Wednesday, July 3, 2024

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.)


Plants for Bee Specialists

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