Integrated Approach to Hemlock Woolly Adelgid Mitigation

Hemlock woolly adelgid (HWA, Adelges tsugae) is an aphidlike, soft-bodied insect, almost too small to see with the naked eye. Native to Asia, HWA was first reported in the eastern United States near Richmond, Virginia, in the 1950s. It has since spread to 20 states and two Canadian provinces. Wind, birds, deer, and humans all contribute to HWA dispersal.

HWA feed on tree sap by inserting their mouthparts at the base of the needles and removing plant fluids. Their feeding causes the needles to turn gray, die, and drop from the tree. Branch dieback progresses from the lower branches to the top of the tree. A tree’s tolerance to the insect’s attack correlates to other stress factors, such as heat and drought. Moderate HWA populations may reduce tree health. Severe infestations may bring about premature needle drop, reduced twig growth, dieback, and tree death.

The most obvious sign of an HWA infestation is masses of woolly white filaments on the underside of twigs at the base of the needles. The masses are produced by the females (there are no males) and are about 2 millimeters long. These woolly masses normally persist throughout the season and into the following year, even after the insects are dead. The white, woolly mass conceals overwintering females and their eggs.

The first generation of overwintering adults lay eggs in large clusters in the woolly masses during warm weather in late winter and early spring. Eggs start to hatch in early April, depending on spring temperatures. A second generation lays eggs in June, hatching in July. The newly hatched nymphs, or “crawlers,” are only mobile for about a week while they look for a place to settle on a hemlock branch before entering a period of dormancy. By mid-September, dormancy is broken and they begin to feed again. First-instar nymphs begin producing wool in late September and spend the winter feeding on trees as mature females.

Under the right conditions, HWA can kill eastern hemlock trees in as few as four years. Cold, hard winters lower HWA survival rates. Extreme cold temperatures, below -15 degrees Fahrenheit, cause widespread HWA mortality. Rising temperatures, due to climate change, will likely allow HWA to expand northward throughout the hemlock’s range and pose a threat by reducing suitable habitat for hemlocks.

Integrated Pest Management Strategies

There are three main tools used to mitigate HWA and its impacts. These include insecticide treatments, biological control agents such as predatory insects, and cultural practices such as breeding hemlock for resistance and regenerating other desired tree species where hemlocks have died. While HWA has caused significant hemlock mortality across much of the tree’s range, many trees are surviving and in need of treatment.

Although HWA is difficult to treat and there are challenges to protecting hemlock stands not yet affected, conservation of this species is still possible. Insecticides are effective in settings from urban landscapes to managed forests. Strategies that focus on chemical control for the short term and biological and cultural control (natural enemies and host resistance) for the long term likely have the best chance for success.

Evaluate Trees

Not all hemlocks can be saved. It is too expensive to do so, as insecticide treatments are necessary every few years. Treat hemlocks that are the healthiest, most vital to the ecology of the landscape or forest, and easiest to reach and treat. Consider removing nearby untreated, symptomatic, declining hemlock trees to eliminate HWA sources that may reinfest treated trees. Do not remove healthy hemlock trees not designated for treatment as these trees may be genetically resistant superior trees capable of surviving HWA.

If trees do not have HWA, they do not require treatment. Insecticide applications are only necessary once a light infestation is present or if adjacent trees are infested. Retreat only if evidence of HWA reappears. Treating infested trees aggressively, while they are in good health, is the best way to maintain them. Severely defoliated trees are unlikely to recover, even with treatment.

Try to minimize tree stress. If possible and practical, water trees during periods of drought, properly mulch around trees, and prevent compacting soil around roots with heavy equipment. Do not fertilize infested trees. This only aids the survival and reproduction of HWA. Prevention alone does not always work. Infested trees will usually die in several years unless an insecticide treatment is applied before the tree declines.

Insecticide Treatments

Insecticide application methods include foliar broadcast sprays, soil and stem injections, soil drenches, soil tablets, and basal trunk sprays (Table 1). They are applied on an individual-tree basis, costly, and labor-intensive. Foliar sprays, soil drenches, trunk sprays, and soil tablets are described here, as these treatments do not require complex, expensive equipment.

Although chemical controls have proved effective in controlling HWA in yards, parks, and forests, the cost and effort associated with insecticide treatments make them impractical for treating large, forested areas, although it can be done. In some state and national parks and forests, hundreds—even thousands—of trees have received treatment.

Until long-term solutions involving biological control agents and/or host resistance are available, insecticide treatments are recommended. Hemlocks should be treated with either horticultural oil, insecticidal soap, imidacloprid, or dinotefuran. The specific application method used depends on several factors, including tree vigor and size, soils, accessibility, and proximity to water.

Horticultural Oil or Insecticidal Soap

Oils and soaps are the safest insecticides for controlling HWA. They are not toxic but kill the insect by smothering it, dissolving its exoskeleton, and/or disrupting cell membranes as the spray dries on the pest. Applications are most effective during two treatment windows when crawler stages of HWA with no protective wool covering are active. The first is following egg hatch from late March through early April; the second is from early September to October before crawlers start producing wool. Treating at other times of year will result in poor control and may result in foliage burn.

For either product, use a 2 percent solution (2.6 ounces of spray material per gallon). There is no residual control with these materials; once dry they will no longer control HWA. Using these materials effectively is difficult if you do not have adequate spray equipment, especially for large trees. Using backpack or handheld sprayers, it is possible to treat trees 10 feet or smaller in height. Larger trees require a high-pressure sprayer, which may involve hiring a commercial pesticide applicator.

For complete coverage, spray until runoff. Drench the entire tree, including the undersides and tops of all limbs. A forceful spray is necessary to obtain adequate coverage. Be sure oils stay well mixed with the water carrier during application. Trees will likely require repeated treatments.

Soil Drenches

This treatment is effective for all size trees. The insecticide is applied to the soil surrounding the tree. Tree roots take up the product and move it to the foliage to achieve control. To be effective, soil drenches require adequate soil moisture in either the spring or fall. The best time to treat is March or April, after the ground has thawed. Fall treatments are made once moisture returns to the soil and should be completed by late October or early November when soils cool.

Label restrictions prohibit application near streams, other water bodies, or where soil is exceedingly rocky. Stem injections or basal trunk sprays (see below) are useful when proximity to water, rocky soils, or other site limitations prevent soil applications.

Most soil drenches use an imidacloprid 2F product (e.g., Bandit 2F, Merit 2F, Imidacloprid 2F, Malice 2F, Mallet 2F, Xytect 2F, Zenith 2F). Landowners can purchase and use these products on their own property in Pennsylvania without pesticide certification. There are other imidacloprid products on the market. Any may be used as long as the label includes the proper application site (e.g., forest, woodland). When applied to soil, imidacloprid moves slowly through the roots to the top of the tree. It may take 3–12 months to move throughout the entire canopy. Protection typically lasts four to six years.

Do not exceed the maximum labeled rate of imidacloprid per acre per year. For any of the 2F products listed above, the maximum labeled amount applied per acre per year is 0.4 pound of active ingredient or 25.6 ounces of product. This maximum rate per acre limits treatment to 256 inches of cumulative trunk diameter, measured at 4.5 feet above the ground—or diameter at breast height (DBH)—applied at the rate of 0.1 ounce per inch of trunk diameter (e.g., 18 trees averaging 14 inches in diameter per acre) or 128 inches of cumulative trunk diameter at the higher rate of 0.2 ounce per inch of trunk diameter. If measuring stem circumference, this allows treatment of 804 inches of total cumulative circumference at the low rate or 402 inches at the high rate (diameter = circumference ÷ 3.14).

To apply a soil drench, pull leaves and other organic mulch material from around the trunk circumference in a 2-foot-wide circle. Follow the label instructions to determine how much product to use for each tree, as the amount of insecticide (0.1–0.4 ounce per inch) is proportional to tree diameter measured at DBH. Mix the insecticide in a bucket containing approximately 2.5 gallons of water and distribute it evenly to the exposed soil within 18 inches of the tree. Refill the bucket with water to rinse out any remaining insecticide and apply around the tree base. The additional water will help move the insecticide to tree roots more quickly.

For soil drenches to work, the trees must be healthy enough to move the product from the roots to the foliage. If trees are already in decline due to HWA, spray as much of the tree as possible with either horticultural oil or insecticidal soap to kill as many HWA as possible. Then treat the following season with imidacloprid.

Dinotefuran (e.g., Safari 20SG, Transtect 70WSP, Zylam), a different active ingredient that is more water soluble and faster acting than imidacloprid, can also be used. Usually, it moves into the tree canopy within two to three weeks and begins controlling HWA in as little as four weeks. Dinotefuran is a particularly good choice when trees are already declining and is indispensable for treating trees with thin crown foliage. It is more commonly applied as a basal trunk spray (see below). Protection typically only lasts for one to two years. Note: Dinotefuran is also recommended for controlling elongate hemlock scale (see below).

Homeowner Insecticide Treatment Option

An imidacloprid product (2.94 percent) specifically for homeowners, Bayer Bio Advanced: Tree and Shrub Insect Control, is readily available from most garden centers. Application involves removing mulch and accumulated duff from around the tree, mixing the insecticide in a bucket of water, and pouring it around the base of the tree on the exposed mineral soil. The insecticide is taken up by tree roots and moved throughout the tree.
Determine the amount to use by measuring trunk circumference at chest height using a flexible tape measure.
Use 0.5 ounce per inch of circumference. Mix the required amount with water and pour around the tree base.
Note: A 1-gallon container (128 ounces) of Bio Advanced: Tree and Shrub Insect Control will treat 256 total inches of trunk circumference. This equals approximately eight 10-inch-diameter trees, each 31 inches in circumference.

Basal Trunk Sprays

Dinotefuran (e.g., Safari 20SG) is labeled for soil drench application, but it is more often applied as a basal trunk spray using a garden sprayer, backpack sprayer, hand trigger sprayer, or pump-up bottle. Using low pressure, apply the insecticide solution to the lower 4 to 5 feet of the trunk, applying the spray to the entire circumference until the bark is wet, just to the point where it is about to drip. Apply slowly to prevent the solution from running down the bark into the soil. Splash back can be an issue with basal trunk applications. If the tree is near flowering plants, it is best to treat once flowering is complete and they are no longer being visited by insects.

It is important to apply the correct amount of dinotefuran insecticide solution to each tree. The Safari 20SG label indicates mixing a 1-gallon solution containing 12 to 24 ounces, dry weight, and apply it at a rate of 1.5 to 2 fluid ounces per inch of trunk diameter.

Calibration is necessary to ensure the correct amount of insecticide solution is applied to the trunk of each tree. The two calibration methods outlined below can be utilized.

• Timing method: Using a rate of 2 ounces per inch of trunk diameter, determine the length of time it takes to spray 2 fluid ounces at a constant pressure into a measuring cup. Do this multiple times and take the average. Measure DBH and multiply the amount of time to apply 2 ounces by DBH. Spray the lower 4 to 5 feet of the trunk for that amount of time.
• Counting method: Again, using a rate of 2 ounces per inch of trunk diameter, determine the ounce count to spray 2 fluid ounces at a constant pressure into a measuring cup by counting (one one-thousand, two one-thousand, etc.). Again, do this multiple times and calculate the average. Measure DBH and multiply your ounce count to apply 2 ounces by DBH. Spray the lower 4 to 5 feet of the trunk for that count.

Calibration Example: Counting Method

16-inch DBH tree
16-inch DBH × 2-ounce rate = 32 total ounces of insecticide solution needed
Ounce count to apply 2 ounces = 6
6 ounce count × 16-inch DBH = 96 total ounce count to apply 32 ounces of insecticide solution to tree

 Determining the amount of insecticide spray solution required to treat a single tree requires some math. Using Safari at the rate of 18 ounces per gallon of water and applying it at a rate of 2 ounces per inch of DBH, use the following equation:

    X ounces Safari = 18 ounces Safari × (DBH × 2) ÷ 128 ounces in a gallon

Example: Treating a Single Tree

20-inch DBH tree
20-inch DBH × 2-ounce rate = 40 total ounces of insecticide solution needed
X = number of ounces of Safari needed
18 = ounces of insecticide per gallon of solution
128 = total ounces in a gallon

Solve for X:
X = 18 × 40 ÷ 128
X = 5.6 ounces (dry weight) of Safari needed

Stir the 5.6 ounces of Safari into a measuring cup containing approximately 30 ounces of water (75 percent of what is needed) until well dissolved. Next, fill to the required 40-ounce mark (DBH × 2). Transfer the solution into your sprayer. Starting at 4 to 5 feet above the ground, apply it to a "band" around the trunk and slowly move down, circling the tree. Continue applying until the sprayer is completely empty, allowing the material to be absorbed into the bark.

The advantage of using basal trunk sprays is that they are safe to use near waterways and on rocky sites since no product contacts the soil; however, as noted above, dinotefuran does not persist as long as imidacloprid and will likely require reapplication within –one to two years to maintain consistent HWA control.

Basal Trunk Sprays with Dinotefuran and Imidacloprid
Some states recommend and allow tank-mixing imidacloprid and dinotefuran for basal trunk applications to take advantage of the quick uptake of dinotefuran and longer efficacy of imidacloprid. Dinotefuran rapidly reduces HWA populations on declining trees and may give them the opportunity to recover enough to take up the imidacloprid for long-term protection.

It is important to note that imidacloprid products do not have labeling for basal trunk spray application. However, in research trials imidacloprid was shown to be effective using this method. Provisions under Section 2.e.e. (Emergency Exemption) of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) allow for certain exceptions regarding the use of a pesticide that may not be listed on the label. One is applying a pesticide at any dosage, concentration, or frequency less than specified on the label, and a second is employing any method of application not prohibited by the label.

Acceptance of Section 2.e.e. provisions are a state-by-state decision and not all states recognize them; Pennsylvania does. Be sure to check with your state pesticide regulatory agency regarding the acceptance of Section 2.e.e. provisions.

Recommendations developed in New York advise mixing 9 ounces (dry weight) of Safari 20SG plus 12.8 fluid ounces of imidacloprid 2F per gallon and applying the insecticide solution at the rate of 2 ounces per inch of trunk diameter. There are 128 ounces in a gallon, so at the labeled application rate of 2 ounces per inch of diameter there is enough solution to treat 64 inches of DBH. The imidacloprid 2F label states that 0.2 fluid ounce is necessary per inch of trunk diameter. Therefore, the amount of imidacloprid necessary to treat 64 diameter inches is 12.8 ounces. Be sure to calibrate sprayer before applying (see the basal trunk spray section above).

Insecticide Precautions
Imidacloprid and dinotefuran belong to the class of insecticides called neonicotinoids. They are systemic insecticides, meaning the chemicals are taken up by the plant and transported throughout its tissues, and have a similar mode of action for insect control. Because they are systemics, insecticide applications can be made to foliage, soil, or bark. Soil and bark treatments are best in forest settings due to reduced likelihood of negative effects to nontarget organisms and water resources. Do not apply to soil near surface water such as streams, lakes, or ponds or where soil is exceedingly rocky.
Read and follow all label recommendations when making any pesticide application. The label contains other important information not included in this fact sheet. The use of any pesticide can have unwanted consequences. Neonicotinoids are toxic to aquatic insects and pollinators such as honey bees. Be sure to follow all directions. Do not exceed the maximum labeled rates of products. Applying a higher rate than labeled will not increase control and can be detrimental to tree health. If mixing and application instructions are difficult to understand, seek assistance from your local extension office or state natural resource or agricultural agency.

Time-Release Soil Tablets

Insecticide tablets containing the active ingredient imidacloprid (e.g., CoreTect, which also contains fertilizer) are available to treat HWA. Insecticide tablets are convenient and effective. Tablet treatments protect hemlock trees for up to five years. Release of the active ingredient from the tablet is dependent on adequate soil moisture. Applications can occur when soils are dry in summer, but imidacloprid release requires tablets to be moistened. If soils are dry at the time of application, watering following treatment, if possible, may be helpful. Do not apply tablets to saturated soils.

Time-release soil tablets are placed 2 to 5 inches below the soil surface and spaced evenly around the tree within 6 to 24 inches of the trunk and along the dripline of infested trees at a rate of two to three tablets per inch of DBH. Once placed, the tablets must be completely covered with soil. Do not apply more than 450 tablets per acre per calendar year (0.5 pound imidacloprid).

Follow-Up

It is not easy to assess successful HWA control. The white, woolly filaments will linger on foliage following treatments. In most cases, it is necessary to use magnification to see if the insects are dead. The best way to know if trees are recovering and treatments were successful is to observe the next flush of new growth, which should be free of the white, woolly filaments. Even when working properly, a soil drench of imidacloprid may take a year or longer to take effect—be patient.

Insecticide treatments can last from one to six years depending on their success and proximity to untreated infested hemlocks. Successful treatments are usually a function of initial tree health and the amount of soil moisture available when treatments were applied. Keep monitoring new growth for the white, woolly filaments of the adults. Retreatment is necessary once adelgids are apparent on numerous branches. A pole pruning saw may be necessary to collect and observe samples.

Large Tracts of Hemlocks

In wooded areas with many hemlocks, it is not possible to save every tree. Determine your budget for materials and treat trees that are most important to the landscape or as future seed trees. Measure and mark treated trees with paint so they can be found for future reapplications. Thin overcrowded trees to improve overall tree health and reduce the number of trees needing treatment. Save small understory trees by spraying with horticultural oil. Do not exceed the maximum labeled insecticide rates per acre per year.

Elongate Hemlock Scale (Fiorinia externa)

Elongate hemlock scale (EHS), sometimes known as Fiorinia scale, is another serious invasive pest of eastern hemlock. This armored scale insect was likely unintentionally introduced into the United States from Japan. It was first observed in Queens, New York, in 1908.

Look for this wax-covered insect on the underside of needles and on new cones. The flattened, elongate, light yellow-brown to brownish-orange waxy cover of the adult female is about 1.5 millimeters long. Males have a smaller, white, waxy cover. Sometimes waxy secretions from settled crawlers may build into a mass of tangled strands. When abundant, these waxy strands give the lower surface of infested needles a white appearance that is sometimes confused with HWA.

EHS also feeds on tree sap by inserting their threadlike piercing-sucking mouthparts into needles and withdrawing nutrients. Excessive plant fluid loss reduces tree growth and health. Feeding injury causes needles to develop a yellow banding on the upper surface. This injury causes needles to drop prematurely, giving the crown of an infested tree a thin appearance.

EHS population densities build more slowly, and its negative effects to eastern hemlock are slower acting than those of HWA. EHS are often found on the same trees with HWA. Research suggests that feeding by HWA allows EHS to reach damaging levels, thus hastening the decline of already weakened trees.

Imidacloprid insecticides are not effective at controlling armored scale insects due to their lack of mobility within the tree. If hemlocks have both HWA and EHS, treat them with products containing the active ingredient dinotefuran (e.g., Safari 20 SG). Dinotefuran controls both EHS and HWA when applied as a basal trunk spray, soil drench, or soil injection. Insecticide applications should occur before bud break and shoot elongation in the spring. Use dinotefuran at 1 to 4.2 ounces of product per 10 inches of trunk diameter. For a soil drench, mix this product per label instructions in enough water to pour around the base of the tree.

Biological Controls

Biological control of invasive insect pests is the most viable alternative for hemlock conservation in forest settings. Once established and reproducing in the field, biological controls require little to no investment of time and money. The best possible outcome is to establish a suite of predatory insects throughout the hemlock range. The Pennsylvania Bureau of Forestry, in cooperation with the U.S. Forest Service and various research institutions, continues to investigate potential biological control agents for hemlock pests.

Since the early 1990s, scientists have evaluated host-specific and generalist predatory insects to control HWA. Several predatory beetles and silver fly species from the HWA native range are currently being released and studied at hundreds of locations with some promising results, but more research on various HWA biological control agents is needed.

Cultural Practices

Reducing environmental stresses, such as during droughts, enables hemlocks to better tolerate HWA infestations. Stress increases the risk and severity of insect attacks. Hemlock mortality increases following mild winters and dry summers. Watering during periods of dry weather and properly mulching are two ways to reduce stress and maintain vigor in landscape trees. Use fertilizers only after adelgids are under control, as nitrogen fertilizers help adelgids thrive and reproduce.

In areas where hemlocks are showing severe decline, consider regenerating the site to desired tree species, particularly native conifers (Table 2). Promote conditions that favor the establishment of desired and appropriately adapted tree species by controlling competing vegetation, invasive plants, and deer overbrowsing. Manage for tree species with seed sources already present in the upper canopy or as advance regeneration on the forest floor, and supplement by planting site adapted species.

Hemlocks with high live crown ratios (the percentage of total tree height that supports live foliage) have better survival rates against HWA infestations. Trees with large live crowns are generally more vigorous and typically survive longer once infested. Hemlocks with live crown ratios of 30 percent or less can be targeted for removal. Targeted removal reduces crowding and enables the remaining trees to maintain large live crowns. Research indicates that thinning uninfested hemlock stands may help boost long-term tree vigor and survival against HWA.

Conifer Alternatives

While reforestation with hemlocks resistant to HWA is the desired goal, it is likely decades away, if ever achievable. In the meantime, consider establishing other conifer species, preferably native, that provide similar conditions and growth form such as shade and dense canopy structure. Encouraging natural regeneration of native conifers and planting conifer alternatives may help mitigate anticipated impacts from hemlock loss. This is particularly important in riparian areas.

Refer to Table 2 for potential conifer alternatives for planting on hemlock sites. This information was compiled from the USDA Forest Service's Silvics of North American Tree Species, Volume 1: Conifers. It compares different coniferous species as a general guide for supplemental plantings. One species not listed as a supplement to hemlock is Norway spruce, a nonnative species planted extensively in the past that is now naturalized across much of the hemlock range. If underplanting or promoting alternative conifer species to replace hemlock, consider sourcing seedlings from more southerly seed zones in anticipation of future climate conditions.

There are two generally accepted approaches for planting conifer alternatives: supplemental plantings under live hemlocks to enhance the conifer component in anticipation of hemlock mortality, or complete reforestation following hemlock overstory mortality.

In Summary

The integration of the pest management techniques covered in this fact sheet outlines a practical and sustainable method for conserving eastern hemlock in the long run. HWA is currently the largest threat to eastern hemlock in North America. Regularly monitor hemlocks to determine HWA and EHS distribution and level of infestation. Prioritize infested sites and individual trees when making treatment decisions. For areas that will not receive treatment or that are of lower priority, consider management actions, either through planting or site manipulation, that will influence which tree species may replace hemlocks if they are lost.

The Pennsylvania DCNR Bureau of Forestry has been actively involved with HWA management since it was first discovered in the state in the late 1960s. Several components of the eastern hemlock conservation strategy are possible through funding from the USDA Forest Service’s Hemlock Woolly Adelgid Initiative. The program reflects the efforts of numerous federal and state agencies, universities, institutions in China and Japan, and private industries. The bureau continues to work with public and private landowners to determine the best ways to protect hemlock trees. As a resource for the public, Penn State Extension actively provides guidance and training for forest landowners and homeowners on hemlock treatment options.

Prepared by David R. Jackson, forest resources educator, Penn State Extension.

Reviewed by James Finley, professor emeritus of forest resources, Penn State; Sandra Feather, green industry educator, Penn State Extension; Sarah Johnson, forest health program specialist, DCNR Bureau of Forestry, Division of Forest Health; and Tim Tomon, forest health program specialist, DCNR Bureau of Forestry, Division of Forest Health.

Portions of this document were adapted from Mark Faulkenberry, Ph.D., Donald A. Eggen, Ph.D., and Ellen Shultzabarger, “Eastern Hemlock Conservation Plan,” Pennsylvania Department of Conservation and Natural Resources, Bureau of Forestry, April 23, 2019. Used with permission.