All right! Today's Twig or Stem All Pecked Up by Birds belongs to elder, Sambucus sp. I ran across this example the other day at Footbridge Farm, while housesitting for friends John and Jana.
Arrows point to holes pecked in the stem. Count them with me now!
Eleven holes can be made out (some more easily than others) from this view...and, counting the ones further up or down the stem, or on the side not visible to the camera, there were 17 total holes! On one stem! That's some serious foraging.
Here's part of another elder stem showing some bird holes in more detail.
One problem with trying to figure out which insect the birds were after when you find something like this is that, well, the birds ate what they were after...so it's not there anymore. And, of course, even when the birds miss a few larvae or pupae in the stem, if the foraging event happened a while ago, say, in early summer -- and any surviving insects have since emerged -- then you're also out of luck. Since the bird holes on these elder stems looked pretty weathered, I figured that second scenario was probably the case here: even the insect survivors had long since departed. But I dutifully opened the stems up anyway, just to see what was in there.
Sure enough, they were mostly empty. I found obvious signs of pith tunneling, probably by larvae of some kind of moth or beetle -- perhaps what the birds had been hunting -- but no actual insects.
Except, that is, for this one.
As you may recognize, this is a caterpillar -- the larva of a moth (order Lepidoptera). Here's a head shot.
This detail of a lateral (from the side) shot shows two of the caterpillar's spiracles, through which it breathes (look closely to see the two small oval openings in the caterpillar's exoskeleton).
Unfortunately, there is something seriously wrong with this caterpillar. For one thing, it's dead. But besides that...can you see how it's sort of swollen?
(The big whitish thing in these shots is a paper point, to which I've glued the specimen.)
If you were to gently squeeze this caterpillar between your thumb and forefinger, you would find it to have no give at all -- it is stiff. Hard. Dry.
It's a caterpillar mummy!
And guess what: another animal made it into a mummy.
Nope, not a spider. We're talking about something way weirder than a spider. Something that's actually still inside the caterpillar.
A mummy-wasp!
There are over 200 North American species of wasps in the genus Aleoides, and they are known collectively as the mummy-wasps. These aren't social wasps like yellow jackets; they don't build big elaborate nests and they can't sting you. Instead, they are specialized parasitoids of caterpillars. Unlike a parasite (a human head louse, for instance), which lives on or in its host without killing it, a parasitoid kills its host to complete its larval development. Many species of parasitoid wasps emerge from their host's corpse as fully grown larvae, and then spin a cocoon externally. Mummy-wasps, however, have figured out how to manipulate their host so that its tissues harden and dry with the immature wasp still inside -- effectively creating a sheltered "cocoon" in which the wasp can pupate and undergo the transformation to adulthood.
More pictures of caterpillar mummies made by these wasps: Mummy #1 | Mummy #2 | Mummy #3
And here's a great image from a scientific paper1 showing a mummy-wasp adult, its host caterpillar (while still healthy-looking), and the mummy (#97) whence the adult wasp emerged.
In most cases just one adult wasp emerges from each mummy, but larvae of Aleoides stigmator mummy-wasps develop gragariously in a host, so that many adults may come out of a single mummy -- leaving it riddled with neatly round exit holes.
That won't be the case with the mummy from the elder stem, though. A. stigmator apparently emerges from its host in summer or fall and does not overwinter inside it. Since I don't see any exit holes, the maker of this mummy is probably some other species of Aleoides -- and it's still inside its host's hardened remains, waiting for spring.2
---
This is all very interesting, but unless we can figure out the identity of the mummified caterpillar, we're no closer to determining which particular insect the birds are looking for in elder canes. And even if we can identify the mummy, there's no guarantee it's what the birds were after. How do we know this caterpillar (pre-mummification) didn't just wander into the cane a few months after the cane was raided by birds? (Many foliage-feeding caterpillars undergo a "wandering phase" just before pupation, and they can wind up in all sorts of interesting nooks and crannies.) So...where do we go from here?
Let's leave the subject of bug-mummies and turn instead to the first photo in this post -- the one with the long section of elder cane, all pecked full of holes. Here's a detail from that image to jog your memory (I've rotated it ninety degrees).
Now, with that in mind, check this out:
OK, so who exactly is "Achatodes zeae"? And what's with that weird font?
Well, the font's not weird, it's just old, that's all. I think old fonts are pretty cool.
To be specific, this font is at least 85 years old, as of this month. February 1933 is, as far as I can tell, the last time anyone snapped a picture of an elder cane raided by birds and then tried to share the photo (and a written explanation) with the world. My über-retro counterpart (if I may call him that) is a certain J.C. Silver...and here's part of the title page from Silver's writeup3:
The "spindle worm" Silver refers to in his title is Achatodes zeae -- the creature those hungry birds were looking for when they tore into the elder stems shown in that black-and-white photo. Here, Silver gives the full story behind the photo.
In this article he also includes an image showing spindle worm larvae inside elder canes. (This animal is also referred to as the elder borer for its habit of feeding inside Sambucus stems.)
As it turns out, MJ Hatfield has collected these caterpillars from elder right here in Winneshiek County, in an attempt to rear them to adulthood.
Pretty striking, eh? Interestingly, the spindle worm bears a strong resemblance to the parsnip webworm -- another insect that bores into herbaceous stems and is there sought out by birds. (I wrote about that bird-plant-insect association in A smart bunch, part 2.)
The likeness is uncanny enough to make me wonder if some sort of mimicry complex might be going on here -- a game of evolutionary bait-and-switch in which one or more animal species evolves to resemble an unrelated model species that predators (like downy woodpeckers) already recognize as distasteful. But that's just speculation....
---
It sure seems like J.C. Silver and I may have both observed evidence of the same kind of insect, albeit eight and half decades apart. But it's important to remember that many plants may have their stems tunneled out by more than one species of insect -- and the sign left behind is not always clear and diagnostic for a particular culprit. (I suspect the mummified caterpillar is something other than a spindle worm -- you may notice it doesn't resemble an A. zeae larva very closely.) We'll have to wait for spring and summer in order to confirm that spindle worms are common enough around here to attract downy woodpeckers to their elder stem hideouts.
For now, I'll just say this appears to be yet another example of how intelligent woodpeckers are. I got some much-appreciated vindication on this point just the other day, when I picked up my housemate's copy of the February 2018 issue of National Geographic. When I turned to the illustration comparing the known or hypothesized "smarts" of various birds, I whooped with joy. There -- right next to the common raven and the gray parrot, two of humanity's most celebrated "bird brainiacs" -- was our very own pileated woodpecker! The illustration ranks birds based on the proportion of their brain devoted to the forebrain, explaining that, according to a prevailing hypothesis, "the bigger a bird's forebrain as part of its entire brain, the smarter the bird." Pileated woodpeckers, having 77% forebrain mass, are neck and neck with common ravens (80%) and gray parrots (79%)...and, among the bird species studied, pileateds ranked second in "innovativeness," behind only ravens.4
Downy woodpeckers were not included in the study; and yes, I know, I know, leave it to humans to rank other animals according to the size of their brains. But this illustration sent my mood sky-high. The more I watch woodpeckers, the smarter they seem...and the more I want their intelligence to be witnessed by others, to be commonly appreciated. When you think about it, what these animals do is remarkable. They survive the coldest, harshest months of the year by locating little insect larvae hidden away behind walls of cellulose. For this they may seem quirky, or odd, or not quite "with it" -- the stuff of Woody Woodpecker cartoons -- but, to me, quite the opposite truth is becoming clear: they absolutely deserve our respect.
---
Notes
1. Shimbori, E.M. and S.R. Shaw. 2014. Twenty-four new species of Aleiodes Wesmael from the eastern Andes of Ecuador with associated biological information (Hymenoptera, Braconidae, Rogadinae). ZooKeys 405: 1-81.
2. However, Aleoides isn't the only genus to which our mystery mummy-wasp could belong. There's a whole subfamily of mummy-wasps -- the Rogadinae -- containing several other genera.
3. Silver, J.C. 1933. Biology and morpohology of the spindle worm, or elder borer. Technical Bulletin No. 345. United States Department of Agriculture: Washington, D.C.
4. https://www.nationalgeographic.com/magazine/2018/02/bird-brains-crows-cockatoos-songbirds-corvids/
Arrows point to holes pecked in the stem. Count them with me now!
Here's part of another elder stem showing some bird holes in more detail.
One problem with trying to figure out which insect the birds were after when you find something like this is that, well, the birds ate what they were after...so it's not there anymore. And, of course, even when the birds miss a few larvae or pupae in the stem, if the foraging event happened a while ago, say, in early summer -- and any surviving insects have since emerged -- then you're also out of luck. Since the bird holes on these elder stems looked pretty weathered, I figured that second scenario was probably the case here: even the insect survivors had long since departed. But I dutifully opened the stems up anyway, just to see what was in there.
Sure enough, they were mostly empty. I found obvious signs of pith tunneling, probably by larvae of some kind of moth or beetle -- perhaps what the birds had been hunting -- but no actual insects.
Except, that is, for this one.
As you may recognize, this is a caterpillar -- the larva of a moth (order Lepidoptera). Here's a head shot.
This detail of a lateral (from the side) shot shows two of the caterpillar's spiracles, through which it breathes (look closely to see the two small oval openings in the caterpillar's exoskeleton).
Unfortunately, there is something seriously wrong with this caterpillar. For one thing, it's dead. But besides that...can you see how it's sort of swollen?
(The big whitish thing in these shots is a paper point, to which I've glued the specimen.)
If you were to gently squeeze this caterpillar between your thumb and forefinger, you would find it to have no give at all -- it is stiff. Hard. Dry.
It's a caterpillar mummy!
And guess what: another animal made it into a mummy.
Nope, not a spider. We're talking about something way weirder than a spider. Something that's actually still inside the caterpillar.
A mummy-wasp!
There are over 200 North American species of wasps in the genus Aleoides, and they are known collectively as the mummy-wasps. These aren't social wasps like yellow jackets; they don't build big elaborate nests and they can't sting you. Instead, they are specialized parasitoids of caterpillars. Unlike a parasite (a human head louse, for instance), which lives on or in its host without killing it, a parasitoid kills its host to complete its larval development. Many species of parasitoid wasps emerge from their host's corpse as fully grown larvae, and then spin a cocoon externally. Mummy-wasps, however, have figured out how to manipulate their host so that its tissues harden and dry with the immature wasp still inside -- effectively creating a sheltered "cocoon" in which the wasp can pupate and undergo the transformation to adulthood.
More pictures of caterpillar mummies made by these wasps: Mummy #1 | Mummy #2 | Mummy #3
And here's a great image from a scientific paper1 showing a mummy-wasp adult, its host caterpillar (while still healthy-looking), and the mummy (#97) whence the adult wasp emerged.
In most cases just one adult wasp emerges from each mummy, but larvae of Aleoides stigmator mummy-wasps develop gragariously in a host, so that many adults may come out of a single mummy -- leaving it riddled with neatly round exit holes.
That won't be the case with the mummy from the elder stem, though. A. stigmator apparently emerges from its host in summer or fall and does not overwinter inside it. Since I don't see any exit holes, the maker of this mummy is probably some other species of Aleoides -- and it's still inside its host's hardened remains, waiting for spring.2
---
This is all very interesting, but unless we can figure out the identity of the mummified caterpillar, we're no closer to determining which particular insect the birds are looking for in elder canes. And even if we can identify the mummy, there's no guarantee it's what the birds were after. How do we know this caterpillar (pre-mummification) didn't just wander into the cane a few months after the cane was raided by birds? (Many foliage-feeding caterpillars undergo a "wandering phase" just before pupation, and they can wind up in all sorts of interesting nooks and crannies.) So...where do we go from here?
Let's leave the subject of bug-mummies and turn instead to the first photo in this post -- the one with the long section of elder cane, all pecked full of holes. Here's a detail from that image to jog your memory (I've rotated it ninety degrees).
Now, with that in mind, check this out:
If you look closely, you can see holes in the stems. Here's the caption that came with this image:
Well, the font's not weird, it's just old, that's all. I think old fonts are pretty cool.
To be specific, this font is at least 85 years old, as of this month. February 1933 is, as far as I can tell, the last time anyone snapped a picture of an elder cane raided by birds and then tried to share the photo (and a written explanation) with the world. My über-retro counterpart (if I may call him that) is a certain J.C. Silver...and here's part of the title page from Silver's writeup3:
The "spindle worm" Silver refers to in his title is Achatodes zeae -- the creature those hungry birds were looking for when they tore into the elder stems shown in that black-and-white photo. Here, Silver gives the full story behind the photo.
In this article he also includes an image showing spindle worm larvae inside elder canes. (This animal is also referred to as the elder borer for its habit of feeding inside Sambucus stems.)
As it turns out, MJ Hatfield has collected these caterpillars from elder right here in Winneshiek County, in an attempt to rear them to adulthood.
 |
| © MJ Hatfield |
Pretty striking, eh? Interestingly, the spindle worm bears a strong resemblance to the parsnip webworm -- another insect that bores into herbaceous stems and is there sought out by birds. (I wrote about that bird-plant-insect association in A smart bunch, part 2.)
 |
| Parsnip webworm © Tarmo Lampinen |
---
It sure seems like J.C. Silver and I may have both observed evidence of the same kind of insect, albeit eight and half decades apart. But it's important to remember that many plants may have their stems tunneled out by more than one species of insect -- and the sign left behind is not always clear and diagnostic for a particular culprit. (I suspect the mummified caterpillar is something other than a spindle worm -- you may notice it doesn't resemble an A. zeae larva very closely.) We'll have to wait for spring and summer in order to confirm that spindle worms are common enough around here to attract downy woodpeckers to their elder stem hideouts.
For now, I'll just say this appears to be yet another example of how intelligent woodpeckers are. I got some much-appreciated vindication on this point just the other day, when I picked up my housemate's copy of the February 2018 issue of National Geographic. When I turned to the illustration comparing the known or hypothesized "smarts" of various birds, I whooped with joy. There -- right next to the common raven and the gray parrot, two of humanity's most celebrated "bird brainiacs" -- was our very own pileated woodpecker! The illustration ranks birds based on the proportion of their brain devoted to the forebrain, explaining that, according to a prevailing hypothesis, "the bigger a bird's forebrain as part of its entire brain, the smarter the bird." Pileated woodpeckers, having 77% forebrain mass, are neck and neck with common ravens (80%) and gray parrots (79%)...and, among the bird species studied, pileateds ranked second in "innovativeness," behind only ravens.4
Downy woodpeckers were not included in the study; and yes, I know, I know, leave it to humans to rank other animals according to the size of their brains. But this illustration sent my mood sky-high. The more I watch woodpeckers, the smarter they seem...and the more I want their intelligence to be witnessed by others, to be commonly appreciated. When you think about it, what these animals do is remarkable. They survive the coldest, harshest months of the year by locating little insect larvae hidden away behind walls of cellulose. For this they may seem quirky, or odd, or not quite "with it" -- the stuff of Woody Woodpecker cartoons -- but, to me, quite the opposite truth is becoming clear: they absolutely deserve our respect.
---
Notes
1. Shimbori, E.M. and S.R. Shaw. 2014. Twenty-four new species of Aleiodes Wesmael from the eastern Andes of Ecuador with associated biological information (Hymenoptera, Braconidae, Rogadinae). ZooKeys 405: 1-81.
2. However, Aleoides isn't the only genus to which our mystery mummy-wasp could belong. There's a whole subfamily of mummy-wasps -- the Rogadinae -- containing several other genera.
3. Silver, J.C. 1933. Biology and morpohology of the spindle worm, or elder borer. Technical Bulletin No. 345. United States Department of Agriculture: Washington, D.C.
4. https://www.nationalgeographic.com/magazine/2018/02/bird-brains-crows-cockatoos-songbirds-corvids/
