Tussock Toxins

We have a nasty little caterpillar that threatens to cause us significant painful itching at this time of year. It is the sycamore tussock moth caterpillar (Halysiodota harrisii). (Does that second Latin word mean “harassing?”) It is a cute little guy—sort of like a fuzzy woolly bear caterpillar that you are drawn to pick up and stroke, like a tiny kitten. It is whitish-yellow, with long decorative tufts of an orange or brown color. Endearingly cute.

But woe unto those who touch this wee caterpillar—which is only about an inch (2-3 cm) long. Those fuzzy hairs are called setae (which originates from the Latin word for “bristle.”) They contain a toxin that evolution has given to the little worm—to ward off potential predators, such as birds. Nature did not endow them with this defense in order to fend off humans, but it works on us too, although we really do not intend to eat them.

The setae, upon being touched, break off and implant themselves in your skin. The hollow setae then empty their toxin, which causes an overreactive human immune system response. As a result, the body gets flooded with a pro-inflammatory compound called histamine—which can incite an array of allergic symptoms. As you might guess, an antidote to the inflammation caused by the histamine is to take a dose of an antihistamine.

The sycamore tussock caterpillar seems to know that it is not a critter to mess with. Unlike other small prey who jump and run when faced with a predator, this guy simply and slowly goes about its business—similar to a skunk. It is fearless. 

My and my wife’s vulnerability to the sycamore tussock moth caterpillar's toxin is heightened, due to the fact that we have several towering sycamore trees overlooking our home. I have written before of how graceful these trees are, and they provide much desired cool shade. But in late summer the caterpillars slowly descend on a silken thread from way overhead—like a circus acrobat in slow motion.

We have become accustomed to looking out for these yellow worms as they dangle in space—giving them a wide berth. Yet, the other night, as I was heating up our outdoor bath, I found one floating on the surface of the hot water, wriggling in discomfort. I picked up the sieve with which I scoop out fallen leaves and other debris, and carefully lifted it from the bath water.

It never occurred to me that its writhing had dislodged some of those nasty setae, which were now floating on the water's surface—awaiting a naked human body to stick to. Unfortunately for my wife, her body was the first one to submerge. Unknowingly, she—and later I—made contact. In a few hours—and certainly by the next morning—we reacted with the usual redness, swelling, itching rash, with welts on various skin surfaces. There's not much that can be done—other than take an antihistamine and suffer for a day or two… or four.

In over two decades of outdoor hot baths, this is the first one that has demonstrated how much suffering a tiny worm can cause in the bath. Was it a one-off? I sure hope so, but in the future I'll be on the lookout for cute yellow caterpillars swimming in the tub, in late summer.