Wildflowers that Keep Out Bees

Evolutionary Biology Program director Irby Lovette highlights a recent study showing why bees can't raid some hummingbird flowers

A Broad-tailed Hummingbird sips nectar from a Penstemon barbatus flower. Photo by J. D. Thomson

People who feed hummingbirds often go to great lengths to keep bees from robbing sugary nectar from their feeders. It turns out that some hummingbird flowers have invented elaborate and highly effective bee guards of their own.

The penstemons, or ?beardtongues,? are a large group of wildflowers that are especially abundant in western North America. Some species of penstemon attract hummingbirds and have bright red flowers with long narrow corollas that mirror the shape of a hummingbird bill. Other types of penstemon are pollinated primarily by bees and typically have small blue or yellow flowers that provide easy access for the insects.

The hummingbird penstemons lure birds by offering a reward of sugar-rich nectar. Although this nectar is costly to produce, the plant benefits when the hummingbird transports pollen from flower to flower as it feeds. Unfortunately, the nectar is also attractive to bees, and studies have shown that bees are much less effective pollen dispersers for these flowers.

We have known for more than a century that many flowers have evolved to attract very specific pollinators. By carefully modifying the shape and orientation of penstemon flowers, Maria Clara Castellanos and colleagues have now shown that flowers have evolved traits that can simultaneously guard against unwanted visitors (Journal of Evolutionary Biology, July 2004).

The experiments showed that some flower attributes deter bees without affecting Broad-tailed and Rufous hummingbirds. The anti-bee traits were especially effective in combination to defend against bee raids. This detailed study of natural ?bee guards? shows how evolution has worked on penstemon flowers to balance the needs of attracting hummingbirds and deterring bees.

—Irby Lovette

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