Monday, August 20, 2012

Parasitoids: Disneyesque, they're not

While exploring a streamside woods in Scioto County last Saturday, our crew happened along this stunningly marked little beetle. Needless to say, we were all quite interested in the animal, and many photos were made. None of us recalled seeing one before - and how could one forget such a stunner! - but we suspected it to be in the genus Calligrapha, so named for the calligraphic-like markings on the carapace.

Sure enough, we had pegged the genus and from there it wasn't hard to determine its identity as the ninebark beetle, Calligrapha spiraea. Going by the scientific epithet, it would seem that the creature was misnamed - its host is the shrub ninebark, Physocarpus opulifolius, which was growing in profusion where we found the beetle. But there is a rational explanation for the apparent misnomer - back when the beetle was originally described and named, ninebark was placed in the genus Spiraea, which is another group of shrubby rose family members.

The beetles lay their eggs on the underside of leaves of its ninebark host plant, and when the larvae hatches, they (anthropomorphically speaking) happily chew leaf tissue. All would seem well. The larva is essentially living on an inexhaustible supply of food, and their tiny size may make them less of a target to the legions of bad guys lurking about.

But beetle larvae eventually morph into beetles, and when they do they become all the more conspicuous. And that's where the trouble lies, once you get on the radar screen of the bad guys. Of course, this beetle is probably fortunate to have made it to this stage - there are enemies that could have picked it off at every point from egg to any of its four instars. All is not Disney's "The Lion King" out here; a lesson this gorgeous little jewel of a beetle is going to learn, hard.

As we moved ever closer to our subject, we saw that it had picked up a most unwelcome guest. A tiny parasitoid wasp has lit on the beetle's shell, and is in the act of depositing an egg. Parasites are generally annoyances, like mites or ticks; parasitoids generally kill their victims, often in gruesome fashion. I don't know the wasp species, or even the family, although I wonder if it may be a chalcid. Considering that the beetle is only about 7 mm in length, the wasp is incredibly small, and photographing it taxed the limits of my 100 mm macro lens. The wasp egg will eventually spawn a tiny grub, which will work its way into the beetle's soft inner tissues, and feed upon its host. Presumably, it - and maybe other wasp grubs also within - will eat their host alive.

Well, that story thoroughly vanquished any happy Lion King views of nature that we may have harbored, so we stumbled over to some nearby lianas of the riverbank grape, Vitis riparia, a plant that often produces interesting caterpillars. We were pleased, and the horrors of the previous tale were driven from our minds, when we found this stunning colony of grapeleaf skeletonizer moth caterpillars, Harrisina americana. The early instars of this beautiful caterpillar are quite gregarious; at least 34 animals can be seen on this leaf. Before we disrupted them, the assemblage looked even cooler, as the cats were lined up side by side, rimming the edge of the grapeleaf.

All seems well, and we were pleased to move in and make interesting photos sans the ghoulish disruptions of parasitoid wasps.

Alas, it was not to be. It didn't take long at all, and we noticed these two tiny parasitoid wasps - presumably braconid wasps? - plying their trade. This elfins are every bit as small as the wasp on the beetle, and would be completely overlooked at a casual glance. Predators such as these, tachinid flies, birds, and numerous other predators are why many moths, beetles, and other insects essentially use a carpet-bombing strategy of egg laying. They try and cast out so many eggs that at least one or a few will make it all the way to the adult reproductive stage. Most don't.

If this wasp were on your fingernail, right now, it would like look a tiny black gnat - scarcely noticeable. Small as it may be, it is more than capable of doing in this much larger caterpillar. Note all of the tiny white oval-shaped eggs that she's already deposited, all within the stiff bristles growing from the black spots. The wasp may be choosing to lay within these clumps of setae as the hairs may afford some measure of protection against other predators that would either lay eggs within the eggs, or eat them.

Same old story here. Those eggs will hatch grubs that'll bore within the caterpillar and eat it alive.
If any of those grapeleaf skeletonizer caterpillars successfully runs the predator gauntlet and makes it to adulthood, it'll look like this. A beautiful moth, which ironically enough, is a wasp mimic.


OpposableChums said...

Yet another in a seemingly unending series of miraculous observations found in the nature right around us. Amazing. Many thanks, Jim.

Sharkbytes said...

I have seen that moth, but never the caterpillars. I'll have to keep looking.

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