A lack of diversity among plants in urban environments may be a key factor in the widespread urban defoliation caused by a kind of caterpillar called cankerworms native to eastern U.S. states, a recent study suggests.
When the caterpillars hatch during the early days of spring, they feast on new spring foliage, created unsightly defoliation that can, in some cases, kill plants, scientists at North Carolina State University say.
The damage is becoming more and more apparent in urban setting, they say.
"We see cankerworms doing more damage to trees in cities than in the wild and examples of widespread cankerworm damage are happening more often," entomology Professor Steve Frank says. "We wanted to know why."
Two features of urban environments that make them different from natural counterparts stood out, the researchers said; less density and diversity of plant life, and more nonnative plant species, including ornamental shrubs.
In the study the researchers focused on what are known as understory plants, species plantedvery near to or underneath trees.
"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 says.
The caterpillars hit native plants exceptionally hard in urban environments, the researchers found, while nonnative species were found to be ignored by cankerworms for the most part.
"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."
Cankerworms as also knows as inchworms, loopers or measuring worms for the distinctive locomotion involved bunching up and then extending their bodies.
They hatch from late April until early May, timed to match the tree budding on the tress that serve as hosts.
Once they mature, the caterpillars can almost entirely consume a large leaf, leaving just the leaf's midrib. Trees that suffer complete defoliation by cankerworms for two years in a row often die.
Trees preferred by cankerworms include ash, beech, maple, and oak, although they will also consume the leaves on apple, elm, birch, hickory and many other hardwood trees.
