Pollination
The vast majority of flowering plants in North America require pollination by some kind of organism, usually an insect. Insects are the vector, transporting the male reproductive genes contained within pollen, from one flowering plant to another (of the same species). For pollination to occur, the pollen must be successfuly deposited on a receptive stigma, a part of the plant's female reproductive anatomy.
Flower Anatomy and Pollination
As a bee forages on a flower for nectar or pollen, pollen grains produced from the flower's anthers collect on the bee's simple and branched hairs. For a flower, the process of pollination is initiated when a foraging bee moves pollen grains from one flower's anthers to another flower's stigma of the same plant species or, in the case of self-compatible flowers, pollen from a flower to its own stigma.
When a pollen grain is deposited on the stigma by a bee, it absorbs moisture from the stigmatic surface, then germinates. A pollen tube grows down the style to the ovary, penetrating through the ovary. Then, two male sperm are released and travel down the pollen tube.
One of the male sperm enters the ovule and fertilizes the female egg cell producing a seed. The other male sperm unites with two polar nuclei to develop into the endosperm (a nutrient-providing tissue surrounding the seed).
Pollination by Bees
The rise of bees from their wasp ancestors coincided with the diversification of flowering plants during the Cretaceous period, approximately 150 million years ago. These events also set the stage for numerous intricate and sometimes mutualistic relationships to develop between bees and flowers.
There are two key bee attributes that increase their effectiveness as pollinators. First, they intentionally collect pollen from flowers to provision their nest cells. Second, they are covered with simple and branched hairs for this purpose, resulting in more pollen carried and transported between flowers. In addition, many male bees are also covered in dense hairs and can inadvertently transport pollen from one flower to another, resulting in pollination. However, while perching on flowers or foliage, males commonly groom pollen off the hairs on their body between flower visits, so their effectiveness as pollinators (compared to females) is reduced.
In addition to pollinating food that humans consume, bees play a crucial role pollinating native plants. These pollination services produce the fruit (and seeds) that feed organisms directly or indirectly in the natural world. Native plants cross-pollinated by bees produce genetically diverse seed with more variability in traits, enhancing the overall resiliency of a plant population. In this way, bees play a crucial role in conserving biodiversity, leading to improved ecosystem functionality and stability.
An Andrena female collects pollen on her hind leg scopae.
An Agapostemon male grooms pollen off his body between flower visits.
Crop Pollination
Bees are responsible for pollinating a significant fraction of the food humans consume, between twenty and thirty-five percent of crops. Many food crops, including grain crops such as wheat, oats, corn, and rice, are wind-pollinated and do not rely on bees for pollination. From a nutritional standpoint, bee-pollinated plants often provide more essential nutrients for bees, including vitamins, and a higher pollen protein content than wind-pollinated plants. Plants such as tomato, eggplant, and pepper belonging to the family Solanaceae require buzz pollination to produce fruit. Their flowers are self-compatible and even without visits from bees can produce fruit, albeit potentially poorly developed fruit. Fruit-bearing trees such as apples, cherries, and plums are self-incompatible and rely upon cross pollination by bees to set fruit. Native bees are important pollinators of small berry crops including blueberries, cranberries, strawberries, raspberries, and currants. Whether a bee-pollinated crop flower is self-compatible or not, pollination by bees typically improves the size, shape, and overall yield of crops.
Apples
Strawberries
Blueberries
Squash
Cherries
Currants
Tomatoes
Peppers
Cantaloupe
Raspberries
Buzz Pollination (Sonication)
Most flowers have anthers that, when fully developed, have openings from which pollen is passively released. The openings are often narrow slits along the length of the anthers. Some flowering plants have inverted anthers with valves or pores, and pollen must be actively released. In addition to plants with inverted anthers, nectarless plants are often buzz pollinated by native bees. For example, Rosa blanda (smooth wild rose), Tradescantia ohiensis (Ohio spiderwort), and Geum triflorum (prairie smoke). To overcome the difficulty of collecting pollen from plants with inverted anthers and pores, many bees have developed a mechanism known as sonication or buzz pollination. To extract the pollen from these flowers, bees employ this mechanism by detaching their wings from their flight muscles while simultaneously clasping the anthers with their forelegs or mouthparts, then vibrating the flight muscles in their thorax at a high frequency, to shake the pollen out of the pores.
Buzz pollinating flowers releases a large quantity of pollen. For bees skilled in the process, it can yield them a quantity of pollen equivalent to that of numerous flower visits without buzz pollination. Bees known to buzz-pollinate flowers include Bombus (bumble bees), Xylocopa (large carpenter bees), Andrena (mining bees), Halictus (sweat bees), Lasioglossum (small sweat bees), and Agapostemon, Augochlora, Augochloropsis, and Augochlorella (metallic green sweat bees). Ericaceous (blueberries, cranberries), Solanaceous (tomato, eggplant, pepper), and some Fabaceous plants such as nectarless Chamaecrista fasciculata flowers require buzz pollination.
A pair of Augochloropsis females buzz pollinate a nectarless Rosa blanda flower.
An Augochloropsis metallica female buzz-pollinates a Rosa blanda flower.
Video © 2024 Jeffrey Karron
A Preliminary List of Buzz-Pollinated Plants in Minnesota
Plant Type | Plant Family | Scientific Name | Common Name | Notes |
|---|---|---|---|---|
Perennial | Commelinaceae | Tradescantia bracteata | Long-bracted Spiderwort | Nectarless |
Perennial | Commelinaceae | Tradescantia occidentalis | Spiderwort | Nectarless |
Perennial | Commelinaceae | Tradescantia ohiensis | Ohio Spiderwort | Nectarless |
Shrub | Ericaceae | Andromeda polifolia | Bog Rosemary | Poricidal anthers |
Shrub | Ericaceae | Arctostaphylos uva-ursi | Bearberry | Poricidal anthers |
Shrub | Ericaceae | Chamaedaphne calyculata | Leather-leaf | Poricidal anthers |
Shrub | Ericaceae | Chimaphila umbellata | Pipsissewa | Poricidal anthers |
Shrub | Ericaceae | Empetrum nigrum | Black Crowberry | Poricidal anthers |
Shrub | Ericaceae | Epigaea repens | Trailing Arbutus | Poricidal anthers |
Shrub | Ericaceae | Gaultheria hispidula | Creeping Snowberry | Poricidal anthers |
Shrub | Ericaceae | Gaultheria procumbens | Wintergreen | Poricidal anthers |
Shrub | Ericaceae | Gaylussacia baccata | Black Huckleberry | Poricidal anthers |
Perennial | Ericaceae | Moneses uniflora | One-flowered Pyrola | Poricidal anthers |
Perennial | Ericaceae | Orthilia secunda | One-sided Pyrola | Poricidal anthers |
Perennial | Ericaceae | Pyrola americana | Round-leaved Pyrola | Poricidal anthers |
Perennial | Ericaceae | Pyrola asarifolia | Pink Pyrola | Poricidal anthers |
Perennial | Ericaceae | Pyrola chlorantha | Green-flowered Pyrola | Poricidal anthers |
Perennial | Ericaceae | Pyrola elliptica | Shinleaf | Poricidal anthers |
Perennial | Ericaceae | Pyrola minor | Small Shinleaf | Poricidal anthers |
Shrub | Ericaceae | Vaccinium angustifolium | Lowbush Blueberry | Poricidal anthers |
Shrub | Ericaceae | Vaccinium caespitosum | Dwarf Bilberry | Poricidal anthers |
Shrub | Ericaceae | Vaccinium macrocarpon | Large Cranberry | Poricidal anthers |
Shrub | Ericaceae | Vaccinium myrtilloides | Velvet-leaf Blueberry | Poricidal anthers |
Shrub | Ericaceae | Vaccinium oxycoccos | Small Cranberry | Poricidal anthers |
Shrub | Ericaceae | Vaccinium uliginosum | Alpine Bilberry | Poricidal anthers |
Shrub | Ericaceae | Vaccinium vitis-idaea | Ligonberry | Poricidal anthers |
Annual | Fabaceae | Chamaecrista fasciculata | Partridge Pea | Nectarless |
Perennial | Orobanchaceae | Pedicularis canadensis | Wood Betony | |
Perennial | Orobanchaceae | Pedicularis lanceolata | Swamp Lousewort | |
Perennial | Primulaceae | Dodecatheon amethystinum | Jeweled Shooting Star | Nectarless |
Perennial | Primulaceae | Dodecatheon meadia | Prairie Shooting Star | Nectarless |
Perennial | Primulaceae | Primula mistassinica | Mistassini Primrose | Nectarless |
Perennial | Rosaceae | Geum triflorum | Prairie Smoke | |
Shrub | Rosaceae | Rosa arkansana | Prairie Rose | Nectarless |
Shrub | Rosaceae | Rosa blanda | Smooth Wild Rose | Nectarless |
Shrub | Rosaceae | Rosa woodsii | Wood’s Wild Rose | Nectarless |
Perennial | Rusaceae | Polygonatum biflorum | Smooth Solomon’s Seal | |
Perennial | Rusaceae | Polygonatum pubescens | Hairy Solomon’s Seal | |
Annual | Solanaceae | Leucophysalis grandiflora | Large False Ground Cherry | Nectarless |
Perennial | Solanaceae | Physalis heterophylla | Clammy Ground Cherry | Nectarless |
Perennial | Solanaceae | Physalis longifolia | Long-leaf Ground Cherry | Nectarless |
Perennial | Solanaceae | Physalis virginiana | Virginia Ground Cherry | Nectarless |
Annual | Solanaceae | Solanum rostratum | Buffalo Bur Nightshade | Nectarless |
Annual | Solanaceae | Solanum ptychanthum | Black Nightshade | Nectarless |
Flower Development and Pollination
Self Pollination
Although most flowering plants are reliant upon an organism to facilitate pollination, a minority of plants have the ability to self-pollinate or are self-compatible. A selfing process includes flower production of pollen, deposition of pollen on the flower's receptive stigma, pollen germination, production of a pollen tube, and the fertilization of the flower's ovule—all on the same plant. Plants that flower in early spring when pollinator visitation is low can employ this self-fertilization process. For example, Viola (violets) produce a second set of flowers (cleistogamous flowers) that lack petals and do not open. Self-compatible and using their own pollen, the cleistogamous flowers fertilize the flower's ovules, ensuring that the plant produces seed that growing season. Sanguinaria canadensis (bloodroot) flowers in early spring. When the flowers first open, the stamens and anthers splay outward away from the stigma and style to limit selfing. If no flower visitation occurs in the following few days, the stamens and anthers flex toward the flower's receptive stigma, depositing pollen on the stigma.
Day 1
Day 4
Cross Pollination
The ideal pollination strategy is cross-pollination, especially with pollen from plants that are geographically distant. Cross-pollinated plants generate robust and genetically diverse seed, helping to suppress undesirable mutations and keeping unfavorable recessive traits from being expressed. Cross-pollinated plant offspring have more variability of traits, resulting in an increase in overall differences and ultimate resiliency in the plant population. Plants prevent self-pollination through self-sterility or self-incompatibility. Self-incompatible plants inhibit the germination of pollen grains on the stigmatic surface or the growth of the pollen tube when pollen from the same flower is deposited on the plant’s stigma.
For plants that are self-compatible, the most common strategy to limit self-pollination is through the staggered development of the flower's male and female reproductive parts, a process called dichogamy. When the male reproductive parts (anthers) mature first, prior to the female stigma becoming receptive, the development process is protandrous; this development sequence occurs in the majority of plants. For example, Geranium maculatum is self-compatible but has a protandrous flower development to limit selfing (self-pollination).
male phase
anthers produce pollen
female phase
style elongates and stigma develops
stigma receptive to pollen
Wind Pollination
The other strategy plants employ to prevent self-pollination is to have male and female flowers on separate (dioecious or unisexual) plants. Many but not all unisexual plants rely upon wind- or water-pollination with the male plants producing a large quantity of pollen (compared to insect-pollinated plants) transported by wind or water from the male plant to the female plant. Salix (willows) are one exception; although unisexual, these woody shrubs and trees primarily rely upon insect pollination. Bees are attracted to both male and female willow flowers, each producing nectar; female bees collect the high protein pollen from male willow flowers, and many bee species in North America specialize on willow pollen.
Woody wind-pollinated plants often flower in early spring prior to leaf emergence. This timing allows the small wind-borne pollen to be carried by wind, uninhibited by foliage. Pollen in wind-pollinated plants is often, but not always, lower in protein than pollen of insect-pollinated plants and can be nutritionally inadequate for developing larvae. For example, the protein content in corn pollen is approximately 15% crude protein; in comparison, the protein content in blueberry pollen is 41%. Many canopy trees in temperate ecosystems rely upon wind pollination. Some such as Quercus (oak) have a high pollen protein content attracting female bees that feed on and collect pollen from male flowers, and also male bees that feed on the pollen. However, bees do not play a role in the pollination of oaks because they do not visit female flowers.
A Colletes inaequalis female consumes pollen of a male pussy willow (Salix discolor) flower.
Flower Advertisements
Besides offering food rewards (pollen, nectar, floral oil) to bees and other flower-visiting insects, flowers have a number of visual, olfactory, and sensory means to attract and guide bee to the floral reources. These include flower color, shape, and pattern; floral fragrance; nectar guides; and floral electric fields. Bees discriminate these advertisements, cues, and signals produced by flowers. They use this information to help them forage more efficiently such as by avoiding flowers that lack rewards. In addition, bees also leave scent marks on flowers, crumb-like cues that alert other foraging bees about resources offered or depleted.
Flower Color
Flowers come in all different shapes, colors, and sizes. During the period that flowers are open and offering resources to bees, parts of the flower may change color after visits by bees or as the flower age. These color changes are visual signals to bees indicating that floral resources are depleted and/or no longer being offered. A result is that color changes can influence bee foraging behavior on a flower head composed of many flowers. For example, Lupinus perennis flowers (family Fabaceae) change color after being visited by a bee. The banner petals darken. A bee approaching a raceme of flowers passes over the dark flowers and moves up the raceme to the first flower that has not been visited. It continues its foraging on the raceme, circling and moving upward, visiting each flower. The age of these flowers on the raceme varies; the newest or freshest flowers at the top are in the early development stage (male phase) offering pollen, the older ones are in the female phase and receptive to pollen. This staggered development of multiple flowers on a raceme helps ensure that the last flowers a bee visits on a raceme are offering pollen, thereby increasing the likelihood that the bee moves the flower's pollen to another plant of the same species (resulting in pollination).
The banner petals of Lupinus perennis (wild lupine) flowers change color (darken) after they are visited by bees.
Fragrance
Flowers produce volatile organic compounds (floral VOCs) that release floral fragrances. These VOCs are emitted from various parts of the flower including the petals, pollen, and nectar. The production of fragrances by flowers can be costly energetically, and the fragrances can potentially attract organisms that may harm the plant. However, flowering plants are usually competing for visits with other coflowering plants. And, for plants that require cross pollination, it is imperative to produce effective signals to ensure that the flowers get pollinated. Bees detect airborne floral VOCs with their antennae and mouthparts, where olfactory sensillae are located. The sensillae are small pore plates attached to olfactory neurons that send signals to the brain. Flower scent may change or diminish as a flower ages, signaling to a bee that rewards have been depleted.
An Andrena female visits the fragrant flowers of Prunus americana (wild plum).
Nectar Guides
Nectar guides are stripes, spots, or color contrasts on flowers that visually guide a bee to the location of the flower's resources. These guides can also influence foraging behavior, such as increasing the likelihood that a bee contacts the anthers while accessing the resources. Color contrasts can occur between pollen and flower petals, among the various flower parts, or between patterns. For example, a bullseye-like pattern, a common trait of plants in the family Asteraceae. These guides can reduce bee foraging time on flowers, making their visits more efficient while conserving energy. Since bees can see color in ultraviolet wavelengths beyond the range of human eyesight, many guides on flowers are not visible to humans. Flower parts may either reflect or absorb ultraviolet light to reveal contrasting colors and complex patterns to pollinators. As flowers age, nectar guides can fade and even disappear in some cases.
Nectar guides on a Phlox pilosa (prairie phlox) flower.
Nectar guides on a Physalis heterophylla (clammy ground cherry) flower.
Nectar guides on a Rosa arkansana (prairie rose) flower.
Nectar guides on a Pycnanthemum virginianum (Virginia mountain mint) flower.
Floral Electric Fields
Clarke et al. (2013 and 2017) determined that bees usually have a positive electric charge and flowers a negative potential in relation to the atmospheric electric field. Also determined was that the floral electric potential is directly affected by pollination (visitation). As bees fly, they develop a positive electric potential. Plants, grounded in the earth, have a negative potential. When approaching a flower on the wing, a positively-charged bee, using their antennae and hairs on their body, can detect whether there is a strong contrasting potential from the opposite charge of a flower (if it has not been recently visited) or a weak potential (recently visited). When a bee lands on a flower not recently visited it discharges the potential, transfering the charge to the flower. In return, the pollen is transfered onto and adheres to the bee. Once a flower is visited, it has a detectable change in potential that influences bee flower preferences.
An Osmia lignaria female approaches a Polemonium reptans (Jacob's ladder) flower.
Explore Bee Families
Citations and Further Reading
Buchmann, S. L., & Hurley, J. P. (1978). A biophysical model for buzz pollination in angiosperms. Journal of Theoretical Biology, 72(4), 639-657.
Clarke, D., Morley, E., & Robert, D. (2017). The bee, the flower, and the electric field: electric ecology and aerial electroreception. Journal of Comparative Physiology A, 203, 737-748.
Clarke, D., Whitney, H., Sutton, G., & Robert, D. (2013). Detection and learning of floral electric fields by bumblebees. Science, 340(6128), 66-69.
Corbet, S. A., Chapman, H., & Saville, N. (1988). Vibratory pollen collection and flower form: bumble-bees on Actinidia, Symphytum, Borago and Polygonatum. Functional Ecology, 147-155.
Knudsen, J. T., & Olesen, J. M. (1993). Buzz‐pollination and patterns in sexual traits in North European Pyrolaceae. American Journal of Botany, 80(8), 900-913.
Portman, Z. M., Gardner, J., Lane, I. G., Gerjets, N., Petersen, J. D., Ascher, J. S., ... & Cariveau, D. P. (2023). A checklist of the bees (Hymenoptera: Apoidea) of Minnesota. Zootaxa, 5304(1), 1-95.
Urban-Mead, K. R., Muñiz, P., Gillung, J., Espinoza, A., Fordyce, R., van Dyke, M., ... & Danforth, B. N. (2021). Bees in the trees: Diverse spring fauna in temperate forest edge canopies. Forest Ecology and Management, 482, 118903.
Willmer, P. (2011). Pollination and floral ecology. Princeton University Press.
Page Illustration and Photography Credits
Photography
Heather Holm
Joel Gardner CC BY-ND-NC 1.0 (Melittidae)
Illustrations
Vector image of flower: © Blue Ring Media, Shutterstock Image ID 141162013
Vector image of bumble bee: © Bourbon-88, Shutterstock Image ID 420754288
Videos
Bumble Bee Ecology Video © 2024 Jeffrey Karron
used with permission