Sarracenia pitcher plants appear to produce specific odors that attract distinct assortments of insect prey, with those species specializing on bees producing floral odors, and those focusing on fruit flies producing fruitier smells
Scent is how plants interact with the world. Many plants use scent to communicate: defending themselves and their neighbors against disease or attack, calling out to predators of insect pests that bedevil them, or inviting pollinators to visit their flowers. Anything that obscures plant odors, such as air pollution, can result a dramatic drop in pollinators that find their flowers, for example (ref).
Despite the importance of scents to the plant world, the odors created by carnivorous plants are especially poorly known (ref). On one hand, it’s been suspected for longer than 150 years that carnivorous plants produce scents that attract insect prey: Charles Darwin was probably the first scientist to propose this possibility in 1875 in his very interesting and readable book, Insectivorous Plants (freely available here), which was the culmination of years of painstaking research.
Despite this, research into plant scents only started when the technology improved within the past 15 years, thereby making a process possible where plants’ odors can be isolated and chemically identified. This process involves sealing plants inside plastic bags and siphoning off air samples for testing of their Volatile Organic Compounds (VOCs), or odors.
A recent study into the possibility that carnivorous plants might communicate using scents was inspired by a group of scientists who wanted to understand why only some insects fall victim to Sarracenia pitcher plants. Do these insects just accidentally fall into traps that are hidden in the surrounding vegetation? Or are they blown into these traps? Or might the traps mimic flowers or fruits by creating visual or olfactory signals that attract a specific assemblage of insects to their deaths?
“Of the signals involved in communication, odor is probably the most cryptic to humans”, co-author, evolutionary ecologist and botanist Laurence Gaume, a CNRS Researcher at the University of Montpellier, said in a statement. “In plants, it is often correlated with other plant characteristics such as nectar, shape and visual signals, which make it difficult to disentangle its effect from others.”
“The smell of Sarracenia pitcher plants is also not very strong, and it takes around two weeks before the smell is at its strongest. While the nuances of the scent might not be very clear to human nose, they can be far clearer to the antennae of insects.”
Dr Gaume and her team of collaborators proposed that scents are how Sarracenia pitcher plants attract certain groups of insects into their traps.
To test their hypothesis, Dr Gaume and her team of collaborators grew purple pitcher plants, Sarracenia purpurea, as well as three of its horticultural hybrids, in the lab. They created a kinship gradient from S. purpurea (which primarily captures ants) towards S. leucophylla (which captures a wide variety of flying insects): S. purpurea (Figure 1A), S. X mitchelliana (Figure 1B), and S. X Juthatip soper (Figure 1C) and S. X leucophylla (Figure 1D).
The team measured several pitcher traits to disentangle the contributions of shape, size and color from odor to prey attraction. They measured VOCs produced by each type of plant by sampling and collecting scents from a total of 39 pitchers on 16 different plants.
Dr Gaume and her collaborators found that the scents produced by all pitchers were similar to those produced by generalist plants that are pollinated by many different insect species, thereby allowing Sarracenia pitcher plants to potentially attract a wide variety of insects, although the research team did detect and measure subtle differences in the VOCs collected from each plant (Figure 3).
According to the findings, pitchers that attract bees and butterflies were rich in compounds such as limonene, a chemical that gives citrus fruits their distinctive smell. This class of compounds is quite common amongst plants: more than two-thirds of all flowering plants use them to attract pollinators.
Dr Gaume and her collaborators discovered that S. purpurea produces an odor that is high in fatty acids known to attract predatory insects and parasitoid wasps. These insects comprise a significant part of this carnivorous plant’s diet, suggesting its scent might be directly targeting them.
Dr Gaume and her collaborators discovered that pitcher dimensions were also important to the sorts of insects they attracted: they found that ants associated with fatty-acid-derivatives and short stubby pitchers; whereas most variation in bee and moth captures were associated with monoterpenes, benzenoids and taller, more slender pitchers, whereas monoterpenes alone explained most variation in Diptera and wasp captures.
Based on these findings, Dr Gaume and her collaborators proposed that the odor of a pitcher in combination with its physical dimensions can be used to predict which insect prey are caught by the plant around 98% of the time. While this isn’t definite proof, it’s a very strong link between the shape and scent of a pitcher plant and its insect prey (Figure 4).
“Our findings are important because they suggest that these carnivorous plants are not passive, but can target their prey”, Dr Gaume elaborated.
“It offers potentially interesting avenues in the field of biological control”, Dr Gaume explained, “and one can imagine drawing inspiration from the olfactory cues of these pitcher plants to control plant pests, for example.”
Future research could help explain how carnivorous plants that are pollinated by insects can attract some species for pollination and others for food.
“However, we remain cautious because our results are currently based on correlations. Even with strong correlations, further tests are necessary to investigate whether the different insect types are indeed attracted to particular scents.”
This is just the beginning of our growing understanding of the relationship between the ecology and evolution in these plants; there is plenty of research remaining to really nail down these findings. For example, the odors must be tested on insects to verify their attractiveness to them. Additionally, no one yet knows if wild pitcher plants create the same scents as those grown in the laboratory or in a greenhouse.
“Our results suggest that odors are key factors of the diet composition of pitcher plants”, Dr Gaume and her colleagues write (ref). “They support the hypothesis of perceptual exploitation of insect biases in carnivorous plants and provide new insights into the olfactory preferences of insect groups.”
Corentin Dupont, Bruno Buatois, Jean-Marie Bessiere, Claire Villemant, Tom Hattermann, Doris Gomez, and Laurence Gaume (2023). Volatile organic compounds influence prey composition in Sarracenia carnivorous plants, PLoS ONE | doi:10.1371/journal.pone.0277603