Every couple of weeks I get a very interesting email message from somebody out there who has thought deeply about monarchs and milkweed. Especially since these exchanges can be insightful and relevant to questions others may be asking, I have decided to start sharing some of these exchanges. Below is a wonderful message from a freelance writer, William Hoover, of Stratford, Wisconsin. My answers are interjected below in green font.
Mr. Agrawal: Good Morning! I checked you book out of our library yesterday and spent the greater part of the evening digesting the greater part of the book. I congratulate you on packing it with great details and keeping it lively. Thank you for your efforts in producing such a fine volume. I am an amateur writer and researcher employed by Christian Light Publications. I am currently writing a science textbook for them on the 5th or 6th grade level.
In the textbook we have a Unit on the meadow, which involves looking at monarch butterflies and milkweed. We don’t dive into great detail; however, the information I do put in I would like to be as accurate as possible. In my research, I was surprised how difficult is was to verify some basic questions:
- Do most birds avoid monarchs?
- Is milkweed actually bitter and poisonous?
- Do viceroy butterflies receive protection by imitating monarchs?
To explain each a bit further:
- Do most birds avoid monarchs? It was frequent on internet sites that they do. However, I was confused by at least one study I read which seemed to indicate that maybe birds do eat monarchs???
- Is milkweed actually bitter and poisonous? Although sites abounded indicating their effects on animals, I was puzzled by the fact that some people claimed to eat it, and even say eaten raw it is not bitter. Is it possible that animals could eat it without deleterious effect?
- Do viceroy butterflies receive protections by imitating monarchs? I was thrown into doubt by the Wikipedia article (citing a study) on viceroys.
In your book, from my perusal last night, I saw you touched on all of these, clearly indicating the poisonous nature of milkweed and its effect as a deterrent to birds from eating monarchs. You also discussed how milkweed can be eaten, and touched on the viceroy connection. Could you shed any light on the seemingly conflicting views presented in the sites given above? I’m aware that science is always a work in progress, and that perhaps not everything is yet definitively assessed. Perhaps birds generally avoid monarchs, but do sometimes eat them. I also noticed in your book how some monarchs are raised on less potent milkweed—perhaps birds eat that type? I read the section on the orioles—but the study above claims even jays would eat them. I find myself a bit confused. I also noticed the study on caterpillars you discussed (with the clay models); it appears birds do attempt to eat them at times, but not as frequently as other types of caterpillars??
Do birds really not eat monarchs? It’s a complicated answer, but the simple answer is: they really don’t eat very many monarchs. The complicated answer is as follows. First off, the toxins that milkweeds produce that are stored by the monarch butterfly are most highly concentrated in the adult wings. Accordingly, often when birds take a bite of a monarch, they take a bite of the wing, and they may be getting a bitter taste or toxicity from the scales on the wings, which have the highest concentration of cardiac glycosides. Interestingly, in the Mexican overwintering grounds, there are two birds that actively consume quite a number of Monarch butterflies. I discuss this extensively in the book. These birds typically avoid eating the wings, and even avoid eating the cuticle of the main part of the body. They primarily eat the innards, which both have the highest protein and fat content, as well as the least amount of toxins. Now in the Northeast and Midwest USA, during the summer, the truth is that most birds avoid most butterflies. The studies on birds of avoiding the coloration of monarchs and the viceroys which mimic the monarchs, is really quite convincing. Birds learn and they prefer not to consume these animals. Nonetheless, as indicated in the article that you shared, it is certainly occasionally the case that birds will eat monarchs and even eat many of them. I have witnessed this myself, and others have written about the occasional consumption of monarch caterpillars or butterflies by birds. I would say this is largely an unresolved issue, but certainly not very common. It may simply be that under the right conditions, when birds are very hungry, or have had something else facilitating in their diet recently, birds may go after monarchs. And it is certainly the case that monarch caterpillars are easy to find and consume, because they are out typically during the day, typically on top of the leaf, and are brightly colored in a way that they’re easy to find. One of the studies that I find most convincing about birds avoiding monarchs is by Bernd Heinrich from the 1970s. Again, I talk about it in my book. Interestingly, Heinrich observed many caterpillar species and directly observed bird predation. He quite convincingly showed that despite the fact that monarchs are among the most apparent, in terms of being out during the day, on tops of leaves, and not blending in, proportionally they were eaten among the least. Additionally, I talked about a graduate student a couple decades later, who isolated the aposematic coloration, the strong contrasting white, yellow and black of monarch caterpillars, as being one of the causal factors reducing bird predation. So overall, I would strongly say that, yes, birds avoid monarchs, but of course there are exceptions, and in my area of science, it’s these exceptions that prove interesting, and keep going after the next advances.
On the edibility of milkweed: I noticed you said boiling it is a key. Perhaps that explains the disconnect between the sites warning of danger to animals and the people proclaiming it as safe. But is it really sweet when raw (see site link above)?? That seems to eliminate the method of making it repulsive to animals.
Oh, yes, the edible milkweed question. The first point, and this is a very important one, is that the common milkweed Asclepias syriaca, which is probably the most abundant milkweed across the US, is among the least toxic. So whatever anybody writes about the edibility of milkweed, it is most often the case that they are talking about the common milkweed. And to be clear, there are many milkweed species (>130 in North America) and some are very toxic, known to kill livestock. But even the common milkweed has abundant toxins in it. And as the great Renaissance scientist Paracelsus noted, “dose makes the poison”. What I’m talking about here is that many animals can consume small amounts of toxins with no ill effect. The common milkweed is indeed typically bitter when you eat the raw foliage or stems. It may be less so under some growing conditions than in others, but most often does contain a bitter taste to humans. I can attest to that. As I and others have noted, the bitterness can be lessened by washing and heating. Nonetheless, much of the bitterness that’s in the common milkweed comes from cardiac glycosides which are rather polar in their chemical nature. This polarity, which is something I go into details in the book, means that these chemical toxins do not easily cross membranes in our animal bodies. That’s a good thing because it means they’re not getting to places like our nervous system where they may cause serious toxicity. The consumption of milkweed is a fun, yet complicated. Overall I would encourage people to eat milkweed, but to read appropriate recipes, as outlined in the book for example. Many plants, including horseradish, chili pepper, and even celery, contain small doses of poisons which typically don’t cause any trouble, but it also must be managed. Great question.
On the viceroy: I suppose it all hinges on whether birds do generally avoid monarchs. If they do, it would seem obvious viceroys receive protection from appearance. However, the study listed above indicated viceroys also taste bad!
The lion’s share of the evidence suggests that, yes, the viceroy does benefit from looking like the monarchs. But as you say in your letter, science is complicated, always advancing, and there are always exceptions. It’s quite clear now that viceroys, on average, are less palatable than many butterflies. In other words they themselves produce some distastefulness that would-be bird predators avoid. Nonetheless, they are far less repulsive than monarchs. And given the answer to the above questions, regarding the avoidance of monarchs by birds, and in particular their visual use of the contrasting orange, black and white coloration, it is quite clear that the viceroy gains benefits from mimicry. A number of very good studies have indeed supported this conclusion. It’s not the case the viceroys are totally edible, but they do benefit from looking like monarchs. I hope that answers your questions, it was fun for me to think about and look at the websites and papers you sent. It’s always nice to be engaged with thoughtful and curious folks out there interested monarchs and milkweed!
Again, I thank you for you excellent book. I am excited to have found an individual with a comprehensive view of this fascinating subject (the monarch/milkweed interplay). Any light you can shed on my questions will be deeply appreciated. Regards, William Hoover