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  • Andy Davis

Food for thought - should heavily-infected monarch populations be enhanced?

(image above is an infected monarch, from Dickinson County Conservation Board, Iowa - link)

Greetings everyone,

Get ready to put your thinking caps on - there have been some recent happenings in the monarch world that involve the parasite, OE, and these have had me thinking. Today, I'm going to provide more details of these events, and then, let you know of my thoughts on this topic. As you can see from the blog title, this is a post that covers a rather controversial topic...

Before I begin, I hope all readers know what OE is by now. I've covered the OE parasite that infects monarchs in many previous blogs, and, there are lots of online resources where people can learn more about it. So at this point, there is no excuse if you don't know what I'm talking about here, especially if you are involved with monarchs. One thing that I will emphasize here though, is that all scientific evidence we have thus far suggests that this is a naturally-occurring parasite, that has likely co-evolved with monarchs for thousands of years. This is important to keep in mind going forward.

This month, there was an interesting new study published on OE in monarchs, that didn't get much press, at least here in North America! In the journal, Ecological Entomology, two scientists (Philip J. Lester, Mariana Bulgarella) published a paper describing OE in monarchs in New Zealand! A link to the article is here, and I think this is fully downloadable.

Yes, monarchs in New Zealand - it's not clear exactly when monarchs colonized this island country, or exactly how big this population is, but monarchs are certainly present there now, just as they can now be found in a number of places around the world. And, these are truly monarchs, i.e. Danaus plexippus. In New Zealand, the authors report that the monarchs breed mostly on introduced hostplants, balloon milkweed and tropical milkweed. In prior genetic studies, monarchs from these outlying populations have been studied, and the evidence shows that the North American monarch population seems to be the "original, ancestral" population, and the rest of these global subpopulations appear to be its offshoots. And, this same research shows that the offshoot populations are not as genetically diverse as the ancestral one. This is especially true for the New Zealand population, which appears to have a low genetic diversity (which is typical of small populations). This is another way to say that there is some inbreeding going on...

This new paper reports on a very new, informal citizen science project that was stimulated by some anecdotal observations by NZ citizens a few years ago. People had been reporting seeing deformed monarchs in their yards, and this apparently motivated the researchers to start this program, as a way to investigate the cause of these deformities. They knew from prior research that the OE parasite can cause deformities, so this program seemed like a logical thing to do. Through social media, they asked people to send them tape samples of monarchs throughout the country. Their program provided instructions for "taping" a monarch, which involves pressing scotch tape to the monarch abdomen to get a sample of the OE spores. Also interesting to note, is that this program really coincided with the COVID lockdown in that country, and so maybe this gave everyone something to do!

I'm going to fast-cut to the results of the project here. After examining over 500 tape samples of monarchs, the researchers report that about 60% of the monarchs in New Zealand were heavily infected! They also report that the OE prevalence across the country did not necessarily predict the prevalence of deformities - surprise! They appear to suggest environmental factors as a cause, though I think more research is needed here.

I'll paste the main figure from the paper below, which shows the prevalence of OE and deformities across the country.

OK, now that I've given you the gist of this paper, I want to talk more generally here about the overall infection prevalence in New Zealand, since this is the number that had got me thinking. Sixty percent is a lot - in other words, 3 in 5 monarchs have a heavy parasite infection in that country. Since we don't have any historical records of OE from there, we don't really know if this number is growing or not. There was a small sample of 6 New Zealand monarchs from some years past that showed 3 were infected (50%), though this sample size is just too small to draw meaningful conclusions. Actually, it doesn't really matter if the number is growing or not, because the real question to ask here, is what should be done about it now? Anything? Nothing? This gets to the question at the top of the blog. While the authors of the study didn't specifically mention this, they did imply that people in NZ are actively trying to bolster the monarch population there, either by establishing hostplants, or even captive-rearing. But should they, is the million-dollar question.

I know there are some people who would take a very simplistic view of monarchs and their conservation, and feel that ALL monarchs should be conserved and ALL populations should be enhanced, period. This may sound good in spirit, but, I tend to think that OE changes things. And, in this case, we're talking about a lot of OE.

Actually, there are a number of places in the world where this same question applies, like in southern California, or the state of Florida. In those places, OE prevalence is through the roof right now: 70-90 percent of all adult monarchs are infected in these regions! And, the same thing is happening there, where well-intentioned people are working to enhance these local populations by adding milkweed, establishing butterfly gardens, or releasing reared monarchs. And in these places, it seems to be working - the local monarch abundance in these areas appears to be increasing (I've seen the data). But on the other hand, so too is the spread of OE. There is no actual historical data to point to here, but I can tell you that in Florida, for example, it used to be that only the southern tip was the region of high infection, but now, it's the entire state! And this spread is undoubtedly because people are helping the monarchs there.

I guess the question I'm getting at, is do we really want these local, highly infected, populations to grow or spread? With New Zealand being an island, their monarchs probably won't spread, but in Florida, this issue is more relevant, because their monarchs can easily spread further inland to the main, mostly-healthy, monarch population.

Think of it this way, when doctors find that a person has a tumor, their first course of treatment is to cut it out, so it doesn't spread. Why isn't this a viable, alternative, conservation strategy for monarchs? I'm not talking about actively wiping out infected populations with poisons or anything, as that would be cruel. But what about a strategy of gradually reducing (as opposed to adding) the available hostplants? Someone else may have a better idea here, and truly, I've not really given this part much thought. And remember, this is really a more of a thought-provoking question, and I have no illusions of being able to convince anyone to actually do this!

One of the problems when talking about OE and monarchs is that people seem to fail to grasp how much the OE is affecting the local monarch population. This is especially true in Florida and California, where people can easily see lot of monarchs flying around, despite the fact that OE prevalence is through the roof. In other words, they can't grasp why this matters so much.

Here is one way to understand how it matters. In 2019, a study was published by some of my colleagues, which was a very technical, theoretical, statistical analyses of OE prevalence data in areas of the southern US where there are local hotspots of high infections. Here is a link. This study addressed a number of research questions, but one important one was asking how OE was affecting the local monarch population size. Through a series of modelling (fancy statistics) the researchers concluded that OE reduces the local monarch population by 50%! In other words, if OE was not present in these areas, there would be twice as many adult monarchs present! Importantly, this same finding would apply to anyplace where monarchs are resident and have a high infection level, such as California, Florida, or New Zealand!

So after hearing this, hopefully you can see where I'm coming from, and then maybe this doesn't sound so crazy. If (big if) we could ever eliminate, or drastically reduce OE in these hotspot areas, the net effect could be an increase in monarchs, which is what everyone wants. But here's the rub, the only way to truly eliminate OE in a monarch population is to first eliminate the infected monarchs!

I've heard many people claim that if we were to do this in heavily-infected areas like Florida, this would effectively eliminate the entire Florida monarch population! This is correct. It essentially would, but, I think the local population would eventually rebound from the simple process of immigration. If you think about it, this is essentially what happens each summer throughout the core eastern population. Every summer, the adult monarch population builds, along with the OE prevalence, and then the monarchs all leave in the fall. The monarchs which return the following year tend to be uninfected, and these rebuild the local population. And similarly, if the diseased monarchs in Florida (or California) were eliminated, there would indeed be fresh uninfected monarchs arriving each fall. We already know this happens in both cases.

While I'm at it, let me also address another commonly-mentioned argument about OE in Florida, California, or other places like New Zealand: people who rear monarchs for release into these populations usually go to great lengths to prevent their reared monarchs from getting infected, by bleaching all the eggs and milkweed they use. This does reduce OE from captive stock, for sure, but there is something important that these people forget here. Even if you release dozens or hundreds of OE-free monarchs into these populations, those healthy females you released will eventually lay their eggs onto OE-contaminated milkweed. When there are so many infected monarchs in a population, that also means that just about every milkweed leaf has hundreds of OE spores. So, even if those females themselves were healthy, their (hundreds of) offspring will be infected. So really, those healthy monarchs you released are only enhancing the local OE spread.

Last thought - recall that I mentioned earlier that we know that OE is a naturally-occurring parasite. On the one hand, one could argue that this means the parasite is not something that we need to actively manage or interfere with (i.e. because it belongs here). Unfortunately, we also now know that we humans have artificially enhanced the spread of this natural parasite, both locally, and throughout the world. Recall the paper a couple years ago showing how OE is being transported around the world by the butterfly house industry (link here for blog). And don't even get me started about how many infected monarchs are being released each year by home-rearing...

So when it comes to heavily infected monarch populations, we have a bit of a conservation conundrum. If people work really hard, they can indeed enhance and help these populations, by supplying habitats, hostplants, and even by adding more individuals, but is all of that really helping? It is helping the infected population, for sure. But there's the rub. Now we have more infected monarchs because of it.

Hope this post got you thinking! What is YOUR answer?

That's all for now.


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