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  • Andy Davis

New study: female monarchs don't know when to quit

Hello everyone,

That's right, a very cool new study has just been published and it is one that I'll be happy to dive into today. Full disclosure, this was a project conducted by a Masters students from here at the University of Georgia, and so of course I'm biased in thinking it's cool. That said, the student in question (Caroline Aikins) really was a fantastic student and did a bang-up job with her project, and, the fact that it was just published in a peer-reviewed journal is icing on the cake.

This project was all about mamma monarch, and her egg-laying behavior, which is something that still has many unanswered questions. One in particular is of interest, do female monarchs have the capacity to judge if their hostplants will be suitable for their offspring? Think about this for a second, we of course know that monarchs lay eggs on milkweed plants, but how many eggs do they lay on each plant? Do they lay more eggs on good plants, and less on bad ones? Do they have the ability to judge the quality of the plant first, before laying? And importantly, do they know if there will be enough plant material for their offspring, especially if there are other eggs already on the plant, or, if there is visible leaf damage from other caterpillars? In theory, if the females saw that there are already lots of other eggs (or caterpillars) already on a plant, they should not lay very many of their own eggs, or better yet, just move on and find a plant that has no eggs or larvae. In other words, do female monarchs look over the plant first before laying?

The paper that described this project was just published in the Journal of Insect Behavior, and here is a link to the journal version. I'll also put in a link here to the personal copy of the paper from the authors, which should be freely-available to download/view.

Let me see if I can walk us all through the project procedures in a nutshell.

So there were two different experiments for this project. Both of these experiments used collections of monarchs that were reared in the lab on greenhouse-grown swamp milkweed, and both involved putting females into tabletop cages with potted milkweed. FYI, the monarchs were the offspring of wild-caught adults. The first experiment was an investigation into whether females could gauge the quality of a milkweed and use this information to guide how many eggs to lay on it. For this, Caroline created three groups of milkweeds (which were in pots in our lab) by manipulating their "quality". She purposely deprived some of water for a few hours so that they would be wilty. For another group, she punched holes in their leaves (with a hole punch), to simulate caterpillar herbivory (see the picture of the holey milkweed). And the last group was a control, which was nice healthy milkweed plants.

She then put these plants into cages with pregnant females and watched what they did. She tracked how much time the females spent investigating each plant, plus how many eggs they laid on each.

The results of this first experiment were as you might expect - females visited all milkweeds equally, but they generally preferred to lay their eggs on the healthy plants, over both the holey milkweed or the wilty one. The wilty and holey milkweeds did have some eggs, mind you, but there were more, on average, on the healthy plants. So, this finding implies they do have the ability to assess the quality of a milkweed plant before laying eggs, though it is not a strong driver, meaning that even the crappy plants got some eggs.

The second experiment was related, and used a similar setup, but this one asked if females actually judge whether a milkweed is suitable by the number of eggs already on it. She did this by presenting naive females with plants that already had eggs on them, or with plants that had no eggs. This question is really interesting if you think about it. For starters, females of other species are known to assess their offspring nest sites based on the presence of other nests in the area, kind of like following the crowd. Perhaps female monarchs see lots of eggs on a milkweed and then this tells them that plant must be good. On the other hand, if a female lays her eggs on a milkweed that already has lots of eggs, those caterpillars will quickly run out of food. So with this notion, females should actually avoid egg-laden milkweeds. Either way, Caroline expected to see some kind of evidence that females would evaluate the presence of eggs.

They didn't.

In fact, Caroline saw no evidence that females were basing their own egg-laying on the presence or absence of other eggs. Females laid just as many eggs on the egg-laden plants as they did on the egg-free plants. So, Caroline figured they either did not see the eggs present (like they could not actually perceive them), or if they could, they didn't care.

Either way, this finding means that female monarchs apparently are ok with "egg-dumping" onto milkweeds, which is actually a terrible reproductive choice. I suspect that many readers here have seen evidence of egg-dumped plants, where there are far too many eggs for the plant to support. And in fact this is exactly what happens - the caterpillars quickly run out of food. So it is actually a terrible strategy.

This finding is actually reminiscent of a prior study that I've blogged about (link here), which was from the Agrawal lab - where the researchers found that female monarchs don't seem to know to lay more eggs on the plants that are best-suited for offspring growth. So combined, these two studies make it sound like female monarchs are really just instinctually programmed to find a milkweed and lay eggs, and there is little actual "choosing," except for ensuring the plant leaves are (more or less) sound.

Finally, if you read the published paper, you can see that they added a small bit of discussion at the end on the topic of the OE parasite, and how these new findings fit in with that. Specifically, they mention that the fact that females don't evaluate plants based on their egg loads means they will also not know to avoid potential OE risks. Or in other words, if there is lots of OE in an area, then a plant with lots of eggs on it will likely be covered with OE spores. If females don't avoid that, then their offspring will be in trouble.

OK that about covers it for this paper. Keep in mind that I didn't get into much detail here, so if you want to know more about a specific bit, just read the actual paper itself.

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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