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Attracting Pollinators

Hummingbirds flock to red, tubular flowers. (Photo by Cathryn Hoyt)

This episode of Nature Notes was previously broadcast on April 21, 2011 and was written by Megan Wilde of the  Chihuahuan Desert Research Institute.

Butterflies throng golden lantana, bees swarm fragrant agarita, and hummingbirds sip from red yucca. These couplings aren’t accidental. They’re the result of marketing wizardry, with each plant vying for the attention of specific passersby who will help it reproduce. How do plants attract pollinators?

Karen Little is a botanist and lab manager at Sul Ross State University. She’s studied the strategies plants have evolved to entice pollinators.

Little: Animals don’t set out to be altruistic and say, “I think I’ll go to a plant and pollinate it because that’s what it needs.” You know, plants can’t get around. They can’t meet each other. So they have to attract something to help them breed. It’s a sexual process. So they offer rewards.

Some plants have an exclusive relationship with one type of pollinator, and they customize their rewards to suit that pollinator’s needs. But many plants hedge their bets by marketing themselves more broadly, offering multiple incentives to tempt multiple pollinators.

Little: The thing about rewards is they are costly to a plant. Just like we would have to go buy perfume. The plant has to use its nutrients to make these rewards. Nutrients it could spend making seeds or growing.

Free meals are a standard reward for a pollinator’s services. Nectar, being mostly sugar water, is an energy-rich fuel and source of nutrients. So plants that rely on pollinators with high-energy diets, like hummingbirds, produce abundant nectar. By producing copious pollen, plants lure bees and beetles.

But what first attracts an animal or insect to visit a flower is often its color.

Hummingbirds flock to scarlet. Bee-pollinated flowers, on the other hand, are typically bright blue, violet, or yellow, but rarely red, a color bees can’t see. Vibrant colors are wasted in the dark though, when nocturnal pollinators, like bats and moths, are drawn to white or pale-colored blooms.

Colorful nectar guides are common on bee- and butterfly-pollinated plants. These patterns often look like runways or targets and show pollinators where there’s nectar. Some are obvious, like an iris’s elegant markings. But others are only visible to bees and insects that can see ultraviolet light.

Besides color, plants deploy scents to grab pollinators’ attention.

Little: Some insects especially will collect scent and use it as part of their mating scent. Other times the scent is just there to say, “Hey come over here! Visit me!”

Sulfur-scented blooms are bat magnets, while moths prefer sweet or fermented aromas. Sweet fragrances also attract bees, as do minty smells. Beetles are drawn to fruity, spicy scents. Bird-pollinated plants don’t usually bother with odors, because their intended audience can’t smell.

Even a flower’s shape can be a marketing tactic. Long, tubular blooms lure butterflies, moths or hummingbirds, who have matching long proboscises or bills that can reach the nectar and pollen inside.

Beetles and butterflies need to perch while they eat, so flowers targeted to these pollinators typically have landing pads. Hummingbirds, bats, and bees, on the other hand, can hover as they feast. So pendulous flowers, like penstemon blossoms, attract these expert aerialists.

Understanding how plants attract pollinators is not only fascinating but useful.

Little: Since I know bees are strongly attracted to blue colors, I don’t plant mealy sage next to my door. Because I know if I do, every time I go in and out my door, bees are going to go in and out with me.

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