Hemlock Woolly Adelgid

The Hemlock woolly adelgid (HWA), Adelges tsugae Annand, is an introduced pest that is deadly to forest and ornamental hemlock trees in the eastern United States. Its presence now encompasses most of the eastern states from Georgia to Maine and as far west as Kentucky and Michigan.

Treatment Strategy

The goal of treatment for hemlock woolly adelgid is to stop feeding activity before the insect populations reach a point where significant damage is sustained. Treatment can be done at any time of the year as long as the soil is not frozen or saturated. Soil moisture will impact the speed at which systemic products are absorbed by hemlock roots and the speed at which they translocate upward in the tree. Uptake and translocation of Xytect from soil applications can take up to 90 days depending on soil moisture. Residual control of Xytect can last for multiple seasons depending on tree size, severity of infestation and the dosage rate applied. Monitor adelgid populations to determine if re-treatment is warranted. Transtect soil applications and bark sprays are typically applied annually each season.

Hemlocks that are showing dieback of more than 1/3rdof the tree are NOT good candidates for treatment. However trees with less than 1/3rddieback of their canopy can often be saved. The dead branches will not grow back and can either be pruned or will eventually fall out. Hemlocks with a large population of adelgids are best treated with Transtect or Transtect Infusible. In some cases it may be best to combine either of these products with Xytect. Transtect treatments result in extremely high levels of active ingredient translocating into hemlocks making it an excellent HWA treatment.

One strategy that is employed is to apply the fast moving highly mobile insecticide, Transtect to control the initial infestation and apply Xytect for long term control. Transtect Infusible or Xytect 10% as a tree injection treatment for trees that cannot be treated with soil applications or systemic bark sprays. Often times these are for hemlocks growing in riparian areas or very close to water areas. Transtect and Transtect Infusible will also be effective on elongate hemlock scale (EHS). Other systemic treatments are NOT effective for this. Applications for HWA can also control EHS if applied at the correct time of year.

Managing Side Effects on Mites

Field reports as well as research suggests that long term use of imidaclopidin conifers reduces the population of predatory mites. This can often create significant and damaging spider mite flare ups. For this reason, if multiple years of applications are necessary adding a miticide such as Aracinate or Lepitect just as spider mite populations begin to build is recommended. Re-apply a miticide if mite activity continues to occur. Heavy populations may require two applications annually to keep spider mite populations under control. Research trials from Purdue University have demonstrated that while Transtect is similar mode of action to Xytect, Transtect does not result in the same mite flare-up issues when applied to hemlocks.

Other Treatment Practices

  • Promote health and vigor with proper irrigation, mulching, proper pruning, root collar excavation and prescription based fertilization practices.
    • Adequate water is a key factor in maintaining stress free plants. A slow, deep watering event once every few weeks during dry conditions will help maintain soil moisture levels and minimize stress.
    • Mulch is very beneficial for all trees because it reduces competition with turf and moderates soil temperature and moisture levels. The addition of 3 inches of wood chips or shredded bark under the drip line can have a very beneficial effect by holding in moisture and promoting healthy fibrous roots.
    • Cambistat can be also be used to promote tree health and reduce the incidence and severity of stress mediated diseases and insect problems on trees growing in urban areas.
  • Manage other pests (insect and diseases), especially mites and Elongate hemlock scale.
  • Horticultural oil spray applications can be applied to assist with control, however, stand-alone treatments are nonsufficient for control alone.

Soil Injection/Drench or Systematic Basal Bark Spray Using Transtect

Tree Injection Using Transtect Infusible

Soil Drench/Injection Using Xytect 2F

Soil Drench/Injection Using Xytect 75WSP

Tree Injection Using Xytect 10%

Soil Injection Using Lepitect

Foliar Spray Using RTSA Horticultural Oil

Expected Results

Transtect and Transtect infusible should be used for infested trees and for rapid quick knock down. These products also accumulate at much higher titer levels than Xytect, making Transtect formulations superior for the initial knock down treatment for HWA. Transtect and Transtect Infusible will also be effective on elongate hemlock scale (EHS). Other systemic treatments are NOT effective for this. Applications for HWA can also control EHS if applied at the correct time of year.

Xytect 2F and XytectWSP: up to 85 % mortality of HWA 9-12 months after applicationo100 % mortality of HWA 24 months after application.

Xytect 10%: may not provide as quick of knockdown of HWA populations as Transtect Infusible if tree injection is the options of choice. Treatments require time to suppress HWA populations (at least one year).

Signs of Damage

  • Symptoms include needle yellowing, needle drop, followed by branch drying of the branches and a thinning crown
  • Limb dieback will occur within two years of infestation on younger trees
  • Decline and death of tree can occur over a period of 4-10 years in Northern Range of HWA and 3-6 years in southern range of HWA
  • Other factors can accelerate the rate and severity of hemlock mortality
    • Drought, construction injury
    • elongate hemlock scale (Fiorinia externa), spruce spider mite
    • root rot disease (Armillaria mellea), and needle rust (Melampsora parlowii)
  • High populations of HWA will decrease the production of buds and new growth the following year. As new growth is reduce the adelgids begin feeding on older growth will contributes to decline and mortality
  • As of 2005, HWA found in 16 states (Maine to Georgia)•HWA is less than 1/16-inch
    • Color varies from dark reddish-brown to purplish-black in color
  • HWA has only female stages and has six stages of development: the egg, four nymphal instars, and the adult
  • Two generations a year on hemlock
    • The winter generation, the sistens, develops from early summer to mid-spring of the following year (June–March).
    • The spring generation, the progrediens, develops from spring to early summer (March–June).
    • Disseminated by wind, birds and mammals and by long distances via nursery stock
  • HWA enters a period of dormancy during the summer months (aestivation period)
  • During cooler periods (October) HWA starts feeding again during cooler temperatures (October) and continue throughout the winter months as it turns into an adult
    • December to March Adults can lay up to 300 eggs each
  • Reddish brown crawlers hatch and first instar nymphs attach their piercing sucking mouth parts to new growth at the base of the needles begin feeding on stored starch
  • Eventually they begin secreting white fluffy wool (ovisac)
    • Ovisacs can be readily observed from late fall to early summer on the underside of the outermost branch tips of hemlock trees
    • Cottony wool can be 3mm long

Photo: Michael Montgomery
USDA Forest Service, Bugwood.org

Photo: John A. Weidhass
Virginia Polytechnic Institute and State University, Bugwood.org

Trees At Risk

HWA develops and reproduces on all hemlock, but only causes damage to Eastern hemlock and Carolina hemlock.


  • All HWA are female and have six stages of development
  • There are two generations a year, but HWA enters a period of dormancy during the summer months.
  • During cooler periods (October) HWA starts feeding again and continues throughout the winter months as it turns into an adult, laying up to 300 eggs from December to March.
  • Reddish brown crawlers hatch and attach their piercing sucking mouth parts to new growth at the base of the needles and begin feeding on stored starch in the tree.
  • Eventually ,the crawlers begin secreting white fluffy wool which can be seen on the underside of the outermost branch tips of hemlock trees.


USDA Forest Service - Southern Research Station

Cornell University Cooperative Extension

University of Massachusetts Amherst

Always refer to product label for rates and approved uses. Some images courtesy of forestryimages.org or Wikimedia Commons. Use of the images does not imply endorsement of treatments.

A Treatment Guide is designed to help you identify common issues and management solutions. Comprehensive Treatment Guide PDFs, which include current products, application rates, and additional information, are available upon request.