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Hemlock Woolly Adelgid Diagnostic Guide

Adelges tsugae

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 StatesIts presence now encompasses most of the eastern states from Georgia to Maine and as far west as Kentucky.

Trees at Risk

Hemlock woolly adelgid (HWA)feeds on hemlock and spruce trees and shrubs. It is only a destructive pest on eastern hemlock and Carolina hemlock.


Signs of Damage

  • Symptoms include needle yellowing and needle drop, followed by drying of the branches and a thinning crown.
  • Limb dieback will occur within two years of infestation on younger trees.
  • Decline and death of a tree can occur over a period of 4-10 years in the northern range of HWA and 3-6 years in the southern range of HWA.

USDA Forest Service – Region 8 – Southern Archive, USDA Forest Service,

Physical Appearance

  • HWA is less than 1/16 inch long.
  • Color varies from dark reddish-brown to purplish-black in color.
  • All HWA are female.
  • Reddish-brown crawlers hatch and the first instar nymphs attach their piercing, sucking mouth parts to new growth at the base of the needles and begin feeding on stored starch.
  • HWA can be identified by 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.
Hemlock Woolly Adelgid signs

Michael Montgomery, USDA Forest Service,

Hemlock Woolly Adelgid signs

John A. Weidhass, Virginia Polytechnic Institute and State University,

Treatment Strategy

The treatment strategy for HWA is determined in part by the severity of the infestation.

Newly infested or Preventative Treatment of Hemlocks

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. Applications of Xytect™ to the soil will provide up to two years of control of HWA. Treatment can be done at any time of the year as long as the soil is not frozen or saturated.

Therapeutic Treatment of Hemlocks With Heavy Infestations

Hemlocks that are showing dieback of more than one third of the tree are not good candidates for treatment. However, trees with less than one third dieback 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 a combination of two products. Begin by applying a fast-moving, highly mobile insecticide like Transtect™, followed up by applying a long-lasting (up to 2 years) product that will protect the tree from reinfestation, such as  Xytect™. Xytect™ can take up to 30-60 days to move into the tree, so it is best supplemented with a fast-acting product like Transtect™.

  • Soil Application using Xytect 2F

    Dosage: Trees less than 15 in DBH: 3 mL/in DBH Trees 15 - 30 in DBH: 4.5 mL/in DBH Trees > 30 in DBH: 6 mL/in DBH
    Timing: Spring or fall
    Re-Treatment: Monitor for pest, re-treat when suppression is no longer noted.
    Xytect 2F
    Xytect 2F
  • Soil Application using Transtect

    Dosage: 6 pkt/gal water (1.5 - 2.0 oz/in DBH) for trees < 24 in DBH
    Timing: Late April/ Early May.
    Re-Treatment: Annually
  • Systemic Bark Spray using Transtect

    Dosage: 6 - 12 pkt water (1.5 - 2.0 oz/in DBH) for trees > 24 in DBH
    Timing: Late April/ Early May.
    Re-Treatment: Annually
  • Tree Injection using Xytect 10%

    Dosage: 1 - 6 mL/inch DBH
    Timing: Late Sept/Early Oct or Late April/Early
    Re-Treatment: Monitor for pest, re-treat when suppression is no longer noted.
    Xytect 10%
    Xytect 10%

Treatment Expectations


  • Up to 85% mortality of HWA 9-12 months after application.
  • 100% mortality of HWA 24 months after application.
  • May cause mite outbreaks.


  • Transtect™ rapidly controls HWA and has shown superior performance on infested hemlock trees.
  • Mite outbreaks have not been associated with Transtect™ treatments.

A Diagnostic Guide is designed to help you identify a pest issue and management solutions. Always refer to product label for all rates and approved uses. Some images courtesy Use of the images does not imply endorsement of treatments by

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