Q&A How monarchs find milkweed

Brandon wrote in recently about a how monarchs find their milkweed host plants… below is his Q and my A:

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Sent: Sunday, June 20, 2021 7:59:59 PM
Subject: Monarch Butterfly Chemoreception

Dr. Agrawal,

My name is Brandon Rosenblatt, I was referred to you during a conversation I was having with Dr. Heather McAuslane at the University of Florida. I asked her a question about Monarch Butterflies and how they “smell” milkweed plants from long distances, and she thought that you may be able to help me. First, let me say that I took a little while to look through your publications and recent blog posts, and I am not entirely sure if you precisely answered my question, but you have gotten close a number of times. I am pursuing a Ph.D. in Physiology, but after being inspired by a few of your lectures on YouTube, I think that I would best fit in the world of Physiological Ecology, as I plan to study how anthropogenic changes to the natural soil in my home state (Florida) affects the natural soil biology, and leads to a cascade of issues in the web of life. I have a feeling entomology will be involved there, so I hope we will be colleagues one day.

My question is, do we currently have an understanding of the mechanism behind the Monarch Butterfly’s sense of smell? Is it truly a “smell,” as in chemoreception, or are there other processes by which the butterflies detect a milkweed plant? It seems incredible to me that a butterfly (or a large population of butterflies travelling together) can detect these plants where they exist in all of the chaos of the modern world. I assume that many wild milkweeds are growing in close proximity to major highways, congested city streets, and so on, which would mean that the butterflies would need to be able to block out the “noise” (smell in chemoreception, or perhaps colors with photoreception) to target a small plant. It’s very fascinating and somewhat unbelievable.

I know that you are very busy, so if you could share any insight, I would greatly appreciate it.

I look forward to hearing from you!

Thanks,
Brandon S. Rosenblatt

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On Jun 21, 2021, at 7:14 AM, Anurag Agrawal <aa337@cornell.edu> wrote:

Hi Brandon,

Thanks very much for your email. Awesome that you’re getting into the study of insect behavior and chemical ecology, I look forward to exchanging information and learning from you in the future.

As far as I understand it, monarchs have at least four mechanisms to chemically sense their environment. First, of course, are their antennae, which are likely important both for short and long distance perception of compounds. In addition, their tarsi or legs, are full of chemo receptors. Adult monarchs will drum and touch their antennae and tarsi to leaves in a “dance” where they’re trying to figure out whether to accept the host plant or not (to lay an egg). It often looks like a drumming of the legs and antennae. In addition, sometimes when on a milkweed plant, monarchs will scratch the surface with their legs and that’s likely to release compounds from the leaf which may be sensed by their tarsi, antennae, or tasted with their proboscis. Finally, monarchs drag the tip of their abdomen against the leaf, and they’re known to have photoreceptors on their abdomen. It’s a little unclear how these photoreceptors are used, but it’s likely some sort of chemical sensing mechanism. I know, unbelievable!

To my mind, the short distance, or touch related chemical sensing of the milkweed plant is certainly very complicated, but perhaps not a giant mystery. What seems to be a bigger question to me is how they find milkweed plants when flying by, often 10 or 50 ft away. I’m guessing that there is both some color sensing (visually, by eyes, of the specific color of milkweed leaves), as well as sensing of volatile compounds with their antennae. As you probably know, insects like monarchs have very strong ability to pull out plumes of compounds that might be floating through the air. What some have speculated about more recently is that the contrast of the smell of milkweed against a very different chemical smell background may be quite useful for finding host plants. For example, if a milkweed plant was growing in and amongst primarily a sea of goldenrod or corn, that might make it stick out (in terms of smell), even if the plant itself was rather hidden visually.

There are some publications on these various issues that I can point you to if you’re interested. Mostly the behavioral work has been very well defined in terms of internal, tarsi, and abdomen touching of the leaves. Less is known about long distance plant location.

In any case, all my best, -Anurag

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Dr. Agrawal,

Thank you so much for the detailed response. I agree, it’s incredible and amazing how these organisms exist and persist out in the world. I am very interested in reading those publications you mentioned, please point me in the right direction.

The long distance sensing of milkweed is where I was hitting dead ends. I haven’t been able to find a good explanation for how this happens, in terms of the physics. Also, I read somewhere that people were making the claim that a monarch butterfly could “smell” a milkweed plant from 2 miles away, which was the thing that seemed really astonishing to me, but I have no idea if the person who said this had any evidence to back up the claim. They certainly didn’t cite their sources.

I know that some scientists are investigating the possibility that vertebrates take advantage of “quantum tunneling” in their olfaction, and was wondering if that theory could be investigated in regards to Monarchs. As I understand it, the standard “lock and key” mechanism of smell doesn’t fully account for the speed at which a smell is perceived, and the “vibrational” mechanism of smell doesn’t account for the fact that enantiomers that share identical vibrational patterns have completely different smells when perceived by organisms. So they’re theorizing that another mechanism, quantum tunneling, gets the molecule in close proximity to the chemoreceptor quicker than would be expected by classical physics. I don’t fully understand the entirety of the theory, but that’s my general understanding of it.

Brandon S. Rosenblatt

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Hi again Brandon,

In speaking with a friend of mine yesterday (who is a lepidopteran behaviorist), it seems very unlikely that butterflies are smelling anything from miles away 😉 Nonetheless, he suggested that they are constantly on alert, sensing smells and other aspects of the environment as they fly. Once they encounter a plume of something potentially interesting, it is thought that they have approaches to turning and orienting as a means to get closer. Then, once in contact with a plant, the tarsi and antennae come into play. In any case, best wishes for your research. Here is the monarch contact behavior paper by Meena Haribal and Alan Renwick I thought you might enjoy.

-Anurag