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  • Andy Davis

Is history repeating itself with neonicitinoids and monarchs?

Hello again everyone,

Last week I posted a blog about the 20-year anniversary of the time when everyone thought Bt corn was killing monarchs. I thought it was interesting to look back at that time in our history and re-read the science, and the factors that led everyone to believe that narrative at the time, which we now know wasn't quite true. In hindsight, it sort of looks like the whole world really, really wanted to believe that big agriculture was imperiling the monarch, and so everyone was quick to jump on that bandwagon. Interestingly, I didn't get any feedback on that blog, and I don't believe it was shared even once on social media, which tells me that even to this day, some of this blog's readers find this topic uncomfortable. If that's the case, you may not like this blog entry either, since a number of recent events have unfolded just now that look very, very similar to what happened 20 years ago. It looks to me like history seems to be repeating itself!

Let me start from the beginning. Neonicitinoid pesticides. For those who have been living under a rock for the last decade or two, neonicitinoids are the latest pesticide that has been developed for farm use, and also for commercial nursery use. It is extremely potent to invertebrates, but not so much to vertebrates, and this potency is supposed to mean that farmers don't have to use as much to protect their crops from insect pests. In fact, these new pesticides are designed to be taken up from the soil by the plants so that the toxin becomes "systemic" in the plants, which means the toxin flows through the plant "veins" and the entire plant becomes toxic. Sometimes this is accomplished by coating the seeds before planting. There are a variety of different types of neonics, including clothianidin, imidacloprid and thiamethoxam, which are the most common. You can also find plants at garden centers (including milkweeds) that have been treated with neonics. They are supposed to have warning labels, but sometimes they don't.

Neonics were first used in agriculture back in the mid-90s, and since then their use has grown. Around the same time, the well-publicized declines of the monarch winter colonies were happening. You can see where this is going. Pretty soon, people started putting two and two together here and started wondering if the rise in neonic use in America, especially in the Midwest, is responsible (or partly responsible) for the decline in wintering monarchs. A number of scientists began wondering the same thing, and started testing and experimenting. This was a logical line of thought - after all, neonics are designed to kill insect pests, they are used in agricultural settings, where there are also milkweeds, and the milkweeds can easily take up these toxins too, especially if it is in the soil, or if the farmers spray it on the fields. And, the rise in their use has coincided with the decline in (wintering) monarchs. So it all made perfect sense.

By the way, the other reason to suspect that neonics were harmful to monarchs was the growing research showing how harmful they are to bees. And this research continues to grow to this day.

Similar to what happened 20 years ago, the narrative that neonics were killing monarchs all began with a single, very preliminary, paper. In 2015 a very short paper was published by researchers from the USDA and South Dakota State University (Pecenka and Lundgren). These authors conducted a very basic experiment where they fed monarch larvae with milkweed that had been treated with one neonic chemical - clothianidin. This is one of the most widely-used neonics out there. I'll put a link here to the original paper if you want to read through it. The researchers conducted a lab study where they allowed young monarch larvae to feed on the treated milkweed for 36 hours, and then tracked their survival. They used a number of different doses of the chemical too, which they claimed were realistic, based on their own chemical analysis and surveys of milkweeds. Sure enough, their experiment revealed that some of caterpillars either grew slowly or died, when fed the neonic-treated milkweeds.

As what happened with the Bt corn story, the results from this one study were then announced very publicly and widely by journalists and media. Here is a link to a story in the Ecologist -

Following this one study, there were a couple more that came out, some just recently, that also seemed to implicate neonicitinoids in the demise of monarchs - or seemingly so... There were at least two studies where researchers did their own toxicological surveys of milkweeds from agricultural areas, sort of like what Pecenka and Lundgren did. One study was published just last year, by Paola Olaya-Arenas and Ian Kaplan, in the journal, Frontiers in Ecology and Evolution. Here is a link. These researchers surveyed milkweeds near farm fields in Indiana, and conducted detailed toxicological screens of the milkweed leaves, to see what chemicals were in them. In the same journal issue, another group of researchers reported on their own toxicology investigation of agricultural milkweeds in California. Here is a link to that study. In a nutshell, all three of these studies (I'm including the Pencenka and Lundren study here) showed that neonics, and other pesticides, are EVERYWHERE. In most milkweeds sampled, the researchers in each study found several different contaminants, including neonicitinoids. The authors of each study then described how their results demonstrate that monarch larvae are consuming contaminated milkweed, just as Pecenka and Lundgren did. However, I note that in both cases, the researchers simply showed that monarchs were eating the chemicals, and they allowed the reader to infer the rest - that the monarchs were harmed by doing so. This is a very important point, especially considering what came next.

Now here is the crazy part. Just this year, TWO OTHER studies have just come out where researchers conducted very, very detailed and sophisticated lab studies to investigate the actual effects of neonics on monarchs, and these studies both appear to cast doubt on the whole story.

One study was conducted by researchers from the USGS and the University of Florida - Timothy A. Bargar​, Michelle L. Hladik, and Jaret C. Daniels, and was published in a journal called PeerJ (link here). These researchers fed young monarch larvae with bits of milkweed that had been treated with neonicitinoids, and then monitored them for survival, growth, pupation success and adult size. The other study was conducted by Paola Olaya-Arenas and colleagues - this is the same scientist who also did the field survey above. Their paper just came out last week in the journal, Nature - Scientific Reports. Here is a link. This paper reports on a similar laboratory study, where monarch larvae were fed milkweed treated with neonicitinoids and several other common pesticides (not all together though). Then, the researchers tracked the monarchs to see if they survived, and if they did, if their growth was impeded in any way.

Bottom line from both of these studies - the neonicitinoids did NOT kill the monarchs, at least at the doses that are commonly found in milkweeds around farm fields. In the PeerJ article, the researchers report that only the very very high doses led to monarch deaths, and these doses are not what real-world monarchs would experience. And I know what you're thinking - how do the researchers know that the larvae actually ate the pesticides? They actually do know - one of the more interesting results from the PeerJ study was that the researchers also quantified how much pesticide was in the adult monarchs, after they had eaten the chemicals during their larval life. It turns out the adults did indeed have a substantial amount of pesticides inside them, and, it didn't seem to harm them. The Nature paper reported that certain of the chemicals did lead to reduction in adult size, but these were not the neonics.

If I sound like I believe these last two papers over the first one, it's because of what I haven't yet told you. I actually conducted my own preliminary lab investigation of neonics and monarchs a couple of summers ago. I had an idea to try to understand if these chemicals were affecting adult flight ability. To do so, I needed to have groups of monarchs that had been reared on different doses of the chemicals. So, after reading the Pecenka and Lundgren paper, I set about trying to expose monarch caterpillars to clothianidin (one of the most common neonics). Note that I said "trying". I fed the larvae the same doses of the chemical that those researchers did, and... absolutely nothing happened! None of the larvae, even at the highest doses, died. There wasn't even any reduction in growth, or change in feeding behavior - nothing. I actually thought I had done something wrong, and I later abandoned the project, thinking that it wouldn't work if I can't expose monarchs to the chemicals.

After reading these last two papers, I now see that I hadn't done anything wrong at all - the monarchs simply didn't care that they were eating neonics!

I'm not sure why the one initial study showed negative effects of the chemicals, while the later two (plus my own) did not. I did notice that the Pecenka and Lundgren study used monarch eggs purchased from a commercial breeder, and then mixed those with some wild-caught eggs. Perhaps those purchased monarchs were inbred? Recall the PNAS study from 2019 that had examined the genetics of commercial monarchs (blog here) - the commercial monarchs were so messed up genetically, the researchers referred to them as "franken-monarchs."

By now you might be thinking this all sounds crazy - how can these chemicals, which clearly have such dramatic effects on bees, and other insects, not harm monarchs? Well, I have a theory, and I see that Paola Olaya-Arenas and colleagues also suggested this too - monarchs are able to tolerate eating poison because they already eat poison - the cardenolides in milkweeds! These compounds are very toxic. So, if monarchs have evolved to eat this toxic plant, then they must have the ability to avoid other toxins too. Maybe they can store the pesticides in their tissues somewhere where it doesn't do anything. I think this theory deserves some study!

Well folks, this is certainly a lot to absorb, and I think I'm still processing these new studies too. To be fair, I think the science still is unfolding on this issue, so it may be too soon to say "case closed" on this. But it is hard to deny how two separate studies (plus my own lab trials) all showed the same thing - monarchs are not harmed by eating milkweeds laced with neonicitinoids. Wow, it even feels weird to write that statement.

That's all for now.

ps - By my count, this is the second time in history where the storyline around monarchs and agriculture has turned out to be untrue...


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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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