Hemlock Woolly Adelgid Diagnostic Guide
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.
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.
- 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.
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™.
- 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 forestryimages.org. Use of the images does not imply endorsement of treatments by forestryimages.org