Hemlock Woolly Adelgid

The Hemlock Woolly Adelgid (HWA) is a tiny, piercing and sucking insect, unseen with the naked eye, that feeds on hemlock twigs at the bases of the needles.

HWA is an invasive species from Asia which has infested the US East Coast hemlock forest from New Hampshire to Georgia, inhibiting twig growth throughout.  It has also recently been found in Michigan and it is believed to be expanding its range due to changes in climate.

HWA has a complex life cycle; its winter generation and spring generation overlap in the spring.  It feeds and reproduces during the colder months, going dormant in the summer.

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  • Common Symptoms
    • The earliest visible sign of HWA is the presence of white, cottony masses, usually located on the twigs and at the bases of the needles. Populations tend to be denser in the lower limbs, but can be anywhere on the tree. Symptoms will progress to fading, thinning and dying limbs, which die off beginning at the base of the tree and moving upwards. Left untreated, the death of the tree is certain.

    • Arborjet recommends a trunk injection of IMA-jet (active ingredient, imidacloprid) insecticide using the TREE I.V. system or using the QUIK-jet or QUIK-jet Air micro-injector.

      The TREE I.V. is designed to work effectively with the hemlock’s primitive tracheid vascular system; it injects high volumes of product under low pressure, resulting in efficient uptake. The QUIK-jet and QUIK-jet Air works best with low volumes of applications in hemlock, and takes only minutes to apply.

      To give the tree a greater health benefit, a follow up application of NutriRoot™ or MN-jet Fe is recommended, the specific formulation dependent upon soil type. Each product is formulated to supply essential nutrients to support foliar development without added nitrogen, which could exacerbate HWA infestations.

    When To Treat
    • Generally, the best seasons for injection are fall and spring, when trees are transpiring. The environmental conditions that favor uptake are adequate soil moisture and relatively high humidity. Soil temperature should be above 40 degrees F for trunk injection.

      In Hemlock, Fall treatments coincide with HWA resumption of feeding. Applications of IMA-jet may be applied in hemlock from September through December, as long as soil temperatures are above 40 degrees F. The second window for application is in the spring months, from March through June.

    What To Expect After Treatment
    • Adelgid mortality occurs after ingestion, generally within 14-28 days, and continues for up to 2 years. Cottony masses remain for some time, but will turn a distinctive gray color. Hemlocks respond positively to treatment with a resumption of twig growth.

    References And Photo Credits
    • Main photo taken by John A. Weidhass, Virginia Polytechnic Institute and State University, Bugwood.org