• Andy Davis

Monarch caterpillar survival is higher on milkweeds with lots of bugs!

Greetings blog readers,

I have a super-cool new study to talk about today. Earlier this year, there was a new paper published in the journal, Insects, which deserves to be highlighted and discussed within the monarch community. I've read through the paper and I'm going to do my best to describe it here, and also talk about some related research too. This blog should be of interest to anyone who has a backyard garden, and for people who take a natural, hands-off, approach to insect conservation, which in my opinion, is the only way to go. See a recent post about the mistakes that can be made when we try to interfere with, and manage nature!

Here is a link to the study, which is free to download. The paper was written by a group of authors who appear to be all based in Texas, at the Dallas Arboretum and Botanical Garden, Texas A&M University, the Houston Zoo, and University of Texas at Arlington. The lead author (who is typically the person who did the most work) is Misty Stevenson. The title of the paper gives you the main take-home message too - "High Survivorship of First-Generation Monarch Butterfly Eggs to Third Instar Associated with a Diverse Arthropod Community". Or in other words, "monarch caterpillars survive better when there are more bugs on milkweeds!"

Lets get into the weeds ourselves here! The study was conducted in Texas in the early spring, and this is a very important point here. The spring migration has been identified by numerous other studies as a critical time of year for North American monarchs. This is when the returning adults recolonize the breeding range as they travel northward. It turns out that the size of the summer breeding population in any given year can be directly predicted by how well the spring migration goes. So this means that we really, really need good information on what is happening in the early spring, especially in places like Texas, where the monarchs are most concentrated! The authors appeared to be aware of this, and indicated this was one of the main rationales for the study.

The study was conducted near Cooper Lake State Park, TX, and I googled this - see the map below. This site is clearly in the region of high importance for the returning adults.

It sounds like the study sites were old fields that contained hundreds of native milkweeds to that region, Asclepias viridis (I think this goes by Green or Antelopehorn milkeed), and I've pasted a picture of this species below, taken from https://www.fs.fed.us/wildflowers/plant-of-the-week/asclepias_viridis.shtml.

This is a low-growing milkweed that thrives in dry areas, and it is a workhorse during the spring migration in Texas. I've visited the Texas area during spring migration, and saw this for myself at Harlen and Altus Ashcen's famous pastures. I recall seeing an egg on just about every plant I looked at in April!

OK, so from my read of the paper, this study was observational in nature (i.e. not experimental). Essentially, the researchers visited these fields daily during the spring months and checked hundreds of these milkweed plants for the presence of monarch eggs and larvae. They did this over three years - 2016 to 2018. When a monarch egg was found, they put a flag next to the plant, and then marked which leaf the egg was on. Then, they rechecked these plants every day after that, to track how long the monarch caterpillars survived. They did this until the caterpillars reached the 3rd instar stage, at which point they stopped watching (apparently the caterpillars tend to wander and can crawl off the plant at this stage).

It sounds like this was a lot of work. Collectively, they report that they tracked 664 eggs on 401 host plants! This is important to consider, as it means that they had a very good sample size for their statistical analyses.

Now, here is the most important part - they also recorded how many other insects and arthropods were present on each milkweed plant. This included aphids, spiders, milkweed bugs, etc. Basically, every other critter they saw, they recorded. There was a big table in the paper where they reported the numbers of bugs they saw, and I'll paste a screenshot of it here. This table is organized by taxonomic group.

As you can see, there were lots and lots of aphids on these milkweeds, and lots of ants too. Then there were other critters like weevils, mites, beetles, and flies. These were all living on the milkweed plants alongside the monarch caterpillars! The rows highlighted in yellow are groups that are known to be predatory, like jumping spiders. This is important too.

After they had collected all of the data on the monarch eggs and caterpillars, they grouped the egg data into two categories - those that survived to 3rd instar, and those that did not. Then, they ran a statistical test to see if the presence of any of the other insect groups in that table predicted whether monarchs survived. You might think that the presence of predatory arthropods would lead to lower caterpillar survival, right? Wrong - there was no statistical link between predatory arthropods (like jumping spiders or ants) and caterpillar survival! Crazy.

After all of the stats were finalized, one group did predict monarch survival - the group labelled "other non-predatory arthropods" listed in the table. This is a catch-all group for any bug or critter that does not prey on monarchs. To see the full list of critters, you need to click the link to the Appendix, which has an even bigger table of critters. I looked at it and there are things like crickets, harvestmen, millipedes, springtails, and lots more. I made a collage of some of these critters in the first image above. Anyway, their analysis showed that when these critters were present on milkweeds, any monarch eggs on that milkweed tended to have a better chance of survival! Their analysis showed that greater abundance of these critters led to higher monarchs survival, and, greater diversity of these critters (more species) also led to greater monarch survival. How cool is that?

If you think about it, these other critters are probably not directly helping the monarchs survive, or protecting them in any way. The authors state that what is probably going on is something called the "dilution effect," which basically means that when there are lots and lots of potential prey items in one area, then any predators searching for a snack would have lots to choose from. Thus, any one critter within that bunch would stand a better chance than if it were alone.

This very thing was actually shown experimentally a number of years ago in a lab setting. In a paper from 2005, in Environmental Entomology (link here), researchers placed tiny monarch caterpillars in a dish with Asian lady beetles. They had some dishes with just the beetle and a caterpillar, and in others they also placed varying densities of aphids. When the caterpillars were alone, they all got eaten by the ladybugs, but when there were lots of aphids around, most of the caterpillars survived! This is the dilution effect.

Speaking of other research, the authors of the Insects paper also describe how their project fits into the prior scientific literature on this topic. And it turns out, there have been a number of prior studies similar to this one, where researchers tracked larval survival on milkweeds. They even have a table in their paper that lists all of the other studies. Some of these studies found a similar result (better survival in the presence of other bugs) and some of them didn't. The key thing to take note of though is that all of the prior studies have been conducted in more northern regions, and in the middle of summer. The authors point this out as a potential explanation for the varying results. They also point out that each time this idea is tested, by various researchers, and different locations, the results may differ because of the characteristics of the site, or the timing, or perhaps even the weather! But either way, I'm inclined to believe the results here because of the massive sample size, and, because they conducted this project over three years, and, the results were the same each year.

If there was one drawback to this study that irked me, it was that the authors did not really state, anywhere in the paper, exactly how much the larval survival improved when other bugs were present. I know the stats say there was a statistically significant effect, but then I was left wondering what the magnitude of the effect was. Does the presence of other critters increase monarch larva survival by 10%, or is it more like 50%? I couldn't tell from the paper, or the stats. I'm guessing it was more on the lower end, based on the reported stats I did see.

The authors end the paper with some thoughts on how this knowledge fits in with current conservation efforts. They indicate that if the goal is to increase monarch larval survival in nature (everyone's goal), then the way to do this is to develop a rich, diverse ecosystem in and around the milkweed patch. The more critters living on the plant, the better for the monarchs. To me, this means simply allowing everyone to coexist on the plants, and not try to remove the bugs you don't like! Or in other words, simply leave the milkweed plants alone!

OK, that about wraps it up. Kudos to the authors for conducting this research. For my readers here, do me a favor and share this post far and wide, please. Everyone needs to see it.



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The science of monarch butterflies

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