraid it's not freely downloadable without a subscription to the journal - https://onlinelibrary.wiley.com/doi/pdf/10.1111/oik.06001.
But wait! - Shortly after this blog post came out, Dr. Agrawal kindly provided a freely-available copy of the entire paper - http://www.eeb.cornell.edu/agrawal/blog/wp-content/uploads/2019/02/Jones_et_al-2019-Oikos.pdf
The title of the paper is very poignant, because unlike most other egg-laying-preference studies out there (there are indeed many), this study examined if the larval performance on different milkweeds matches up with the mother monarch's egg-laying preference. In other words, if a female lays a lot of eggs on one type of milkweed, is it because she knows that is the best one for her offspring? A very interesting question, indeed! Think about this - that milkweed in your backyard that tends to get the most eggs - is that really the "best" milkweed?
The authors tackled this by conducting a series of controlled experiments. They used 4 different types of milkweed for these experiments (all grown in an enclosure in pots): Asclepias perrenis (aquatic milkweed), tropical milkweed (A. currasavica), and two varieties of swamp milkweed, Ascepias incarnata incarnata, and Ascepias incarnata pulchra (I'll just call this one pulchra). Apparently these two are distinct subspecies of swamp milkweed. These four milkweeds all have different characteristics, including leaf shape, thickness, trichome density (those hairs on the leaves and stems), and latex (that milky sap). The plants also differ in cardenolide content.
The first experiment was the female choice test. They put female monarchs individually in mesh cages with four plants, representing the 4 milkweed types, and allowed her to lay eggs for one hour. Then, they removed the plants and counted the eggs to see which plant type she "preferred". They did this for 12 different females.
Next, they conducted a separate experiment that examined larval performance on the different milkweeds. When those eggs from the first experiment hatched, they placed single caterpillars onto clean, potted milkweeds, and did this for each of the four plant types. In total they had 29 caterpillars per host plant species, so by my math this was 116 plants, each with a single caterpillar on it! It sounds like this was done outside in a field enclosure setup. They allowed the caterpillars to eat for a week, then brought them into their lab, where they put them in containers and fed them cuttings of the milkweeds, so that they could keep track of them (larvae like to crawl around when they get big). Importantly, the authors weighed the caterpillars on day 9, to see how big they had gotten.
There were also some nifty experiments done where they examined how much latex each of the plant types produces, and also how dense their trichome hairs were. These were done to try to sort out some of the results from the larval feeding experiment. I'll gloss over these a bit here for the sake of brevity.
So here is what they found:
Below is a graph showing how many eggs were laid on average by females on the four milkweed plants. The y axis shows the average number of eggs laid (in one hour) across the four females.
This graph shows that the females preferred to lay their eggs on the pulchra subspecies of swamp milkweed. Females laid about 9 eggs per hour on this plant compared to 5 eggs per hour on the others. So this result is pretty clear - the females had a "preference" for pulchra.
Now, here is the part that is very interesting - the next graph shows the results of the larval feeding experiment, or essentially, how big the caterpillars were on day 9 of their development.
This graph shows that the pulchra milkweed was actually the worst of the bunch, in terms of larval performance. The caterpillars on this plant were way behind the larvae that were on the other plants! And remember, they were all the same age (9 days old)!
The other thing to note was that the researchers also kept track of how many larvae survived to pupation on the different milkweeds, and this too was interesting. Monarch larvae had the lowest survival on pulchra (22% survival). Compare this to 39% for tropical milkweed, 43% for the other swamp milkweed, and 42% for aquatic milkweed. Recall that the larvae had been allowed to feed on potted plants placed outdoors in some kind of netted enclosure, so there likely weren't any predators eating the larvae. This means the survival results here reflects how many larvae survived the plant defenses - the latex and trichomes (see below).
If you put all this together, you can begin to see that the female monarchs were actually choosing to lay eggs on the one plant that is the worst for their offspring! Wait, what the...?
Recall that the researchers looked into the latex and trichomes of the different milkweeds, to see if that could explain the survival and growth results. In short, it did - apparently, pulchra exuded over 20% more latex than any of the other plant species. Plus, pulchra had 250% more trichomes on its leaves than the other 3 plant types! Having more latex and trichomes are both significant impediments to monarch caterpillar feeding.
So that all brings us to the big question - why? Why would a female monarch specifically choose to lay more eggs on a plant that is apparently perfectly designed to fend off caterpillars? Shouldn't she want to lay all of her eggs on the plants that will lead to the highest larval survival? From my read of the paper, it doesn't seem like the authors had a solid explanation to this question. That's ok, this is the way science works - each study is supposed to lead to further questions that need to be answered. I did see that the authors hypothesized that the female choice for pulchra could be related to the high trichome density of that plant. Even though the larval survival was low on this plant type, the trichomes perhaps act to shield the baby caterpillars from predators, they suggested. They also suggested that perhaps there is no explanation for the disconnect between female choice and larval performance, and maybe the females don't really know what they're doing (my words, not theirs).
Let me riff on this last part for a second here. I wonder if there is something else going on? Let's assume the female monarchs do know what they're doing, and that they actually chose the hardest, toughest, milkweed for a reason. Maybe this was to give their larvae some toughness, so to speak. Even though very few larvae survive on this plant, maybe those survivors are stronger because of this? Just a thought. The authors did actually examine the mass of the pupae from these four plant types, and there was no statistical difference among them. So this might blow my theory. But, I wonder if the authors have examined anything else about those monarchs that survived, like their wing toughness, or the shade of their orange pigmentation (redder shades are better), or even their flight performance? I could go on with other possibilities, but you get my point.
OK, so to sum up here, this study showed that if you give a female monarch a choice between four milkweed species that all vary in characteristics, that she will choose to lay the bulk of her eggs on the one milkweed that is the worst for her offspring! In other words, the number of eggs laid by female monarchs on a particular milkweed (her "preference"), does not mean that that is the "best" milkweed. In fact, it seems that the opposite is true - the best milkweed is the one with the fewest eggs! Go figure.
That's about it for now. Thanks for reading.
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