Elm trees throughout parts of North Carolina have recently begun to appear brown, or dead, as if they’ve been scorched by fire. Having been reported by many counties across the state, these ‘scorched’ elm trees have primarily been seen throughout the Central Piedmont. Upon closer examination, the leaves are skeletonized, meaning the tender green portions of the leaf have been consumed by an insect, leaving the tougher leaf veins intact. What is causing this?
Staff with the N.C. Forest Service (NCFS) Forest Health Branch investigating these reports have found a native forest insect, the larger elm leaf beetle (Monocesta coryli). Forest Health staff have also discovered that American elm trees have been the primary target while winged elm trees and slippery elm trees have not been damaged as severely. Although reports have not been received to this point, nonnative ornamental elm trees such as the Siberian elm and lacebark elm may also be infested.
Like many native forest insects, larger elm leaf beetle populations fluctuate depending on environmental conditions and natural cycles, including the animal populations that feed on them. It’s not uncommon for these types of insects to impact large areas as forest health officials have found insect populations, including the larger elm leaf beetle, disturbing elms at various times. Locations observed over the last 20 years include the Roanoke River Basin in eastern North Carolina, the northern part of the Central Piedmont and Wake County. Larger elm leaf beetles emerge during the spring to lay eggs, which later hatch into the larvae causing the defoliation we’re seeing. The larvae then crawl to the ground and overwinter in the soil where numerous insects may congregate underneath affected trees.
The insects can easily be identified. The orange adults with blue metallic stripes running across their backs are one of the largest leaf beetles and the only member of its group in the U.S. The larvae also sport a yellowish orange look, particularly on their underside. Both life forms exude an orange liquid as a defense mechanism when handled.
One of the most common questions NCFS staff are asked with respect to this insect is, “will my elm tree die?” Typically, defoliation in the earlier part of the growing season is more problematic than those late in the season after the tree has had time to establish root reserves. However, repeat defoliations can cause mortality. This insect is typically brought under control by natural predators and often does not lead to tree death. Although, elm trees in the home landscape may be treated with one of several common insecticides, including Orthene and Sevin. Remember, any insecticide must be used in compliance with its label. Some landowners experiencing defoliation have applied sticky bands to their trees to entrap adults emerging the next spring to prevent repeat defoliation.
With the recent emergence of the elm zigzag sawfly (Aproceros leucopoda), the continued presence of the nonnative elm leaf beetle (Xanthogaleruca luteola) and Dutch elm disease (Ophiostoma ulmi), native elms in North Carolina could potentially be impacted by various assailants. Fortunately, the larger elm leaf beetle is part of the common cycle of insect life in our state. People who suspect there’s an infested tree in an area near them should contact their NCFS county ranger. To find contact information for your county ranger, visit https://www.ncforestservice.gov/contacts.