An invasive caterpillar species poses a serious threat to a valued native moth, even though the two species never directly compete for food, according to new research from ecologists at the University of Wisconsin–Madison.

The gypsy moth caterpillar, introduced from Europe, has defoliated entire stands of trees in the United States in periodic outbreaks over the past two decades. While these population surges come and go, the voracious caterpillars can appear in mass numbers and decimate local forests.

In the summer of 2021, a research team led by University of Wisconsin–Madison entomology professor emeritus Rick Lindroth encountered a severe gypsy moth infestation on an aspen research plot near the university’s agricultural research station.

“They are the most destructive forest insect in North America,” said Lindroth “Newsweek.”And they can clear hundreds of square kilometers of forest.”

The ecologists had planned to conduct new experiments, but the extent of the invasion forced them to change course.

“There were spongy masses of moth eggs everywhere. We wanted to start some experiments, but there were too many invasive insects to continue. There was no way to remove them. We thought we were doomed,” Lindroth said in a statement.

However, the team saw this as an opportunity to investigate the consequences of the destruction caused by the gypsy moth.

Lindroth’s previous work had looked at how plants defend themselves against herbivores. The researchers suspected that the aspens’ defenses, ramped up to ward off the gypsy moth, might inadvertently harm a native species that has evolved to feed on the trees’ normal chemical properties.

And indeed, when the defoliated aspens sprouted leaves a second time, the team found that the new foliage had an eightfold increase in defense substances – substances similar to aspirin that act as a poison against insects.

The scientists then fed the caterpillars of the Polyphemus Moth, the second largest moth species in North America. Less than 18 percent of native moth caterpillars survived on the heavily defended leaves, compared with a much higher survival rate among those fed on leaves from uninfested trees.

“We are witnessing an invasive species harming a native species – a valued, charismatic, beautiful moth – by altering the quality of its food plant. Without the two ever encountering each other,” said Lindroth.

“The message to take from this is that we know that insect populations are declining worldwide, and that this is happening in novel ways through invasive species,” he said.

“For insects, it’s like death by a thousand cuts.”

Although gypsy moth caterpillars have not yet reached all parts of the United States, their success in new habitats means it is only a matter of time before they do.

“Gypsy moths have a large number of host plants compared to many other moth species that may only feed on one or two closely related plant species,” said Anna Platoni, a consulting entomologist at Forest Research who was not involved in the study. News week.

“This means that when they are introduced into a new area, they always find a suitable host. In addition, the eggs laid by the females, which are protected by yellow-brown hairs, are difficult to detect and can therefore easily be displaced unintentionally.”

The results published in the journal Ecology and Evolutionsuggest that the effects of the gypsy moth invasion could affect the entire forest ecosystem.

“We have described at least half a dozen other species that we know from studies in my lab are negatively affected by high concentrations of these toxic compounds,” Lindroth said.

In addition to insects, mammals and even the trees themselves can be affected.

Diverting resources to chemical defenses also resulted in reduced growth of affected aspens, potentially compromising the trees’ ability to sequester carbon and mitigate climate change.

“The aspen is the most widespread tree species in North America,” Lindroth said. “Every breath we take contains oxygen molecules produced by the aspen. It is a very important forest species, and it is amazing to see the impact an invasive insect has on the forest community by changing the toxicity of the food landscape.”

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References

Lindroth, RL, Zierden, MR, Morrow, CJ, & Fernandez, PC (2024). Forest defoliation by an invasive outbreak insect: catastrophic consequences for a charismatic megamoth. Ecology and Evolution14, e70046. https://doi.org/10.1002/ece3.70046