A brutal fate (or why you shouldn't come back as a caterpillar)

Last Saturday, August 13, the Midwest Native Plant Society sponsored a one-day workshop on pollinators. It was well attended, with over 140 participants. The speakers were great, fun was had, and there were field trips following the indoor sessions.

The venue was the U.S. Army Corps of Engineers' fabulous visitor center at Caesar Creek Lake in Warren County. We really like holding events at this place, as the conference room is perfect for groups of up to 175 or so people, AND one only need step outside the doors to get into interesting habitat. As proof, one of our field trip leaders, John Howard (striped shirt, back to camera) shows a group a huge female Dobsonfly on the wall by the doors. There was also a stunning Cloudless Sulphur butterfly in the raised flower bed, near that rock.

At the end of the day, a couple of us were poking around the woodland edge near the visitors center when I spotted a Redbud, Cercis canadensis. As this tree is often productive for finding various slug moth caterpillars, we went over to inspect its foliage. It didn't take long before I spotted one of our most striking caterpillars, the Saddleback, Acharea stimulea. It's always a crowd-pleaser.

NOTE: The Saddleback in this photo is a proxy for the actual specimen that we found. I imaged the animal above two years ago in southern Ohio. Scroll on and you'll see why I used a substitute image to show what one of these cats looks like.

Here is the actual Saddleback that we found on the Redbud last Saturday. It's dead, and its body bristles with the cocoons of a Braconid wasp. There are so many cocoons festooning the unfortunate cat's body that it took me a minute to realize what species we were looking at. To top off the predatorial assault, there are even a few tachinid fly eggs on the body for good measure.

Braconid wasps and tachinid flies, among others, are parasitoids. Parasitoid insects such as these generally kill their hosts. One of these flies or wasps lays its eggs on - or injects them into - the victim. The eggs hatch quickly, and the tiny grub begins boring its way through the host's body, consuming non-vital tissues at first. Obviously, as evidenced by the photo above, many such larvae might occupy a host. The parasitoids cleverly avoid consuming the organs that allow the caterpillar to continue to remain mobile and thus better elude other predators, such as birds. Finally, in a grisly last hurrah, the larvae finish off the victim and burst from the dead husk and spin tiny cylindric cocoons.

It takes a few days for the wasps to pupate and emerge from their cocoons. I saw an opportunity to attempt to make images of the emergence, and took the caterpillar and its gruesome hitchhikers and placed the whole shebang in a clear vial. I tried to check on the occupants every hour or so to see if anyone was hatching. Today (August 15) was the day.

I had the vial in my office, and when I checked it around 10 am, a few wasps were flying about within. An inspection of the cocoons revealed that most appeared ready to hatch, but in spite of regular monitoring, no more emerged during the day.

I left the office around 5:15, vial and camera in hand. When I reached my car in the parking lot, I took one more look before stowing everything. Probably two dozen wasps were flying about! Perhaps my moving the vial jostled them into action, I don't know, but I rushed back to the building to create some images of the wasps emerging.

It doesn't take long for one to pop out. As the pupa matures into an adult wasp, the outer edges of the summit (lid) of the cocoon get thinner and more fragile. The wasp then apparently chews a very neat incision around the summit of the cocoon, and when the time is right, it pushes up and pops the lid off. From the time that I could detect an obvious movement of the lid in the case of the wasp above, it was probably only 30-45 seconds before it had broken completely free of the cocoon. The three cocoons to the right of the wasp still have their occupants. Other cocoons have the lids popped and we can see the almost perfectly circular shape of the lid.

Just seconds after emergence, a braconid wasp takes stock of its new world. After a brief run-around, it took wing and vanished.

Making images of these insects was not easy. The wasps are probably only 2-3 millimeters in length - smaller than can be effectively dealt with by using a "normal" macro lens such as my Canon 100 mm. Thus, I resorted to the tricky but useful Canon MP-E 65mm "mega" macro lens, which magnifies up to 5x life size. But this lens is not forgiving, and very hard to use on objects that are moving. It has no focusing system; the operator just moves the camera back and forth to focus. Depth of field is extremely shallow, even at f/16 (what these images were made at), and front of the lens flash is essential. And it is nearly essential that the camera be firmly braced in some way.

But if all goes passingly well, we can see into a micro world that is far too small to be visible to the unaided eye.

A wasp gives brief pause after a few wing shakes and a spot of grooming. An instant later it was gone. The spearlike spikes to the right of the wasp are part of the caterpillar's armament of stinging spines. Saddlebacks are said to have a very painful sting, but while such weaponry may work on birds and other larger predators, it is not effective against small parasitoid insects.

The wasp family Braconidae is immense, and undoubtedly includes scores of species as yet undescribed. I have no idea what species these wasps are, or even the genus. Identification of species in this group is the bailiwick of specialized experts, and I suspect specific identification isn't even possible in most cases without a specimen in hand. If anyone reading this can shed more light regarding identification, please let me know.

I captured this wasp just as it was ready to take wing. Note the dark stigma along the leading edge of the forewings.

A fate such as documented above is very common in the caterpillar world. Those of us who regularly hunt for caterpillars are quite used to seeing victims bristling with wasp cocoons, and/or the shells of empty tachinid fly eggs stuck to their exteriors.

Very little in Nature is Disneyesque.

Saddleback caterpillar

A Saddleback caterpillar, Sibine stimulea, in repose. This one was feeding on Sugar Maple, but these caterpillars are broadly polyphagous - they can eat seemingly anything. I've found them on a variety of woody plants including Witch-hazel and Redbud, and they've even been reported feeding on corn. Saddlebacks are not infrequent in gardens, occupying ornamental plants, and perhaps you have seen one.

This caterpillar is a thing of great beauty, and the swan in a reverse ugly duckling story. The caterpillar is fantastically ornamented with dense fascicles of stinging spines, and no matter how great the urge to stroke one of these, I would resist temptation. Some authorities say that Saddlebacks pack among the greatest punches of any North American caterpillar. Those stinging spines will leave a blistering rash that will smart for quite some time. So, admire the bristly little beast wearing the lime-green horse blanket from afar. The adult phase - the moth - probably wouldn't interest you nearly as much as this larva. It is a rather plain brown Jane.

Amanda Duren spotted this animal on a nocturnal foray in Ashtabula County, Ohio, last Wednesday night. Of course, we briefly detained the animal for a photo shoot before returning it to the maples. Neither caterpillar or photographers were harmed in the process.

Whoa! Seen up close and head on, the Saddleback takes on an entirely new look, and a scary one at that. It sort of resembles a manically evil clown. Caterpillars are never boring to shoot. From one angle, something might resemble little more than a tubular bag of goo; seen from another perspective the cat might look quite cool indeed.

Saddleback bites the dust

Not long ago, I was delighted to come across a small redbud tree filled with caterpillars. There were at least five species noshing away, including three saddleback caterpillars, Acharia stimulea. Saddlebacks grow up to be rather ordinary little brown moths, but they're truly spectacular as larvae. It's as if a sea slug was saddled with a tiny lime-green horse blanket.

However, I noticed that the individual in these photos was not looking especially lively. Even though the caterpillar looked pretty good - slightly faded, perhaps - something was amiss. So in we go for a closer inspection.

Uh oh. While making macro photos - those little columns of spines will give you one heckuva sting by the way - I noticed a major problem for the poor little cat. That tiny white cylindrical object is an egg from a tachinid fly. These bristle-bodied parasitoid flies are among a caterpillar's worst nightmares. In this case, I think that's probably the old case of the egg - as we shall see, it looks as if the fly has already spun its gruesome magic.

Tachinid flies, which rather resemble house flies, are major caterpillar predators and attack the cats by sticking an egg to the the larva's exterior. In short order, the fly grub hatches from the egg and bores directly through the caterpillar's skin and into its interior.

Once within the caterpillar, the ever-growing fly grub begins consuming non-vital hemolymph fluids and tissue. Clever parasitoids that they are, it would not behoove the grub to kill its victim until the last possible moment, as a living host is better able to move about and better avoid other predators such as birds. In a final frenzy, the fly grub goes ape and eats all of the innards before bursting from the caterpillar and going off to pupate.

In this photo, we can clearly see what must be the grub's exit hole. What a show that must have been, and I'm sure you wish I had caught the grub bursting from the caterpillar's husk on video so you could enjoy that bit of cinematic loveliness as you ate your breakfast. But such a video was not to be - I suspect that these tachinid fly grubs tend to emerge under cover of darkness.

As I was making these photos, this tiny chalcid wasp alit on the caterpillar husk and began looking around. This little wasp is looking to parasitize the fly by laying eggs on either the fly grub or its puparium. The world of parasites and parasitoids (the latter generally kill their hosts) is truly strange and multidimensional.

I made this photo a few weeks ago. While photographing this caterpillar - a yellow bear, Spilosoma virginica, I believe - I noticed a tachinid fly perched nearby. The fly watched the bristly caterpillar's every move, occasionally shifting position to get other viewing angles. Yellow bears are heavily beset with stiff hairs, and the fly was undoubtedly waiting for an opportune moment to get at some unprotected part of the caterpillar. I watched for a while, but had to leave before the fly moved in. I wish I had had the time to stick around and film the actual attack.

A stinger and a looker

A recent foray to southernmost Ohio resulted in many nice finds of both animal and vegetable matter. Two organisms that fall in the former category are detailed below.

Impossibly cute, as caterpillars go, is the Saddleback Caterpillar, Acharia stimulea. Looking like a tiny sea slug draped in a lime-green horse blanket, the little beasts march about the foliage boldly, even in broad daylight.

You see, they have an effective defense, and know it. All of those columns of bristles may look cool, but this is heavy artillery. The bristles are called urticating hairs (Urtica = a genus of stinging nettle), and they pack a whallop. Handle one of these things and you'll get a big dose of experiential learning that'll probably stick - you won't grab another one. Should you suffer this fate, it is allegedly a good idea to find some sticky tape and plaster it over the stinging hairs that are lodged within your flesh. Then rip the tape off and with it perhaps the hairs. The tape-ripping process will hurt, too.

Anyway you slice it, this little 2 cm bag of colorful goo will cause pain should you molest it.

We were very excited to find this orange and black beauty. It's a long-horned beetle, but we all were flummoxed by its identity. And I was with some people that have long explored the haunts that we were in, and only one of us had ever seen one and then only once, but had not figured out what it was.

Searching my guides yielded no good answer, so finally in desperation I tossed a photo out to the boys on Bug Guide.net. The answer came back quickly - we had found a beautiful specimen of the Amorpha Borer, Megacyllene decora. The specific epithet, decora, means showy and the moniker is well deserved.

The borer was feeding on the nectar of Late-flowering Thoroughwort, Eupatorium serotinum. I have always thought of this native plant is rather coarse and weedy, but have revised my opinion. It formed a large stand on the banks of the Ohio River, which is where we found the beetle, and the collective blossoms emitted a pleasant, sweet-smelling aroma. The odor must be a good lure, because the patch was lousy with bugs drawn in by the fragrance.

The first part of the common name of this borer, Amorpha, refers to its host plant, False Indigo, Amorpha fruticosa. This woody shrub reaches its northern limits in Ohio, and is thought to only be native along the banks of the Ohio in our state. Populations to the north of that mighty stream are non-native, spread from cultivation or otherwise introduced.

So it makes sense that the beetle would occur where we found it. But I wonder how rare it might be, as it seems hardly anyone has seen one, at least in these parts. It may be that their span as an adult is a short one, and the window is short to go out and find one. Or perhaps they truly are rare here, scattered populations at their northernmost outposts, living where the scattered patches of indigo grow.