Fungi as Fixers

The plants in this restored prairie at Elm Creek Park Reserve are mostly mycorrhizal. Fungi live with their roots in a mutually beneficial relationship.

The last post introduced mycorrhizae, the fungus-root associations that benefit both partners. The fungus transports nutrients and water to the plant, while the plant gives sugars and other carbon-containing molecules to the fungus. Most plants are mycorrhizal and are dependent on this symbiosis for their best growth.

On a larger scale, mycorrhizae are also important in ecological restoration, the practice of regenerating native plant communities after they’ve been degraded or destroyed. They may also help with carbon sequestration, in this case the storage of carbon in fungal mycelia, the underground bodies of fungi.

Mycorrhizae and Ecological Restoration

In a 2023 review article, a team of scientists led by Lisa Markovchick wrote about a gap between the science and practice of using mycorrhizae in restoration projects. Although research supports such use, practice lags, in part because of negative views of fungi as only pathogens.

To counter this perception, Markovchick and her collaborators offer insights from research and tips for both protecting mycorrhizal fungi and deploying them during restoration projects. A few of those insights and tips are below. Links to Markovchick’s full paper and a webinar summarizing her work follow the list.

According to Markovchick and her collaborators:

• Mycorrhizae perform many functions, such as promoting water infiltration and retention, preventing erosion, and boosting plant nutrition, survival, and resilience.
• Mycorrhizal fungi also have roles in providing ecosystem services, such as responding to disturbance and providing habitat for other organisms, thereby enhancing biodiversity.
• Change in land use, drought, invasive plants, and other disturbances can deplete mycorrhizal fungi or change the fungal species present. Even some necessary practices, such as applying herbicides to invasive plants, can affect mycorrhizae.
• The benefits of mycorrhizal fungi are clear from many studies. For example, they can significantly increase species richness (the number of different plant species in a community), and plant biomass, and their effects tend to grow with time.
• There must be a good match between plants and mycorrhizal fungi when both are used to restore a community. Mass-produced mycorrhizal fungi may not provide that important pairing, leading to neutral or negative results. Introducing fungi from a native community near the restoration site has proven most beneficial.

References

The Gap Between Mycorrhizal Science and Application. Wild Earth Guardians YouTube video featuring Lisa Markovchick. 53:46.

The gap between mycorrhizal science and application: existence, origins, and relevance during the United Naton’s Decade on Ecosystem Restoration. Lisa M. Markovchick, Vanessa Carrasco-Denney, Jyotsna Sharma, and others. Restoration Ecology Vol. 31, No. 4. May 2023.

 

Mycorrhizae and Carbon Sequestration

Potential solutions to a warming climate include nature-based options such as protecting forests, grasslands and wetlands. Protecting and enhancing mycorrhizal growth could be another solution, because plants transfer carbon-containing compounds such as sugars to below-ground fungal bodies (mycelia) and the roots they support.

The carbon that builds those compounds comes from atmospheric carbon dioxide captured during photosynthesis, and it can be a significant amount. In a 2023 review article, Heidi-Jayne Hawkins and others estimate that, globally, about 13 gigatons of carbon dioxide equivalents are transferred to the mycelia of mycorrhizal fungi each year. That amounts to about 36 percent of the carbon dioxide emissions from fossil fuels in 2021.

Living mycelia can also promote long-term carbon storage by releasing sugars and acids from their hyphae (the fungal strands that constitute the mycelium). These compounds eventually lead to the formation of mineral-associated organic matter, or MAOM. In this type of soil organic matter, carbon compounds are bound to clay, silt, or other mineral particles in soils. MAOM is slower to decompose, in part because it’s protected inside mineral aggregates that are harder for decomposers to access.

Even after fungal mycelia die, they can support carbon storage. Their organic matter is added to the soil, where it can attract soil particles and form the enlarging aggregates that stabilize carbon as MAOM.

The authors emphasize that there is more to understand about the flow of carbon into and through mycorrhizae and its effects on carbon sequestration. Still, they consider mycorrhizal fungi “a major carbon pool.”

Reference

Mycorrhizal mycelium as a global carbon pool. Heidi-Jayne Hawkins, Rachael I.M. Cargill, Michael E. Van Nuland, and others. Current Biology, Volume 33, Issue 11. June 5, 2023.

How can we support mycorrhizal fungi?

 Lisa Markovchick’s article recommends several actions and tips to improve the diversity and function of mycorrhizae, specifically in natural areas. A few of them are below. For a full list, see the link above.

• Protect source populations of mycorrhizae. Native communities with little or no history of disturbance are refuges for these fungi.
• When restorations are planned, include steps for soil conservation.
• Choose mycorrhizal fungi that are appropriate for the plant species being restored. This could be accomplished by introducing the “full diversity” of fungi from nearby native communities like the one being regenerated.
• Plants and their mycorrhizal fungi won’t associate unless both are alive and come into direct contact. Timing and placement are important, as is the source of fungi. Commercial mycorrhizal products may be a poor choice.

Nature's Ancient Engineers

Young mushrooms of fly agaric, Amanita muscaria, in a northern Minnesota mixed coniferous-deciduous forest.The caps of the mushrooms will flatten and expand up to 10 inches wide.

Chances are, you recognize this mushroom. Fly agaric has long been known for its colorful cap, its hallucinogenic effects, and its supposed ability to attract and kill flies. It has also found its way into popular literature and media; It’s the Alice in Wonderland mushroom, the place where Smurfs live, and the Super Mushroom in Super Mario Bros. games. Unfortunately for some, it’s also poisonous, even deadly (1, 2).

Less known but much more significant is what fly agaric does below ground. The mushroom is just the visible part, the body that produces spores for reproduction. Underneath it is an extensive network of thread-like strands called hyphae (HY-fee), together called the mycelium (my-SEE-lee-um). The hyphae are like hunter-gatherers; they run through the soil and absorb water and nutrients to support the rest of the fungus.

Fly agaric also needs organic (carbon-containing) molecules, such as sugars and amino acids, the building blocks of proteins. Many fungi obtain these by breaking down dead organic matter in the soil. Fly agaric, though, goes about it differently: It barters with living plants.

The one above likely does business with white pine (Pinus strobus), red pine (Pinus resinosa), paper birch (Betula papyrifera) or quaking aspen (Populus tremuloides) in the northern Minnesota forest where it grew (3). Beneath the surface, its mycelium wraps around the tips of the trees’ roots, forming a sheath. Some of the hyphae in the sheath grow into the root and form a net around some of the cells. That’s where the exchange occurs: The fungus gives water and nutrients to the plant, while the plant gives organic molecules to the fungus. 

Illustration of an ectomycorrhiza, which forms a sheath around root tips. Hyphae cross the epidermis and enter the root cortex. There they form a net, called a Hartig net, between the cells. The unit of measurement in the upper right scale is 100 microns. One hundred microns is the approximate width of a human hair. The photographs are colorized images produced by a scanning electron microscope, or SEM. Illustration by Atrebe10, licensed under Creative Commons Attribution-Share Alike 3.0 Unported license, via Wikimedia.

This symbiotic relationship is called a mycorrhiza (MY-co-RY-za), literally “fungus root.” The sheathing type, called an ectomycorrhiza, is relatively new, about 200 million years old give or take a few million years. An older type, roughly 400 million years old, doesn’t sheath a plant’s roots but instead grows inside the root cells themselves, where the exchange of nutrients, water and organic compounds occurs. This type is called an endomycorrhiza or arbuscular mycorrhiza, the latter named for the tree-like growth, called an arbuscule, the fungus forms inside the cells.

Development of mycorrhizae (plural, ending in -zee) was critical to the transition of plants from water to land hundreds of millions of years ago (4, 5 7). These early land plants had no roots or vascular (conducting) tissues. Those that eventually developed symbiotic associations with fungi had an advantage, because mycorrhizae could extend the plant's reach into the soil to obtain water and nutrients. Mycorrhizae were so beneficial that they developed independently many times over, involving different plants and fungi (6).

Given that far-reaching history, it’s no surprise that there are more types of mycorrhizae than the two described above. Ecto- and endomycorrhizae are the most common, but there are at least two more types (6). Whatever the type, in most cases both partners benefit and often are dependent on each other for survival. Without their association, neither partner would thrive. Their ecosystems wouldn’t, either.

That’s because approximately 80% of modern terrestrial plants are mycorrhizal, although the percentage of such plants in a community can vary (7). Prairies, deciduous forests, coniferous forests, and other terrestrial ecosystems are generally dominated by mycorrhizal plants. They are the foundation of these systems, supporting all the trophic (feeding) levels above them. A mycorrhizal white pine, for example, grows the cones that hold the seeds that feed a variety of birds and mammals, which in turn have their own roles in their communities.

Mycorrhizae are so important to ecosystems that restoring highly disturbed places, such as mines and abandoned agricultural fields, can be helped by adding mycorrhizal fungi to soils or seeds when restoration begins. Mycorrhizae have also been suggested as possible remedies for high carbon dioxide levels in our atmosphere, because they can help move carbon below ground. These practices will be the subjects of the next post.

References

1.      Two cases of severe Amanita muscaria poisoning including a fatality. Ethan M. Meisel, MD, and others. Wilderness and Environmental Medicine, Vol. 33, No. 4, 2022.

2.     Notes from the field: Acute intoxications from consumption of Amanita muscaria mushrooms — Minnesota, 2018. Joanne Taylor and others. MMWR Morb Mortal Wkly Rep 2019, Vol. 68: pp. 483–484, 2019.

3.     Fungus associates of ectotrophic mycorrhizae. James M. Trappe. Botanical Review, Vol. 28, No. 4: pp. 538-606, 1962. Available for reading with a free JSTOR account at https://www.jstor.org/stable/4353659.

4.     The origin and evolution of mycorrhizal symbioses: from palaeomycology to phylogenomics. Christine Strullu-Derrien and others. New Phytologist, Vol. 220, No. 4: pp.1012-1030, 2018.

5.      History of mycorrhizae. Jake Sun, Lindenwood University. The Confluence, Vol. 1, No. 2: Art. 2, 2022.

6.      Mycorrhizal Fungi. Society for the Protection of Underground Networks. Website accessed February 3, 2025.

7.      Coevolution of roots and mycorrhizas of land plants. Mark C. Brundrett. New Phytologist, Vol. 154, No. 2: pp. 275-304, 2002. 

Are Fungi Plants?

In early classification systems, these morel mushrooms (Morchella esculenta) were included with plants.

 
At one time, morel mushrooms and other fungi were considered plants. That was when all life was classified as either plant or animal, and morels sure didn’t look like animals. They emerge from the soil, they don't move, and they produce “fruiting” bodies (mushrooms) for reproduction. In addition, its cells are surrounded by rigid walls, as are the cells of ferns, grasses, trees and other living things that are true plants. (Most bacterial cells also have walls, but that's another topic.)

It wasn’t until the 1960s that fungi were placed in their own kingdom. Ecologist Robert Whittaker thought they should be separated from plants because they are decomposers, not producers. In other words, they break down and absorb organic matter, whereas plants make organic matter by photosynthesis. In ecologists’ terms, fungi are saprotrophic (literally, rot feeders) whereas plants are autotrophic (self-feeders). There are exceptions in each group, but they’re in the minority.

Other differences became apparent as the decades went by. Fungal cell walls, for example, are made of chitin, long chains of modified glucose molecules. That’s the same material that forms the exoskeletons of insects. Plant cells, however, are mostly cellulose, twisted and bundled chains of glucose without the modifications present in chitin.

Other differences appear inside their cells. Fungal cells have no chloroplasts, the organelles where photosynthesis takes place. They also lack chlorophyll, the green pigment primarily responsible for absorbing light energy. That makes sense, since fungi aren’t photosynthetic.

Plant cells (left, from a moss) contain chloroplasts, the green organelles where photosynthesis takes place. Fungal cells (right, from a Morchella, or morel, mushroom) do not. This is one difference between plants and fungi. Moss cells: Kelvinsong, CC BY 3.0, via Wikimedia Commons. Morchella cells cropped from image by Marc Perkins CC BY-NC 2.0, via Flickr.

There are other differences, but maybe these are enough to show that fungi and plants aren’t closely related. Even so, the line between them can be muddy. Some older botany textbooks include at least a chapter about fungi. And as mentioned above, mushrooms and other large reproductive bodies are still called “fruiting” bodies, even though they’re not at all like the fruits produced by flowering plants.

It can be confusing. What’s not confusing or unclear is that fungi, though not plants, are important to plants for many reasons. For example, fungi made it possible for plants to colonize land, a step in development not only for plants but for entire terrestrial ecosystems. More on that in the next post.

The Color of Survival

These blackberry leaves (Rubus species) displayed multiple colors this fall.

If conditions are favorable, fall gives us a brilliant display of yellow, orange, red and almost purple leaves. As green fades and other colors appear, leaves are also revealing what makes them tick. The pigments in their blades aren’t just for show. They’re workhorses, and their tasks are critical to a plant’s survival. 

The following sequence shows a leaf of highbush cranberry photographed from September into November. As the season progresses, the blade changes from green to orange-red to dark red, each phase produced by a different set of pigments, each set with a different purpose. The leaf was photographed in 2008.

September 20

The leaf is green from the pigment chlorophyll. Two nearly identical forms of chlorophyll, called chlorophyll a and chlorophyll b, absorb mostly red and blue light and reflect green. These pigments are vital for photosynthesis, the process that converts light energy into chemical energy in the form of glucose. 

October 11

As day length shortens and temperatures fall, chlorophyll production slows. As green fades, yellows, oranges and scarlet reds appear. These colors are from carotenoids, pigments that have been there all along but have been masked by chlorophyll. Carotenoids are accessory pigments in photosynthesis; they absorb blue and green light, expanding the wavelengths available to power this process.

 

October 19

As chlorophyll continues to be degraded, more carotenoids are visible. The leaf blade is distinctly pale green with larger areas of light red. This trend continues as the days pass.

 

October 27

Nearly all chlorophyll is gone and more carotenoids are revealed. At the same time, darker red pigments called anthocyanins begin to form. These pigments need sugars to develop, so their deep red to purple hues are muted when fall months are cloudy and rainy and photosynthesis is limited. Some plant species produce only small amounts of anthocyanins. Highbush cranberry, dogwoods, red oak, red maple and sumac are among those that produce higher amounts.

November 1

Abundant anthocyanins now mask the carotenoids. Anthocyanins absorb yellow, green and ultraviolet light but are not involved in photosynthesis. At one time they were considered a useless waste of a plant's energy. Now, though, they're thought to block excess light energy from damaging leaf tissues, similar to sunscreen. Anthocyanins can also form in other stressful conditions, such as drought, high salinity, and nutrient deficiency.

Chlorophyll, carotenoids and anthocyanins may be present in other parts of plants, too. Flowers, fruits, stems and roots -- carrots, for example -- bear many kinds of pigments, and their roles in those organs may be different from those in leaves. The color of survival is complex. 

Learn more about leaf pigments and photosynthesis:

Absorption of light. LibreTexts Biology. 

Leaf Pigments. Harvard Forest.

When and why leaves change color. Minnesota DNR.

Plant Profile: Zigzag Goldenrod

 Solidago flexicaulis | Family Asteraceae (Aster) 

Zigzag goldenrod along a woodland edge in August.

Zigzag goldenrod, also called big leaf goldenrod, is a native perennial of forest edges and openings. It’s literally a late bloomer, flowering from August into October with narrow, upright clusters of yellow-gold flowers at the top of the plant and from the upper nodes.

Each “flower” is actually a group of small flowers in a head inflorescence, an arrangement typical of plants in the aster family. The central flowers, called disk flowers, have small, recurved, yellow petals that are easiest to see with a magnifying lens. Around them are several ray flowers, so called because each bears a single, petal-like ray. Zigzag goldenrod heads typically have 3-5 rays at their peak.

Left: A single head of small flowers. Two rays are visible. The "spears" emerging from the flowers are stamens and pistils, the reproductive parts of the flower. Right: A single head dissected to show disk and ray flowers. The white threads at the base of the flowers are a group of modified sepals called a pappus.

At the base of either kind of flower are white, thread-like, modified sepals, together called the pappus. Unlike the leafy or petal-like sepals many plants have, these tiny filaments persist after flowering. They are attached to the top of small, linear fruits called achenes (ah-KEENS). A single plant produces hundreds (thousands?) of them, each carried by wind with the help of its parachute-like pappus.

Left: The fruits on this zigzag goldenrod are ready to catch the wind.
Right: Individual achenes, each just a millimeter or two long and topped with a spreading pappus. 

Even before it flowers, zigzag goldenrod is easy to recognize. As its name suggests, the stems typically zig and zag from one node to the next. The pattern is subtle, but it’s still a good identifying characteristic.

The name “big leaf” refers to the lower leaves, which are egg-shaped and up to 4 inches wide and 6 inches long. Their margins (edges) are coarsely toothed and their petioles are winged, especially where they meet the leaf blades. Farther up the stem, the leaves are smaller and lance shaped.

Left: A stem with a typical zig zag pattern. Upper right: Lower and middle leaves are egg shaped and sharply toothed.
Lower right: Upper stem leaves are lance shaped, becoming smaller up the stem.

Zigzag goldenrod spreads not just with seeds but also with rhizomes to form colonies. It isn’t as aggressive as Canada goldenrod, but patches will expand noticeably in a few years. That’s helpful where cover is desired but not so helpful in formal gardens, where plants are often preferred to stay in place.

Zigzag goldenrod is pollinated by a variety of insects, including bees, flies, wasps and butterflies. Goldenrods in general, along with asters, are important sources of food for pollinators late in the season. Goldenrods also host insect larvae, such as the colorful caterpillar of the brown-headed owlet moth.

Left: A bumble bee pollinates zigzag goldenrod while gathering nectar and pollen.
Right: A caterpillar of the brown hooded owlet moth feeds on zigzag goldenrod.

It’s hard to fault zigzag goldenrod for anything, but goldenrods in general have a reputation for causing seasonal allergies. Their pollen, however, is relatively heavy and sticky, ideal for attaching to insect bodies but not for catching the wind. The pollen of common ragweed and giant ragweed, however, is light, dry and wind-borne, and it’s released from mid-summer to mid-fall, about the same time as goldenrods. Also, ragweed grows in the same dry, sunny habitats that favor some goldenrods, so the latter gets a bad rap.

It’s undeserved. No need for tissues to enjoy zigzag goldenrod. Just a semi-shady spot and an appreciation for this golden yellow pollinator magnet.

References

Minnesota Wildflowers

Board of Water and Soil Resources

Blue Thumb

Illinois Wildflowers

Plant Profile: Wild Carrot

 Daucus carota L./Carrot family, Apiaceae

Wild carrot, Daucus carota, growing along a roadside near Maple Plain, MN, on August 10, 2024.

Wild carrot, also called Queen Anne’s lace, is a Eurasian biennial introduced to North America by colonists for food or medicinal use. It produces basal rosettes of carrot-like leaves the first year and leafy stems topped with umbels (umbrella-shaped clusters) of small, white flowers the second year. It blooms in mid- to late summer, typically in the dry soils and full sun of disturbed sites, such as abandoned lots, old fields, rail corridors, and roadsides. It spreads by seeds, and they are abundant. Because a single plant blooms continuously in its second year, that plant can produce thousands of seeds before it dies.

As the name “wild carrot” suggests, this is the ancestor of cultivated carrots. Wild carrot’s taproots are carrot-like in shape and smell, although they’re narrower than cultivated carrots and become bitter and woody with age, especially in the plant’s second year of growth.

Although some sources caution against eating wild carrot, it’s generally considered non-toxic and edible. Large quantities can raise blood pressure or make it difficult to regulate (1). In some people, sap from the plant can irritate the skin when it’s exposed to sunlight, a condition called phytophotodermatitis, literally “plant-light-skin inflammation” (2, 3, 4). In addition, carrot greens, presumably from cultivated carrots, are listed as mildly toxic on the list of poisonous plants from the MN Poison Control System, now the Minnesota Regional Poison Center. Wild carrot greens may have the same effect.

Deadly Look-Alikes

The much greater danger in foraging wild carrot is mistaking deadly look-alikes for this plant. Water hemlock (Cicuta maculata) and poison hemlock (Conium maculatum), also in the carrot family, resemble wild carrot and have accidentally been added to salads or sampled in the field or garden. The result can be severe illness and even death caused by the plants’ alkaloids, compounds many plants produce to deter herbivores. All parts of these plants are toxic.

Three look-alikes. From left: Wild carrot, water hemlock, poison hemlock. Water hemlock photo by Rob Routledge, Sault College, Bugwood.org. Poison hemlock photo by Eric Coombs, Oregon Department of Agriculture, Bugwood.org.

Accurate Identification is Crucial

Several resources help identify wild carrot and its look-alikes. A plant identification sheet from the University of Minnesota Extension Service contrasts poison hemlock with ten other plants, including water hemlock and wild carrot. An article by the Clearwater Conservancy in Pennsylvania includes a table contrasting the characteristics of seven members of the carrot family, including those featured here. The Minnesota Department of Transportation also has a guide to identifying poison hemlock and its look-alikes..

Here are some additional tips.

Tip 1: Wild carrot umbels sometimes have a dark red or purple flower in the center. Beneath the umbels are long, branched bracts. After flowering, the umbels curl up and in to form a bowl or nest. That’s why wild carrot is sometimes called bird’s nest. Neither water hemlock nor poison hemlock have these characteristics.

 

Left: After flowering, wild carrot umbels turn up and in to form a small nest. Notice the long, branched bracts below the umbel. Right: A purple flower in the center of a wild carrot umbel (arrow). Not all umbels have them.

Tip 2: The plants are hardest to distinguish in their first year of growth, when they produce only basal leaves. All three have pinnately divided leaves, but wild carrot leaves are narrower in outline and more finely divided into narrower leaflets. The leaves of both water hemlock and poison hemlock are broader, triangular in outline, and have wider leaflets. Poison hemlock leaves are often described as fern-like.

 

From left: Leaves of wild carrot, water hemlock, and poison hemlock. Not to scale. Water hemlock photo by Steve Dewey, Utah State University, Bugwood.org. Poison hemlock photo by John Cardina, The Ohio State University, Bugwood.org

Tip 3: Habitat and flowering phenology differ somewhat among these plants, but because they overlap, they are less reliable characteristics. In general, wild carrot thrives in sunny, well-drained, disturbed places such as pastures, old fields, roadsides, and railway corridors. In Minnesota it flowers as early as May and as late as October, but more typically from July to September.

In contrast, water hemlock is an obligate wetland plant. It needs the moist soils of wet prairies, wet meadows, marshes, and streambanks. It flowers from June to September. Poison hemlock also prefers wet or moist soils, but it will tolerate drier conditions. Its habitats include streambanks, ditches, roadsides, and pastures. It flowers in June and July.

If You Find Poison Hemlock

Because poison hemlock, an introduced plant, is so hazardous and because it’s not yet so widespread in Minnesota that it can’t be controlled, its locations should be reported to Report A Pest or EDDMapS. Do not remove it without taking serious precautions. Better yet, hire a professional (5).

Cited References

1)     Wild Carrot. Missouri Poison Center. Accessed August 11, 2024.

2)     Wild Carrot. Minnesota Department of Agriculture. Accessed August 11, 2024.

3)     Phytophotodermatitis Clinical Presentation. William P. Baugh, MD. Editor William D. James, MD. Medscape. Updated November 4, 2021.

4)     Queen Anne’s lace (Daucus carota). Minnesota Department of Natural Resources. Accessed August 12, 2024.

5)     Poison hemlock. A. Gupta, A. Rager, and M. M. Weber. University of Minnesota Extension Service. Reviewed in 2020. Accessed August 12, 2024.

Additional References

Daucus carota (Queen Anne’s Lace). Minnesota Wildflowers. Accessed August 12, 2024.

Water Hemlock. G.D. Bebeau, The Friends of the Wildflower Garden, Inc. Accessed August 12, 2024.

Cicuta maculata (Water Hemlock). Minnesota Wildflowers. Accessed August 12, 2024.

Poison hemlock (Conium maculatum). Minnesota Department of Natural Resources. Accessed August 12, 2024.

Poison Hemlock. Minnesota Department of Agriculture. Accessed August 12, 2024.

Poison hemlock (Conium maculatum). Minnesota Department of Natural Resources. Accessed August 12, 2024.

Minnesota Noxious Weed List. Minnesota Department of Agriculture. Accessed August 12, 2024.

Chadde, S.W. 2012. Wetland Plants of Minnesota. 2^nd ed. A Bogman Guide.

Plants for Bee Specialists

Jerusalem artichoke, Helianthus tuberosus, is one of several sunflower species favored by the sunflower mining bee, a specialist pollinator.

The sunflower mining bee, Andrena helianthi, has discriminating tastes.

This native bee is a specialist, gathering pollen primarily from plants in the aster family, Asteraceae (formerly Compositae, also called composites). To be more specific, it favors pollen from plants in the genus Helianthus, the sunflowers, to feed its larvae. (1).

In pollinator terminology, the sunflower mining bee is oligolectic, meaning “few chosen.” It’s far from alone in having narrow food preferences. According to the recent Minnesota Statewide Bee Survey (2), about 30% of the nearly 360 bee species confirmed in the survey are oligolectic.

Benefits and Drawbacks of Oligolecty

Given that high number, there must be advantages to oligolecty. One possibility is that the bees co-evolved with a few host plants that offer more digestible pollen (3). Entomologists at the University of Wisconsin found that the larvae of the blueberry mason bee, Osmia ribifloris, thrived when fed preferred host pollen that also included the microbes naturally found in that pollen. In contrast, larvae fed microbe-free pollen from the preferred host plant were much less fit, and larvae fed pollen from non-host plants had intermediate fitness (4).

Plants benefit from the relationship, too. Species visited by oligolectic bees have dedicated pollinators that transfer pollen among only a few kinds of plants, which makes successful pollination more likely. The plant loses less pollen to insects that carry it to a wider variety of plants, most of which can’t use it.

The potential disadvantage for both oligolectic bees and their plant hosts is that if either one becomes rare, its partner could become rare, too. A spiraling decline of both bees and plants happens when, for example, a plant population is displaced by an invasive species or fails to thrive in a warmer or wetter environment. As the plant becomes less abundant, so does the oligolectic bee that depends on it. In turn, as the oligolectic bee becomes less abundant, so can the plant that depends on it for pollination. If fewer seeds are produced, the population may decline further, with consequent effects on the oligolectic bee, and so on. It’s a vicious circle that can be difficult to interrupt.

The Habitat Solution

Difficult but not impossible. The answer is to provide habitat, including preferred host plants, for the sunflower mining bee and other oligolectic species. Fortunately, there are several resources to learn which plants or groups of plants benefit which bee species.

Entomologist and ecologist Jarrod Fowler compiled a list of bee specialists documented in the Central U.S., a region that includes Minnesota, Iowa, Wisconsin, North Dakota, and South Dakota (3). He also tabulated their preferred plant(s) and found that species in the aster or sunflower family, Asteraceae, and the bean or pea family, Fabaceae, were visited most frequently by specialist bees in this region.

In addition, he noted the top 25 genera that support oligolectic bees. The genera found in this region include Helianthus (sunflowers), Heterotheca (false goldenasters), Solidago (goldenrods), and Symphyotrichum (asters).

Although Helianthus is a favorite of sunflower mining bee, the insect has also been collected from the flowers of (l to r) cup plant (Silphium perfoliatum), New England aster (Symphyotrichum novae-angliae) and goldenrod (Solidago) species, here showy goldenrod (S. speciosa).(1)

The Minnesota Department of Natural Resources’ Minnesota Bee Species List is another useful resource (5). The list of around 460 bees includes both those that collect pollen and those that are parasites on other bees’ nests. For those that collect pollen, the table provides the species’ lecty (range of pollen preference, either oligo- or poly-) and its nesting habitat, if either is known.

The species list in the Minnesota Statewide Bee Survey (2) includes not only the bees’ names and lecty, if the latter is known, but also the ecological province(s) where each species was found. The report includes distribution maps of the bee species as well as their conservation status, or S-rank, which can range from S1 (critically imperiled) to S5 (secure).

The 2024 Featured Plant series from the Board of Water and Soil Resources (6) highlights several plants that support specialist bees or other insects. A Featured Plant is posted online at the beginning of each month.

If you observe and photograph bees visiting plants, consider submitting your records to iNaturalist. Several bee-related projects are hosted on this online platform, including Minnesota Native Bees. To find other projects, go to the iNaturalist website, choose Projects from the Community drop-down menu, and type “bees” into the search box.

Who knows, maybe the sunflowers you watch this summer and fall will host sunflower mining bees. Although the bees were uncommon to rare in the state’s bee survey, you could be the lucky one who spots this specialist pollinator.

References

1)  Andrena helianthi, Robertson 1891. Discover Life. Website accessed July 3, 2024.

2)   Minnesota Statewide Bee Survey 2014-2023. Minnesota Department of Natural Resources.

3)   Pollen Specialist Bees of the Central United States. Jarrod Fowler, 2020.

4)   Dharampal, P.S., Hetherington, M.C., and Steffan, S.A. 2020. Microbes make the meal: oligolectic bees require microbes within their host pollen to thrive. Ecological Entomology 45: 1418-1427. DOI: 10.1111/een.12926. https://par.nsf.gov/servlets/purl/10253224

5)    Minnesota Bee Species List. Minnesota Department of Natural Resources, August 2023.

6)     Board of Water Resources Featured Plant series, 2024. (Red-berried elder is the July featured plant; plants featured in earlier months are in the Featured Plant archive.)