Using iMapInvasives to Help Control Hemlock Woolly Adelgid

Eastern hemlock (Tsuga canadensis) is Pennsylvania's state tree and is often found growing in old growth forests in the Commonwealth. Hemlock trees provide crucial ecosystem services and essential habitat for a variety of native birds, mammals, and insects. Hemlock canopies host an incredible amount of insect and arachnid diversity and uniquely influence soil and stream chemistry, reduce soil erosion, and help to regulate stream temperatures. The beauty, majesty, and silence of hemlock forests is considered unparalleled by many.

Unfortunately, this beautiful and vital tree is threatened by an invasive insect: hemlock woolly adelgid (HWA; Adelges tsugae). It is an aphid-like insect from Japan that was accidentally introduced to the eastern United States in the 1950s and was found in Pennsylvania in 1967. Hemlock woolly adelgid can be identified by the unique woolly ovisacs at the base of needles on the underside of hemlock branches, hence the name "woolly. This unique structure can be confused for a number of things including spiderwebs, spittlebugs, dried tree sap, and bird droppings. Hundreds of adelgids feeding on a tree’s fluids slowly drains away sugar and nutrients. Tree decline can occur over the course of many years, and as the tree declines, the canopy thins. Tree death can happen quickly if other stress factors are present, or the tree is already in poor health. Healthy trees on higher quality sites can persist with HWA for over 20 years. Hemlock woolly adelgid does not fly tree-to-tree in the United States; instead, it gets around by hitching a ride on people, animals such as deer and birds, or wind currents.

HWA can be controlled through the application of insecticides; the two commonly used active ingredients are imidacloprid and dinotefuran. Dinotefuran is sprayed on the trunk of a tree or applied as a soil drench, which rapidly takes it up, spreads it throughout the tree, and killing any adelgids. While, dineotefuran provides rapid protection, it only protects a tree for a year. Imidacloprid is applied to individual trees as injections into soil, injections into the tree trunk, soil drenches, tablets, or basal bark spray. The tree takes up the chemical in solution, which is then moved throughout the tree, so when an adelgid attacks the tree, it consumes the pesticide(s) and dies. With one treatment, HWA can be controlled for 3-7 years depending on the active ingredient, formulation used, and annual weather.

The impacts of hemlock woolly adelgid are prominent in southern Pennsylvania, where hemlocks grow on lower quality sites and winters are milder. Warmer winters allow HWA populations to exponentially increase, especially in areas where hemlocks are still healthy.

Figure 1. The current distribution of hemlock woolly adelgid in Pennsylvania (Image credit: DCNR, accessed August 27, 2021)

The hemlock woolly adelgid is found on hemlocks across Pennsylvania including state forests and parks. When found in these places, the Pennsylvania Department of Conservation and Natural Resources (PA DCNR) Bureau of Forestry attempts to manage it. To protect a forest by controlling this insect, foresters and other DCNR staff map outbreaks and then implement Integrated Pest Management (IPM) plans at priority sites. This is an enormous task because of the scale of the forests in Pennsylvania; currently 60% of the state is forested. The Bureau of Forestry takes this unique, yet aggressive threat very seriously by annually treating 100,000 inches of tree diameter (an estimated 8,000-10,000 individual trees) across 250+ sites statewide. 

To better understand the threat that hemlock woolly adelgid poses, the DCNR needs help from the public. Citizen/community scientists can take part in saving hemlocks by mapping outbreaks. This helps scientists and managers know where outbreaks are occurring, are likely to occur, and what the trends in movement are. To take part, sign up for a free iMapInvasives account and download the iMapInvasives mobile app to a smartphone or tablet. In the app, preferences can be set and users can start capturing observation data by taking pictures of any HWA outbreaks they see on hemlocks in Pennsylvania. These photos are then verified by experts to ensure accuracy. The data from citizen/community-scientists is used to make better maps, improve management, and conduct research. For more information on this collaboration, see the article “DCNR’s Hemlock Woolly Adelgid Management Program".