Hemlock Woolly Adelgid: The Invisible Threat to Hocking Hills

The most significant ecological threat facing Hocking Hills is not development, erosion, or overcrowding. It is a tiny insect — barely visible to the naked eye — that is slowly killing the trees responsible for the region's most defining feature: the cool, dark, mist-shrouded gorges.

The hemlock woolly adelgid (HWA) was detected in Hocking Hills State Park in 2013. This invasive pest, originally from East Asia, attaches to the base of hemlock needles and feeds on the tree's stored starches. Infested trees gradually decline over 4–15 years as they lose the energy to produce new growth. Left untreated, HWA kills nearly every hemlock it infects.

Why Hemlocks Matter to Hocking Hills

Eastern hemlocks are not just trees in the Hocking Hills landscape — they are foundation species that create the microclimate defining the entire gorge ecosystem. The dense, year-round canopy of hemlock needles blocks sunlight, traps cold air, and maintains humidity levels that support species found almost nowhere else in southern Ohio.

The gorges of Hocking Hills run 10–15°F cooler than surrounding ridgetops. This temperature differential exists primarily because of hemlock canopy. Remove the hemlocks and the gorges would receive direct sunlight for the first time in millennia. Temperatures would rise. Humidity would drop. The mosses, ferns, and liverworts that carpet the sandstone walls would dry out and die. The cool, mist-shrouded atmosphere that draws millions of visitors would fundamentally change.

These hemlocks exist in Hocking Hills because of a geological accident. During the last ice age, the glacial boundary ran just north of the region, pushing Canadian-affinity species — including eastern hemlock — southward. The hemlocks found refuge in the deep gorges of the unglaciated Allegheny Plateau and never left. They coexist here with southern species like rhododendron, creating an ecological overlap zone that is extraordinary for its latitude.

What ODNR Is Doing

The Ohio Department of Natural Resources' Division of Forestry finalized a conservation plan in 2017 and actively treats priority hemlocks with injected systemic insecticides (primarily imidacloprid and dinotefuran). These treatments are effective — a single injection can protect a tree for 5–7 years. However, treating every hemlock in the region is logistically and financially impossible. ODNR focuses on the most ecologically significant trees: those lining the main gorges and trail corridors.

As of 2024, most hemlocks in Hocking Hills State Park remain healthy thanks to active treatment. But decline is expected in untreated areas, and the long-term trajectory without sustained intervention is loss. ODNR is also researching biological controls — predatory beetles that feed on HWA — as a more sustainable long-term solution.

What Visitors Can Do

Stay on designated trails. Off-trail foot traffic compacts soil around hemlock root zones and can physically damage the shallow root systems that these trees depend on.

Do not transport firewood. HWA and other invasive pests spread through firewood movement. Buy firewood locally or use the kiln-dried bundles available at gas stations and grocery stores near the park. Ohio law restricts transporting firewood across county lines.

Report infestations. If you spot woolly white masses on hemlocks in areas that appear untreated, report the location to the Hocking Hills State Park office or ODNR's Division of Forestry. Early detection helps prioritize treatment resources.

What Hocking Hills Would Look Like Without Hemlocks

If HWA ultimately succeeds, the gorges would not disappear — the 340-million-year-old Black Hand sandstone is permanent. But the experience of walking through them would be unrecognizable. Open canopy would replace dense shade. The gorge floors would dry out, warm up, and lose the species adapted to cool, moist conditions. The fern-and-moss-carpeted sandstone walls would become bare rock. The atmosphere — that feeling of entering a different climate zone when you descend into the gorge — would diminish. Hocking Hills would still be beautiful. It would no longer be the same.