Written by Mary Free, Certified Master Gardener
The shape, color, structure and odor 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 the service of pollination, the flower provides a reward: usually food such as nectar and/or pollen. Thus plants and their pollinators enjoy a mutualistic relationship.
For example, nectar-rich flowers that are brightly colored: red, orange, pink and yellow are especially attractive to hummingbirds. Nodding flowers and those with long corolla tubes are a good fit for their long beaks and tongues. When hummers 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. Wild columbine and cardinal flower are examples of two flowers suited to and primarily pollinated by hummingbirds.
Many flowers with the same bright colors, including red and purple, and with narrow tubes and spurs that attract hummingbirds also attract butterflies. Unlike hummers that feed while hovering, butterflies perch or walk around as they feed, so they prefer flowers arranged in clusters like Asclepias (milkweed), Lantana, Verbena, Buddleia (butterfly bush) and Solidago (goldenrod) or with wide landing platforms like composites such as asters and Echinacea (coneflowers).
Flowers paler in color and stronger in fragrance attract moths. Like hummingbirds, some moths hover to drink nectar. (In fact, some sphinx moths, known as hummingbird moths, are sometimes mistaken for hummers because they exhibit similar characteristics as they move among the flowers.) Or, like butterflies, moths may perch on flowers to feed. Also, like butterflies moths prefer flowers that are clustered or have landing platforms.
For bees, flower fragrance is the 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 red flower nectar. 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. (To see what bees see, visit:
http://www.naturfotograf.com/UV_flowers_list.html#AMARYLDX) Once pollinated, the ultraviolet coloring of some flowers changes or fades away.
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. Some female bees like honey bees and bumble bees have “pollen baskets” (corbiculae) on their hind legs in which they carry the pollen that they collect. Other bees, like sweat bees, leaf-cutter bees and mason bees, have specialized hairs on their legs and/or abdomens called scopae that transport the pollen.
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.
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. Some studies2 conclude that the negative consequences of nectar robbing do indeed outweigh any positive effects. However, other studies3 suggest that in some instances robbers may actually have a positive effect on seed production and the relationship between plant and robber may be mutualistic. So it appears that it is yet to be determined whether a nectar robber truly deserves the negative reputation that its label implies.
1 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.
2 Impact of Nectar Robbing on Estimates of Pollen Flow: Conceptual Predictions and Empirical Outcomes at:
3 Are nectar robbers cheaters or mutualists? at: http://faculty.salisbury.edu/~jemaloof/manuscripts/Ecology.htm