Madison Reporter

Trees stripped by invasive caterpillars muster defenses that can harm native insects

August 19, 2024

By Chris Barncard

Rick Lindroth, entomology professor emeritus, and Patricia Fernandez, an ecologist and professor visiting on a Fulbright Fellowship from the University of Buenos Aires.

An invasive insect with an insatiable appetite can cause serious problems for a favorite native moth that likes the same food source — even though the two are never in direct competition for a meal, according to new research from University of Wisconsin–Madison ecologists.

Since the early 2000s, spongy moth caterpillars, an import from Europe, have flexed their gustatory muscle in Wisconsin by stripping entire stands of trees of their leaves during late spring and early summer in remarkably destructive feeding binges. While spongy moth outbreaks wax and wane, the caterpillars can show up in significant numbers, seemingly out of the blue.

That was the case in 2021 when Rick Lindroth, entomology professor emeritus, and members of his lab trekked into a research forest of quaking aspen trees Lindroth had planted in 2010 just a few miles from UW–Madison at the university’s Arlington Agricultural Research Station. The team was excited to begin planned research after losing much of the 2020 fieldwork season to the COVID pandemic.

“There were spongy moth egg masses everywhere!” Lindroth says. “We wanted to begin some experiments, but there were too many invasive insects to proceed. There was no way they could be removed. We thought we were sunk.”

In a stroke of good fortune, Lindroth says his lab had an extra pair of hands that summer: Patricia Fernandez, an ecologist and professor visiting on a Fulbright Fellowship from the University of Buenos Aires. Lindroth and Fernandez formulated a new research plan. They knew the spongy moth caterpillars were likely to chomp up every leaf on their trees, so why not study the aftermath?

One aspect of Lindroth’s prior work had been exploring how different genetic and environmental factors shape the way plants defend themselves against attack. The researchers wondered whether an aspen tree’s defenses — rallied against marauding, invasive spongy moth caterpillars — might do unusual harm to native species that have evolved to feed on trees with baseline levels of chemical protection.

“Any wild plant is defended against an array of herbivores via a number of mechanisms, and chemistry is one of the most important,” Lindroth says. “Aspen trees — like their relatives, willow trees — produce a suite of salicylate-like compounds, close to aspirin, that act as toxins to protect the trees from many insects.”

Because the spongy moth caterpillar ends its leaf-eating spree relatively early in the aspen tree’s growing season, defoliated trees produce a second flush of leaves to capture enough energy to survive (if not necessarily thrive) through winter and into the next growth year. As expected, that second green canopy of leaves appeared by early July in Lindroth’s forest plot. But this backup crop was different in at least one important aspect.

“Those trees had cranked up their defenses,” Lindroth says. “By mid-summer, they produced an entirely new set of leaves that had, on average, an eight-times higher concentration of defense chemicals.”

By then, the spongy moth caterpillar culprits are pupating and changing into moths. But mid-summer is when caterpillars of the striking native polyphemus moth — second largest moth in North America — are hatching and trying to fill their own rumbling stomachs.

In their lab, researchers fed some polyphemus caterpillars leaves from the spongy-moth-infested plot and other polyphemus caterpillars leaves from a stand of aspen trees untouched by spongy moths (despite growing just six kilometers away).

Less than 18% of native moth caterpillars fed on high-toxin leaves left in the spongy moth’s wake survived while polyphemus caterpillars fed from undamaged trees were about four times as likely to survive to their next life stage.

“We’re seeing an invasive species harm a native species — a cherished charismatic beautiful moth — by changing the quality of its food plant without ever meeting each other,” Lindroth says. “That’s where this study is unique. And it suggests defoliation from an invasive species could impact a whole community via increases in toxic plant defenses. This research has identified yet another factor contributing to worldwide insect decline.”

The results published today in Ecology and Evolution also suggest that diverting resources to chemical defenses reduces tree growth relative to normal years. This hampers forests' ability to increase woody mass and sequester carbon mitigating climate change.

“Aspen is North America's most widely distributed tree species,” Lindroth says. “In every breath you take there are molecules produced by aspen trees. It’s very important forest species; seeing effect ripple out through changing toxicity astounding.”

This research was supported by grants from U.S Department Agriculture (WIS03003 & 2016-67013-25088), National Science Foundation (DEB-1456592), Fulbright Visiting Scholar Fellowship.