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  • Andy Davis

Very interesting study on monarch predators recently published

Greetings all,

I just saw a brand new study published late last month that I thought was very cool. It just came out in the Journal of Insect Conservation, and I thought I'd share my thoughts on it for this week's blog entry. From reading the author info, it was spearheaded by Shaun McCoshum, who was a student of Dr. Kristen Baum at Oklahoma State (now a postdoc at Cornell). Other authors were from Karen Oberhauser's lab in Minnesota. The title was "Species distribution models for natural enemies of monarch butterfly (Danaus plexippus) larvae and pupae: distribution patterns and implications for conservation." Here is a link to the published paper.

So, before I begin let me say that I know the issue of predators of monarchs is a sensitive one, so not everyone may share my feeling that this was a cool study. I know there are those folks out there in the monarch world who want nothing more than to squash every predator and enemy of monarchs. If you are one of these, please keep in mind that monarchs are part of the food chain, plain and simple. They have always been part of the food chain. Monarch females lay hundreds of eggs because their larvae are part of the food chain. The bottom line is they have lots of natural enemies, and there is nothing wrong with that. Now, back to the science.

So the goal of this new study was to map the distribution of all known monarch predators, including tachinid flies and wasps, fire ants, and a few other (invertebrate) critters. That is, all of the beasties that prey on the larval and pupal stages of monarchs. There is actually a surprising number of species that are known to prey on monarch caterpillars, and they listed a lot of them in the paper. The researchers did not include any of the adult predators, like preying mantis, or those birds in Mexico that eat the overwintering monarchs (orioles?). Nor did they include the protozoan OE parasite that affects monarchs. The list of critters they did look into included tachinid flies, a Chalcicoid wasp (a parasitoid), wheelbugs, ladybugs, little black ants, spiny soldier bugs, Polistes wasps, and fire ants.

To start with, the authors got their hands on a number of large-scale datasets where citizen-scientists have been reporting when and where they see these critters. Some were from the monarch larva monitoring project, others were from museum records, and other databases they could access. It looks like they had data for 6 species of predators. Then they input these data points into a special computer program (MaxEnt, for the science folks) that maps the predicted range for these species, based on their specific environmental requirements. As I understand it, the program looks at where the critters can or do live already, then predicts where else they could survive based on the known climatic conditions across the continent.

I won't get into the rest of the nuts and bolts of the methods, and I'll just skip to the results here. In the results of this study, the authors provided detailed maps of each predator's predicted range, then one large combined map for all (or most) of the monarch predators. I think the combined map is most interesting, and I've copied it below.

As you can see, this map shows that the majority of monarch predators and parasitoids are concentrated in the southeastern United States, with another small pocket in the Pacific Northwest. Who knew there was so much overlap in predator ranges?

So what does this map mean? Good question. In the paper I saw that the authors did not have a clear answer for this either. They briefly mentioned that this might affect conservation efforts. I guess that would be true if you are trying to create habitat for monarchs - in this case, if you live in the southeast, you would probably need to plan for the fact that a lot of immature monarchs on your backyard milkweeds would be killed by these predators. Meanwhile in the Upper Midwest (and Canada), this would not be the case, or at least, the predation pressure would be less. In this regard, I suppose this information could be useful for habitat planning purposes.

Other than that one small drawback, I feel like this was a very cool paper, and one that adds a new element to our knowledge about monarch biology. What makes it so cool is that before now, we didn't really know the range of these various critters, and more importantly, that they have this high degree of spatial overlap (i.e. the southeast). Imagine the competition that must occur (amongst the predators and parasitoids, that is) for monarch larvae in that region! And from the monarch larva perspective, this begs the question, do southeastern monarch larvae need to have enhanced immune systems or predator avoidance behaviors than their Midwestern cousins? Perhaps this question could be examined in the future, now that this study has laid the foundation, so to speak.

The other reason this paper is significant is that it shines a much-needed spotlight on the large (but normal) impact of predation to the monarch. Some folks might have the idea that monarchs are protected against predation because they ingest toxins from their milkweed hostplants, which in turn makes monarchs toxic to predators. This is only partly true. Yes, the monarchs are toxic, but only to vertebrate predators. As this paper shows, there is a whole other world of invertebrate critters that can eat (or parasitize) monarchs, and who aren't affected by the toxins. To me, this further emphasizes that monarchs are part of the food chain of nature.

Let me end by reiterating my view of monarch predators, in case it wasn't clear from this article already: whether they be pupal parasitoids, stinkbugs, or preying mantis - they are not the enemy, and please don't kill them. They have every right to exist as do monarchs. Just because the monarch looks pretty does not give it more value than the stinkbug, or the parasitoid wasp, or whatever else makes its living by eating monarchs.

ps - the exception to this may be fire ants, which are invasive and just plain nasty.

That's all for this week!


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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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