Invasive spongy moths are defoliating aspen trees and making their leaves regrow with high levels of toxins, harming a native insect, according to recent research from the University of Wisconsin-Madison.
In 2021, Rick Lindroth, UW-Madison entomology professor emeritus, began noticing this effect. After his research group originally planted aspen trees in 2010 to conduct other experiments on the trees at the Arlington Agricultural Research Station, a spongy moth outbreak occurred following their absence during COVID-19 in 2020.
The research team — made up of Lindroth, visiting scientist Patricia Fernandez, former graduate student Clay Morrow and post-doctorate Mark Zierden — had to take a different route according to Lindroth.
“When we walked into the aspen stand in spring 2021, we realized there were thousands of egg masses — each egg mass contains 400 to 600 larva — and we were not anticipating that. I knew right then that we were doomed to have a major defoliating outbreak of spongy moths,” Lindroth told The Daily Cardinal.
Spongy moths, which arrived in North America from Europe in 1869, only entered Wisconsin in the early 2000s, according to Lindroth. They feed on more than 300 species of trees and can defoliate trees for miles, which can harm entire forests, he said.
When the outbreak occurred on aspen trees at the Arlington Agricultural Research Station, the researchers instead decided to study the consequences of spongy moth defoliation on the native polyphemus silk moth, the second-largest moth in North America.
“What we were looking at is the legacy effect of the defoliation that carried on in terms of food quality for the native polyphemus moth,” Lindroth said.
Although these two species don’t directly interact, both moths feed on aspen trees at different times of the year.
“Spongy moths feed early in the season and then they’re done. Polyphemus moths feed mid- to late summer so anything the spongy moths do to alter food quality will impact the polyphemus moth,” Lindroth said..
Aspen trees produce salicylate-like compounds, which can be highly toxic at high levels, according to Lindroth. The compounds are similar to aspirin as they are made up of a salicin derivative and are known as salicinoids, he noted.
“A spongy moth caterpillar feeding on a tree with normal levels of these salicinoids would consume, in human terms, 5-7 pounds of aspirin in a day. Imagine what that would do to your gut,” Lindroth said.
When the spongy moth eats aspen leaves in the spring, the aspen trees respond by reflushing a new set of leaves. But these leaves have eight times the levels of salicinoid defenses that they previously had, according to Lindroth.
And although spongy moths and polyphemus moths have both adapted to eat these toxins at normal levels, when aspen trees increase their defenses following the spongy moth’s feeding season, it overwhelms polyphemus caterpillars’ ability to detoxify them, Lindroth said. This can result in lesions in the gut, septicemia, hemorrhaging and death, he said.
Spongy moth outbreaks don’t only negatively affect the polyphemus moth. They also degrade aspen trees.
Aspen trees that have lost their leaves even once have a dramatic reduction in growth for the year, according to Lindroth. At the Arlington plot, he said their aspen trees were more susceptible to disease and drought years later, causing some to die.
Additionally, Lindroth told the Cardinal aspens are the most widely distributed tree species in North America and therefore aid a great deal in carbon sequestration, the process of capturing carbon dioxide from the air, which helps combat climate change.
Other research done by Lindroth has shown that when trees photosynthesize, they can use the energy produced to either grow or make defense compounds. Growth creates carbon sequestration, he said.
“I think we can make the argument that invasive species like spongy moths can negatively affect the capacity of aspen forest to sequester carbon by removing leaves so the trees aren’t photosynthesizing, and when the leaves reflush, they’re busy making defense compounds rather than growing,” Lindroth said.
Lindroth underscored that there is currently no experimental evidence for this, but it has potential to be a serious implication for the spongy moth outbreaks.
To combat this, the Wisconsin Department of Agriculture, Trade and Consumer Protection is part of a federal “Slow the Spread” program that aims to prevent spongy moths from traveling westward and to aid in treatment of isolated outbreaks. Treatments include aerially spraying a bacterial insecticide or spraying wax droplets containing pheromone to stop male spongy moths from finding females.
Although some insecticide sprays may be able to treat larger outbreaks, Lindroth said that once they’ve been established in an area, there’s not much that can be done to contain the outbreak completely.
Lindsay Pfeiffer is the science editor for The Daily Cardinal.
