Snakeroot's Secret

White snakeroot, Ageratina altissima, flowering in a woodland edge in August.

In the fall of 1818, a 35-year-old pioneer woman fell ill and took to her bed in a crude dwelling near Pigeon Creek in Indiana. She had been caring for her sick relatives and a neighbor before she came down with the same symptoms: lethargy, abdominal pain, fever, nausea, and worse.

She had no medical care, so her health declined quickly. In a matter of days, she slipped into a coma, but before she lost consciousness, she called her two children to her side. When she died, her nine-year-old son, Abraham, is said to have been devastated. He would later write that his mother, Nancy Hanks Lincoln, made him all that he was.

Called sick stomach and later milk sickness, the mysterious illness was a menace on the 1800s wooded frontier. It sickened and killed thousands and terrified thousands more, because its cause was unknown. Faced with the agonizing and unexplained deaths of their family and friends, many pioneers abandoned their settlements for what they hoped would be healthier locations. In some cases, entire towns were deserted, as told by a writer to the Farmers’ Register in 1834:

A Village Depopulated by the Milk Sickness

The following extract is of a letter from a traveler dated at St. Louis:

A few miles below Alton, on the Mississippi, I passed a deserted village, the whole population of which had been destroyed by the “milk sickness.” The hamlet consisted of a couple of mills and a number of frame houses, not one of which was now tenanted; but the dried weeds of last year choaked [sic] the threshold of the latter, and the raceways of the mills were lumbered up with floating timber, while the green slime of two summers hung heavy on the motionless wheels. Not an object but ourselves moved through the town; and the very crows themselves seemed to make a recruit around the fatal place when they came in view of the thickly sown burial ground on the skirts of the deserted village. (1)

Although they often found the illness again in their new homes, the settlers’ knowledge was building. They recognized that cattle stricken with “the trembles,” a shaking weakness that progressed to more severe illness, could cause a similar condition in people who drank the cows’ milk or ate their beef, butter, or cheese. The illness tended to appear later in the season, from mid-summer through fall, and it was worse in dry years. Newcomers to areas stricken with the illness were advised to avoid eating beef or dairy products from July to the first frost.

That good advice likely prevented many cases of illness, but the ultimate cause of milk sickness remained unknown, or at least debated, for decades. In hindsight, it didn’t have to be. Unfortunately for many who would later become ill, an early and accurate warning was largely missed, in part because it came from a woman. Actually, from two women.

In Illinois around 1830, Anna Pierce Hobbs Bixby, a nurse and midwife called Doctor Anna, was grieved about the cause of milk sickness. It had killed her mother and sister-in-law and her father had developed a chronic, disabling form of the illness called “the slows.” She suspected the cause was something cattle were eating, so she followed them into their wooded pasture to record what they ate.

While she was there, she is said to have met an elderly Shawnee woman hiding from forced relocation to a reservation in Kansas. After the elderly woman learned what Doctor Anna was looking for, she identified white snakeroot as the plant that was making animals and people sick. The women parted, and the fate of the Shawnee elder is lost in history.

Now known by the scientific name Ageratina altissima (formerly Eupatorium rugosum, E. ageratoides, and E. urticaefolium), snakeroot’s phenology matched the seasonality of the sickness: It flowers in mid-summer into early fall, coinciding with the time milk sickness tended to occur. Its habitat was another good match. Snakeroot grew in woodlands, including the forested pastures where cattle then commonly grazed, and it persisted in drought. When nothing else was available, cattle had to eat snakeroot.

White snakeroot range in North America (left) and the upper Midwest (right). USDA NRCS 2023.

With this new-found knowledge, Doctor Anna began experimenting. She fed the plant to animals, including calves, and found that they developed the trembles. Convinced that she had found the cause of milk sickness, she spread the word. She grew a garden of white snakeroot to teach others what it looked like, and she urged farmers to pull it out of their pastures. They did, and her advice is thought to have saved many lives, at least in southeastern Illinois.

But that’s as far as it went. Whether her work was dismissed or not widely published or both, it didn’t get much traction. Instead, physicians and settlers alike continued to speculate about the cause of milk sickness. They blamed all kinds of things: arsenic or other metals, bacteria, bad water, poison oak, poison ivy, and other agents. Some thought the cause was miasmas, supposed poisonous exhalations from the earth that misted the vegetation and sickened the cattle.

As the debate continued through the 1800s, milk sickness nearly vanished. That was another mystery, although a welcome one. Two hundred years on, we know why it disappeared: Cattle came to be pastured not in the woods but in cultivated pastures where snakeroot was excluded, and commercial operations combined and diluted milk from many sources. If the contaminant was present in the milk, it was at lower concentrations, too low to produce the severe illness caused by chronic consumption of tainted meat and dairy products.

Even as milk sickness waned, research continued into its cause. The poisonous-plant hypothesis eventually held after other possibilities were eliminated, and snakeroot was finally confirmed as the cause of the illness in the early 1900s, almost 100 years after the Shawnee woman and Doctor Anna warned of its dangers.

In 1928 or 1929, James F. Couch, a chemist with the USDA, identified the toxin in snakeroot that had caused so much suffering. He described it as “a viscous . . . oil with a pleasant aromatic odor” and named it tremetol after the tremors it caused (2). The compound is present in all parts of the plant and is also found in rayless goldenrod, aka jimmyweed (Isocoma pluriflora), a plant native to the Southwest.

Milk sickness, or chronic tremetol poisoning, is rare now, but the University of Minnesota includes snakeroot among the plants known to be poisonous to livestock. While there is some concern that a return to small-scale, “natural milk” could result in cases of (now treatable) milk sickness, today white snakeroot is more often appreciated as a late-season source of nectar or pollen for bees, wasps, and flies and as a likely host plant for moth larvae (3). It’s available from many native plant nurseries – with some history attached.

To learn how to identify white snakeroot, see this page from the Friends of Eloise Butler Wildflower Garden.

Cited References

(1) A Village Depopulated by the milk sickness. Farmers' Register. Oct1834, Vol. 2 Issue 5, p308-309. 2p. [Obtained through the Hennepin County Library’s database of  American Antiquarian Society (AAS) Historical Periodicals Collection: Series 2.]

(2) Trembles (or milk sickness). James F. Couch. Circular No. 306, United States Department of Agriculture. 1933. https://archive.org/details/tremblesormilks306couc/page/n1/mode/2up

(3) White snakeroot. Illinois Wildflowers, website accessed 9-17-23. 

Additional References

Milk Sickness. Curtis Wood, NCPedia, 2006.

The “Slows”: The Torment of Milksickness on the Midwest Frontier. Walter J. Daly, Indiana Magazine of History 102 (1): 29-40, March 2006.

The Death of Nancy Hanks Lincoln. Philip D. Jordan, Indiana Magazine of History 40 (2): 103-110, June 1944.

Religion and Removal among the Shawnee from Ohio into Kansas. Brady DeSanti, International Journal of Humanities and Social Science 3 (4): 46-56.

How an 1800s Midwife Solved a Poisonous Mystery. Will McCarthy, Smithsonian Magazine, July/August 2023. [Note: A photograph in the article incorrectly labels white snakeroot flowering in spring. It flowers in mid-summer to fall.]

USDA, NRCS. 2023. The PLANTS Database (http://plants.usda.gov, 09/17/2023). National Plant Data Team, Greensboro, NC USA.

Once Upon a Milkweed

A sweat bee (genus Lassioglossum) on swamp milkweed (Asclepias incarnata) is in a precarious position. 

Milkweeds are familiar to many as essential for Monarch butterflies, but there’s much more to their story. A close look at their flowers shows an intricate structure with a tricky way to snag insects – literally.

The flowers of swamp milkweed (Asclepias incarnata), like many other milkweeds, are composed of five reflexed petals, five upright hoods, and five narrow horns around a gynostegium, a central column of fused stamens and pistils. The bases of the hoods hold nectar, and between them are narrow slits bordered by two “guide rails.” Each slit leads to a chamber that contains the reproductive parts of the flower, including the stigma, the part that receives pollen. For that reason, it’s called the stigmatic chamber.

What’s missing from the flowers are anthers shedding dust-like pollen grains. Unlike typical flowers, milkweeds don’t offer individual grains for insects to carry away. Instead, their pollen is packed into waxy sacs called pollinia. Each chamber holds two pollinia connected by a pair of arms and a central gland or corpusculum, Latin for “little body.” The whole structure, called a pollinarium, looks like a small pair of winged maple seeds.

Pollinia are rare. Only orchids also make them. The advantage of packing pollen grains together is that they can be carried as one unit to deliver hundreds of grains to the stigma of another flower. That improves the odds that every ovule in an ovary will be fertilized and develop into a seed. 

[Sidebar: In seed plants, ovules contain egg nuclei and develop into seeds. One or more ovules reside in an ovary, which sits at the base of a pistil, the “female” reproductive part. Above the ovary is a neck-like style and the stigma, the surface that receives pollen. Ovary walls develop into fruits.]

The challenge for milkweeds is to somehow get the pollinia out of the chamber and onto another flower. To accomplish this, milkweeds rely on bees, wasps, flies and butterflies as carriers. The insects visit the flowers to get nectar, and in the process, take the pollinia. And that’s where it gets tricky.

As an insect walks across the waxy surface of a milkweed flower, a leg or other body part can accidentally slip into one of the slits between the hoods. The corpusculum then catches its leg, forcing the insect to pull hard to get it out. Sometimes the insect doesn’t succeed, and it either leaves behind a leg or dies trying to get it loose. But if the insect can manage, it extracts its leg with the pollinarium attached. Then it’s off to another flower and perhaps another slip into a chamber, where the pollinia are deposited and the pollen can reach the stigma.

Pollinarium with dangling yellow pollinia on the tarsus (lowest leg segment) of a digger bee. 
Photo by Allan Smith-Pardo, Bees of the United States, USDA APHIS PPQ, Bugwood.org.

That’s a lot of effort, for both the insect and the plant. The reward for the insect, if it isn’t snagged forever in a milkweed flower, is a source of nectar that is almost pure sucrose, the same as in your sugar bowl. The reward for the plant, as mentioned above, is an abundant and directed source of pollen. No other plants but milkweeds can receive pollinia, so little pollen is lost on plants that can’t use it. Even a different milkweed species is unlikely to accept pollinia from, say, a swamp milkweed, because the size and shape of the receiving chamber may not fit the arriving pollinia. Hybrids are therefore uncommon.

To listen to an ecologist talk about milkweed pollination and why it’s so unusual (and cool!), see this video by Dr. Thomas Rosburg of Drake University for Iowa PBS.

To see milkweed pollination in action, see this video from the Master Gardeners of Northern Virginia (scroll down at the site) or another video from Monarch Butterfly USA. Some of the terms differ, but the process – and the pitfalls – are the same. 

To learn more about swamp milkweed in particular, see this page from Minnesota Wildflowers.

References

Minnesota Wildflowers

Illinois Wildflowers

Milkweed pollination biology. By Eric P. Eldredge, USDA NRCS. November 2015. 

Milkweed pollination: A series of fortunate events. By Chris Helzer in The Prairie Ecologist, January 2021. 

Wyatt, R. and Broyles, S. B. 1994. Ecology and evolution of reproduction in milkweeds. Annual Review of Ecology and Systematics 25: 423-441. https://www.jstor.org/stable/2097319

Plant profile: Jumpseed

Persicaria virginiana, formerly Polygonum virginianum, Antenoron virginianum, Tovara virginiana

 

Jumpseed, also called Virginia knotweed or woodland knotweed, is a perennial herbaceous plant native to the eastern U.S., including southeast and east central Minnesota. It thrives in the damp soils and part shade of deciduous woods and edges, often where there has been some disturbance. These plants were growing along a trail through a woodland.

From July into September, the plants produce long, slender racemes bearing tiny, whitish flowers, each just a few millimeters across when fully open. After pollination by honeybees, bumble bees, leaf-cutting bees, and other bees and wasps, the flowers form small fruits that are deflexed – they angle downward on their short pedicels (flower stalks), a tensioned position that needs only a slight touch to be released. When it is, the fruits “jump” off the plant. Their hooked ends, formed by remnants of their styles, can latch on to fur or clothing and help the seeds travel farther from their parents.

Jumpseed flowers in mid to late summer, producing slender racemes up to 16 inches (40 cm) long. The small flowers
have four sepals, but no petals. Fruits have hooked tips that can latch on to passing animals, including humans. 

Jumpseed also spreads by rhizomes, underground stems that form patches of plants. Where the plants aren’t desired, this can be a problem, because they tend to persist even after pulling. The same is true for eastern jumpseed, an introduced plant. Once considered a variety P. virginiana called filiformis but now recognized as a separate species, Persicaria filiformis, it is beginning to develop a reputation as invasive because of its rhizomatous habit. It differs from P. virginiana in having pink to red flowers and often variegated leaves, characteristics that make it popular in the horticultural trade. Several cultivars of eastern jumpseed, such as ‘Painter’s Palette,’ ‘Lance Corporal,' and ‘Batwings,’ are offered for sale from some nurseries.

The last two cultivar names come from colorations on jumpseed’s leaves. In spring, the leaves are marked with maroon or dark green chevrons, upside down V’s that resemble military insignia or wings. The chevrons on Virginia jumpseed disappear by the time the plants flower, but those on eastern jumpseed leaves may persist.

Jumpseed has alternate leaves and hairy ocreas.
Notice the swollen nodes. 

Jumpseeds belong to the buckwheat or knotweed family, Polygonaceae (poly-gon-AY-see-ee). A typical trait of the family is a thin sheath called an ocrea (or ochrea) above each node. In formal terms, the sheath is described as scarious, meaning it is dry and membranous. The ocrea is formed from stipules that, sometime in the long history of knotweeds, fused to form a tube around the stem. 

Another trait that identifies this family is its knobby or swollen nodes. They resemble knees, if you use your imagination. Polygonaceae means “many knees.”

References

Minnesota Wildflowers

Illinois Wildflowers

Maryland Invasive Species Council (Persicaria filiformis)

John Philip Baumgardt. How to Identify Flowering Plant Families: A Practical Guide for Horticulturists and Plant Lovers. Timber Press, 1982. 

Plant Profile: Michigan Lily

 

Michigan lily flowering in mid-July in Hennepin County, Minnesota.

This lone Michigan lily (Lilium michiganense), barely out of reach of the ditch mower and surrounded by invasive reed canary grass (Phalaris arundinacea), grows at the edge of a wetland. That’s typical for this native perennial, which is also found along streambanks and shores and in wet meadows, prairies, bogs and woodland edges and openings.

Michigan lily looks much like Turk’s cap lily (Lilium superbum), another native that grows in the same habitats farther south. In fact, some references call Lilium michiganense Turk’s cap lily, a mix-up that shows why scientific names are helpful. They may be tongue-twisters, but unlike common names, scientific names are usually the same no matter where you are (or what you’re reading), so there’s less confusion.

According to most references, this one is almost certainly Michigan lily. It’s 3–4 feet tall, well within the 3– to 6–foot height typical of this species. Turk’s cap lily tends to grow taller, usually 5–7 feet.

Their flowers also differ, but the differences are subtle. Both species have umbels of nodding flowers with orange-red, dark-spotted tepals (similar petals and sepals) that are reflexed, bending back toward the base of the flower. Large stamens and a long pistil emerge from the center of the flowers and hang downward.

Umbels of Michigan lily flowers (left) and sets of whorled leaves on the stem (right).

In Michigan lilies the tips of the tepals are said to reach the base of the flower, but not much farther. In contrast, the tepals of Turk’s cap lily reach so far back they go beyond the base of the flower and may touch each other. In addition, their anthers, the pollen-bearing tips of the stamens, differ in length. According to several references, those of Michigan lily are never more than ½ inch long, whereas those of Turk’s cap lily are at least ½ inch long or longer.

If you don’t have a ruler handy, there are other differences to look for. If flower buds are present, look at their shapes. Michigan lily buds are more or less round in cross section, whereas Turk's cap lily buds are triangular. Open flowers also display differences. The pistil of Michigan lily is orange-red, whereas the pistil of Turk’s cap lily is greenish white to whitish orange. Looking deep into the flower, you’ll sometimes see a green, star-shaped center in Turk’s cap lily, but not in Michigan lily. 

Another look-alike is the introduced tiger lily, Lilium lancifolium, a garden favorite. It differs from both Michigan and Turk’s cap lilies in that it has alternate, not whorled, leaves, and small bulbs in the leaf axils.

Growing near this Michigan lily was yet another look-alike, Tawny day lily, Hemerocallis fulva. Also called ditch lily for its common habitat, it has orange-red flowers that open upward and have streaked but not spotted tepals. Day lilies have strap-shaped basal leaves but no leafy stems.

Tawny day lily flowers on leafless stems amid strap-shaped basal leaves. The flowers open upward, their red-orange tepals streaked but not spotted.

Michigan lily ranges throughout the upper Midwest and the Great Lakes region and less commonly farther east and south. In this region, it flowers in July. Pollinators are thought to be hummingbirds, butterflies and moths.

Michigan lily's range in North America (left) and the Upper Midwest (right). Maps from USDA Plants Database.

References

Minnesota Wildflowers - Michigan Lily

Friends of Eloise Butler Wildflower Garden - Turk's Cap Lily

Illinois Wildflowers - Michigan Lily

Illinois Wildflowers - Turk's Cap Lily

Flora of North America – Michigan Lily

Flora of North America – Turk’s Cap Lily

USDA, NRCS. 2023. The PLANTS Database (http://plants.usda.gov, 07/18/2023). National Plant Data Team, Greensboro, NC USA.

Plant profile: Cow parsnip

Caution: The sap of cow parsnip and other members of the carrot family can cause skin rashes and blisters. See below for more information.

Cow parsnip, Heracleum maximum, is a big plant with a big name. This 4- to 8-foot-tall native plant of damp meadows and fields, streamsides, ditches, and low, open woods is named for Heracles or Hercules, the mythic Greek hero of superhuman strength. Even the species name, maximum, hints at its size. Few herbaceous plants are as robust.

A biennial or short-lived perennial in the carrot family, cow parsnip has alternate, divided leaves up to 2 feet across, with smaller, undivided leaves higher on the stem. The leaflets are irregularly lobed, coarsely toothed, and hairy. The petioles of lower and middle leaves are 3 to 10 inches long with sheaths where they meet the stem. Stems are stout, hollow, ridged, and hairy.

Lower leaves of cow parsnip are divided into three lobed, toothed leaflets. This leaf blade (left) is about 18 inches long. Petioles are long with sheaths where they meet the stem (right).

Cow parsnip flowers from late May into early July. Small, white, 5-petaled flowers are in flat-topped, compound umbels, clusters that resemble a collection of little umbrellas. (See Flower Parts for Plant ID to learn more about types of flowers and flower clusters, called inflorescences.) The flowers are pollinated by honeybees and many kinds of native bees and flies, the variety so great that some consider cow parsnip a pollinator magnet.

Cow parsnip flowers are arranged in compound umbels. Each spoke of the umbel extends to another, smaller umbel, called an umbellet. The tiny, white flowers have five petals. 

Fruits are flattened, up to 1/3 inch long and 1/4 inch wide, and brown with dark, vertical lines when they mature in late summer or early fall. They are winged on their edges and so can be dispersed a limited distance by wind. Eventually the fruits split apart and release two seeds. Cow parsnip reproduces only by seed.

These immature fruits of cow parsnip will turn brown when they mature in late summer.

Although the plant is native here and in much of North America, in some states it’s introduced and invasive. In North Carolina, for example, cow parsnip is described as aggressive and insidious and is classified as a noxious weed.

Be Careful

Cow parsnip is rarely considered an ecological problem here, but it should be handled with care. The leaves and stems contain furanocoumarins, compounds that sensitize skin to ultraviolet light. If skin is exposed to plant sap and then sunlight, blistering rashes may result. The condition, called phytophotodermatitis, can take weeks to heal. Always wear gloves, long sleeves and pants when handling this plant, and wash off and cover skin if exposed to the sap. The sap can also damage the eyes, so it’s best to wear eye protection if the plant will be disturbed.

Watch Out for a Look-alike

Giant hogweed (Heracleum mantegazzianum), a similar but much larger plant, is introduced and invasive. So far, this 10- to 15-foot-tall plant hasn’t been found in Minnesota, but it is present in Wisconsin, and it’s known to be especially hazardous. The Minnesota Department of Agriculture includes giant hogweed on its list of noxious weeds that should be eradicated if found. Report the plant’s location using Report a Pest or EDDMapS and handle it using extreme caution.

Invasive Plants in the Carrot Family

Cow parsnip isn’t one of Minnesota’s “bad carrots,” but several other species are, including not only giant hogweed but also wild parsnip, burnet saxifrage, Queen Anne’s lace, and several others. A guide to identifying these plants is available in the Downloads tab.

 

References

Friends of Eloise Butler Wildflower Garden

Minnesota Wildflowers

BWSR Featured Plant: Cow Parsnip

Illinois Wildflowers

USDA, NRCS. 2023. The PLANTS Database (http://plants.usda.gov, 06/21/2023). National Plant Data Team, Greensboro, NC USA.

Giant hogweed – Minnesota Department of Agriculture

Giant hogweed (Heracleum mantegazzianum) – Minnesota Department of Natural Resources

Plant Profile: Starflower

Starflower blooming in late May in a mixed coniferous-deciduous forest in north-central Minnesota.

Starflower (Trientalis borealis, aka Lysimachia borealis) is a spring-blooming, perennial wildflower of coniferous and deciduous forests. In early spring, stems emerge from overwintering tubers and grow 4–8 inches tall, their slender stems bearing six to eight lance-shaped leaves of unequal size. In May and June, one, two, or rarely three flowers grow from the leaf axils. Each flower is about ½ inch wide and typically has seven white, pointed petals and orange anthers that later turn brown.

The flowers are self-incompatible, so they can’t pollinate themselves. To form seeds, they must receive pollen from another patch of starflowers, delivered primarily by native mining bees (andrenid bees), sweat bees (halictid bees) and hover flies (syrphid flies). If the bees are present, if the patches are close enough for the bees to transit, and if pollination is successful, small seed capsules eventually form at the tips of the stems.

That’s a lot of ifs and little assurance of a next generation. Starflower doesn’t depend only on seeds for reproduction, however. In fact, very little of its energy is dedicated to flowering and seed set. Most of its reproductive effort is spent on rhizomes, underground stems that extend the plant’s reach and give rise to new plants. It’s a faster way of reproducing, and in a stable environment, it’s more reliable. The downside is that the parent plant and its vegetative offspring are genetically identical, so if the environment changes, the plants may not have what it takes for a population to survive.

If conditions remain favorable, though, the rhizomes grow and form patches of new plants. By midsummer, tubers begin forming at their tips. Aided by the cool nights of late summer and fall, they fill with starch to fuel next year’s growth. Rhizome connections then wither and the leaves yellow and fall. Bare stems topped with capsules are all that remain above ground, while tubers below ground carry their incipient roots and shoots through winter, ready to resume growth in spring. Starflower seeds also overwinter, but they don’t germinate until fall of the second year.

Starflower range in North America (left ) and the Upper Midwest (right). Maps from USDA Plants Database.

As with many plants, Starflower is facing challenges brought by climate change. The cool nights needed for maximum tuber development are warmer now, and researchers have found that flowering and seed set lessen toward the southern edge of the plant’s range. These changes raise questions and concerns about whether the species can adapt, because in some places, it isn't. Starflower is state-listed as endangered in Georgia and state-listed as threatened in Illinois.

Populations in Minnesota and other northern locations are responding to warmer May temperatures by flowering earlier. That may or may not be beneficial, but so far, starflower seems to be holding its own here. The species name borealis, meaning “of the north,” may be truer than ever.

References

Minnesota Wildflowers

Illinois Wildflowers

USDA, NRCS. 2023. The PLANTS Database (http://plants.usda.gov, 06/03/2023). National Plant Data Team, Greensboro, NC USA.

Roger C. Anderson. June 1970. The role of daylength and temperature in tuber formation and rhizome growth of Trientalis borealis Raf. Botanical Gazette, Volume 131, Number 2, pp. 122-128.

Roger C. Anderson and Orie L. Loucks. July 1973. Aspects of the biology of Trientalis borealis Raf. Ecology, Volume 54, Issue 4, pp. 798-808.

Roger C. Anderson and Michael H. Beare. March 1983. Breeding system and pollination ecology of Trientalis borealis (Primulaceae). American Journal of Botany, Volume 70, Issue 3, pp. 408-415.

Emily Dangremond. No date. Climate change and starflower in the Midwest. Illinois Native Plant Society.

Emily Dangremond, Christopher H. Hill, Shahd Louaibi, and Ivette Muñoz. 2021. Phenological responsiveness and fecundity decline near the southern range limit of Trientalis borealis (Primulaceae). Plant Ecology, Volume 223, pp. 41-51.  

Linda G. Chafin. 2020. Trientalis borealis Raf. Georgia Biodiversity Portal, Georgia Department of Natural Resources/Wildlife Resources Division.

Got Garlic Mustard? Look for Aphids

These sap-sucking insects traveled with their invasive plant hosts and could offer natural control. Researchers need help learning where they are and what damage they cause.

Garlic mustard, Alliaria petiolata, flowering in mid-May in southern Minnesota.

Garlic mustard, Alliaria petiolata, is an invasive plant of woodlands and woodland edges. Since it was introduced to the US from Europe in the 1800s, it has become widespread, especially in the eastern US. In Minnesota, the plant has spread throughout the southern two-thirds of the state and even to some far northern counties (EDDMaps).

Garlic mustard emerges in spring, and like other invasive plants, it’s bad news for the communities it occupies. The plants can form dense patches that shade out and displace other plants, and they can release chemicals into the soil that inhibit other plant growth. For these reasons, garlic mustard is a restricted noxious weed in Minnesota. That means its propagating parts, such as seeds or live plants, can’t be imported or sold and they can’t be transported without a permit (MDNR, MDA, UMN).

Garlic mustard isn’t reported to be invasive in its home range (CABI Compendium), but here, it behaves differently. There are many possible reasons. One is that it arrived without the insects or diseases that control its populations. What a surprise, then, when garlic mustard aphids (Lipaphis alliariae), also called grenade aphids, were first confirmed in 2021 on plants in Ohio. These garlic mustard specialists have since been documented in several more states, including Minnesota. So far, they’ve been found in St. Louis, Anoka, Washington, Dakota, Winona and Olmsted counties (EDDMapS).

Although it's usually worrisome to find another introduced species that's naturalizing, this one may offer some hope. Garlic mustard infested with these aphids develop deformed leaves and seed pods, and researchers want to know if this could be a natural control for the plants. If it is, it could be more effective than hand-pulling, spraying or other methods of garlic mustard management, especially in large invasions.

To start, we need to know where the aphids are, and if they’re present, what damage they may cause. That’s where citizen scientists come in. The Midwest Invasive Plant Network (MIPN) and its partners need volunteers to look for the aphids during garlic mustard pulls or more casual activities. This is an ideal time to search, so below are resources to identify garlic mustard and its aphids and instructions on how to report what you find. If you find garlic mustard but no aphids, that's important, too, and is helpful to report. 

Garlic mustard identification

Minnesota Department of Agriculture

Minnesota Department of Natural Resources

University of Minnesota Extension Service

University of Wisconsin Extension Service (video)

Garlic mustard aphid identification

EDDMapS

Bugwood’s garlic mustard aphid ID cards (print and cut; especially good for group pulls)

How to report garlic mustard aphids

The MIPN has prepared an informational flyer explaining how to report the aphids if they’re found. Negative reports – reports of their absence – are also important. Volunteers use EDDMapS, an early-detection reporting tool available as an app and online. Users must set up an account, but it’s free. See the flyer for more information.