HWA was introduced into the US from Asia in the 1920s, first in the Pacific Northwest, and then in Virginia in the 1950s. That was the start of its foothold in the east, and it spread to other states from there, including Pennsylvania and West Virginia. It didn't reach Ohio until 2001, and has spread into about 27 counties since then.
Unfortunately, in 2012 it was detected in my home county, Washington County. It was found in Belpre and Marietta, probably due to natural spread from nearby areas--possibly from across the Ohio River: Wood County, West Virginia has reported HWA. The Ohio Department of Natural Resources held an informational meeting about the HWA yesterday in Marietta to talk about its biology, risk to hemlock trees, and what the state is doing to stop its spread, so I attended and took some notes. The next few paragraphs are a quick summary of the meeting.
Eastern Ohio is where most of our hemlocks (T. canadensis) grow. Eastern hemlock prefers rocky sandstone gorge type habitats and is very shade-tolerant (so think Hocking Hills region). It can often develop into pure stands where it grows, but is also planted as an edge tree in more urbanized areas. It can act as a foundation species for an ecosystem, supporting many bird species (nearly 100 have been identified as being associated with eastern hemlock in some way) and it can also strongly regulate aquatic ecosystems by shading streams.
Ohio's forests don't have many conifers. Most of the forests consist of deciduous trees, but eastern hemlock is one of our common conifers, which means HWA poses a real threat to taking out a good-sized chunk of Ohio's conifer trees. HWA can cause tree mortality in 4-10 years, less if the tree is stressed by factors such as drought. HWA feeds on the tree's leaves and can be detected on a tree by the white, waxy residue that covers its body (hence the "wooly" adjective in its name). Essentially, it will look like cotton candy or spider webs on the tree.
HWA has a complex life cycle, and only the first instar (life development stage) is very mobile. There are two generations per year, and the winter generation can produce egg sacs with up to 300 eggs. The complex life cycle makes it difficult to control by introducing predators, but there has been some success on that front and beetle introduction is seen as a long-term solution. Another control option is the use of insecticides.
When it comes down to it, the state agencies involved in invasive species management need the help of the public to do their job. Tackling the HWA is no different. This includes keeping an eye on the trees and reporting the movement of wood products in counties with quarantines. Otherwise, HWA can spread under the radar. Whenever it's reported from a county, it's usually because someone noticed something strange on their trees and called it in. For example, an infested site in Marietta was found because a college student fell out of a tree and brought down some infested branches with him.
There's not nearly enough funding devoted to detection and eradication of invasive species, so we have to do the best we can with the resources we have. For more information on the Hemlock Wooly Adelgid, check out the links below. And if you notice something on any hemlock trees that looks out of the ordinary, report it!
Ohio Division of Forestry Fact Page
Ohio Department of Agriculture Fact Page