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  • Andy Davis

Monarch caterpillars become frightened when touched by humans


UPDATE Feb 2020 - This blog entry was written ~2 years ago, using a small collection of monarch caterpillars, which were not directly handled. This is NOT a description of the latest research project from the Davis lab examining effects of handling on monarchs.


Greetings blog readers,

Are you read to have your mind blown? Get ready...

Today's post is about a topic that is of immense importance to the issue of captive-rearing, which has been hotly discussed as of late. Folks have been debating the pros and cons of this practice (hint - there are more cons than pros), with most of the talk being about the rates of tag recoveries, and migration success. I noticed that a lot of the discussions have revolved around whether or not captive rearing harms monarchs, and on this, no one seems to have an answer. Today's post may just change that. I'm going to tell you about some data I collected in the last year that were published in the recent study I spearheaded on road noise (Davis et al 2018, Biology Letters). The data I'm going to highlight were largely overlooked in all of the media frenzy around that study, probably because they were only presented in the supplemental file that accompanied the publication. Even so, keep in mind that these results are peer-reviewed, published science.

If you recall from a few months ago when this paper came out, my colleagues and I had set out to measure the effect of highway noise on monarch caterpillars - specifically on their heart rate! Elevations in heart rate are a universal signal of stress in the animal kingdom, even in insects! So my colleagues and I wanted to see if road noise causes stress in monarch larvae, by examining their heart rates. I had written a summary of that paper in a previous blog. Basically, I have developed a technique in my lab to monitor, in real-time, the beating hearts of caterpillars, using a very cool electronic device attached to a computer. It doesn't hurt them, because it senses the heart movement from about a half-inch away from the larva. I'll show a picture of this gizmo below, in case you didn't see the previous blog.

The device reads the beatings of the caterpillar's heart, and displays them in real-time on a computer screen, so it's a lot like those screens you see next to a hospital bed. In essence, I use it to take the caterpillar's pulse!

Recall that in that study, we determined that larvae exposed to road noise at realistic loudness causes their hearts to race (there was a 16% elevation in heart rate), which is a sign of a physiological stress reaction. In other words, the larvae were frightened by the noise.

Now, in the course of doing that project, we also conducted a side-experiment, to determine if monarch larvae become stressed simply by picking them up and handling them, and more importantly, how long does it take for their heart rates to increase. This was an important question, because we needed to know if the act of handling them compromised the data from the noise experiment. This is the part that was published in the supplemental file.

To answer this question we conducted a very simple experiment - at least, it was simple to do after we had all of the gear set up and running. We examined twelve 5th instar larvae for this experiment. They had all been reared in the lab (by us). They were reared from eggs in plastic containers, fed greenhouse-grown swamp milkweed, and most importantly, their handling was minimized. That means we did not physically handle the larvae whenever their food needed to be replenished in their containers. An in fact, we only changed out their food every other day so as to minimize disturbance.

The procedure we used to assess their stress reaction was pretty simple - we had the larvae containers in front of us to start - then we picked up a larva by pulling out the leaf it was sitting on, held it in front of us for 5 seconds, and then placed it (the leaf plus larva) on the heart monitor to get readings. As an aside, you will be surprised to know that the larvae usually hold still under the device if we allow them to hold onto their leaf. Every now and then one decides to wander off during the procedure, so then we just use another one. Anyway, this "procedure" was nothing more than us picking up the caterpillar.

Once the heart readings began, we allowed the device to continue monitoring for at least 10 minutes. We let it run for so long because we wanted to know how long it took for the heartbeats to speed up, or in other words, how long it took for the larva to be stressed from being picked up. We actually suspected that it might take a while, because I had been doing some similar research on a beetle species that showed it can take up to 8 minutes for their heart rates to max out after a stressor (link here to see that paper).

OK, now that I've told you about the project, and the procedures, now let's get to the results. Below is the primary graph from this experiment, showing the heart rates of the caterpillars over time. Each point here is the average of the 12 larvae.

There are a couple of things to point out here. First, there was indeed an elevation of heart rate - the initial rate of 46 beats per minute was typical for a "resting" heart rate of monarch caterpillars (I know this from experience in the lab). And recall that this initial rate was obtained pretty quickly after picking up the larva, so we can be confident that it was not itself elevated. Next, see that the heart rate maxed out at 5 or 6 minutes, at around 53 beats per minute. That time frame is pretty much what I expected, based on the work I do with other species. Importantly, the maximum elevation is an increase of about 15% over the baseline heart rate. I can tell you (also from experience) that a 15% elevation is consistent with a stress reaction in an insect. In fact, recall that when larvae are exposed to road noise, their heart rate goes up by 16%. Actually, if you're wondering, when humans become stressed, our heart rate increases between 15-20%.

Establishing this timecourse of the monarch caterpillar stress response was very important to our road noise study. This graph told us that the cardiac stress reaction in monarch larvae takes a few minutes to kick in after you pick up a given larva. So we figured that if we could work quickly enough during testing, then we should be able to obtain the heart rates of our test larva before they became stressed from our handling. But this timecourse is a little tangential to the fact that there WAS an increase, which is of importance here.

For the current discussion about captive rearing, these results are extremely relevant. In fact, these may change the way you think about how you interact with monarch larvae (if at all). What these data are telling us is that monarch caterpillars become frightened when someone picks them up. Let me repeat this, and more clearly - monarch larvae experience a stress reaction if a human picks them up. This result is pretty undeniable - we saw this pattern in all twelve of the larvae we tested. There was some variation in the magnitude of the elevation, but the point is they all became stressed.

Now, before you ask, we didn't continue monitoring the caterpillars after 10 minutes, so we don't really know how long the stress reaction lasts (that wasn't our intent). But I can tell you that in other insects, acute stress usually lasts for about a half hour. So that means that stressed monarch larvae probably return to normal after a stress event like this, provided they are returned to their milkweed where they feel safe. BUT (and this is a big but), in most captive rearing scenarios, larvae need to be repeatedly disturbed, moved, and/or picked up, typically on a daily basis for about a week. What happens over time to these larvae? Do they eventually acclimate to this repeated stress? Don't know. And if they do, does this mean they become acclimated to humans? Don't know. And is this daily handling stress similar to what they would face in the wild? Don't know.

There is one other thing that is important to discuss here. Recall that when we "picked up" the larvae to start the testing, we actually didn't even touch each one, but rather we pulled off the leaf it was sitting on, and carried the larvae and leaf to the device. And the heartrate device also didn't physically touch them. At no time was the larva ever directly touched. And yet it still became stressed. That means that the mere act of moving the larva away from its habitat, holding it in front of you, and then exposing it to an unfamiliar environment was enough to frighten it. Given this, one can imagine that the stress reaction would be even more pronounced if a larvae were actually handled.

So let's sum this up here. I've just presented results from a project that is up-to-date, peer-reviewed science, which shows that monarch larvae become stressed when picked up by a human. That's another way to say that they are frightened, or scared, when they are handled. This stress is probably transient, and likely goes away after 30-40 minutes (we don't really know), but it also probably happens each time larvae are handled. And we also don't know what the effect of repeated stressing is to monarchs in the long run. So this research really opens up a lot of questions - questions I hope to tackle in the coming years.

Now, back to the original question of this post - does captive-rearing harm monarchs? I guess that depends on what your definition of harm is. If by harm, you mean, do they die, then that's one thing. But if you consider scaring them (on a daily basis) to be harmful, then the answer is yes.

Before I sign off, please don't email me telling me that YOUR monarchs are not stressed. You can't tell just by looking at them.


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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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