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Perfume for Pollinators—Using Fragrant Plants to Lure Insects and Other Critters Into the Garden
Plants & Gardens News Volume 20, Number 2 | Summer 2005
by Janet Marinelli
And we think we invented scratch 'n' sniff, perfume strips, and other forms of olfactory advertising. The truth is, plants beat us to it by at least a hundred million years.
The purple trillium (Trillium erectum) perfumes its blossoms with the odor of rotten meat to attract pollinating flies. (Photo courtesy www.delawarewildflowers.org)
Plants advertise with fragrance in various ways. For example, they perfume their blooms to seduce the insects, bats, and other critters they need to pollinate their flowers. Floral fragrance is a kind of olfactory come-on that proclaims to a potential flower fertilizer, "Come hither, honey, 'cause there's scrumptious pollen and sweet nectar hidden inside these pretty petals." In addition to a full belly, the pollinator leaves with pollen attached to its body in a bundle or dusted on its fur. When the pollinator lands on another flower while looking for its next meal, cross-fertilization can occur.
Scientists believe that one reason plants are in big trouble around the globe is because their pollinators are disappearing. A major factor in their decline is loss of habitat to farms and urban development. Without their pollinators, the flowers of many species don't get fertilized. If they don't get fertilized, they don't set seed and can't reproduce. In a pinch, some plants can pollinate themselves, but this often causes inbreeding and other genetic problems that ultimately threaten the species' survival. By creating gardens that feature a variety of fragrant flowers to attract a diversity of pollinators, we gardeners can help compensate for the loss of habitat and lend plants and their partners a helping hand—and as a bonus get whiffs of heady scent ourselves.
Signature Scents
Plants employ not just scent but also visual cues like flower color to facilitate reproduction. But it appears that during the early days of floral evolution, fragrance, not color, was the principal allure. Beetles achieved the pinnacle of insect evolution during the late Jurassic and early Cretaceous periods, when flowering plants were first evolving. Although these primeval pollinators are virtually colorblind, they have a great sense of smell, so it's no big surprise that the magnolias and other primitive flowers they still pollinate today pack powerful perfumes.
A black blister beetle (Epicauta pennsylvanica) pays a visit to a goldenrod flower. (Photo courtesy the Hilton Pond Center for Piedmont Natural History)
Floral fragrance is far from an antiquated trait, however. Even the most highly evolved flowers, such as orchids, use it to captivate their reproductive partners. In fact, although flowers can be identical in color and shape, no two floral fragrances are alike. Every plant has its own signature scent, a complex mixture of volatile organic compounds that easily turn to gases and waft through the air. Some 1,700 compounds have been identified in flower fragrances so far, according to Natalia Dudareva, a molecular biologist at Purdue University whose specialty is floral scents. An orchid can produce a hundred different volatile compounds, she points out, while a snapdragon produces seven to ten.
Dudareva recently isolated the gene for one of these compounds, methyl benzoate. Some 30 to 40 commercially important plants—including snapdragons, flowering tobaccos, and petunias—use this same fragrance-generation system. Intensive breeding for bigger, more colorful, and longer-lasting blooms during the past few decades evidently has deactivated the gene, which is why so many modern varieties are disappointing in the scent department. A plant that is pouring so much energy into producing flashier-looking flowers, Dudareva hypothesizes, is in essence too pooped to make perfume.
To date, little is known about how pollinators respond to the individual compounds found in flower scents. But it is clear that they are capable of distinguishing among complex scent mixtures and therefore among plant species—their schnozzolas steer them to the ones that provide the most delectable nectar or pollen. Hence, over the eons, plants have evolved floral fragrances that best cater to the olfactory proclivities of their most efficient pollinators.
You could say that it's all in the proboscis of the beholder (or antennae, the olfactory organs of bees, beetles, and moths). Pollinators are very picky about flower odors. Bees, for example, prefer the sweet scent of plants like snapdragons and sweet peas. Beetles are partial to flowers with fruity and spicy scents, such as magnolias.
An evening primrose (Oenothera glazioviana) is visited by a hummingbird moth. (Photo by Robert B. Lyon)
Moths, which are mostly nocturnal, are attracted to flowers such as jasmine, which advertise their presence under the cloak of darkness with strong, sweet perfumes. Moths have a keen sense of smell and have even been known to pick up the scent of an enticing plant from 900 feet away. Bats are also night flyers with good noses, but they favor blooms with musty aromas. Most bats in the United States are insectivorous, but three flower-eating species migrate from Mexico to pollinate dozens of agaves and giant cacti in the desert Southwest. Lesser long-nosed bats, for example, take a predictable path in spring, following blooming cacti northward through the Sonoran Desert.
Most people don't think of flies as pollinators, but they play a critical role in the fertilization of some flowers. Flies fancy blossoms that emit the essence of carrion or dung and look like lumps of rotting flesh. Among their favorites are our native red trilliums, which early naturalists christened "stinking benjamins" because of their stench.
Birds and most butterflies are olfactory challenged, so the flowers that depend on them for pollination don't waste time and energy on smelling beautiful.
Within the various groups of flower fertilizers there are generalists, which have cosmopolitan floral tastes, and specialists, which have a monogamous relationship with the blossoms they visit. Among the ultimate pollinator specialists are the moths that fertilize yuccas, which typically send up stout stalks of white flowers. Yuccas, including about 30 species native to North America, such as the Spanish bayonet (Yucca schottii) and the Joshua tree (Y. brevifolia), are pollinated only by yucca moths, and 70 percent of yucca moth species visit the flowers of only one yucca species.
False Advertising
Pollinators aren't always too bright about using fragrance to find the flowers that offer the best rewards, and some flowers exploit their gullibility by resorting to false advertising. Consider jack-in-the-pulpit, skunk cabbage, and other aroids, whose tiny flowers, massed together along a fleshy pole partially surrounded by a leaf, smell of stinking fish and feces. Flies arrive with great expectations of finding some rotting tissue in which to lay their eggs. They get trapped inside a chamber at the base of the leaf that protects the fertile flowers and remain incarcerated until the flowering pole wilts. In the process of trying to escape, they pollinate the plant.
False or not, when it comes to advertising scent, timing is everything. Different flowers flaunt their fragrance at different times. According to Dudareva, snapdragons release four times more scent during the day, when their bee pollinators are busy foraging, than at night. By contrast, nicotianas are most fragrant after dusk, when their moth pollinators are out and about. What's more, flowers show off their perfumes only when they are good and ready for fertilization. Newly opened blossoms don't produce as much scent as mature ones do, and fertilized flowers not only make less fragrance but also lower-quality perfume.
It's useful to keep these things in mind when using fragrance in the garden to help nurture plants and their pollinators:
- Invite a variety of pollinators into your garden by offering a large and diverse mix of fragrant species, both day and night bloomers. See "Flowers for Scent-Savvy Pollinators" for some selections.
- Plant favored species in drifts of at least three to five to attract pollinators and make it worth their while to visit.
- Choose old-fashioned varieties and native flowers whenever possible, because breeding has caused some modern-day blooms to lose their fragrance, and even those that still have scent may lack the nectar that pollinators need to thrive.
- Plant a mix of flowers that cater to both generalist and specialist pollinators, so all can partake of the feast.
- Design your garden so that there is a continuous succession of plants in bloom to provide nonstop food sources for hungry pollinators.
- Avoid using pesticides, even nonchemical ones such as Bt (Bacillus thuringiensis), which can decimate pollinator populations.
Janet Marinelli is director of Publishing at BBG. Her new book, Plant, which has just been published by Dorling Kindersley, showcases 2,000 species worldwide that are threatened in the wild but alive and well in cultivation. Royalties from the book will help support urgent plant conservation efforts at botanic gardens around the globe.