Return of the Slug Moth Caterpillars
I have spent the last week in regular visitation to our switchgrass (Panicum virgatum). Not because I have a perverse fascination with switchgrass, per se, but because a plethora of Saddleback Moth caterpillars have been reducing the blades to nubs. [There are three in the photo above.] Many, many photographs have been taken, which I will subject you to here. It was during one of these exercises that I discovered photography can be painful, but more on that later.
Switchgrass is a native grass much safer for home landscapes than the invasive Miscanthus species. Miscanthus sinensis, commonly known as Chinese Silvergrass or other, similar names, was introduced as a "low maintenance" grass by the landscape trade. It was also adopted by many Departments of Transportation for use at ramp or median planting projects, and has since been adding itself to the invasive species lists of many states, due to its success in propagating itself wherever the wind carries the seeds. You can find out more about the dangers of the grass here, but suffice it to say that you shouldn't plant it anywhere you don't want to provide a fuel source for fire. It's also a bit explosive, due to the oils in it. Did I mention you shouldn't plant it?
Switchgrass is a prairie grass species, but it has done well for me here in the clay of Western North Carolina, as well. It is a known larval host for most of the banded skippers and most satyrs, according to the Lady Bird Johnson Wildflower Center. The variety our caterpillars are on is called "Shenandoah," which I have linked below. This hybrid is one of the more petite varieties of switchgrass, not getting much more than 24" wide and only 36" to 48" tall, as opposed to other six-foot versions like "Heavy Metal." It also has a lovely red to purple hue that stays with you all year long. It is deer resistant, and grows so well that at least any incidental munching won't kill the plant. We planted ours four years ago, and have given divisions of the plant twice. It's a great plant for lending some height behind border plants. Usually trouble-free, resistant to disease, takes drought or rain. In the case of ours--well, you may be able to make out the aberrant growth on this blade close-up!
The caterpillars in question are a species of slug moth, so named for their mode of locomotion. Some sites will say these caterpillars resemble slugs. Personally, I think they look more like cactus. They do, however, move in a somewhat slug-like fashion. Instead of putting one foot down and moving along with the prolegs typical of caterpillars, slug moth caterpillars move along in a wave-like manner. You can see the wavy nubbins in the photo at left. You can also see the folds covering the mouth of our saddleback here as he munches down on some lovely red switchgrass. AND you can see the little urticating bristles all over him!
Now might be a good time to mention that "urticating bristles" are very effective (at least in my opinion) spiny things that stick out from species as a form of defense against prying photographers and other critters. In my case, the collision with urticating bristles resulted in stinging pain for about ninety minutes (I applied an antihistamine cream at about the 10 minute mark, may or may not have helped). This is apparently a fairly mild reaction, as such things go. Others have described rash, swelling and nausea, so maybe my collision was more like a fender-bender instead of a real collision. Regardless, you will want to approach very cautiously!
I was fortunate enough to catch one of these guys shortly after climbing out of his too-small skin. It appears, and I have been unable to find confirmation of this, that he shed the exoskeleton from back end to front end. On the saddleback moth caterpillar, the big white "eye" spots on the end are the back end. The mouth end is covered with extra bristly-nubby parts and folds that protect and hide his munching parts. In this image, you can see the white spots on the shed skin. I was not able to stay and see whether this guy ate the shed skin, which some caterpillars will do.
This next image shows--this is the same caterpillar, different angle--that in their earlier development, they don't necessarily eat the whole leaf. Here (the brown, skinny parts on the right side of the image) the midrib has been left, and only the tender side parts were eaten. As they get older and larger, they eat the whole thing, usually from the underside.
As I found the last time we had saddleback moth caterpillars, many of our little guys were parasitized by braconid wasps. This one left the rest of the tribe and moved over to some iris next to the switchgrass. A couple of days later, he was covered in cocoons.
Most only ended up with only a few cocoons. Just as lethal, I'm sure, but it was interesting to watch the behavioral difference--none of the rest of the caterpillars left the switchgrass. So did this guy get victimized more severely *because* he left the switchgrass, or is that *why* he left the switchgrass? The pursuit of science. Endless questions.
I should mention that the adult Saddleback Moth isn't nearly as astounding as the larvae. Small, furry, basic brown. Looks like a dried up bit of leaf. But as a larval specimen--striking! And well-designed for the repelling of mammalian species. You can find these caterpillars on any number of different host plants--evidently, they are not very picky eaters. From BugGuide: "Many trees, shrubs and grasses including apple, asters, blueberries, citrus, corn, dogwoods, elms, grapes, linden, maples, oaks, Prunus species, sunflowers and viburnums" . . . and switchgrass!
And now for your final glamour shot. Hope you have enjoyed our tiptoe through the switchgrass!