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Pitcher plant’s pitch a sweet one

Work being reported today in Biology Letters about pitcher plants would please Charles Darwin. This curious naturalist was so intrigued by carnivorous plants that he wrote a 400-page book, Insectivorous plants, detailing his thoughts and observations on these unusual species. He was fascinated by the similarities in the insect traps of unrelated plants and demonstrated that these plants had the ability to digest and absorb the prey they caught. This week, researchers report on how one species, a pitcher plant found in Canada and the eastern United States, snags its dinner. At issue was whether a red color or a sweet treat is what makes this death trap so appealing.

In 2007, H. Martin Schaefer of the University of Freiburg in Germany evaluated the role of pitcher color in luring insects. Using 20 pitcher plants (Nepenthes ventricosa) from Southeast Asia, his team painted half the pitchers green and half of them red, then placed the plants outside, counting the number of trapped insects after 1 and 2 weeks. They found that red plants caught more prey, particularly flies. They assumed that the plants smelled alike—like paint—to the insects. Others had suggested that the UV marks or visual stripes on pitchers were needed to reel insects in, but the opaque red pitchers were quite effective without such decorations, they reported.

But Aaron Ellison of Harvard University wasn’t satisfied with this finding, given that the plants were not in their native environment, where typically they trap ants and termites, not flies. He wondered whether pitcher plants were taking advantage of a sweet tooth in their victims. Pitchers produce excessive amounts of nectar, separate from what the flowers make. And earlier this year, Ganesh P. Bhattarai and John D. Horner of Texas Christian University in Fort Worth had suggested that odors, including sweet ones, could be important attractants.

Ellison recruited Katherine Bennett, a local elementary school teacher who had become interested in science through a Harvard program for schoolchildren to help scientists gather data. Bennett suctioned prey carcasses from pitchers of 25 Sarracenia purpurea pitcher plants growing in a Massachusetts bog, then checked again 3 days later for new prey. After several rounds of suctioning and inventorying the plants, she and Ellison collected the plants and assessed their redness by measuring spectral reflectance. Next, they made 70 plastic centrifuge tubes into “pseudopitchers” painted either green, red, or green with red stripes. Ten were left unpainted. They streaked half the pitchers with thickened corn syrup and embedded all the tubes at angles in a bog close to the plants surveyed earlier. Several times, they counted the trapped insects, comparing the pseudopitchers’ body count with the real plants’.

They retrieved about 350 prey each from real and nectar-streaked fake pitchers, whereas the unsweetened pseudopitchers garnered only about 60, they report in Biology Letters. Ants were the most susceptible to the nectar lure. The amount of red didn’t seem to matter much at all. “We only manipulated the color of pitcher plants,” comments Schaefer. “This study goes one step further and [shows that] the effects of sugar rewards outweigh those of color. This experimental support has been lacking.”

As for Bennett, she says, her experience has “transformed the way I teach science.”