How far would leaf-eating insects go to dine on their favorite food? Perhaps the other side of the world, according to researchers at Penn State who found insect damage on fossil leaves from South America that is nearly identical to what is seen today on those trees’ living relatives in Australia.
The findings, recently published in the journal New Phytologist, suggest multiple insect herbivore lineages have fed off gum trees, which are plants in the well-known Australian genus Eucalyptus, for at least 52 million years and in locations across the Southern Hemisphere.
“The big stunning result is that every type of insect damage on the fossils survives today on Eucalyptus,” said L. Alejandro Giraldo, a doctoral student in the Department of Geosciences at Penn State and lead author of the study. “It suggests that the insects that were eating the Eucalyptus plants in the past tracked their plant food through time.”
Fossil plant and insect relationships can provide important information about how terrestrial ecosystems evolve through geologic time, but fossil evidence that directly links associations from deep time to the modern-day has been scarce, the researchers said.
“There is an evolutionary arms race between plants and insects,” Giraldo said. “Plants evolve new defenses, insects find a way to go around them to continue to feed, and this cycle repeats through time. This work shows nicely that the evolutionary association between herbivorous insects and their plant food can be stable through time. We are showing that all the damage you see in the fossils you can see in the present, so those relationships may be fairly consistent.”
In prior work, Penn State scientists and their international colleagues found that plants considered native to Australia and Southeast Asia—like the Eucalyptus—existed millions of years ago in modern-day Patagonia, Argentina. The plants may have spread when the planet was much warmer and Australia, Antarctica and South America remained connected before the final breakup of the Gondwana supercontinent.
The new research suggests that insects may have followed their food source.
“We’ve been able to tell this very interesting story of plants like Eucalyptus surviving through time and space because people have been working on them for a long time—but when it comes to the very diverse insects that were eating those plants, we’re not quite as sure,” Giraldo said.
Giraldo studied insect damage on 284 fossil Eucalyptus leaves from the 52-million-year-old Laguna del Hunco fossil rainforest in Argentina and compared the damage patterns with over 10,000 modern Eucalyptus samples from Australia and Southeast Asia, preserved in Australian and U.S. collections.
The ancient Eucalyptus plants appeared to be a popular food source. The researchers identified multiple insect damage types, including examples of external feeding, galls and mines. Galls are tumor-like growths that occur when insects feed or lay eggs on leaves, and mines are tunnels left behind by insect larvae as they wiggle around eating plant material.
“It’s just like today—if you go to a park and pick up a bunch of leaves, you’ll probably see chew marks, galls or mines,” Giraldo said. “We can see these same insect-feeding traces in the fossils, and by doing so we can take a glimpse into the ecology of ancient forests.”
The specific insect culprits responsible for the damage seen in the fossils are not preserved as fossils and remain unknown. Giraldo said estimates indicate as many as 15,000 to 20,000 insect species feed on Eucalyptus today—but the majority have never been described.
According to the team, this work could provide an opportunity to identify new insect species because it provides scientists with a list of modern Eucalyptus species, the location of their populations and the associated insect damage that matches that observed in the fossils.
“We think that some of these insect lineages followed plants through time and space, but there is a limit to what we can see through fossils, and we cannot assign the damage to a culprit straight up,” Giraldo said.
“What we do have are the locations of the modern plants we studied. So, we can say, if you’re interested in describing a potentially new insect species, you can go to this site and look for this plant, and it’s very likely that you could find a new insect species if you track the same feeding traces we observed in the fossils. I think that’s a neat perspective in how studying the past also allows you to better understand the present.”
Peter Wilf, professor of geosciences at Penn State and Giraldo’s adviser, also contributed to the study. Also contributing were Michael Donovan, paleobotany collections manager, Field Museum of Natural History; Robert Kooyman, honorary research fellow, Macquarie University, Australia; and Maria Gandolfo, professor, Cornell University.
More information: L. Alejandro Giraldo et al, Fossil insect‐feeding traces indicate unrecognized evolutionary history and biodiversity on Australia’s iconic Eucalyptus, New Phytologist (2024). DOI: 10.1111/nph.20316
Journal information: New Phytologist
Provided by Pennsylvania State University