Invasive insects have greatly changed the forest makeup in Sullivan County over the past forty years. Emerald Ash Borer are responsible for the loss of thirty-two ash trees on my property alone. But the trees most affected by insect infestation are none other than the once abundant Canadian Hemlock (Tsuga canadensis). 

Canadian hemlock helped put Sullivan County on the map when back in the late 1700’s tanneries started to dot the landscape. The early 1800’s marked explosive growth, fueled by the Delaware and Hudson Canal, opening in 1828, which slashed shipping costs for bark and hides to New York City markets. By 1830, entrepreneurs recognized the county’s dense hemlock forests as a goldmine as the bark’s high tanning content yielded reddish, supple leather prized for soles, harnesses, and boots. Leather tanned with hemlock proved to be stronger than oak tanned alternatives and the industry grew rapidly. Tanning boomed with the Civil War demand, employing thousands in facilities along almost every stream in Sullivan County and producing 103,198 pounds of sole leather in 1865 alone. Sullivan County led New York, by tanning nearly one-third of the state’s output of over two million hides annually by 1868. Villages like Livingston Manor and Liberty thrived on the trade, with bark-peeling crews cutting hemlocks in spring, stacking bark for drying, and hauling it to the steam powered mills. 

Yet prosperity was fleeting. By the 1880’s, hemlock scarcity and competition from chrome tanned imports in Pennsylvania and the Midwest triggered decline. The industry, once Sullivan County’s lifeblood, spurring population growth from 6,000 in 1810 to 25,000 by 1850, faded quickly, leaving ghost towns, polluted streams, and echoes of a once great industry. 

After the tanning days however, the hemlock once again started to reforest the region right up until another threat introduced itself in the mid-1980’s. That threat was none other than the Hemlock Woolly Adelgid (HWA, Adelges tsugae). What started in the warmer counties of Orange, Rockland and Westchester, quickly moved up the Delaware River Basin and by the mid-1990’s my customers from the Barryville, Eldred and Long Eddy areas already being affected. 

Woolly Adelgid feeds on the nutrient rich tissues at the base of hemlock needles using piercing mouthparts, depleting the tree’s stored energy and causing needle loss, branch dieback, canopy thinning, and eventual tree death within four to ten years if untreated. 

Treatment for small trees, less than ten feet is possible, but for large trees the cost is just too much. A systemic soil drench is the best defense against woolly adelgid, but to treat one tree, less than ten feet tall, will cost around fifty dollars and it may need to be done every three to four months. Not something that most people want to do. I moved from Swan Lake to Eldred a couple years ago and there are no hemlocks left in the Eldred area. Because of the woolly adelgid problems, most nursery growers in the northeast have stopped carrying hemlocks. We haven’t been able to get them since the early two thousands. 

As of 2025, woolly adelgid affects 28 counties in New York, with Sullivan County in the heavily infested Catskill PRISM (Partnership for Regional Invasive Species Management) region. But a new treatment for woolly adelgid could mean a bounce back for the hemlock population. In 2025, silver flies were released and established statewide in New York, including many Catskill’s region sites. 

Native to the Pacific Northwest of North America, Silver Flies have emerged as a promising biological control agent against the invasive hemlock woolly adelgid, the sap-sucking pest that threatens hemlock forests across the eastern United States. Silver Flies are specialist predators, meaning they primarily target adelgids (aphid-like insects) and do not pose risks to non-target species, making them ideal for classical biological control programs. It will take ten or more years before the Silver Fly population grows large enough to contain the woolly adelgid, but for the first time in forty years there may be a way to help the once prevalent Canadian Hemlock gain popularity again in Sullivan County.