I spoke to someone recently who said that the most unpleasant feeling in the world is itchiness. Many of their fellow humans might agree; a 2011 study at the Emory University School of Medicine compared the impacts of chronic pain and chronic pruritus (itchiness) on quality of life, and found that the average itchy patient was willing to give up 13% of their life expectancy to live itch-free. It’s not surprising, therefore, that so many plants and animals utilize itch-inducing adaptations to keep us the heck away from them . . . or do they?
Although a Poison Ivy rash can feel like a personal attack, the chemical urushiol, which causes dermatitis in humans, likely evolved as an antimicrobial defense agent. The chemical is actually being studied for its potential as an ecofriendly antimicrobial coating. It has other fun applications too; it’s used as a traditional lacquer in Japanese urushi artwork, and it’s even been considered as a sustainable underwater adhesive. If cured properly, it won’t cause a rash. Leave it to humans to find novel uses for a substance that causes us misery!
The unfortunate reaction in humans is likely collateral damage. Most other animals that come across Poison Ivy aren’t allergic. Deer and rabbits routinely chow down on it, though Guinea pigs, most primates, and sometimes pigs and dogs also have a reaction. I’m sure Guinea pigs will figure out a use for urushiol one of these days!
Poison Ivy takes many forms, and there’s even been a debate about whether it’s one species or several. Because its morphological characteristics change according to habitat and growing conditions, scientists have previously named more than 30 species of poison Ivy. The most recent research proposes one species, Toxicodendron radicans, with several subspecies. It’s not uncommon to see two different leaf shapes growing on the same plant, so it’s easy to understand the confusion. It can be a shrub, a vine, or creep along the forest floor. Its leaves can be lobed, or not. There are abundant resources online for identifying Poison Ivy, and luckily it’s one of the first plants to turn red in the fall, which helps it stand out.
If you do come into contact with Poison Ivy, there may be a remedy close by. Common Jewelweed (Impatiens capensis) is a native understory plant with small, orange, bell-shaped flowers. We’ve known for thousands of years that rubbing crushed jewelweed over skin that’s come into contact with Poison Ivy can prevent a rash. Researchers have speculated that it could be the saponins in jewelweed that accomplish this. Saponins are chemicals known for their ability to form stable foams (sapon is Latin for soap). It’s thought that the surfactant properties in saponins lift sticky urushiol from skin. Jewelweed is commonly found in low-lying shady areas near Poison Ivy (which can grow in lots of habitats).
Another plant that I get questions about is Poison Sumac. We have a little cathedral of Staghorn Sumac (Rhus typhina) along the northwest edge of the West Meadow at Gilsland Farm. This is not Poison Sumac, and you can touch it all you want! Staghorn Sumac (which is native to Maine and the northeast) has pleasantly fuzzy branches, like velvet on a stag’s horns.
Poison Sumac is in the same family but a different genus as Staghorn and Sicilian Sumac (the spice popular in Mediterranean cuisine). It is therefore “sumac” in name alone. It is, however, in the same family and genus as Poison Ivy and Poison Oak, Toxicodendron (“poison tree.”) All three contain urushiol. There are a total of thirty iNaturalist observations of Poison Sumac in Maine, so this is not a plant you are very likely to run into. This is probably because it grows primarily in wetlands, which are not the best places to go walking in the first place. The plant’s small, plump, whitish-yellow berries are food for birds and small (non-Guinea pig) mammals.
And then there are the allergenic caterpillars, walking dermatitis. The hairs on these furry larvae are technically setae, modified scales made of chiton, the same material that insect exoskeletons and mollusk shells are made of. Setae are only occasionally toxic, and serve a variety of functions. Earthworms have setae which keep them from sliding backwards in loose substrates and make it harder for birds to yank them from the ground. Geckos have microscopic setae on their toe pads that allow them to cling to surfaces. Some species of moth larvae (caterpillars) have special barbed setae called urticating hairs that embed in skin and cause irritation. Some are hollow and contain toxins, like tiny needles. This is the armor of the infamous browntail moth, an introduced species identified by the two reddish-orange dots towards the rear of the caterpillar. These guys molt multiple times as they grow, shedding hairs each time. The hairs stick around in the environment and can cause irritation when kicked up or blown by wind.
We also have several nontoxic native caterpillars that can cause rashes when handled, such as the various species of tussock moth (pictured above). Whereas browntail moth hairs can find you, dealing with these other allergenic caterpillars is usually as easy as not picking one up. You might assume that such a potent defense would keep birds away too, but where there is juicy protein in nature, hungry organisms have likely found a way to access it. Setae, even when nontoxic, can disrupt digestion, so some birds simply eat fuzzy caterpillars in extreme moderation. Many of those beat caterpillars against a branch to dislodge the bristles. Baltimore Orioles, with their pointy bills, eat the innards and leave the hairy outside behind. The two species of cuckoos native to Maine, Black-billed and Yellow-billed Cuckoos, can gobble loads of them right up. The hairs form mats on their stomach lining, and when it gets dense enough to obstruct digestion, they shed the lining and grow it anew. We owe cuckoos a big “thank-you” for keeping populations of eastern tent caterpillars in check, and for being an ally in the fight against invasive browntail moths. If you run into any furry caterpillar, it is wise not to handle it, and in fact most caterpillars would actually prefer that you not handle them, so it’s a good policy all around. If accidental handling causes a reaction, tape can remove embedded hairs.
I decided to look into itch-inducing species after my fiancé ended her weekly farmer’s market routine because of the rash that would consistently appear on her arms the next day, probably from lingering browntail moth hairs. After a summer of long-sleeved shirts and hydrocortisone cream, it’s been nice to remember that these irritants evolved as defense, not offense, and usually not even against us. After all, we are a mere 300,000 years old, and Poison Ivy has been dealing with threats from microbes, and caterpillars threats from birds, for a lot longer than that. That being said, it’s convenient to keep us away; we are big lumbering mammals capable of trampling plants and insects without even knowing. To keep yourself safe and comfortable outside, I advise you to be mindful, but not fearful! Very few organisms are actually out to get us. Maybe some day I’ll write a blog about the few that are, but for those that aren’t, being aware of your surroundings, learning what toxic species look like and where they’re found, and using physical barriers like long sleeves and pant legs can get you by most of the time.
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