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  • Andy Davis

Another damning research paper comes out on tropical milkweed - have you ripped yours out yet?

Hi Folks,

You may not know about it, but last month, there was a very quiet scientific study that slipped into the world of monarch science, and it shows, yet again, how dangerous it is to plant tropical milkweed.

The paper, written by soon-to-be Doctor Dara Satterfield, as well as my partner in crime, Dr. Sonia Altizer, recently came out in the journal Integrative and Comparative Biology. As I always do, I'll put a link to the paper here, but it may require a subscription to download. It was titled, " Migratory monarchs wintering in California experience low infection risk compared to monarchs breeding year-round on non-native milkweed". The full list of authors on the study included Dara Satterfield, Francis Villablanca, John Maerz, and Sonia Altizer.

(This is just a pretty picture of a larva munching on a tropical milkweed plant that Sonia took)

Let me see if I can see the stage for this new paper. I'll assume my readers already know about OE in monarchs - if not, you can find out more information about it at Sonia's parasite page -

So a lot of previous work has shown - quite convincingly - that long-distance migration weeds out most of the sick monarchs (i.e. those infected with OE) from the migratory journey, which tends to lower overall infection levels in migratory populations (this is a good thing). As a testament to this, in the eastern monarch population that has the longest migration of all monarch populations, infection levels have traditionally been less than 5%. But in populations that don't migrate at all, infections tend to be high, because there is no 'weeding out' process. For example, in South Florida there is a non-migratory population with super-high OE levels (70-90% I think). You might recall the previous paper by Dara and Sonia, from 2015 - link here - which looked into the OE infection levels of those pockets of monarch that winter and breed in the southern US rather than migrate to Mexico. As expected, Dara found that most of these wintering monarchs were highly infected - 50-100% of the monarchs were sick with OE. Importantly, all of the places where monarchs were wintering in the US were places with tropical milkweed planted, so the connection was made between tropical milkweed and high OE levels. This plant tends not to die back in the winter in places where it doesn't freeze, which allows them to build up OE spores on their leaves. Since the climate is warm, and there is year-round milkweed available, this makes monarchs want to stay and breed throughout the winter, instead of migrating to Mexico (or coastal California in the west). So because the monarchs lose their migratory urge, and also because of the high build-up of OE spores on the plants, this creates local hotspots of highly-infected monarchs - all because of tropical milkweed.

You may also recall all of the hoopla that ensued over these findings. Up until that time, people were joyously planting tropical milkweed, thinking they were 'helping the monarchs,' when in fact it turns out they were actually hurting them. I remember there was a flurry of press with crazy headlines too - the most high-profile one was "Plan to save monarch butterflies backfires" in the journal, Science. I saw this one article getting posted and re-posted on facebook, and it is still being posted even now. I recall Dara getting some angry phone calls from people who read about the study, or who just read those headlines. I believe she was very diplomatic when she spoke with people - and I know she still is when she talks with the gardeners. This is certainly a very touchy subject with people - everyone just really, really wants to help monarchs by planting milkweed, and they sure as hell don't want any goddamn scientists telling them they're doing it wrong!

So the gist of this new paper is very similar to the 2015 study, except that it was based entirely on western monarchs, i.e. monarchs in California. While the western population of monarchs doesn't get as much press and exposure as does the famous eastern one, monarchs west of the Rockies do undertake an annual migration to and from wintering sites in California. It's a shorter migration for sure, and there is much we don't know about it, compared to the eastern one, but they migrate, nonetheless. We also know that western monarchs are prone to infections with OE. Historically, the levels had been much higher in the west, compared to the east - something like 30% of western monarchs used to be infected in any given collection of wintering monarchs, although lately the levels in the west have been way down - to less than 10%. So there should still be a 'weeding out' process during the western migration, similar to the east. We also know that more and more gardeners in California are planting tropical milkweed, and just like in the Gulf region, this creates local populations of year-round resident monarchs (non-migratory). So what does this mean for OE infection levels at these sites? That's essentially what they tested in this new paper.

The way they did this was to gather 'tape samples' of monarchs from specific sites in California (see Sonia's OE webpage for more info on the tape sampling method - The sampling was done by the authors themselves, or from volunteers in the MonarchHealth and MonarchAlert programs. They specifically sampled monarchs at roosting sites during the winter - i.e. at those places along the California coast where monarchs cluster in the trees (these are migratory monarchs), and they also sampled monarchs at places where monarchs hang around all year - i.e. at sites with tropical milkweed (these would be resident monarchs). They also threw in a few sites where monarchs exhibit seasonal breeding. I posted a screenshot of their figure from the study below. In the end they sampled a total of 2135 migrant monarchs and 1290 resident monarchs, and I believe all of the sampling was done from 2013 to 2016. All of the tape samples were examined back in the lab at the University of Georgia.

So what did they find? You guessed it - the resident monarchs that hang around tropical milkweed all year all had very high infection levels compared to the migrants. About 3 out of 4 year-round residents were infected (74%), compared to only 8% infection rates in migrants (that's like 1 in 10 monarchs that are infected). Statistically speaking, this was highly significant. This result is similar to what they found previously in the Gulf region, but in fact the California results are even stronger. So this new paper leaves little room for doubt - if you plant tropical milkweed where it can grow year-round, you will have highly-infected monarchs.

Here's another very important result of the study that folks should know about too. There were several (not many) tropical milkweed sites in the study where the homeowner had been cutting the milkweed back each fall for a few years prior to the study. This is a practice that was recommended to help keep OE spores from building up on the leaves. This recommendation was created to allow people to keep planting tropical, but to be able to feel good about it. But (big but here), when Dara looked at the monarch samples from these sites she found they were still highly infected - 72%! So cutting back the tropical milkweed did little to reduce the high OE infection levels at these sites! It seems that the problem is that there are just too many places with tropical milkweed, so that when a gardener in one neighborhood cuts back their tropical milkweed for the winter, the monarchs just go to the next patch down the block where the milkweed is not cut back. In other words, for the cutting-back thing to work, everyone has to do it.

So what should people do? Well, there are some big differences in opinions over this issue among the scientists I know. I know Lincoln Brower thinks people should just rip their tropical milkweed out of the ground. Sonia and Dara have a slightly softer, more diplomatic, message - that folks 'try not to plant it, and replace tropical with natives whenever possible'. I think I'm with Lincoln here. In fact, I'd say people should rip it out, and burn it for good measure. This plant just has too many strikes against it to justify keeping it around, and I actually know that there is more research on the way that will add more fuel to this fire (get it?).

I'm going to add a couple of helpful links here, for folks who want to read or hear more about this ongoing issue, and decide for themselves what to do. One is the recent webinar Dara and Sonia (plus another grad student in the lab who is studying this - Ania Majewska) put on about this issue -

The other is the position statement about tropical milkweed that was written by Sonia and Karen Obserhauser, which is on the MJV website -

I'm going to end with this one final note - if you're reading this now and you currently have tropical milkweed in your garden, you have some serious thinking to do.



Direct link to this blog article:

The science of monarch butterflies

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

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