All About HWA
What is HWA?
Hemlock woolly adelgid (HWA) (Adelges tsugae) is a small, aphid-like insect that attacks and eventually kills Eastern hemlock (Tsugae canadensis) trees. There are at least ten distinct species of hemlocks around the world, some of which coexist with HWA.
HWA is native to Asia and the Northwest USA and there are several genetic lineages of HWA around the world. According to the NYS Department of Environmental Conservation website, "HWA was introduced to the western United States in the 1920s. It was first observed in the eastern US in 1951 near Richmond, Virginia after an accidental introduction from Japan. HWA has since spread along the East Coast from Georgia to Maine and now occupies nearly half the eastern range of native hemlocks." HWA first arrived in the Adirondack Park in 2017. HWA is now well established in the Lake George watershed.
HWA Identification
HWA is a very small, approximately 1.5 mm in length, insect. Its body is brown to black in color and in the fall/winter months is covered with a white waxy wool substance, giving the insect its namesake. HWA has a unique lifecycle, making it easier to find and identify at certain times of the year.
Spring/Summer
In the spring and summer, HWA enters a period of rest known as aestivation. At this time, insects are most difficult to detect with the naked eye but can still be found by a trained observer. Look for small black spots surrounded by a faint halo of white wool. Insects will be settled at the ends of the twig (Photo Credit: NYS Hemlock Initiative).
Fall/Winter
HWA ramps up production of its white wool in the fall and winter months
and will be most visible from November to March. This is a great time
to survey for the insect! Look for small spots of white wool one the
underside of the twig.
What are HWA's Impacts?
HWA feeds by inserting its mouthpart into a hemlock twig near the base of a needle. The tree responds by walling off the wound, essentially creating a small amount of scar tissue. The collective impact of the small scars resulting from a heavy infestation will reduce the tree's ability to transport nutrients, which in turn prevents needle production and eventually leads to death of the tree. The time from infestation to tree mortality varies significantly based on several factors including the tree's health prior to infestation, growing conditions, annual minimum winter temperatures, and the number of adelgids present.
In some areas, trees will survive with some infestation of HWA for 20+ years, while other areas have seen significant mortality in less than 5 years. The loss of hemlock trees presents a myriad of negative ecological, economic, and societal impacts. As a foundation species, eastern hemlock plays a critical role in forest ecosystems. It provides food and habitat for many species of wildlife; moderates stream flows and provides shade to keep waters cool; and grows on steep slopes where it helps to stabilize soil. Hemlock is one of the most common trees found in Adirondack forests and forms the backdrop of many stunning scenic vistas. It helps fuel our local timber and tourism economies. For more information about HWA and the role hemlocks play in our forests, see the NYS Hemlock Initiative website.
HWA Prevention
- Clean your boots, clothing, and gear before and after recreating outdoors. This is particularly important if you have visited an infested area when HWA is in its "mobile" (crawler) stage in the spring, generally April to June. This is the only time of year that HWA are mobile. In addition, do not move infested branches or twigs.
- Use local nursery stock from a reputable supplier for gardening and landscaping projects. Inspect the plants carefully for any signs of wool before introducing them to your property.
- Avoid hanging birdfeeders in or near hemlock trees. Birds can transport HWA crawlers on their feet during the spring, potentially over long distances.
Survey and Reporting
HWA is visible all year, but certain times of the year are better for surveying. From mid-fall to early-spring, HWA is covered with its characteristic white, woolly coating. In the summer, HWA is resting (aestivating) and will not be covered with a thick coating of wool. Instead, it will appear as a small black spot surrounded by a faint halo of white wool. Take caution or avoid surveying for HWA in the mid-spring to early-summer (April to June) when HWA crawlers are active. At this time of year, HWA can be inadvertently moved from infested stands.
You can help survey and report HWA as an APIPP Forest Pest Hunter. Learn how to get started here.
How to Report HWA
If you think you have found HWA, take a picture and report your findings. You can submit your observation to iMapinvasives using the mobile app or website. Photos can also be submitted directly to APIPP.
HWA Management
Once present in a forest, HWA is difficult or impossible to eradicate. However, management can effectively reduce HWA populations and maintain the health of hemlock trees. A combination of chemical, biological, or silvicultural practices can be applied.
Chemical Control
Pesticide Products
There are two pesticide active ingredients commonly used in NY to control HWA: dinotefuran and imidacloprid. Both are systemic insecticides meaning they are absorbed and work within the tree. However, that have different chemistry and may be used alone or in combination to achieve various management objectives. Key differences between active ingredients are provided below.
- Dinotefuran: A fast-acting pesticide that will control adelgid within the same season of application, but it does not provide long term protection. It is used when quick adelgid control is required. For example, when a tree is in poor health and a reduction of pest load is required for the tree to survive. Dinotefuran is also effective against a secondary invasive hemlock pest, the elongate hemlock scale (EHS) (Fiorinia externa). Using a tank mix of dinotefuran and imidacloprid will control both EHS and HWA in a single application. It is marketed under the trade name Safari 20 SG and is only available to commercial pesticide applicators in NY.
- Imidacloprid: A slower acting, but long lived product. It can take a full year after application to take effect, but once present in a tree imidacloprid can provide protection from HWA for 5-7 years. It is marketed under many trade names and formulations. Imidacloprid is the only pesticide available to homeowners (non-certified applicators) in NY. Using the lowest labeled rate, only 256 diameter inches of trees can be treated per acre per year.
Application Methods
Applications methods vary by site, product, and applicator certification status. Techniques used in NY include:
Basal Bark Application
Basal bark is the most common application technique used by conservation professionals in NY. It involves the application of a diluted solution of pesticide to the lower ~4-5 feet of a tree’s trunk using a backpack sprayer. Pesticide is absorbed through the tree bark, translocated throughout the tree’s vascular system, killing any insects feeding on the tree. Basal bark can be used to apply imidacloprid, dinotefuran, or a mix of both products. This technique is only available to certified pesticide applicators.
Tree Injection
Direct tree injection or stem injection uses a specialized tool to deliver a small dose of concentrated pesticide directly into the trunk of a tree. Small holes are drilled around the circumference of the tree near its base and specialized pesticide is injected into each hole under pressure. Some tools require holes to be plugged with a plastic cap, while others use a “plugless” design. The injection method is costly and time consuming, and is best used for trees close to shorelines or other areas where basal bark or soil application is not appropriate. This technique is only available to certified pesticide applicators.
Soil Application
Soil application or soil drench involves the application of a pesticide solution to the soil surface around the base of a tree. The product leaches into the soil and is absorbed by the tree’s roots. Soil application is the only application technique available to non-certified pesticide applicators (i.e. homeowners). The equipment required to perform a soil application is minimal and pesticide products are available at many home improvement stores. Note that soil applications can leach more rapidly where soils are thin or sandy and take caution in these areas. In addition, extra caution should be exercised if water or wetlands are present near your application site. In these areas you may wish to hire a certified pesticide applicator who can inject the trees. Always read the pesticide label before use and abide by all recommendations. A list of soil application instructions and supplies is available from the University of Georgia (Photo Credit: Hemlock Restoration Initiative).
Biological Control
Biological control (or biocontrol) is the use of natural enemies to control a pest population. Biocontrols undergo extensive laboratory testing and field evaluation before being released into the environment to ensure they selectively control only the target pest. The long-term goal of biocontrol is to establish self-sustaining populations that provide landscape level suppression of the target invasive species.
Biocontrol research in NY is led by the NYS Hemlock Initiative at Cornell University. Visit their website for in-depth information about ongoing biocontrol research. Multiple biocontrols are currently being researched and developed for HWA. They are not yet available for commercial distribution, but experimental releases are underway.
Laricobius nigrinus
Laricobius nigrinus (Laricobius) is a 2-3mm long, black beetle from HWA’s native range in the Pacific Northwest of the United States. Laricobius beetles feed on nymph and adult HWA in the fall and winter months. The first releases of Laricobius in the eastern United States occurred around 2003 and they have been released in NY by the NYS Hemlock Initiative since 2008 (Photo Credit: NYS Hemlock Initiative).
Learn More
Leucotaraxis spp.
Leucotaraxis spp. or silver flies are 1-3mm long, silver flies from HWA’s native range in the Pacific Northwest of the United States. Two species of silver flies are being investigated and released in New York Leucotaraxis argenticollis & L. piniperda. Leucotaraxis feed on the eggs of HWA during the spring season. Leucotaraxis have been released in NY by the NYS Hemlock Initiative since 2015 (Photo Credit: Katja Schulz/Flickr).
Learn MoreSilvicultural Options
Silviculture (i.e. forest management) will not directly control HWA but can be used to promote overall stand health and resilience. Thinning a stand can promote hemlock growth and increase a trees tolerance to HWA and other stressors. However, once invaded, even healthy trees will eventually succumb without additional intervention such as chemical control.
Researchers from Harvard Forest and UMass Extension have identified several management options for hemlock-dominated and mixed stands. For more information, visit Silvicultural Options for Managing Hemlock Forests Threatened by Hemlock Woolly Adelgid. Preemptive harvest of hemlocks prior to the arrival of HWA is not recommended, as wholesale removal of trees may result in the loss of potentially resistant individuals. If you do harvest hemlock in an area infested with HWA, try to avoid harvesting from April to June as it is possible to inadvertently spread HWA on equipment and during log transport at this time of year.