Brilliant sumac keeps most herbivores at bay

By Jeff Mitton

The eastern face of Flagstaff Mountain lights up each fall as clones of smooth sumac, Rhus glabra, turn brilliant red.

Fall brings out the best in clonal species — quaking aspen, gambel oak, chokecherry and smooth dogwood. I hiked up the Flagstaff Mountain trail to get a closer look and found that the yellow of small ash trees and the orange of chokecherry complemented the red sumac.

A chemical analysis of leaves revealed that smooth sumac has a large arsenal of chemical deterrents, including phytol, vitamin E, several fatty acids and several tannic acids. The tannic acids are at very high concentrations, from 10 to 16 percent of the dry weight of leaves. So deer, elk, small mammals and most insects will not eat sumac leaves.I met some hikers on the trail, and although they enjoyed the sumac color, they were apprehensive because they thought it was poison sumac. I assured them poison sumac looked quite different and much more similar to eastern poison ivy, Toxicodendron radicans, or western poison ivy, Toxicodendron rydbergii. All of the Toxicodendrons synthesize urushiol, which induces the dreaded itchy rash; smooth sumac produces many defensive compounds, but not urushiol.

I walked among the sumac, enjoying the color, and came away with the impression that few herbivores grazed on it and the suspicion that it has an effective chemical defense. A chemical analysis of leaves revealed that smooth sumac has a large arsenal of chemical deterrents, including phytol, vitamin E, several fatty acids and several tannic acids. The tannic acids are at very high concentrations, from 10 to 16 percent of the dry weight of leaves. So deer, elk, small mammals and most insects will not eat sumac leaves.

Given that species survive for millions of years and that the world is crawling with potential herbivores, it is not surprising that one or a few species would evolve adaptations to breach the chemical defenses of a well-defended plant. For smooth sumac, the most successful herbivore has not only evolved to tolerate the defenses, it has evolved to specialize on smooth sumac and a few other sumacs in the genus Rhus.

The sumac flea beetle, Blepharida rhois, relies heavily on sumac, feeding on it throughout both the adult and larval stages. The larvae appear in spring on sumac foliage, and their mode of feeding leaves a characteristic pattern of numerous small holes. The larvae are soft and slow, and they do not synthesize any defensive chemicals, so they are potentially easy prey for other insects and birds.

Some species of herbivores, such as monarch butterflies, are able to sequester, or store consumed defensive chemicals in fatty tissues, turning plant defenses into herbivore defenses. But sumac flea beetles are not able to sequester the phytol or tannic acids that they consume.

Sumac flea beetles defecate the phytol and tannic acids that they consume, but they have a noteworthy trick for recycling plant chemical defenses. A flea beetle's anus is not on the bottom of the body, or at the very end, but on the top of the body near the hind end. When a flea beetle defecates, it maneuvers its body to coil the feces on top of itself; some flea beetle larvae are able to cover their entire upper body, from stem to stern.

The coiled mass of fragrant feces is called a fecal shield because it is an effective deterrent to predators. When predators such as ants come in contact with the fecal shield, they back off immediately, grooming frantically to clean off all of the feces. Laboratory studies have shown that ants have the same response to purified phytol and tannic acids, indicating that these plant defensive compounds are the active ingredients in fecal shields.

Studies of flea beetles and smooth sumac have reported that some sumac clones are heavily infested while adjacent clones might not have a single beetle. When the chemical profiles of heavily infested clones are compared to clones free of beetles, concentrations of phytol and tannic acids differ.

Infested clones have five times as much phytol as uninfested clones. Sumac flea beetles prefer to consume foliage from and lay eggs on clones with higher concentrations of defensive chemicals, and their preference provides their offspring with more potent fecal shields.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

Oct. 24, 2014