Carnivorous pitcher plants have long been known to capture prey using their unique pitfall traps, but researchers have discovered that the plants’ odours play a key role in determining their diet. The study by Dupont and colleagues, published in PLOS One, reveals that pitcher plants emit distinct scents that attract specific prey, providing new insights into the olfactory preferences of various insect groups.
Sarracenia plants may share the same method of trapping prey, but they don’t share the same prey. Until now, these differences have been primarily attributed to the plants’ morphology. However, the team hypothesized that the plants’ odour profiles also play a crucial role in determining the prey they capture.
To test their hypothesis, the researchers compared the odour and prey compositions of different Sarracenia taxa grown together. They analyzed a kinship gradient from Sarracenia purpurea, known to primarily capture ants, to horticultural hybrids Sarracenia × mitchelliana, Sarracenia × Juthatip soper and Sarracenia × leucophylla. They also measured several pitcher traits to discern the contributions of morphology and odour to prey variation.
The study found that the pitcher odours were as diverse as those of generalist-pollinated flowers but with notable differences among the different taxa, reflecting their relatedness. Volatile organic compound (VOC) similarity analyses revealed taxon-specific odour profiles, which were mirrored by the prey similarity analyses.
For example, Sarracenia × leucophylla was found to be more specialized in capturing flying insects like bees and moths and released more monoterpenes, compounds known to attract flower visitors. Sarracenia × Juthatip soper trapped a similar number of bees but fewer moths, with sesquiterpenes contributing less to its scent. Ants and Diptera were the primary prey of the other two taxa, which had scents dominated by fatty-acid-derivatives.
The researchers revealed two syndromes in the plants: ants were associated with fatty-acid-derivatives and short pitchers while flying insects were associated with monoterpenes, benzenoids and tall pitchers. In Sarracenia × leucophylla, the emission rate of fatty-acid-derivatives and pitcher length explained most variation in ant captures. In contrast, monoterpenes and pitcher length explained most variation in bee and moth captures, while monoterpenes alone explained most variation in Diptera and wasp captures.
The study’s findings suggest that odours are key factors in determining the insect biases in carnivorous plants, where the plants take advantage of insects’ innate preferences for certain scents to lure them in. Dupont and colleagues write in their article that humans’ relatively poor nose for scent, and reliance on eyes, may have led botanists to ignore an important part of an insect’s experience of a plant.
Of the signals involved in communication, odour is probably the most cryptic to humans, who primarily use their visual sense and it may thus have been largely overlooked. In addition, odour is often correlated with other plant characteristics, making it difficult to disentangle its effect from others. Furthermore, it cannot be precisely described without a technological tool, i.e. the GC-MS. Its effect may also be not easily pointed out because different compounds that form the blend can have opposite effects on different guilds of insects. Sarracenia plants are not spared from these issues. The positive correlation revealed in this study between certain odour components such as the amounts of benzenoids and terpenes with pitcher length could explain why the effect of odour could have remained unnoticed. Indeed, pitcher morphology, a set of traits easier to measure, was often shown to be “the” driver of prey abundance.Dupont et al. 2023
The researchers believe that their work provides new insights into the olfactory preferences of insect groups, which could have broader implications for understanding insect behaviour and plant-insect interactions.
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Dupont, C., Buatois, B., Bessiere, J-M., Villemant, C., Hatterman, T., Gomez, D. and Gaume, L. (2023) “Volatile organic compounds influence prey composition in Sarracenia carnivorous plants,” PLOS One, 18 (4): e0277603. Available at: https://doi.org/10.1371/journal.pone.0277603.