Plants that devour insects could be luring prey with signature scents

By Kate Golembiewski, CNN

Sarracenia pitcher plants, found in bogs throughout eastern North America, look like trumpet-shaped flowers, often in purplish or reddish hues. But looks can be deceiving. The striking “flowers” are actually modified leaves, forming a cup that contains digestive enzymes.

When a small animal — usually an insect — wanders too close, the pitcher functions like a pitfall trap, complete with slippery sides that make it hard for the critter to climb back out. The carnivorous plant then digests the insect, gaining nourishment that can be difficult to get from the nutrient-poor soil in its boggy home.

Different kinds of Sarracenia pitcher plants tend to eat different kinds of insects — some species trap more ants, while others feast on bees and moths. Pitcher color, size or shape might play a role in the plants’ ability to attract prey, according to different hypotheses floated by scientists. A new study, published Wednesday in the journal PLOS ONE, points to another weapon in the pitcher plants’ arsenal, which may help explain the different species’ diets: odor.

Dr. Laurence Gaume, the study’s senior author and a researcher at the French National Centre for Scientific Research, or CNRS, and the University of Montpellier, has studied carnivorous plants for more than 20 years.

In previous work on pitcher plants from Southeast Asia, which are only distantly related to North American Sarracenia, she and her colleagues found that different species there also caught different proportions of ants versus flying insects. The research team learned that the smells emanating from the different Asian pitcher plant species attracted different kinds of insects, so she wanted to see if the same was true for North American pitchers.

Species-specific scents

For us humans, it’s hard to pick out the subtle differences in scent from one Sarracenia species to another, especially, Gaume said, when the plants’ characteristic smells are overpowered by their meals.

“The odor of organic matter in putrefaction … can be strong in pitchers filled with insects,” she said. But recently opened pitcher plants that don’t stink of rotting bug carcasses provide an opportunity to identify the scents, she explained.

“We can note different odors according to the species, which go from the odor of green grass to floral or even fruity odors.”

These odors are caused by volatile organic compounds, or VOCs, produced by the plants. In the new study, Gaume and her colleagues, including CNRS and University of Montpellier doctoral candidate and first author Corentin Dupont, selected four kinds of Sarracenia pitcher plants (two found in nature and two human-created hybrids) and examined the VOCs they produce.

The study team placed each plant in an airtight bag and pumped purified air through it. The odor-causing molecules produced by the plants were trapped on filters for the researchers to analyze and identify. The team found that the different pitcher plant species each produced their own unique bouquet of VOCs.

Possible prey specialization

Dr. Claire Villement, an entomologist at the French National Museum of Natural History in Paris and coauthor of the study, examined the insects caught by 41 pitchers of the four species in the study. While the pitchers contain digestive enzymes that break apart the insects’ bodies, Gaume explained that it’s safe for humans to handle the pitchers and their partially digested prey: “There is no danger, the enzymes cannot eat your hands.”

As predicted, the different pitcher species trapped different kinds of insects.

The researchers then compared the VOCs produced by each plant with the types of insects it caught. The species that produced more floral scents trapped more bees, moths and other flying insects; pitchers that produced more fatty acids caught more ants. These results held up even when the researchers took into account the way that the pitchers’ size and shape might limit what sorts of insects they could easily trap.

Gaume was careful to note that the study doesn’t definitively prove that these different scents attract the different kinds of insects; it just shows a strong correlation. Still, it makes intuitive sense that pitchers that smell like flowers might attract pollinators that fly from bud to bud, whereas ants, which rely heavily on fatty acid chemical signals, might gravitate towards those odors.

“These findings are important because they suggest that these carnivorous plants are not simple passive plants with random captures, and that they can target their prey,” Gaume said.

Vulnerable habitat

Dr. Aaron Ellison, a retired senior research fellow at the Harvard Forest in Petersham, Massachusetts, who was not involved with the project, said: “It’s an interesting study. I think that the conclusions they come to are solid relative to the data that they have. The biggest challenge with the study, honestly, is that they did it in France,” rather than in the Sarracenia pitcher plants’ native North American bogs.

He also noted that the study features horticultural varieties in addition to wild-growing plants, which might behave differently. “I’d love to see this kind of work being done in the field in the southeast US with the plants growing in their normal habitat,” he said.

These natural habitats are under threat due to the climate crisis, pollution and land development, said Dr. Phil Sheridan, the president and a director of the Meadowview Biological Research Station in Woodford, Virginia.

He was not involved with the study but praised it as “a key paper” providing “compelling evidence” that pitcher plants’ scents, along with their physical structures, influence the insects they’re able to catch. It’s important to study pitcher plants due to the unique role they play in their increasingly fragile ecosystems, he added.

“They grow in very sensitive habitats, the water is very clean where they grow and if you pollute it, you’re done. It’s finished,” Sheridan said. “They’re beautiful plants, and once they’re gone, you don’t get them back.”

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Kate Golembiewski is a freelance science writer based in Chicago who geeks out about zoology, thermodynamics and death. She hosts the comedy talk show “A Scientist Walks Into a Bar.”