Just came across this really interesting study by the University of Kentucky, studying the effect that garden milkweed arrangement has on the abundance of monarchs. They found that milkweed planted on the edge/perimeter of the garden had 2.5 to 4 times more abundant eggs and larva than milkweed plants surrounded or intermixed in a garden.
https://www.frontiersin.org/journals/ecology-and-evolution/articles/10.3389/fevo.2019.00474/
by goblin-fox
17 Comments
So…when milkweed is apparent and accessible, and not hidden in a cloud of non-native plants, the monarchs do better?
Is this a principle we could apply to other beneficial insects and pollinators?
🤔🤔🤔
[Edit: I know these results would very likely repeat even if milkweed was in a cloud of native plants. I’m just having fun at the expense of non-native plants as the opportunity allows. Though I’m curious if some native plants which often grow alongside milkweed would provide a signal to the monarchs that milkweed is likely present here.]
They should redo the study with native plants. It’s a good concept but failed execution. All this tells us is that nonnatives make natives harder to find.
The other critique of the study I’d see is that they planted an extremely well behaved milkweed.
We have common milkweed in our yard. That stuff is going to be everywhere and there’s nothing you can do about it.
Very interesting study about how particularly in the Midwest, milkweed in cornfields and in other crops such as soybeans and surrounding them produced more eggs and caterpillars compared to non agricultural land.
“We found immature monarchs in cornfields throughout their breeding season, even though the corn is up to 200 cm taller than the milkweed by the end of the season (Fig. 2). In the four regions studied, per ramet densities of monarchs were as high or higher within cornfields as in other habitats on many monitoring dates. Egg densities on milkweed in agricultural fields were even higher relative to nonagricultural habitats at the end of the summer in the upper Midwestern sites (Fig. 1 and Table 2).
Our data suggest a significant proportion of the monarchs that originate in the Midwestern U.S. come from agricultural habitats. We estimate that cornfields and soybean fields together produce 78 times more monarchs than nonagricultural habitats in Iowa, and that cornfields produce 73 times more monarchs than nonagricultural habitats in Minnesota/Wisconsin. Nonagricultural habitats in the upper Midwest tend to produce more monarchs on a per area basis, but they are relatively rare, comprising <1% of the total potential monarch breeding habitat (Table 4), and thus are less important for overall monarch production.”
“These findings demonstrate that practices affecting milkweed densities in agricultural habitats are likely to have large impacts on monarch abundance. Farming practices such as tillage, herbicide use, and cropping choices may affect milkweed abundance and thus monarch numbers.”
Just highlighting the conclusions that I found most interesting/important but feel free to read the rest of the study [here](https://www.pnas.org/doi/10.1073/pnas.211234298)
Sounds like because it’s just easier to find, whether located near native or non-natives and the survey is for small urban gardens. Wish I had more land!
I imagine having milkweed be the only thing sticking out would make it more noticeable, also bunching plants.
Interesting study! There were a few points that I found notable:
1. In citizen (AKA home) gardens, the strongest predictors of monarch egg + larvae count were **physical separation of milkweeds** from other plants and **unobstructed north-south access** (presumably since monarchs migrate in the north/south direction). Both strongly influenced counts. Factors that did not influence counts included garden size, density of either nectar or milkweed plants, and 360* access.
2. In citizen gardens, ***A. incarnata*** **and** ***A. syriaca*** **hosted more than** **ten times more monarch eggs/larvae** compared to *A. tuberosa*. Butterfly milkweed overall was a poor host with an average of 28 ramets (stems) needed for 1 monarch egg/larvae compared to ~3 ramets per egg/larvae for either *A. incarnata* or *A. syriaca*, despite having a relatively equal abundance of each. This wasn’t included in the model so there’s no way to tell if this was influenced by other factors (for example, are people less likely to physically separate *A. tuberosa*?). I’d like to know more about the 2 gardens had no monarch eggs or larvae at all for the entire season despite an abundance of milkweed.
3. As you noted, the perimeter gardens yielded more eggs/larvae than the interior or intermixed plans, with relatively little difference between the interior vs the intermixed plans. They also separately showed that **milkweed closely surrounded by tall native* grass yielded almost no eggs** (*actually a nativar *Panicum virgatum* “Shenandoah”). I think this has significant implications for the Oudolf “matrix meadow” style gardens which call for plants to be intermingled in a matrix of grasses. This style is very popular especially for large-scale public gardens. It would be interesting to see if modifying the design to incorporate some physical separation and ensuring north-south access, or perhaps surrounding milkweeds with a shorter matrix, would improve counts.
Personally I think I will be incorporating some of these points. I have two main gardens, one in the front yard with relatively unobstructed north-south access, and one in the back yard on the south side of a tall fence. Both would be considered “intermingled.” I have been planning to add more milkweeds and will likely focus on adding more swamp milkweed to the front garden with some physical separation.
How interesting! Thanks for sharing OP
It’s worth noting that established clumps of milkweed can get quite large and form thick colonies. They aren’t typically seen as individual plants dispersed among a prairie.
Cool! Thanks for sharing this. Going to try to remember this for if I ever have a yard someday.
I look forward to reading the study later. In the meantime, I thought I’d add an unscientific anecdote from observations on my property.
For several years, I maintained a thick patch of milkweed (essentially a monoculture) in an area that would otherwise have be free of plants (landscape rock). From that patch, I collected a number of caterpillars gathered at various development stages, and the success rate was dismal (maybe 50% if I had to guess?). All made it to the chrysalis stage, but when they failed, I believe it was due to parasitoid wasps or flies (there was a silky string dangling from the failed chrysalises).
My theory is that overly dense plantings may successfully draw in a higher egg rate but also signal predators more successfully – similar to the rationale that diversifying garden planting reduces pest pressure as insects try a few plants in an area and move on if they don’t find a suitable host. I removed the thick patch of milkweed and started planting into my prairie area. Of the caterpillars I’ve reared from those plants, the success rate has been much higher (maybe 10% failure?).
Again, I want to emphasize that this is purely anecdotal, but it would be interesting to test more rigorously at some point. My hypothesis assumes that the monarchs are going to find the plants one way or another (yes, this assumption is doing a lot of heavy lifting). If you make it easy and they fill up a dense planting, they aren’t physically diversifying the egg laying which makes it easier for the predators to wipe out a local caterpillar population. Additionally (and I’m wandering even further into the realm of speculation here), I’ve heard anecdotal evidence of monarchs frequenting the same tree year after year which may imply some pheromone marking or something. If that concept can extend to a large milkweed patch that is maintained for many years either from marking a tree or structure or whatever that’s in close proximity, the monarchs may concentrate their egg laying on that patch further exacerbating the predator problem over time.
To add relevant context to the above – as I’m sure you’ve all noticed, you get much better at finding caterpillars the longer you’ve been at it. So my anecdote is almost certainly skewed by the fact that my more recent caterpillars were reared from an earlier stage – i.e. egg – 2nd instar. The parasitoid wasps/flies may not find them until a later stage, so I may be suffering selection bias or something. Even the 50% fail rate from my dense patch is better than the 95% natural failure rate I read somewhere.
Oh, and to address 2 other items (again, both anecdotal): 1) I do seem to find more caterpillars on plants at the prairie edges, but that could also just be due to convenience (mine) – i.e. not wanting to wade through tick town to find cats. 2) A Coworker that planted swamp milkweed was getting a crazy number of caterpillars on his one plant. I had read and assumed it to be largely true that monarchs won’t lay more than 1 egg on a plant, but he had a single plant that hosted maybe 10 in a summer – so obviously several concurrently. They dig the swamp milkweed.
I wonder how this effects predation?
Very informative, thanks for sharing! This gives credence to me leaving the milkweed near the property line. I was doing it especially this year to thumb my nose at the asshole neighbor who in previous years mowed down all the milkweed along our house. I love that he has to look at it everyday and can’t do anything about it (its all safely behind a small decorative metal garden fence along with native planting signs). I was debating trimming some back as all my native shrubs fill in but have been thrilled with all the baby volunteers being living green mulch. I’ve seen at least 1 monarch everyday for the past few months, some bigger than my hand!
The author of this study was on the Backyard Ecology podcast to discuss this several years back. It was interesting to listen to the conversation. I only skimmed the study and it’s been a while since I listened to that episode, so I don’t know if he presents any other info or responds to questions/critiques, but it is worth a listen if you are interested in it.
I’ve planted a cluster of various milkweed species (e.g., whorled, swamp, butterfly weed) at the front of my garden on the perimeter. It’s been incredibly popular, but wasps have been stalking each plant
So glad you shared this! I saw it a few weeks ago, perfect timing as I finally fridge-stratified a few showy milkweed seedlings successfully and had no idea where to put them! This fall I’m going to make room around the south side perimeter of our meadow/pollinator garden for them and I am excitedddd.
This is kind of a game changer for me. I’ve always been afraid to plant it at the edges, because people run their mowers close and I don’t want the vibrations, smells, sounds bothering the monarch caterpillars. I’ve also seen a coworker knock some off with a leaf blower, and the babies went splat on the pavement. I wonder if this study includes properties with mowers and blowers near the beds, or without?