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Sex in the Garden
Page 2

Photo of a white-lined sphinx mothMy friend Rob Raguso, a biologist at the University of South Carolina, has been studying sphinx moths for over a decade. In 1993, he placed electrodes on their antennae to record their responses to different odor compounds. He wondered what, exactly, these moths could smell. It turned out they could smell everything, at least every floral odor from sweet to spicy that Rob could produce. Rob also wondered how a sphinx moth, with its sesame-seed-sized brain, knew when to stop and feed. How does a moth experience the world?

Photo of a sacred datura blossomAt the Arizona-Sonora Desert Museum in Tucson, Rob manipulated sacred datura plants. Flowers covered with dyed cheesecloth bags could be smelled but not seen. Flowers with plastic bags over them could be seen but not smelled. White paper funnels with nectar tubes simulated real flowers but without fragrance. In the end, he realized that sphinx moths were fairly discriminating. In order to be fully seduced -- to slow down and uncurl their proboscis -- the moths need both the right smell (that lemony scent) and a visual display (those creamy-white petals tinged with lavender).

Rob is studying the white-lined sphinx moth's courtship with evening primroses in Utah. Wild evening primroses are delicate flowers with yellow or white petals that seem tissue-thin, veined like the skin of the very young or old. Female sphinx moths lay eggs on these plants while drinking nectar. When the eggs hatch, the larvae eat the flower buds and petals. "I'd like to look at the costs and benefits," Rob says, "of having white-lined sphinxes as your pollinator."

In other words, why attract a suitor whose offspring will turn around and eat you up? Perhaps the key is timing. If you have already sent your pollen into the world, if your eggs have been fertilized and gone to seed, then your job is done. You don't mind being nibbled to death.

Rob's study was inspired by his observation that primrose flowers being eaten by sphinx moth caterpillars produce a fragrance different from that of flowers from an undamaged plant. Like the sphinx moth, Rob specializes in smell: where smells are produced on a flower, how they evolve in a plant, what role they have in pollination and plant-insect interactions. Like a perfumer, he has trained his nose to recognize hundreds of floral scents. "Instead of smelling like Earl Grey tea," he says, "the flowers of these wounded plants smell faintly like cream."

When being eaten by caterpillars, crop plants like corn and cotton produce flowerlike fragrances from their leaves. This chemical cry for help drifts through the air and attracts insects such as wasps, which hurry over to eat or parasitize the caterpillars. From the plant's point of view, these wasps are the cavalry. Rob wants to know what happens to plants such as evening primroses that already produce a flowerlike fragrance. Is their change in odor a similar cry for help? If not, what are their strategies against pollinators who have become abusive?

In truth, pollination often involves abuse, a range of violence and deceit. Some orchids imprison their pollinators in a carnival fun-house of chutes and cages. Sometimes these flowers eject their pollinium (a disk of pollen with a stem attached) into a sphinx moth's eye, an experience akin to having a hockey stick attached to your face. Eventually, the vision-impaired insect may starve if it cannot feed itself. In the meantime, it might pollinate a few more flowers. Flowers pretend to have nectar when they don't, or they exaggerate the richness of their pollen with bright yellow coloring. In the common milkweed, pollen can stick so persistently to a visiting bumblebee that, as the bee flies away, its legs tear off. This doesn't matter so much to the milkweed if the bee has already brought some pollen to fertilize its eggs.

By comparison, the interaction between a sacred datura and a white-lined sphinx moth almost seems romantic. At least no one is getting hurt. In my garden, after the big night, the datura flower hangs limp, wilted, hopefully fertilized. The white-lined sphinx moth is somewhere around, waiting and sheltering against the day's heat. If it is a male sphinx moth, he waits to find a female to mate before he dies. If it is a female sphinx moth, she waits to mate and then lay her eggs before she dies. In any case, her life span as a moth measures three to ten days. Meanwhile, the datura plant is an important food source.

I like this pollination story in part because it is a common one. The sacred datura is classified as a weed and the white-lined sphinx moth as a pest, not endangered, not threatened (although heavy uses of herbicides and insecticides could change that). Their lives are entirely their own. They are indifferent to me, my children, my garden. They care, really, about one thing only. You know what that is. The white trumpet unfolds. The proboscis uncurls. It's as complicated as that.

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Photos: moth, Richard Seaman; flower, Tom Till

OnEarth. Winter 2004
Copyright 2003 by the Natural Resources Defense Council