A lack of plant diversity may be a key contributor to the widespread defoliation caused by cankerworms in cities, according to a recent study.

Fall cankerworms are caterpillars that are native to the eastern United States and hatch in early spring. They defoliate trees and other plants, eating new leaves as they emerge -- which is both unsightly and can ultimately kill the plants, according to researchers from North Carolina State University.

"We see cankerworms doing more damage to trees in cities than in the wild and examples of widespread cankerworm damage are happening more often," Dr. Steve Frank, author of the study and an assistant professor of entomology, said in a statement. "We wanted to know why."  

For the study researchers looked at two aspects of urban environments that distinguish them from natural environments: the fact that urban environments have less diversity and density of plant life; and the fact that urban areas have more nonnative plant species, such as many ornamental shrubs.

They also evaluated the impact of diversity and nonnative species on cankerworm damage by focusing on the damage cankerworms did to understory plants -- those plants that grow near or under trees.

"I found that plant diversity plays a significant role," Frank said. "Cankerworms did more damage in simple urban environments, where the understory consisted of only a few shrubs, than they did in more complex environments with greater plant diversity."

Researcher also found a distinction between the impact on native and nonnative plant species.

They found that native plants were hit particularly hard in simple urban environments. They benefited significantly from complex environments that more closely resembled natural habitat. Nonnative species were largely ignored by cankerworms, regardless of the setting.

"This does not mean that everyone should plant nonnative species," Frank said. "The take-home message is that we need to take steps to make urban environments more like natural environments in terms of plant diversity."

The findings were recently published in the journal Urban Ecosystems.