How the Monarchs Die

2010 September 11

This was a good year locally for Monarch butterflies. We have a lot of milkweed growing around the place, and in particular there is a smooth-leaved variety growing back in the woods that the Monarchs really like. Around the middle of July, it was to the point that practically every milkweed plant had at least one Monarch egg on it, and the young caterpillars were everywhere[1].

And yet, there were not nearly as many adult butterflies as there were caterpillars. There was obviously a lot of attrition between the dozens of eggs laid by a given butterfly, and the relative handful of adult butterflies that appeared. What happened to them?

(A) The Red Death
Well, the attrition starts right away. S_ found this tiny mite (maybe one or two millimeters long) on a milkweed plant near one of our old outbuildings, and noted that it was eating a monarch egg[2]. These red mites are pretty common, so they probably account for a large fraction of the eggs. This is probably also one of the reasons[3] why the mother Monarch only lays maybe one or two eggs per milkweed plant: if the eggs were laid in a cluster, things like these mites would probably just find the cluster and gorge on them.

(B) The Invisible Death
And, once they hatch out, there are other predators. This one is being eaten by what I think is a crab spider [4]. The excellent camouflage of the spider, along with its ability to quickly snatch insects before they can react, means that the caterpillar probably never had a chance.

(C) The Black Death
At one point, we noticed that a number of the caterpillars we saw back in the woods were significantly darker than the others. Their black banding was wider and darker than normal. Compare these two:

Dark Caterpillar:

Normal Caterpillar:

At first, we thought that these were just a darker color phase, and that the reason we were seeing several of these dark ones was that they were all from the same mother. But, the one that we brought home then promptly turned even darker and died.

S_ did some checking online, and found some other people who have reported finding darker-than-normal Monarch caterpillars that then died. It appears to be an infection by one of the Pseudomonas bacteria that kills them. S_ has also found some black, foul-smelling Monarch chrysalises that were probably killed by either the same bacteria, or by nuclear polyhedrosis virus. Given the number of infected caterpillars we were seeing on the plants back in the woods, I’d estimate that at one point the infection rate was probably close to 25%

So, there’s three confirmed ways that they can fail to grow up – egg eaters, predators on the very young, and disease. And then, once they grow up, there are a few birds that eat the adults in spite of the toxins in them. Plus the ones who are killed by damage due to high winds, or being struck by cars. Overall, the fraction who actually survive to migrate to Mexico and lay eggs in the spring is probably very small. Which is why they have to lay lots and lots and lots of eggs.
[1] We have raised up several Monarch caterpillars to adulthood every year since Sam was old enough to appreciate them. This year, we started with three, but it turned out that when we went out to get fresh milkweed leaves to feed them, these leaves frequently had eggs or very young caterpillars on them that we didn’t see at first. By the time the season ended, we had raised up about five, even after giving three of them to the public library for the children’s section. The same thing happened at the library: I understand that their three caterpillars ultimately became seven butterflies after all of the accidental introductions on the fresh milkweed. The kids at the library loved them, by the way.

[2] The primary defense of Monarchs against getting eaten is the foul-tasting and somewhat toxic chemicals that they concentrate in their bodies from the milkweed that they eat. But, the eggs haven’t eaten anything yet, so they are probably completely lacking in defensive chemicals.

[3] The other reason for only one or two eggs per milkweed plant, is that the caterpillars eat a lot. To the point where one caterpillar can eat about a third of the leaves on an average milkweed stalk, and two or three can strip it down to a bare stick. It’s a good thing that milkweed is a noxious weed rather than a cash crop, because otherwise Monarchs would be considered a major agricultural pest.

[4] The very young monarchs evidently don’t contain as high of a level of the defensive chemicals as the older ones do. They probably can’t quite tolerate the chemicals themselves at that point. The fresh-hatched caterpillars have a way of eating the milkweed leaves that minimizes the amount of the toxic milky sap that they eat. Of course, that makes them much more edible themselves than they will be later.

9 Responses
  1. September 11, 2010

    What a great job of photojournalism catching all those predations. Outstanding as usual.

  2. September 12, 2010

    The interesting thing is that we weren’t actually looking for predations or disease, these are just the ones that we stumbled across accidentally while looking for milkweed leaves to feed the caterpillars we were raising. This implies that there are a lot of predators out there that we didn’t document, and that the caterpillars are, overall, having a pretty rocky time of it.

  3. Della3 permalink
    September 19, 2010

    Great picture of the red mite. I never knew they were hairy!

  4. September 19, 2010

    Great post. The red mite is a member of the Erythraeidae, probably a generalist, so only bad luck for the Monarch rather than searching them out. There is a related mite that attacks the eggs of the endangered Richmond Birdwing in Queensland, but it’s a tough life being a bug, even if you are a butterfly. As I recall, a certain percentage of the first instar larvae actually drown in the milky sap of the milkweed too – the ones that can’t cut the veins fast enough to stop the plant’s defensive bleeding.

  5. September 20, 2010

    Dave: Thanks for the mite ID. We have a lot of little red mites around, and I rarely get a good enough photo to identify any of them.

    And yes, I agree that it’s not easy being a bug.

  6. Lyle Laylin permalink
    March 8, 2018

    I can add a death dealer for Monarchs
    Assassin bugs
    I have photos from over at Pictured Rocks of one sucking a Monarch dry

  7. Lyle Laylin permalink
    March 15, 2018

    Ah HA!
    After digging through my old photos.
    It was NOT an Assassin bug it is in fact the same as your Red Stinkbug nymph.
    My photos are almost exactly the same, except you can see that wicked proboscis in action.

  8. August 17, 2018

    I’ve raised a few monarchs that as caterpillars had darker banding, not too different than the one you show, and they didn’t die or get sick from black death.

  9. She Cossa permalink
    September 25, 2019

    Here in Plymouth, MN, I saw wasps going after monarch cats in several different places. Apparently, they feed them to young wasps.

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