The scent of blooming flowers and fresh plant life is not just a perk of springtime; it is a key driver in the survival and evolution of butterflies and moths. New research led by scientists at Penn State reveals how the daily cycles of plant aromas are linked to the dietary habits and evolution of the winged insects collectively known as Lepidoptera.
In a recent study published in the journal Proceedings of the Royal Society B, an international team of researchers tested a new hypothesis for why some Lepidoptera have very specific diets, feeding on only a few types of plants, while others are far less picky.
The new idea, called the Salient Aroma Hypothesis, suggests that the smells plants release play a crucial role in determining how specialized a butterfly or moth’s diet becomes. The researchers found that greater availability of plant aromas during the day provides more chemical information for day-active insects to use to locate and specialize on particular host plants, while the decrease in plant aromas at nighttime means night-active Lepidoptera have to take what they can get and have a more varied diet.
“This idea provides a new perspective on why some butterflies and moths are picky eaters while others are not,” said Po-An Lin, an assistant professor at the National Taiwan University who launched the research while earning his doctoral degree from Penn State and continued the work as a postdoc in Taiwan. “It also highlights the critical role of plant volatiles, or scents, in shaping insect-plant interactions and evolutionary adaptations.”
To determine whether plant scent may have driven adaptation, the researchers looked at the insects’ primary organs for smelling — the antennae — and compared the antennal size of 582 specimens from 94 species of butterflies and moths.
The Penn State team collaborated with a team at Harvard that found that female Lepidoptera that are active during the day tend to have larger antennae relative to their body size than those active at night.
This might suggest that having better “smelling” equipment is more beneficial when there are more smells to detect, explained Gary Felton, the Ralph O. Mumma Professor of Entomology at Penn State, co-author on the paper and Lin’s research adviser. Similarly, specialist female Lepidoptera — those that feed from only a few types of plants — often have larger antennae than generalist females, possibly because they need to be very good at detecting the specific aromas of their host plants.
“The relationship between antennal size and host plant breadth was very strong,” Felton said. “Larger antennal sizes have been associated with a greater number of sensilla, the sensory structures involved in the sense of smell, thereby increasing the surface area for sensory receptors. The enhanced capacity may be a key adaptation for how certain Lepidoptera have evolved to feed on a limited and specific range of plants.”
The findings suggest a potential link between the availability of plant aromas during the day and an evolutionary investment in olfactory structures in the insects, particularly in females that engage in host plant selection by laying their eggs on the plant, Lin explained.
“This finding demonstrates how the availability of chemical signals influences the evolution of sensory organs in insects,” he said. “It provides a fascinating example of how plants, through their chemical emissions, have played a direct role in shaping the evolution of the insects that rely on them.”
Lin and colleagues at Penn State used a combination of approaches to investigate the link between plant aromas and Lepidoptera diets. They first conducted a meta-analysis of existing scientific literature to confirm that plants generally release more diverse and abundant volatile organic compounds, or aromas, during the day versus the night. Then they studied the Lepidoptera family tree to analyze the relationship between the insects’ activity patterns — day or night active — and their preferred host plants, using statistical models that account for evolutionary relationships.
“Our analyses showed a significant correlation between being active during the day or night and the diversity of host plant species that Lepidoptera consume,” said Naomi Pierce, professor of biology at Harvard University and co-author on the paper.
The researchers found that day-active Lepidoptera, like monarch butterflies, have more opportunities and more specialized organs to detect plant aromas and, as such, have evolved to be picky eaters. On the other hand, night-active species, like the Polyphemus Moth, encounter fewer and less diverse plant aromas. With less clear chemical information available, it might be harder for them to be so selective, potentially leading them to have more generalized diets, feeding on a wider range of plants.
“Insect herbivores, such as butterflies and moths, must find the right plants to feed on and, in the case of females, to lay their eggs,” Lin said. “This is a crucial decision because caterpillars depend entirely on the selected plant for survival. Unlike humans, who eat a wide variety of foods to stay healthy, many insect herbivores specialize in feeding on only a few plant species. The Salient Aroma Hypothesis helps explain why some insects are highly specialized while others are more flexible in their diet.”
The other authors on the paper are Wei-Ping Chan and Even Dankowicz of Harvard University; Liming Cai of the University of Texas Austin; Yun Hsiao of National Taiwan University; and Kadeem Gilbert of Michigan State University.
The U.S. National Science Foundation, Taiwan’s National Science and Technology Council and the Yushan Fellowship Program from the Ministry of Education of Taiwan funded this work.
