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  • Andy Davis

Why do we think the American Midwest produces lots of monarchs? A look at an influential study

These days, if you ask anyone what's happening with monarchs they will tell you that monarchs are disappearing from the American Midwest because of the increasing use of roundup-ready crops and/or higher agro-chemical usage in this farm-heavy region. And, this is devastating to monarchs because the Midwest is the breeding center for the entire population. That's the news story that has been told and re-told in hundreds of newspapers and blog posts, etc., for the past few years. But today I'd like to hone in on this story a bit more to trace it back to its origins, and discuss some implicit assumptions that we've made to get to this point, that may or may not be valid.

At the core of this narrative is the idea that there are lots and lots of monarchs in the Midwest (more than anywhere else), because they use the abundant milkweed found within corn and soybean fields, or at least they used to, before the farmers began eradicating it. Now, just to be clear, it is well-established that the milkweed in these fields has diminshed substantially - there is scientific research backing this up, and this is not in dispute. However, the idea that the American Midwest produces MOST of the eastern monarchs is an notion that seems to have originated from a key paper that came out in 1998, and there are some things about this paper that should be discussed.

The paper was by Len Waasenaar and Keith Hobson, and it was in a very big journal - PNAS, for the science nerds reading this. Essentially, the scientists collected monarchs from the Mexican overwintering sites in one winter, and did some very sophisticated chemical analyses on them which allowed them to predict which region of the continent they came from. This approach (using stable isotopes to infer natal origins) is a method these scientists basically pioneered, so they are the world experts, and therefore these data can be considered really good. The entire study boiled down to one key map, which I copied below. This map shows what region of the continent those overwintering monarchs came from, which at the time (late 90s) was in the heart of the American Midwest. In other words, 50% of the overwintering monarchs came from the Midwest.

At the time, this map was (and still is to some people), a key piece of information that suggests that most monarchs in the eastern population are produced in the agriculturally-intense region in the American Midwest. Since this region is comprised of a lot of agricultural land, most people assumed that this map links monarchs with farm fields. But does it really show that? A very careful read of the 1998 paper is revealing. Nowhere in the paper does it specify WHERE in the Midwest the monarchs came from, just that they came from that region of the country. So that was assumption #1 about that paper.

A second assumption we (scientists and the public) all made at the time was that the size of the overwintering colony was a general reflection of the breeding population in the previous summer. The recent study by Leslie Ries and colleagues showed this is not neccessarily true (see previous blog post) - the overwintering numbers appear to be declining while the numbers of breeding adults does not seem to be. How can this be, given the clear declines in agricultural milkweeds? Here's my current thinking on this - and people are free to disagree with it - we all assumed the 50% range in the map above meant that this region PRODUCED 50% of the eastern monarch population, but in retrospect, maybe it means that monarchs from the midwest were simply more likely to reach Mexico in the fall migration. See the difference? If you think of the Mexico colony as the remnant survivors of a very long journey that few survive (which it is), those butterflies that have the easiest travel route would logically make up a larger proportion of the survivors. If you consider the position of the Midwest in the flyway, monarchs that are born there at the end of the summer would have a direct southern flight southward with no major water crossings on their way to the overwintering site. Meanwhile, monarchs in the northeast US have to travel about a third more distance, and would probably experience higher mortality along the way (and ultimately make up a small proportion of the overwintering colony.

So that's the big question I'm raising today - does this map above indicate that 50% of all eastern monarchs are produced in the Midwest, or does it show that the midwestern monarchs have an easier migration route (which means they make up a larger proportion of the overwintering colony)?

It may help to look closely at tag recoveries to sort this out a bit. About 14 years ago, Chip Taylor produced the map below that pointed out where tagged monarchs at the overwintering sites came from (based on one or two seasons I think). I pulled this map from one of his newsletters - 2001.

Look closely at the center of the country, where there is a distinct band of tagging sites all pointing straight south. This is basically the central flyway of the monarch migration, and it is clear that those monarchs tagged along this flyway are more likely to reach Mexico (than monarchs tagged in Tennessee, for example). Getting back to the original issue then, you can ask yourself at home, do these flyway areas PRODUCE more monarchs, or do monarchs from these places have HIGHER MIGRATION SUCCESS?

I don't have the answer to this question, but I do know that of these two possible scenarios, there is only one that is being touted.

That's all for now. If this blog post made you think critically about what we think we know about monarchs... mission accomplished.


The science of monarch butterflies

A blog about monarchs, written by a monarch scientist, for people who love monarchs

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