Test Your Knowledge - November

An infestation of hemlock woolly adelgid
An infestation of hemlock woolly adelgid
Chris Evans, River to River CWMA, Bugwood.org

Yes, this is an infestation of hemlock woolly adelgid!

The hemlock woolly adelgid (HWA) was first reported in the eastern US in the mid-1950’s. Since that time it has spread northeastward to Maine and southwest to Georgia. It is a threat to eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana) trees.

Map of states infested with HWA
Hemlock woolly adelgid infestations in 2007

The hemlock woolly adelgid (HWA) is a small (1/32 inch), reddish purple, oval shaped, aphid-like insect. This insect has six stages of development (egg, four nymphal instars, and adult). During spring and early summer months the immature (nymph) crawler stage is easily dispersed by wind, birds, wildlife, and people. Once the crawler locates a feeding site it inserts its piercing-sucking mouth parts at the base of a hemlock needle and enters a sleep period during the summer months. In fall they “wake up” and begin to feed and secrete a fluffy “wool” to protect its body. By the following spring, infested trees are more obvious with the bright, white cotton masses visible under the needles. There are two generations of HWA per year, eggs are laid in the fall (November/ December) and spring (April/ May).

Eggs of hemlock woolly adelgid
Hemlock woolly adelgid eggs
Pennsylvania Department of Conservation and Natural Resources - Forestry Archive, Bugwood.org

Hemlock woolly adelgid crawlers
Hemlock woolly adelgid crawlers
Pennsylvania Department of Conservation and Natural Resources - Forestry Archive, Bugwood.org

Hemlock woolly adelgid adult
Hemlock woolly adelgid adult
Michael Montgomery, USDA Forest Service, Bugwood.org

The HWA adult and nymph feeding causes needles to turn yellow and fall from the tree. Over a two year period the tree will start to have branch die-back and crown thinning. Mature trees can die in 4 to 10 years depending on the infestation level, tree size, and environmental stresses.

Hemlock trees in decline from HWA infestation
Hemlock trees in decline from HWA infestation
William M. Ciesla, Forest Health Management International, Bugwood.org

Prevention and Control

Avoid bringing HWA to your landscape by inspecting trees before purchase and regularly checking the hemlock trees in your landscape for signs of infestation. Inspections may be difficult for homeowners from late June through September because the nymphs have no wool covering to make them stand out. Prevent the dispersal of HWA crawlers by removing birdfeeders from the vicinity of hemlock trees. Even before a hemlock woolly adelgid infestation is detected, it is best to minimize the stress level of the tree. This can be done by applying a 3-inch layer of organic mulch under the canopy of the tree to retain moisture and protect roots from extreme heat and cold. Also, avoid excessive levels of nitrogen fertilizers to minimize succulent new growth that nymphs prefer. Under drought conditions provide supplemental irrigation (1 inch of water per week) when possible.

Control of the HWA can be challenging due to the woolly covering protecting the insects and the height of the tree. If hemlock woolly adelgid populations are isolated to a small section of a tree, then infested branches can be removed to reduce further spread of the insect. Horticultural oil (not dormant oil) and insecticidal soap are low toxicity products that can be applied as foliar treatments with high pressure sprayers. These products provide little risk to the applicator, children or pets, and are best applied September through October or April to May when crawlers are present. Horticultural oil can be applied at a rate of 1% (2 1/2 tablespoons per gallon of water) in May and September and a 2% rate (5 tablespoons per gallon of water) in October and April. To prevent damage to leaves, avoid spraying horticultural oils when new leaves are emerging and when temperatures reach above 90°F or below 45°F. Avoid drift to streams and ponds. To reduce separation of horticultural oil and water in the sprayer, shake the sprayer periodically during application. NOTE: Horticultural oil provides no residual control, spraying trees before there is an infestation will not prevent hemlock woolly adelgids.

Due to the height of the tree a soil drench treatment of imidacloprid, a systemic insecticide, may be the most practical. This active ingredient is available to homeowners in products such as: Bayer Advanced Garden Tree and Shrub Insect Control® and Ferti-lome Tree and Shrub Systemic Drench. For best results, water the hemlocks one to two days prior to treatment. Rake back leaf litter or mulch one to two feet from the trunk before application. Do not apply imidacloprid near streams and ponds. As with all pesticides, read and follow all label instructions and precautions. If treatment is not practical for the homeowner to perform, then an International Society of Arboriculture Certified Arborist should be contacted. See ISA website for certified arborists in your area: http://www.isa-arbor.com/findarborist/findarborist.aspx

Biological Control

Biological controls are being examined for forested areas since chemical applications cannot be done practically. Research is being done with several predatory beetles. Of these, Sasajiscymnus tsugae appears to be most promising although they will not work alone. Sasajiscymnus tsugae are highly mobile and feed on all stages of HWA. Three other predators are being released and several others are being studied in lab or field settings.

For more information see:
Hemlock Woolly Adelgid
Introduced Biological Control Agents for Hemlock Woolly Adelgid (HWA)

Millie W. Davenport
HGIC Extension Agent

Page maintained by: Home & Garden Information Center

This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.