One of the most commonly encountered polylectic bees, a metallic green sweat bee (likely Augochlora pura), on tall American bellflower (Campanulastrum americanum). Photos courtesy of Josh Klostermann.

August 1, 2023

Mono/Oligo/Polylecturing: A crash course into the pollen specialization of bees

Every August sweltering sun rays untangle the ultraviolet blooms of ironweed out of roadsides and meadows. This marks the return of just one of many crystalized dances between plants and insects on the landscape. With a still approach to one of these plants, one might first hear an unfamiliarly high-pitched buzz, a quick peripheral dash of commotion, and then catch a glimpse of a frosty blue-eyed beast mid-tango with the deep purple disk florets of ironweed (Vernonia sp.). That little beast is likely the long-horned bee Melissodes denticulatus, a pollen specialist of ironweeds and arguably one of the region’s most beautiful bees.

A small, black and white bee called the long-horned bee <i>(Melissodes denticulatus)</i> on a purple ironweed flower.
Long-horned bee (Melissodes denticulatus) on ironweed.

‘Pollen specialization’ or the restriction of a bee’s diet to the pollen of one species or a few different genera of plants is a common trait that can be found across all bee families. Examples of pollen specialization represent intimate relationships between two distantly related organisms that have been dragged through deep time and still occur today. The goal of this article is to provide a broad overview of the different degrees of pollen specialization, highlight some pollen specialists from Illinois, and stress the importance of preserving and restoring natural plant communities.

There are likely around some 500 species of bee that can be found in Illinois, all of which can be placed into one of three boxes depending on their pollen collecting behaviors; monolectic, oligolectic and polylectic (see the pun in title).

Monolectic bees are the pickiest of eaters. They have evolved to strictly collect pollen from only one species of plant (known as their host plant). Oligolectic bees are slightly less picky but still collect pollen from only a few similar and closely related host plants (such as Melissodes denticulatus above). Polylectic bees don’t really care at all and will collect pollen across many unrelated plant species.

The degree that a bee specializes on pollen can also provide insight into other aspects of their life. Polylectic bees generally have a longer flight period, as they are able to switch between plants, learning to pig out on what comes into bloom as the growing season progresses. On the other hand, monolectic and oligolectic bees have a shorter flight period that is hardwired to be correlated with their host-plants bloom period.

Perhaps the most commonly encountered native polylectic bee is the ligated furrow bee (Halictus ligatus). They will collect pollen from many different plants and can be found flying throughout the growing season. Bumble bees (Bombus sp.) are also familiar polylectic bees. This collage photo shows the common eastern bumble bee (Bombus impatiens) collecting pollen from a variety of plant species.

 A collage of four photos. Four large, yellow and black Eastern bumble bees <i>(Bombus impatiens)</i> collect pollen from four different types of unidentified flowers.
Eastern bumble bees (Bombus impatiens) collect pollen from a variety of plants across many families.

Polylects are much more readily able to handle changes in the environment due to their ability to collect pollen from many different plants. For this reason, they are abundant in urban landscapes where remnant plant communities have been disturbed and replaced by introduced plant communities. It is likely that each reader usually passes at least a handful of polylectic bees in a day. A few other notable polylects that one may commonly encounter are the pure green sweat bee (Augochlora pura), the introduced European honeybee (Apis mellifera) and some leaf-cutting bees (Megachile brevis, etc.).

Mono/oligolectic bees aren’t exactly less common than their generalist counterparts but are surely more restricted in where they occur and are localized to areas where their host plants still grow. In Illinois, you can easily see pollen specialists where intact native plant communities still exist. Places like nature preserves, conservation areas and state parks are crucial habitats for specialist pollinators because they protect and preserve the communities of plants that are found within their boundaries. But these are not the only types of spaces native plant communities can still exist. Often forgotten places, such as rural roadsides, the large fallow lots at the edge of town, and powerline easements, foster crumbs of native habitat that are sprinkled across the landscape. These overlooked spaces are vital because they are a refugia for bees that have survived the past centuries of changing land use. They also act like a web that connects the larger patches of preserved habitat together. Unfortunately, excessive mowing and herbicide use to eliminate ‘weeds’ has and is causing many of these small populations of plants to disappear, further imperiling the insect pollinators that rely on them.

Although the webbing that holds the protected land together is rapidly vanishing, a good number of mono/oligolectic bees still occur in Illinois. Many species in the genus Melissodes are oligolectic, such as M. denticulatus (ironweed longhorn bee on ironweed), M. desponsus (thistle longhorn bee on thistles) and M. druriellus (sunflower longhorn bee on asters).

Left: Thistle longhorn bee <i>(Melissodes desponsus)</i> on a thisle. 
Right: Sunflower longhorn bee <i>(Melissodes druriellus)</i> on an aster.
Left: Thistle longhorn bee (Melissodes desponsus) on a thisle.
Right: Sunflower longhorn bee (Melissodes druriellus) on an aster.

The bee genus Andrena (mining bees) displays a wide variety of pollen specialization and has many notable oligolectic representatives. Andrena rudbeckiae is an oligolectic bee that collects pollen from black-eyed Susan’s and gray-headed coneflowers. This bee can be found at prairie restorations across the state. The more western oligolect Andrena helianthiformes is only known from Illinois in areas where remnant populations of its coneflower (Echinacea spp.) host plant still exist.

Left: Andrena rudbeckiae on a gray-headed coneflower. 
Right: Andrena helianthiformes on a coneflower.
Left: Andrena rudbeckiae on a gray-headed coneflower.
Right: Andrena helianthiformes on a coneflower.

A rare monolect from Illinois is Perdita gerhardi (spotted horse mint fairy bee), which specializes only on spotted beebalm (Monarda punctata) and has such an intimate relationship with this plant that it has evolved a similar coloration to be camouflaged when within its flowers.

Spotted horse mint fairy bee <i>(Perdita gerhardi)</i> on spotted beebalm <i>(Monarda punctata)</i>.
Spotted horse mint fairy bee (Perdita gerhardi) on spotted beebalm (Monarda punctata).

The persistence of specialist bees depends on the preservation and restoration of natural plant communities. Once the host plants disappear so do the bees that rely on their pollen. As the plant communities of Illinois are continually disregarded for developments, bee species will continue to be extirpated. The shadows of bygone bees darting between bluestems loom over the Prairie State. At times the situation seems dire, and I am awed by the loss. If we wish to reverse the damages done to plant and animal communities we need to start small. The webbing that exists between what remains can be strengthened by each of us with seed and shovel. Manicured ditches and lawns can transform into meadows and meadows can transform people.

I hope this article inspires the reader to go out and find the little patches of ditch sunflower still left, and join the ages-old dance between bee and plant by respectfully, and only with permission from the landowner, collecting some seed, and extending that web into their yard.

Josh Klostermann is a student of botany (plants) and melittology (wild bees) with the goal of being a regional expert in these fields. His current research is focused on the effects of landscape disturbances and restoration on bee and wasp communities. He is an avid wildlife photographer and all-around bug nut.

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