Women know that just the right amount of perfume can drive men wild, but too much can drive them away. Ancient plants called cycads take advantage of this age-old principle, too. By changing the amount of an odor they produce, the plants drive tiny pollinators called thrips out of male plants and attract them to the females.
Plants tend to depend either on wind or on bees, birds, or other animals to carry pollen to female flowers, or in the case of pine trees and other gymnosperms, to female cones. Many species have evolved sophisticated chemical cues for attracting just the right pollinator. Botanists had thought that some cycads, which can look a lot like ferns or palms, depended on wind pollination. But behavioral ecologist L. Irene Terry of the University of Utah, Salt Lake City, suspected otherwise, as the cones' scales are too tightly layered to let the pollen blow in. In 1999, she discovered that the Australian cycads she studied were pollinated by thrips, 1- to 2-millimeter-long insects commonly found in flowers.
Terry also noticed that the male cones often exuded a sharp odor reminiscent of turpentine. In 2004, she and her colleagues demonstrated that cones rev up their metabolism and heat up for a few hours midday, reaching 12 degrees Celsius above ambient temperature and, as a result, emitted much higher concentrations of the smelly stuff. The heating and "take-your-breath-away" odor coincided with a massive exodus of thrips, which otherwise spend their lives inside the cones of male plants, mating, laying eggs, growing, and feasting on pollen. In seeking out new cones, some thrips visited female plants, pollinating those cones. Terry and her colleagues wondered whether the insects left the male cones because they got hot or because they didn’t like the smell.
The incentive to pack up, they learned, is more smell than heat. To show that, the researchers put the bugs in Y-shaped tubes so that they could choose to go toward or away from a particular odor emitted into one arm of the Y. They found that the insects were attracted to low concentrations of the cycad cone odor but repelled by high concentrations, which actually proved toxic to the thrips. Female cones emit these low concentrations. Come afternoon at certain times of the year, thrips flee their usual digs in male cones and are subsequently lured to female cones, at least temporarily, Terry's group reports in the 5 October issue of Science.
What's novel is that the plant repels pollinators to achieve its aims. "That's not something that's been on the radar before," says Olle Pellmyr, an evolutionary ecologist at the University of Idaho in Moscow. Pellmyr says that a close look at other plant-pollinator relationships, such as that between wasps and figs, might reveal similar sorts of complex chemical manipulations.