Monday, August 15, 2016

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.

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