Wednesday, July 20, 2011

Don't come back as a caterpillar

What a day yesterday was. After attending a conference all day in downtown Cincinnati, I hustled off to meet one of Ohio's preeminent herpetologists. He showed my what is now the Queen City's most famous reptile, and I've got photos of that excursion to share later.

Following that, it was off to Adams County and the Edge of Appalachia preserve for an 8 pm meeting. Why so late? Caterpillars. We had enticed a major newspaper to do a story on caterpillars and their importance to the environment, and as most caterpillars are active nocturnally we had to adopt an owl's schedule. Thanks to whiz-bang naturalists John Howard, Rich McCarty, and Mark Zloba for agreeing to help the reporter and photographer find some cats and learn about their fascinating habits. I'll share more about the article once it emerges.

John, Mark and Rich had been keeping a watchful eye for caterpillars all week, and capturing various interesting ones. By doing so, we'd be assured of having lots of interesting goodies to show off, should our nighttime search efforts not bear much fruit. This is one of the cooler cats: a spotted apatelodes, Apatelodes torrefacta. Apatelodes are common, and generalists that feed on the foliage of a wide variety of common trees.

As you've noticed, this silky cat is heavily beset with long setae, or hairs, and looks like a tiny, tubular Pomeranian dog. There are probably two primary reason for all of the "fur". One, it makes it tougher for birds to snack on Apatelodes, as all of those bristles are unpalatable and it's just not worth the bother for most bird species. Two, all of the hair may make it tougher for the parasitoid insects - flies and wasps - to get at the caterpillar's body and attach their eggs. And as we shall see, the latter fate is a caterpillar's worst nightmare come true.

As we moved through the dark forest, searching the plants with lights for foraging caterpillars, John Howard spotted this spectacle. It is a large sphinx moth caterpillar, covered with the coccoons of braconid wasps.

The Braconidae is an enormous family of parasitoid wasps - some authorities believe there are in excess of 100,000 species worldwide - and we've got scores of them in Ohio. A parasitoid should not be confused with a parasite. The latter is something like lice or ticks. They're a bother, and they take from their host, but they don't kill it. Parasitoids such as braconid wasps ultimately kill their victims.

The female wasp has a long stingerlike ovipositor, and she uses it to inject eggs on or into the tissue of the caterpillar (or other insects, depending on the species of wasp). Along with the eggs comes a special cocktail known as a bracovirus, which serves to infect the caterpillar's immune system and mask detection of the invading eggs. Once the wasp grubs hatch, they begin feeding on non-vital innards within the caterpillar - eating it alive, essentially. It is in the wasp grubs' best interest to keep the caterpillar alive as long as possible, so they have fresh food to see them to maturity, and also a living caterpillar is better able to hide and avoid other predators such as birds.

Finally, when all is ready, the wasp grubs burst through the skin of the caterpillar and form the cyclindrical little coccoons that we see in these photos. Amazingly, this caterpillar was still alive, although barely. Perhaps even by now, a day later, the wasplets have emerged from their coccoons, leaving the dried husk of the sphinx caterpillar. A seemingly terrible fate, but one shared by scores and scores of caterpillars.


rebecca said...

A coworker and I attempted to rear a couple tussock moth caterpillars down in Georgia - both made coccoons, but while one coccoon produced a male moth, the other hatched out a tachinid fly! It was quite cool, although I did wish we'd gotten a female tussock, moth since they're apparently wingless and that would have been interesting to see.

Jim McCormac said...

That would have been cool to see, Rebecca. I kinda wish I had kept the sphinx in my photos, and photodocumented the emergence of the wasps. That'd make for an interesting series of shots.

Jack and Brenda said...

I think I've shown you this photo before, but here is a photo I took of a Tomato Hornworm with wasp cocoons. You can use it in your article if you'd like.


Rebecca said...

Jim, you are awesome. My 4 year old and I were trying to figure out what kind of moth he caught and you answered all our questions and more. Thank you for your research... we enjoy it!