Predatory Bug Committing Regicide

2022 October 23
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On July 2, 2022, we happened to notice this little drama taking place on one of the milkweed plants next to our back door. A predatory bug had killed a monarch caterpillar[1], and had already sucked out a significant fraction of its bodily fluids.

It wasn’t really in a good location for photography, and while trying to manipulate the milkweed plant for a better view, the bug moved off of the caterpillar corpse. It sill kept close enough to continue sticking in its proboscis and sucking out more material from the caterpillar.

We can see how the proboscis is quite large around, allowing fluids to be drawn through it quite quickly. It is also jointed so that it can be extended out and poked into the prey easily. This is quite different from the proboscis that you see on plant-feeding bugs, which are typically thinner and not jointed [2].

The wings haven’t grown in yet on this bug, so it is clearly still a nymph. Although, with this gigantic meal, it probably is ready to finish growing into an adult now.

While I have found predatory stink bugs nymphs before, the ones I saw previously were much more brightly colored, while this one is quite subdued. I expect it is one of the Podisus species, although to find out which one I should have finished rearing it to adulthood.

While poking around on BugGuide, I saw that a lot of people posted pictures of predatory stink bugs eating monarch butterfly caterpillars. This kind of suggests that these bugs are one of the primary predators of monarchs. Which brings up a point: birds and mammals don’t generally eat monarchs, because they are full of “cardiac glycosides” that the caterpillars collect from the milkweed that they are eating. This makes them taste bad, and in high enough dosages can even cause heart failure. This is why the caterpillars are so prominently colored. It is a warning to things with sufficient brains to have a memory, that these caterpillars are Not Good To Eat. And yet, this bug can apparently eat them with impunity. Why?

As near as I can find out, the reason is probably that monarch caterpillars mainly sequester the milkweed toxins into, or just under, their skins. They do this because having the toxin all through the caterpillar’s body would poison the caterpillar, too. This is fine against predators that eat the whole caterpillar, like birds and wasps and predatory beetles. But, these predatory bugs poke right through the poisonous skin and feed directly on the caterpillar’s largely toxin-free body. So, the part of the caterpillar that is not toxic is what these predatory bugs eat. This would explain why, when I have seen monarch caterpillars being preyed on in the past, it has been by predators that suck juices out of them, and not generally the ones that just straight-up devour them. Although, I suppose that a sufficiently discriminating wasp could peel them like a banana and just eat the insides.

While we are on this topic, another thing that can get past the skin to prey on monarchs is apparently parasitoid flies. This caterpillar had four little eggs laid on its head end, which were going to burrow through its skin once they hatched. Again, this would give them access to the non-toxic insides in spite of the toxins in the skin.


[1] “Regicide – the act of killing a monarch”. Well, that’s certainly what this is, yes?

[2] The thin, unjointed proboscis of plant-feeding bugs probably makes it easier to drill into tough plant tissues. The differences between the predatory bug proboscis and the herbivorous bug proboscis is probably mainly due to the fact that animals try to run away when something tries to eat them, making speed a priority. Plants, on the other hand, can’t attempt to escape and so the plant-feeding bugs can eat at their leisure, without a proboscis that emphasizes speed.

One Response
  1. December 23, 2022

    The regicide was inspired.

    I would imagine that the beetle would have to make several meals of this caterpillar, waiting for it’s relatively small stomach to empty.

    Great analysis of the puncture predators versus the devour predators!

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