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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

The 'sting' of nettle is a chemical combination believed to contain histamine and acetylcholine designed to cause burning or itching in animals that browse on the plant. © Kelly Kearns

June 2005

A nettlesome question

Many butterfly larvae put the bite on this stinging plant.

Anita Carpenter

The "sting" of nettle is a chemical combination believed to contain histamine and acetylcholine designed to cause burning or itching in animals that browse on the plant.

© Kelly Kearns

In the moist soils of lowland woods, dense stands of a nondescript, non-showy plant thrive. The waist-high vegetation is often overlooked unless you carelessly brush exposed skin against its bristle-covered leaves. Then stinging nettle makes its presence known. Within moments, your skin burns, followed by intense itching. Prudence suggests avoiding a stroll through stinging nettle, but this plant need not be feared nor should it be dismissed.

Stinging nettle, Urtica dioica, is an herbaceous perennial that prefers low, wet habitats but also grows along fencerows, the edges of upland forests and railroad rights-of-way. Stinging nettle may form large, almost impenetrable colonies. Each year the colony enlarges as the plant sends up new shoots from spreading underground rhizomes. The fibrous, unbranched stems grow three to four feet high. Two- to three-inch serrated leaves that resemble elm leaves grow from regular intervals along each stem.

Stinging nettle flowers from July into September. Slender non-showy yellow-green flowers grow in clusters where the leaves attach to the stem. Some clusters are composed entirely of female flowers, some are totally male flowers, while others contain both types. The anther-tipped filaments bend over, much like a person trying to touch his toes. When the flower matures, its covering dehisces or breaks open and the spring-like filaments straighten up, hurling pollen aloft. Drifting on summer winds, the pollen settles on receptive female flowers. Only one small achene or fruit, resembling a miniature tan pumpkin seed, is produced per female flower but each plant may produce several hundred seeds. The seeds remain on the plant until the first frost, then fall to the ground.

Stinging nettle receives its common name from the action of many minute stinging hairs that cover its leaves and stem. Each hair resembles a hypodermic needle composed of a sharp-pointed tip, a capillary tube through the bristle and a fluid-filled sac at its base. When the hair is touched, the tip bends and effectively penetrates the skin. The action puts sufficient pressure on the sac to force fluid up through the tube into the skin. The chemistry of the fluid is not fully understood, but the irritating chemicals are believed to be histamine and acetylcholine. The body's response to injection by these foreign substances is intense burning and itching. Very sensitive individuals may develop red, raised welts. The irritation is transient, its duration depending on each person's sensitivity, but may be bothersome and put a damper on the day's outing.

Several butterfly larvae, such as the question mark butterfly, are immune to the nettle's sting and rely on it as a host plant. © Don Blegen
Several butterfly larvae, such as the question mark butterfly, are immune to the nettle's sting and rely on it as a host plant.

© Don Blegen

From the plant's perspective, the stinging hairs are a good defense mechanism against browsing herbivores and curious naturalists. From our perspective, the plant is irritating. Our initial reaction is to get rid of it. After all, what good is it? Before you dash for your gloves and loppers, understand that several native butterfly species are dependent on stinging nettle as their larval host plant, that is, the plant upon which their caterpillars feed. Without the host plant, the butterflies would not survive.

Milbert's tortoiseshell, Nymphalis milberti, a two-inch brown butterfly with an orange-yellow band highlighting the outer third of its wings, is totally dependent on stinging nettle as a host. Unlike most other butterfly species which lay single eggs on many plants, a female Milbert's tortoiseshell lays her eggs in large clusters on the underside of nettle leaves. After hatching, the young larvae feed on nettles within silky communal webs. As the young grow and molt, they wander to neighboring nettle plants to feed alone and rest in folded leaf shelters. When it's time to pupate and transform into adult butterflies, the fully-grown larvae crawl off the nettles, never to return.

Red admiral butterflies, Vanessa atalanta, can't survive Wisconsin winters and must, as migrants, recolonize the state each year from the south. Some years red admirals are plentiful, other years they are scarce. In their wanderings, red admirals search for stinging nettle and wood nettle. Females lay their eggs singly on nettles and the larvae feed on the tender leaves.

Larvae of several orange and dark brown anglewing butterflies feed on nettles as well as other plants. Question mark larvae, Polygonia interrogationis, feed on elm, hackberry and nettles. The eastern comma, P. comma, lays her eggs singly or in a short stack directly on nettles, elms or hops. The rare satyr comma, P. satyrus, of northern Wisconsin uses nettle as its only host plant. The pale-green eggs are laid on the lower surface of nettle leaves. Like other anglewings, the young caterpillars rest inside folded leaf shelters. It appears the larvae are unaffected by the stinging hairs, though scientists don't understand how they are protected from the nettle's chemical defenses.

Learn to recognize stinging nettle, walk around it and think twice before cutting it down. Some people even plant stinging nettle in butterfly gardens along with common milkweed, everlasting and violets. All the butterflies that feed on nettles are wanderers so you're more likely to have these colorful visitors find your garden than many other butterfly species. As an added bonus, study your growing nettle patch to discover other creatures that live and dine on this delicacy.

Anita Carpenter sidesteps the nettles on walks near her Oshkosh home.