New York State’s wild forests are under threat by a tiny insect with the power to kill hemlock trees and harm drinking water for millions of people [because] erosion-prone slopes will be washed into New York City reservoirs. The Hemlock Woolly Adelgid is currently a severe threat in the Castkills and has begun to make its way toward the Adirondacks. – ADK Executive Director, Neil Woodworth

New York has more hemlock trees than any other state in the nation. They are threatened by the continued expansion of an invasive insect pest, the hemlock woolly adelgid, or HWA. First found damaging hemlocks near Richmond, Virginia, in the mid-1950s, HWA has since spread from Georgia to Maine and has been causing widespread hemlock mortality.

In New York, HWA was first detected in the lower Hudson Valley in the late 1980s and has spread through the southern Catskills and Finger Lakes west to Letchworth and Allegany State Parks. Hemlock mortality has been dramatic in the Hudson Valley and is mounting in southern Catskill watersheds and the Finger Lakes.

HWA has recently reached the southern Mohawk Valley near Schenectady, but has yet to be found in the Adirondacks, where hemlock stands are iconic, representing almost ten percent of the forest. When HWA becomes established in the dense hemlock forests of the southern Adirondacks, it will be unstoppable.

Although the experience from the Appalachians is bleak, recent developments are demonstrating that HWA can be managed. There is still hope to mitigate the impacts of HWA in New York.

Once invasive forest pests like these become established in new locations, they leave their natural enemies behind. Also, new host trees have no resistance. Long-term management of HWA will depend on both of these factors.

Resistance breeding is a very long and involved process and should be initiated as soon as possible, but is best addressed with resources and expertise at the national level. Establishment of biological control, on the other hand, although laborious and requiring years for predator populations to build, is more rapid and feasible at the state level. Biocontrol should therefore be the focus of our immediate efforts at long-term HWA management.

As we work on biological controls, we will need to protect the aesthetic and genetic resources of hemlocks in areas where HWA is already established, such as state parks and ecologically important forests. Systemic insecticides have been used for years in national parks and other priority areas farther south and have been demonstrated to be effective, inexpensive, and environmentally benign.

Biological Control

No predators native to the East Coast have been found to feed on HWA, so it is necessary to implement what is referred to as classical biological control. This technique involves investigating the biocontrol agents, or predators, where HWA is native and under control, such as in the Pacific Northwest and Eastern Asia. A classical biocontrol program sponsored by the USDA Forest Service has been underway since 1993 and a number of predators have been evaluated. The most promising has been a small beetle from the Pacific Northwest, <itals Laricobius nigrinus>. New York State has seen 17 releases since 2009, but with limited quantities available, the number in each release has ranged from 200 to 600, far from ideal. In order to implement an effective HWA biocontrol program in New York, we need to have a constant supply of large quantities of predators.

There are two methods of obtaining HWA predators: wild collections in locations where predators are established, and rearing predators in a laboratory. While wild collections are generally less expensive, populations fluctuate from year to year and quantities can be limited. More effective is laboratory rearing. Labs in the southern Appalachians have been developing effective rearing techniques since 2000 and have produced thousands of <ital Laricobius> for release.

We need this kind of predator production in New York to effectively implement an HWA biocontrol program. However, it is important to consider that predator populations take years to build, spread, and become efficacious. In areas of the state where HWA is already well established, we face the possibility of substantial mortality before predators can become effective. We need to consider other options in order to conserve our hemlock resources.

Systemic insecticides.

The efficacy of systemic insecticides to control HWA is well established. These have been widely used by state and federal agencies as well as individuals to keep hemlocks alive. Their use is facilitated by low cost, ease of application, and efficacy that lasts for up to seven years. Systemic insecticides have been used to protect scenic resources in parks, valuable genetic resources, and hydrologic function of watersheds, and to mitigate the economic impact of having to remove large numbers of dead trees near human infrastructure. Great Smoky Mountains National Park personnel have been using them to try to preserve a small portion of the hemlocks across that landscape. Researchers found minimal concentrations of insecticides in stream water, well below limits allowed by the U.S. EPA Aquatic Benchmark.

One of the most important tactics to forestall HWA population growth at the leading edge involves early detection of spot infestations and treatment of those spots, as well as a generous buffer of surrounding trees. This tactic may be an important tool in areas where HWA is not already generally present, such as the Adirondacks. We can use this tactic to keep populations in check and slow the spread, allowing valuable time for the biological control program to take hold.

HWA is already well established in parts of New York State. We need to move rapidly to implement biological controls for the long term and use systemic insecticides to protect valuable priority hemlock resources in infested areas.


Mark Whitmore is a forest entomologist in the Department of Natural Resources at Cornell University. For the last 10 years he has studied invasive non-native insect species that are changing the face of North America’s forested landscape.