SRS research entomologist Bud Mayfield was relieved to find that defoliation on an American chestnut planting site was not as severe as expected. Mayfield and SRS research forester Stacy Clark are coauthors on a paper in the Journal of Insect Science that describes a study they conducted with Ashley Case, an adjunct lecturer at the University of North Carolina at Asheville.
The research could aid the collaboration between the U.S. Forest Service, the University of Tennessee (UT), and The American Chestnut Foundation with reintroducing the once abundant American chestnut tree throughout eastern forests.
In 2009, thousands of young chestnut trees–including genetic hybrids created from a backcross breeding program using American chestnut and Chinese chestnut, an Asian tree species resistant to the blight fungus–were planted across three national forests in the southern Appalachian mountains.
SRS scientists have been monitoring their growth and resistance to the deadly chestnut blight fungus that functionally eradicated the trees in the 20th century. In the fall of 2011, Clark noticed a lot of leaf damage on the young chestnut trees and contacted Mayfield for help identifying the culprit.
Mayfield recognized the blocky, narrow defoliation pattern: Asiatic oak weevil (Cyrtepistomus castaneus). This nonnative insect is one of many found on trees in the family that includes beeches, oaks, and chestnuts, but its impact on American chestnut trees was not well understood. Previous research has shown that oak leaf damage by these insects can compromise plant growth and seed survival and can lead to increased seedling mortality.
Mayfield and Clark collaborated with UT and Case–at the time a student in their Department of Forestry, Wildlife, and Fisheries–to collect field data at two planting sites in North Carolina and Tennessee the following summer. In North Carolina, the forest canopy above the chestnuts was dominated by tulip poplar and red maple. At the Tennessee site, chestnut oak and eastern white pine were dominant.
From May through September, the team recorded the dates of weevils emerging from the soil, the number of weevils resting or feeding on chestnut trees, and the amount of leaf damage the weevils inflicted.
The peak of oak weevil emergence happened in July, and the scientists found more weevils at the Tennessee site, under the oak overstory. More weevils were found on the American chestnut and its most closely related hybrid than the Chinese chestnut. It could be that the Chinese chestnut tree and the Asiatic oak weevil evolved together, giving the tree opportunities to develop chemical or physical defenses against the weevil.
The oak dominated site in Tennessee also saw more defoliation by the weevils, although both sites experienced less than 25 percent damage. “Because the defoliation we measured was below a 30 percent significant damage threshold, it wasn’t likely to significantly impact growth of the saplings,” says Case. “The use of insecticides or other control measures wasn’t warranted.”
“Leaving oaks in the overstory may make American chestnuts and their closest hybrids more susceptible to Asiatic oak weevil damage,” adds Mayfield. “Weevils probably aren’t going to wipe out the chestnut, but they are an unforeseen part of a larger pest complex that should be considered in American chestnut restoration efforts.”
The scientists will continue to monitor the juvenile American chestnuts with hopes that these trees will evade the Asiatic oak weevil–and other invasive pests–and return to their historic prominence in the South’s native forest ecosystems.
For more information, email Bud Mayfield at firstname.lastname@example.org.