Be on the Lookout for Hemlock Woolly Adelgid
Jonathan Ferbrache, PLA, CPESC, Resource Specialist, Fairfield Soil and Water Conservation District
In partnership with Tom Macy, Forest Health Program Administrator, ODNR Division of Forestry
As winter approaches and you see that first dusting of snow or heavy frost on the landscape, you might want to take a second glance at one plant specifically around your home.
The eastern hemlock (Tsuga canadensis), sometimes referred to as Canadian hemlock, is a popular landscape tree that is often planted as a single specimen or as a hedgerow. This tree also is native in parts of the eastern half of Ohio, typically in moist, sheltered ravines and slopes.
Eastern hemlock has significant ecological and economic value in Ohio. It is one of the most long-lived – individuals can survive for more than 600 years – and shade-tolerant tree species in eastern North America. Eastern hemlock forests provide critical habitat for many birds, mammals and amphibians, and the shade they create cools streams, creating valuable habitat for certain fish and aquatic macroinvertebrate species.
These forests are economically important as well. Areas such as Hocking Hills and Mohican state parks are extremely popular outdoor recreation destinations and owe much of their scenic beauty to eastern hemlocks.
Unfortunately, this valued tree species is now threatened by a non-native insect called hemlock woolly adelgid (pronounced uh-DEL-jid), or HWA. This small, less than one-sixteenth of an inch, aphid-like insect is native to hemlock forests in Asia and the Pacific Northwest. It was first discovered in the eastern United States near Richmond, Va., in the 1950s and was probably imported accidentally on Asian hemlock trees.
The insect attaches itself to the base of eastern hemlock needles, where it feeds on the tree’s stored carbohydrates. Trees usually die from HWA infestation in 5-15 years.
The insect is most readily visible from fall (October) through spring (May), when maturing larvae surround themselves with what looks like fluffy white wool, giving them a “cotton swab” appearance. In the summer months, HWA enters a dormant state and has no woolly covering, making it nearly invisible to the naked eye.
HWA has caused major mortality of eastern hemlocks, both in forest ecosystems and landscape settings, in the southern Appalachian region and Smoky Mountains and northward into New England. This insect is transported by wind and wildlife, especially birds, as well as humans moving infested plant material.
It was first discovered in Ohio in Shade River State Forest (Meigs County) in 2012. Since then, it has been discovered in six additional southern Ohio counties, including in the Hocking Hills region. The Ohio Department of Natural Resources – in partnership with Ohio Department of Agriculture, The Ohio State University Extension and U.S. Forest Service – has been surveying for HWA throughout the state and treating infestations when they are found.
There are several treatment options available to protect eastern hemlock trees from HWA. A number of chemical insecticides, when applied to the infested tree, can be effective in preventing HWA damage.
There are also several beetle species that have been found to feed exclusively on HWA. These beetles are very important for controlling HWA in forested settings, where treatment with chemicals on an individual tree basis is not always feasible.
While it is not realistic to attempt to eradicate this pest from Ohio’s forests and landscapes, the use of chemical and biological controls can help keep specific eastern hemlock trees and forests alive. It is very important that landowners do not treat their hemlock trees without first verifying the presence of HWA. This advice should also be followed for any other landscape plant. Unnecessary use of pesticides is a waste of time and money, and could negatively impact non-target organisms.
It is recommended that eastern hemlock trees be inspected at least once a year, between fall and spring, for HWA on the underside of branch tips. Any findings of suspected HWA infestation should be reported to the ODNR Division of Forestry at 614-265-6694 or the Ohio Department of Agriculture Division of Plant Health at 614-728-6270.
