By Mary Free, Extension Master Gardener
The shape, color, structure, and scent of a flower usually determine the type of pollinators it attracts.1 A flower requires a pollinator that will visit it regularly and successfully transfer pollen in and/or between it and other flowers of its species to ensure fruit and seed production. For this service, animal pollinators usually receive a reward of food in the form of nectar and/or pollen. Thus plants and their pollinators enjoy a mutualistic relationship.
Hummingbirds are attracted to bright colors like red, pink, orange, and yellow but the flowers they ultimately favor are those with high nectar flow and with nectar high in sucrose content.2 Nodding flowers and those with long corolla tubes, such as Aesculus pavia (red buckeye), Lonicera sempervirens (trumpet honeysuckle), Monarda didyma (scarlet beebalm), Impatiens capensis (jewelweed), and Lobelia cardinalis (cardinal flower), are a good fit for their long beaks and tongues. When hummingbirds feed from these flowers, pollen from the flowers’ long stamens attaches to their throats and heads and is carried from flower to flower as they feed.
Many flowers with the same bright colors, including red and purple, and with narrow tubes and spurs that attract hummingbirds, also attract butterflies. Unlike hummingbirds that feed while hovering, butterflies perch or walk around as they feed, so they prefer flowers arranged in clusters like natives Asclepias (milkweed), Eupatorium perfoliatum (common boneset), Eupatorium hyssopifolium (hyssop-leaf thoroughwort), Eutrochium (Joe-pye-weed), and Solidago (goldenrod) or with wide landing platforms such as composites like coneflowers (Echinacea and Rudbeckia) and Symphyotrichum (e.g., blue wood aster and New England aster).
Flowers paler in color and stronger in fragrance attract moths. Moths process smells through two olfactory “channels”–one for their favorite flowers and the other for alternative nectar sources.3 Like hummingbirds, some moths hover to drink flower nectar. (Those sphinx moths, known as hummingbird moths, are often mistaken for hummers due to their plump bodies, fan-like tails, flying movements, and audible hum). Like butterflies, some moths perch on flowers to feed. Also, like butterflies, moths prefer flowers that are arranged in clusters or have landing platforms. Certain yucca plants and yucca moths display obligate mutalism, depending on each other for survival. Each of these yucca species can be pollinated only by one species of yucca moth, which in turn uses only that plant as host for its larvae.
For bees, flower fragrance is a primary attractor. [Because bees have a keen sense of smell and may react to certain odors, refrain from using colognes and scented lotions, etc. when working or wandering in flower gardens.] Unlike hummingbirds and butterflies, bees cannot see the color red and often overlook red flowers that appear similar to surrounding greenery; therefore, they do not usually compete with hummers or butterflies for the nectar of red flowers. Instead, bees tend to pollinate flowers that are bright white, yellow, or blue. Petal patterns and ultraviolet color at a flower’s center guide the bees to the flower’s nectar and pollen. Once pollinated, the ultraviolet coloring of some flowers changes or fades away. (To see what bees see, visit Flowers in Ultraviolet Arranged by Plant Family.)
As bees feed on nectar, they also collect pollen for their developing larvae. Their body hair attracts the pollen grains electrostatically and they carry the grains from flower to flower making them very effective pollinators. Female sweat bees, leaf-cutter bees, and mason bees have specialized hairs on their legs and/or abdomens called scopae that transport the pollen. Female honey bees and bumble bees have “pollen baskets” (corbiculae) on their hind legs in which they carry the pollen that they collect. (Learn more about Bee-havior: Gathering and Transporting Pollen.)
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Sometimes though, certain birds or insects obtain nectar in an unconventional manner—they bypass the reproductive parts of the plant and apparently fail to collect or deposit pollen. They are called nectar robbers. For example, short-tongued bees that cannot reach the nectar of flowers with long corollas (petals) instead use their proboscis or mandibles to cut a hole through the base of the corolla to get at the nectar. Carpenter bees, known for their strong mandibles—after all they core tunnels in wood to make their nests—often make the initial hole and are called the “primary nectar robbers.” Honey bees and bumble bees that take advantage of the pre-made holes and use them to remove nectar are referred to as the “secondary robbers.”
Robber holes are often seen at the base of flowers like columbine and salvia. You may have seen flowers with such holes in your own garden and wondered if they had been attacked by some garden pest. Instead, it is likely that birds or insects made the holes in search of nectar. (Watch the video Nectar Robbing Monarda didyma ‘Jacob Cline’ to observe bees actively stealing nectar.)
For many years, nectar robbers were considered “cheaters” because they were thought to receive the reward of nectar without providing the pollinating service. It was assumed that their actions were detrimental to fruit and seed production. However, as scientists have studied the effects of nectar robbing on flower pollination, they have found that the relationship between robbers and the plants that they rob is more complex. Researchers have discovered that particular flowers deploy defense mechanisms such as replenishing their nectar to remain attractive to pollinators4 or producing nectar alkaloids that are more distasteful to robbers than pollinators.5 Although some studies concluded that the negative consequences of nectar robbing do indeed outweigh any positive effects,6 others showed insignificant negative consequences on reproductive success7 or suggested that in certain instances robbers may actually have a positive effect on seed production and the relationship between plant and robber may be mutualistic.8 It appears that there is still much to be learned…
- The US Forest Service “Celebrating Wildflowers” Web site at: http://www.fs.fed.us/wildflowers/pollinators/syndromes.shtml has a pollinator syndrome table to identify the potential pollinators associated with different flower types.
- Taste Preferences, Color Preferences, and Flower Choice in Hummingbirds at: https://www.researchgate.net/publication/237597952_Taste_Preferences_Color_Preferences_and_Flower_Choice_in_Hummingbirds
- Moths wired two ways to take advantage of floral potluck at: https://www.washington.edu/news/2012/12/06/moths-wired-two-ways-to-take-advantage-of-floral-potluck/
- Nectar replenishment maintains the neutral effects of nectar robbing on female reproductive success of Salvia przewalskii (Lamiaceae), a plant pollinated and robbed by bumble bees at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5604579/
- Distasteful Nectar Deters Floral Robbery at: https://www.sciencedirect.com/science/article/pii/S0960982217308709
- Impact of Nectar Robbing on Estimates of Pollen Flow: Conceptual Predictions and Empirical Outcomes at: http://www.dartmouth.edu/~
- Effects of nectar robbing on male and female reproductive success of a pollinator-dependent plant at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4724041/
- Are nectar robbers cheaters or mutualists? at: http://faculty.salisbury.edu/~jemaloof/manuscripts/Ecology.htm