A smart bunch, part 2

A thought inspired by the loveliness of this blue-sky January day:

What I lack is a context for beauty.
What does it mean, that the world is this beautiful?
What do we do, given that the world is this beautiful?

---

In part 1 of this post, we considered a few examples of bird sign on dead wood and on stems of two herbaceous plants, figwort and jewelweed.  Here we'll see examples from the stems of two other herbs.




COW PARSNIP

Heracleum maximum is a sort of coarser, jollier, more badass cousin to the ubiquitous wild parsnip (Pastinaca sativa).  Cow parsnip and wild parsnip are easier to tell apart during the growing season, but there are obvious differences between their dead stems in the dormant season too.  Above, you can see a dead stem of wild parsnip on the left, and one of cow parsnip on the right.  Below, fruits: wild parsnip at left, cow parsnip at right.

While visiting some friends early this past summer, I noticed that stems of the cow parsnip growing in their yard had been attacked heavily by some critter trying to get in.  It wasn't until many months and two seasons later that I finally returned to Lindy and Lee's in order to document this damage.  The plant stems, long since dead and dried, had broken off and fallen to the ground, and some were buried by snow, but I gathered what I could.  The attackers' work was still evident.

Now, as with jewelweed, the scientist in me insists on mentioning that I have not directly observed the animal responsible for this.  However, based mostly on the form and appearance of the holes, I assert here that a bird is responsible.  Someday one of us will catch the culprit in the act!

Unlike jewelweed and figwort, the target insect does not overwinter in the stem.  Rather, its stem-dwelling stage -- the larva and pupa -- occurs in the late spring and early summer, when cow parsnip is maturing (and right around the time I first saw damage to these stems at Lindy and Lee's).  This insect is the parsnip webworm, Depressaria radiella.  Though named, and thus clearly known, for its habit of spinning webs in the flower heads of cow parsnip and wild parsnip, some larvae of the parsnip webworm enter their host plant's stems, which, for cow parsnip at least, are naturally hollow.  (Presumably the larvae cut holes in the stems in order to get in and out; this could account for a few of the stem holes evident in the first stem-damage photo above.)  Furthermore, some of the larvae will actually pupate inside the stems -- and I suspect it's this pupal stage that the predators were after in the plants at Lindy and Lee's.

Before pupating inside its dark and hollow stem-home, a parsnip webworm larva apparently first etches an oval scrape on the inside of the stem.  The larva either eats this excavated material, converting it to frass (caterpillar poop), or simply shaves it off the stem, resulting in an accumulation of sawdust.  Either way, it collects the granular byproduct and incorporates it into a domelike cocoon, which it spins over itself just prior to pupating.  The results (but with the culprits long gone) are shown below.

First, a piece of stem interior on which four larvae pupated.  The most obvious oval scraped areas are on the far left and far right; two more are clustered at center.  I've removed the domed cocoons that were spun over three of the scraped areas; the fourth cocoon (at far left) has been flipped over to reveal the scraping underneath it.

 

 Two cocoons, one in the top stem, one in the bottom stem (and part of a third cocoon, I think, at upper right).  Larvae scraped the stem underneath these cocoons,  processed it into "sawdust" or frass, and finally incorporated it into the cocoons.

 

 Cocoon

 Wall of cocoon flipped over, revealing shed skins of larva and pupa, indicating successful emergence of an adult moth

 Detail from inside the above cocoon, taken with my "bug scope."  After spinning the cocoon (top), the larva sheds its skin (right) to become a pupa.  When the adult eventually emerges from the pupa, it leaves the pupal skin behind (left).  (And by "skin," I mean exoskeleton.)

Here's a sequence showing another cocoon, its leftover (spent) contents, and the adult that emerged from it:

The adult's wings are improperly formed in this case.

If this moth looks vaguely familiar, it may be because you've seen it in your house!  I saw one in my bathroom last week.  And, under her image of a nice adult specimen on Bugguide, my friend and collaborator MJ Hatfield writes, "Yes, there is a lot of parsnip in the neighborhood.  But still, why do more of these show up in the house than any other moth?  Perhaps under bark of firewood? I don't know."  Neither do I, MJ!

WILD LETTUCE

One winter day, while visiting friends John and Jana, I heard someone a-tap-tap-tapping away in the yard over by the guest cabin.  It was unquestionably a woodpecker, but the tapping had a sort of treble quality to it that suggested the bird was tapping on something other than wood.  Walking over to investigate, I frightened a downy woodpecker from a dead stem of a tall, weedy herb.  The stem belonged to a wild lettuce, Lactuca biennis.  Here's a different stem of the same plant species, also from John and Jana's, with the author attempting to provide scale but mainly just looking like a super dork.  As you can see, this herb gets to be quite tall!

Seedhead of Lactuca biennis, with most of the seeds already gone

Well, when I got close to examine the downy woodpecker's wild lettuce stem, I was astonished and delighted at what I found.  A significant portion of the plant stem had been stripped or pecked away.  (I stripped away more as I investigated, then took the photo below.)  This activity had exposed the pale inner pith of the hollow stem, and this exposed pith, all up and down the stem, had been thoroughly pecked over by birds.  Below, the whole dead stem is shown (at left), along with two details of affected portions (at right).

Here's a detail of an affected area near the base of the stem:

At first, I thought all those brown specks were simply peck marks left by birds.  As I looked closer, I noted that the birds had indeed excavated the pith in these areas.  But their pecking hadn't caused the brown specks.  In fact, the "specks" were little round chambers in the pith, hundreds of them, each one emptied by the birds of whatever had been inside it.

To illustrate what I mean, here's another dead stem of a wild lettuce plant attacked by hungry birds.  I found this one on a property in Clayton County, IA where friends Joe, Conor, and I spent some time last year.

As they peck and probe the pith, the birds' bills leave small indentations (labeled, at center of inset).  However, this sign is quite distinct from the brown "specks" -- more clearly identifiable here as chambers (see label at upper right of inset) -- that are constructed in the pith by the insect that the birds are harvesting.  As it turns out, before it gets raided by a bird, each of these little brownish chambers contains a single cream-colored insect larva:

This is the larva of a tiny, black, non-stinging wasp in the family Cynipidae.  Animals of this kind are known as "gall wasps" for their habit of growing to maturity inside the swellings they induce in plant tissue.  Not all plants affected by gall wasps show obvious swelling; this one didn't.  However, the tattered wild lettuce stem at John and Jana's did have some galls on one spot along its length:

The birds, of course, noticed.

 A cross-section of the swollen stem reveals two gall wasp chambers (arrrows).  I think the larvae fell out when I cut the stem open, which is why they aren't visible here.

Another gall on the stem, sectioned, showing larvae in their chambers

At the time, I didn't know exactly what this insect was, so I collected a few pieces of affected stem from John and Jana's and put them in a jar.  That was early April; by late May, adult gall wasps in the genus Aulacidea had begun to emerge.

These images, of a point-mounted adult in less than ideal lighting, really don't do it justice, though.  This is a lovely creature with which we are fortunate to share this Earth (and, honestly, it's kind of sad to see it dead in these photos I took!).  I wouldn't have known about it were it not for that enterprising downy woodpecker, so the bird deserves my gratitude too.

---
 
A final note: When I came across this bird-insect-plant association, it wasn't just the insect I couldn't identify; I wasn't quite sure about the plant, either.  As you might expect, identifying plants by their dead stems can be challenging.  However, it's been so useful to me as an amateur entomologist that I have spent the last few dormant seasons learning to do it, and it's not only possible, but also satisfying and enlightening.  Along the way I've picked up a trick or two.  One of these involves the leaves.

Though many flowering plants shed their leaves in fall, some leaves stay attached, especially on certain herbaceous plants, where they simply shrivel up and hang from the stems.  If the dead stem of a mystery herb has any such leaves still hanging on, try collecting a few of them (carefully, as they are quite fragile) and placing them in a shallow, flat-bottomed container of water.  The shriveled leaves will gradually rehydrate, and once they're waterlogged, you can (carefully) use a toothpick or other tool to gently pull them open and flatten them out (while they're still in the water, ideally).  This will reveal the general shape, veination, and other details of the leaf useful for making an identification.

I tried this rehydrated leaf method with a few leaves from the then-mystery plant occupied by the then-mystery insect at John and Jana's.  Here's the result:

Compare with the image here of a living leaf of Lactuca biennis.  It takes a bit of effort and patience, but this method works!

A smart bunch, part 1

Some birds find food by opening stems of herbaceous plants.

So much possibility
Yes there is, yes there is... --Anonymous

It's hard to call it a "dead stem" when it's so full of life. --MJ Hatfield



Driving along Iowa's state highway 9 a while ago, I found my eye caught by contrast: a little patch of lightness on an otherwise rather dark wooded slope.

A pileated woodpecker tree  -- one of those recently dead snags all torn up by the Upper Midwest's "Lord God bird" -- is a common enough sight around here...but this one seemed to be on a whole 'nother level.

 

That bird wasn't messing around!

People say pileated woodpeckers eat mostly carpenter ants, which may be true, but I've noticed they also seem to have a penchant for trees inhabited by wood-boring horntail wasps and their parasitoids, the giant ichneumons.  Here's my friend and collaborator MJ Hatfield, deftly showing off these and other insects in a pileated woodpecker tree from her backyard.

Smaller woodpeckers leave sign on trees that's less drastic but no less interesting.  This "woodpecker flute" has been in my natural-things collection ever since I found it on the campus of my alma mater, St. Olaf College.

Not long after I collected the "flute" I sawed off a centimeter or so on both ends to make them flat.  As you can see, this also emphasized the fact that the twig -- collected from a pine tree -- is hollow all the way through.  Presumably an insect (some wood-boring beetle?) hollowed it out or lived inside a pre-existing hollow in the twig.  I don't know exactly which type of insect, but clearly -- judging from the amount of effort involved -- the bird really wanted that bug.

Though it's not as widely recognized, some birds are also adept at locating insects hidden within the stems of herbaceous (non-woody) plants.  The classic North American example is a downy woodpecker or black-capped chickadee opening up a goldenrod gall, which contains a fly larva.  (Links take you to images of the birds at work on this task.)  But there are other examples, too.  In this two-part post, we'll explore a handful of obscure but fascinating associations between bird, insect, and herb.

JEWELWEED

With mesmerizing orange or yellow flowers (depending on species), and droplets of rain or dew beading readily on its leaves, jewelweed (Impatiens spp.) may entrance you.  In winter its dead stems are less likely to impress, but their unique pinkish or rusty color and weak knobby "joints" are still distinctive.

Inspecting such patches, you may find this:

 Note the ragged, rough appearance of these holes, suggesting they were made after the plant had dried down to its currently brittle state (rather than while the plant was still succulent and green).  It took me only a few minutes of searching this jewelweed colony on my friends' property (thanks, Lindy and Lee!) to find several more examples.  Look carefully, and you'll see at least one hole gouged in each stem piece in these images.

Though I've not yet had the privilege of watching it happen, I believe these stems were raided by birds -- for the purpose of harvesting a particular type of insect.  A mouse or other rodent could perhaps have left this sign, but to me, the small size and splintered appearance of the holes indicate they were made by a sharp, narrow object (a bird's bill or beak).  A predatory wasp might hypothetically chew into a stem to harvest insects inside, but its tiny mandibles should leave a much more delicately fashioned opening.

Assuming for now that this is a bird, what exactly is it looking for?  In my experience, herbaceous stems are not indiscriminately opened by birds hoping by dumb luck to find tasty things inside.  Rather, birds -- being the smart bunch that they are -- have learned to recognize and exploit specific associations between plants and insects.  In this case, the target is a caterpillar -- the larva of a certain species of moth (order Lepidoptera).  Larvae of this moth species overwinter in dead stems of jewelweed, apparently emerging as adult moths sometime in spring.  (I've tried to rear them to adulthood but haven't succeeded, so the species is unidentified as yet.)

These larvae are actually rather easy to find inside jewelweed stems at this time of year.  Try finding them yourself!  (Caveat: I haven't yet learned to tell the jewelweed species apart from their dead stems, so I'm unsure if it's both of our commonly encountered Impatiens species, or just one of them, that plays host to the larvae.)

FIGWORT

Plants in the genus Scrophularia, commonly called figworts, are related to garden snapdragons.  In fact, their plant family is known as Scrophulariaceae (say it with me!  Skroff - you - lair - ee - AY - cee - ee), which, while cumbersome, shows the importance figwort held to the people who named the family.  It's a surprise, then, that figwort is not better known.  I'll leave it to a couple of North America's great wildflower websites -- both Midwestern, by the way -- to help you recognize Scrophularia marilandica and S. lanceolata, which (I think) can both be found here in northeastern Iowa.  Let's get to the bird damage!

 And, inside a figwort stem* not yet raided by birds:

This insect species, once again in the form of a caterpillar that will one day be an adult moth, has been dubbed the "verbena bud moth" (scientific name: Endothenia hebesana).  It's an extreme generalist, known to feed on many different plant species.** Biologists refer to this habit as polyphagy, loosely translated as "many eater" or "varied eater."

However, just because this bug is flexitarian does not mean its relationships with plants are haphazard.  You can reliably find its larvae in stems of certain species of plants (and not others) at certain times of year -- which means the birds can, too.  That deserves emphasis: Even polyphagous insects may establish patterns of association with particular plant hosts -- and birds, humans, and anyone else who's interested can learn to recognize these patterns.

Consider a human analogy.  You may be willing to visit just about any c-store, diner, or coffee shop to get your daily caffeine buzz, but you probably have just a few places you prefer to go, and you probably go there at certain times and order certain drinks.  An undercover cop trailing you to document your nefarious coffee-drinking could learn your top haunts and favorite brews easily enough.  Same idea with our ecological example: Figwort stems in winter are one of this insect's preferred places...and the birds have found out!

---


If you're a caterpillar of the verbena bud moth who makes it safely through the winter inside a figwort stem, you'll most likely make a cocoon there, in which you will shed your final larval "skin" (exoskeleton) and become a pupa.  In the pupal stage, you wear a unique skin that looks a bit like a caterpillar, a bit like an adult, but not exactly like either.  Beneath this skin, much of your body is more or less dissolved and rearranged into a nearly-adult moth-to-be.  When you're ready, you thrust yourself out of the stem, break your pupal skin, and emerge as a newly minted, scale-winged flying creature.  (Lepidoptera, or "scale wing," refers to the microscopic scales that slough off the wings like dust when you handle an adult moth or butterfly.)

Finally an adult, you now get to wear a sort of gray-green camo on your brand new wings, and two little fireburst ornaments of rusty orange on your back.  Pretty cool!

---

In part 2 we'll examine two bird-plant-insect connections involving the plants cow parsnip (Heracleum maximum) and wild lettuce (Lactuca spp.).

A smart bunch, part 2

---

NOTES

* You may notice that, aside from the color differences, there's at least a passing resemblance between the larvae in jewelweed and figwort.  Indeed, it's possible the jewelweed moth larva may turn out to be Endothenia hebesana, too.  The easiest way to tell is to raise the larva to adulthood.

** If you go checking figwort stems for evidence of bird damage, you should know that verbena bud moth larvae actually make holes in figwort stems too.  However, they do this as part of their routine stem-dwelling activities, and their holes are quite different from those made by birds.  I've documented a larva-made stem hole and its architect here, on Bugguide.