What monarch caterpillars need to do to survive
- Andy Davis
- 11 hours ago
- 9 min read
Hi blog readers and fans of monarchs,
This was going to be a simple facebook post at first, but the more I dived in to the subject, the more complex it got, so I turned it into a blog post. This is also what I think you should take from this post if you read it fully - that it is much more complicated than you think.
Let's start with some context. As we all know, the vast majority of monarch eggs and larvae never make it to the adult stage in nature. This is by design. That's right, mother nature's plan is for most monarch larvae to die before they even get to the pupa stage. This is not something that is bad, or that needs to be fixed by us humans. This is the way monarchs (and most insects) work. There is a reason why females lay 500 or more eggs in their lifetime. Really, she only needs a couple to survive to replace herself, and her mate, in the population. So keep this in mind going forward here. In this post I'm going to go over a whole suite of "dangers" that monarch larvae face, but the vast majority of them are completely normal. So don't get upset, and don't take this post as a reason why monarchs need to be raised in captivity.
Another way to think about this is with the phrase that you've all probably heard, that nature is "survival of the fittest." This is true for monarch caterpillars too. Only the best, brightest, strongest, etc, survive to become adult butterflies, and really, this is a good thing. This "natural selection" process works to keep the entire population healthy and fit.
So, I'm going to order this post by topic, and cover some of the basics in each topic. This list will not be complete, by any means, and I may add to it later even. These are just the topics I could think of right now. And, while I won't include references here, know that the explanations of the topics come from my knowledge of the scientific literature around monarchs, and also from observations of other species.
Surviving the milkweed defenses
First off, when caterpillars hatch from their eggs, one of their first tasks is to reduce the milkweed "sap" on the leaf they are on, or else the sap will literally kill them! They do this by digging a trench around themselves, which cuts off the latex supply to that area. See the picture below. But if they aren't careful with this step, they can get the latex in their jaws and it gums them shut, preventing their eating afterward, and thereby killing them. Or, if they don't stop the latex flow completely, they will get stuck in the sap and die. This is all during the tiny first instar stage too! When they get bigger, they still have to deal with the latex, sometimes by notching the leaf stem at the base. In the picture below, note how small the caterpillar is compared to the latex sap. You can see how easy it would be to get stuck in that sap and die.
Then there is the cardenolides themselves in the milkweeds. These chemicals are designed to make the milkweeds toxic, which then make the monarchs toxic too. Monarchs have evolved to live on milkweeds and incorporate these chemicals into their bodies. But, these chemicals are difficult to process internally, and apparently doing so requires a great deal of energy. There have been multiple studies showing how monarch caterpillar survival and growth are lower on milkweeds with high cardenolides. The upside is that monarchs that do survive on these plants have a higher toxicity level when they become adults. The bottom line here is that monarchs must have a strong "internal" physiology to be able to deal with the very thing they eat. If they don't, their growth would be slowed, leaving them exposed for longer to predators, for example.
Surviving predation
This is the topic that everyone naturally thinks of. While the milkweed toxins do provide some protection from some predators, not all predators are affected, or care. This is especially true for invertebrate predators, like stinkbugs, or preying mantises, or wasps. Each of these will gladly take a monarch caterpillar, though keep in mind what I said before - this is normal, and a natural part of nature's design.
Believe it or not, monarch caterpillars do have some defenses against these threats. First, they can fight back, literally. This is more pronounced (and visible) at the later, 4th and 5th instar stages, but it involves thrashing their bodies when the predator attacks. This is either to "swat" the predator, or to try to get away, by thrashing so much that they drop off of the leaf. I'm not aware of any statistics to show how successful these behaviors are, but it is something that happens, and caterpillars that mount a strong attack might be able to thwart the predation. Also, larvae can adopt a curled-up posture when attacked or threatened, which is an attempt to protect themselves, or simply to hide. Any larvae that fails to elicit these behaviors (throughout their larval life) would be toast.
Another behavioral response to the threat of predation is something called "thanatosis," which basically means freezing in place. When monarch caterpillars sense an aerial predator, they will stop eating or moving and freeze in place, in hopes of avoiding detection.
Now, let's talk about parasitoid predators, like tachinid flies! These hated species get no love, but really they should, since these native flies are better pollinators than monarchs! But their life cycle is weird (maybe even gross) to us humans, plus they take out the caterpillars, and so they get no love...sigh. Anyway, as we all know, tachinid fly larvae develop as maggots within the caterpillars, utilizing the resources and tissues of the host for their own development. When they are big enough, they dig their way out of the host, often at the time the host tries to pupate, and this tends to kill the host.
But here's the thing that people don't realize - monarchs can combat this parasitoid with their own immune system. Their immune system is designed to take out anything inside them that is foreign, including parasitoids. This is more effective in the early stages of the parasitoid infection. But, monarchs with a strong immune system can kill the parasitoid and live to tell the tale. But, there is another but - having a strong immune system comes at a cost. It is an energy drain on other bodily functions. Mounting a strong immune response sometimes means the larvae experience slow growth, or, reduced immunity against other threats, as I'll mention next.
Surviving infections and disease
There are a suite of pathogens that can kill monarch larvae, including bacteria, viruses, and the dreaded OE parasite. I won't get into these in any detail here. Most of these have been studied in lab settings, where experiments can be performed, and we certainly know most about the OE parasite (see www.monarchparasites.org for more info) compared to the other diseases. But from all of this work, it is clear that the monarch immune system needs to be robust and functioning to defend against these infections. Those monarchs that have weakened immunity will be at greater risk of perishing before they even get to the pupal stage.
Surviving the weather
That's right, monarch caterpillars have to deal with the weather. Strong rains can wash the caterpillars off the milkweeds, and if they are too weak to get back on the plants (or if they get lost on the ground), they'll starve. The same goes for strong winds that knock the caterpillars off. In either case, caterpillars must either hold on tight with their feet, or maybe move to a protected side of the plant. Remember, they are in this caterpillar stage for up to a week, and any one storm during that time can be fatal.
Extremes of heat and cold can affect caterpillars too, but they have a built-in defense for this, in their body design. The black and while (and yellow) stripes of monarch caterpillars serves not only for advertising their toxicity, but also helps them deal with temperatures. Specifically, the black stripes can become larger or smaller, so as to either absorb more solar energy, or to help avoid the solar energy. In other words, caterpillars growing in a cold environment (or season) tend to be more black, and vice versa. But, if the caterpillars don't do this effectively, they could either heat up too much, or get too cold. Added onto this is another defense that people sometimes forget: caterpillars can move themselves around on the milkweed to deal with temperatures too. They can crawl under the leaves to avoid the heat, or, move to the top of the leaves to get the sun's warmth if it becomes too cold.
Surviving their comrades
In the wild, there are sometimes multiple caterpillars on the same milkweed, and when that happens, there is some drama. If there are too many caterpillars on one plant, they can quickly eat the plant down to the stem, and then everyone runs out of food (and dies). So, monarch caterpillars seem to instinctively eat faster when there are other caterpillars around. This has been shown in lab settings multiple times. It appears to be a means of beating the other guys to the punch.
Then, there is the more awkward part - monarchs can become cannibalistic at high densities. For example, the very tiny instars will consume some or all of the unhatched eggs nearby, possibly as a way to reduce their competition. If they don't do this, then they run the risk of running out of food. So in this case, it is a matter of "survival of the cannibals!"
At later development stages, monarch caterpillars are known to take bites out of nearby chrysalids, sometimes killing them. This is often seen in lab settings, or close quarters, and often when the milkweed supply runs low. But if you think about it, this behavior can help the cannibal to survive, because it provides them with an added snack, thereby helping it to reach their own pupal stage.
So for monarch caterpillars, eating your friends is a strategy for survival.
Surviving human-caused threats
Up until now, I've talked about the natural threats to caterpillars and how they deal with them, but as we all know, we humans have affected the landscape in many ways, and effectively added yet more threats to caterpillars. But keep in mind here the research showing how the monarch breeding population is stable - meaning that despite these added threats, monarchs are still doing ok. Again, these added threats are NOT a reason why we humans have to step in and rear the monarchs in captivity.
The first is man-made chemicals and pollutants. Think here of pesticides, which are by default, designed to kill insects. These chemicals get into the milkweed plants, especially if they are in agricultural landscapes. Believe it or not though, monarchs can tolerate some amount of chemicals, more so than other insects, but this still comes with a cost in terms of bodily energy drain. And for some reason, monarchs are champs at surviving neonicotinoid pesticides - really!
Another threat in this category is roadsides, which are highly polluted environments. Fumes from the cars, runoff from the roads, etc., all wind up in the nearby milkweeds, which the caterpillars consume. Multiple studies have shown that the caterpillars in these environment contain all of these toxins. Also, in the northern areas, all of the salt used to keep the winter roads safe eventually winds up in the summertime milkweeds, and then the monarchs themselves! So, monarch caterpillars living next to roads must be able to tolerate all of these chemicals to survive to the adult stage. Again, their immune system is of importance here.
Another aspect to consider here is the noise of the roadsides, which has been shown to stress monarch caterpillars (by my lab). While this threat is not fatal, it can be energetically taxing, which would take away energy from other important bodily functions, like the immune system, or digestion. We really don't know enough about this stress response in monarchs to be able to tell how much it impacts them. But think also of the noise in your own backyard - do you ever run a lawnmower right next to your milkweed bed? How about a leafblower? What does that noise do to the monarchs?
Lastly, I'll put in a section here about something that I recently blogged about, which is electromagnetic radiation. This is a human-caused threat, not only to monarchs, but to us humans! As I mentioned in the blog, this threat is everywhere, and we don't really know what it is doing to monarchs. Read the prior blog article to learn more about this one.
Concluding thoughts
I'm reaching the end of what I can put down off the top of my head here, so I'll wind down for now, and leave you with this thought. If you've come away with anything here, it should be that the life of a monarch caterpillar is filled with dangers and stressors. Any ONE of these issues above can take out a caterpillar. And, keep in mind that they often face multiple threats or issues at once. Really, it is no wonder that so many perish.
You should also come way with this - to survive to the adult stage, caterpillars must be smart enough, strong enough, wily enough, tough enough, AND have the best bodily functions, to be able to handle all of these "threats." Plus, have a little bit of luck. Given all of these, you can see that by the time the caterpillars have turned into butterflies, these individuals really do represent the best of the best.
This is natural selection.
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