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Lack of Plant Diversity Spurs Cankerworm Damage in Cities

Research from North Carolina State University finds that a lack of plant diversity is a key contributor to the widespread defoliation caused by cankerworms in cities, and highlights the role that increasing diversity can play in limiting future damage.

A fall cankerworm. Image free for use by news media. Photo credit: Steve Frank. Click to enlarge.

A fall cankerworm. Image free for use by news media. Photo credit: Steve Frank. Click to enlarge.

Fall cankerworms (Alsophila pometaria) are caterpillars that are native to the eastern United States and hatch in early spring. The cankerworms defoliate trees and other plants, eating new leaves as they emerge – which is both unsightly and can ultimately kill the plants.

“We see cankerworms doing more damage to trees in cities than in the wild and examples of widespread cankerworm damage are happening more often,” says Dr. Steve Frank, author of a paper describing the work and an assistant professor of entomology at NC State. “We wanted to know why.”

Frank 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.

To evaluate the impact of diversity and nonnative species on cankerworm damage, Frank focused 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 says. “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.”

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

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 says. “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 paper, “Bad neighbors: urban habitats increase cankerworm damage to non-host understory plants,” is published online in the journal Urban Ecosystems. The work was supported by the U.S. Department of Agriculture under grant number 2013-02476. Additional support came from the Department of the Interior’s Southeast Climate Science Center, which is based at NC State and provides scientific information to help land managers respond effectively to climate change. An overview of recent NC State research on controlling cankerworm populations is available here.

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Note to Editors: The study abstract follows.

“Bad neighbors: urban habitats increase cankerworm damage to non-host understory plants”

Author: Steven D. Frank, North Carolina State University

Published: online April 2014, Urban Ecosystems

DOI: 10.1007/s11252-014-0368-x

Abstract: Plants growing in vegetationally diverse habitats or near taxonomically distinct neighbors often experience less herbivory than plants in more simple habitats. When plants experience more herbivory in these situations it is called associational susceptibility and is most common when herbivores spill from their preferred plant host onto neighboring plants. Cankerworms are common pests of urban trees that have been shown in forests to disperse from preferred to less preferred hosts. I found that two common characteristics of urban habitats, low vegetational diversity and exotic plants, affect cankerworm herbivory of non-host understory plants. In an urban landscape I measured cankerworm herbivory on native dogwood trees growing in the open and below cankerworm host and non-host trees. Herbivory of native dogwoods was ten times greater below cankerworm hosts than on trees below non-hosts or in the open. At an arboretum I measured herbivory of native and exotic plants growing below cankerworm hosts in simple landscape plantings and in natural forests. Associational susceptibility of native dogwoods and Rhododendron spp. disappeared when they were growing in complex natural forests even though cankerworm hosts were more abundant. Cankerworms consistently preferred native plant species more than exotic congeners in laboratory experiments. As such, exotic plants experienced very little herbivory regardless of habitat. Herbivorous pests are often more abundant on urban plants than plants in natural habitats. My research shows that, although some plants experience more herbivory when growing near cankerworm hosts, increasing urban habitat complexity could reduce pest damage overall.

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