Hemlock woolly adelgid control


Meeting the Threat of the Hemlock Woolly Adelgid

ENTFACT-452: Meeting the Threat of the Hemlock Woolly Adelgid | Download PDF

by Lee Townsend, Extension Entomologist & Lynne Rieske-Kinney, Associate Professor, Forest Entomologist
University of Kentucky College of Agriculture

The hemlock woolly adelgid (HWA), a native of Asia, is a 1/32 inch long reddish purple insect that lives within its own protective coating. White, woolly masses that shelter these sap-feeding insects are at the bases of hemlock needles along infested branches. The presence of these white sacs, which resemble tiny cotton balls, indicate that a tree is infested.

Hemlock woolly adelgid on hemlock branches

HWA is a threat to eastern hemlock forests, and eastern and Carolina hemlock of all sizes are susceptible. Kentucky has a significant hemlock component throughout its eastern forests, all of which could become infested. In addition, ornamental plantings in urban settings are equally susceptible. HWA feeding reduces new shoot growth, and causes grayish-green foliage, premature needle drop, thinned crowns, branch tip dieback, and eventual tree death.

Control Options

Homeowners and private landowners have two treatment options: 1) spray foliage with insecticidal soap or horticultural oil at the proper times during the HWA life cycle, or 2) use a systemic insecticide that moves with the tree sap and is consumed by the adelgids as they feed.

Tree sprays

Several brands of insecticidal soaps and horticultural oils are available at garden centers or from retail stores that sell pesticides. These are contact insecticides that must thoroughly cover the insects to kill them so the foliage, twigs, and branches must be sprayed almost to runoff. Neither product leaves a toxic residue so several applications may be needed. The soaps and oils can be applied to small trees with pump-up garden sprayers but power sprayers are needed to treat larger trees. If coverage is not thorough or trees are too tall for the equipment that is available, surviving HWA soon will reinfest the trees.

These applications will be most effective only during two treatment windows when stages of the HWA that do no have a protective covering are active. One is following egg hatch from March through April, the other is from September to October. Read the product label carefully before purchasing and applying an insecticidal soap or horticultural oil. In some cases, there will be precautions against spraying when temperatures are above or below certain limits, on windy days, or in late spring when the new growth is present on the tree and has not hardened to the dark shade of green.

Insecticidal soaps and horticultural oils can be used in sensitive sites near houses, creeks, or ponds without risk to non-target organisms or the environment. Foliar sprays with other insecticides can be effective but some pose risks to the environment.

Systemic Treatments

Systemic insecticides containing the active ingredient imidacloprid can be used as soil drenches or injections to control the HWA. Brand names include Bayer Tree & Shrub Insect Control Concentrate or Merit Insecticide. Imidacloprid, which is very soluble in water, is taken up by tree roots and moved in the sap to sites where the adelgids are feeding. This approach eliminates the problem of spray coverage, especially on trees that are too tall to treat using available equipment. This treatment should be applied during spring (mid-March to early June) and fall (mid-September to mid-November). The ground should not be frozen or waterlogged but the soil should be moist. Research indicates that the treatment requires at least 30 days to be taken up by the tree, but it can kill adelgids for about 24 months. Homeowners with infested hemlocks may consider soil treatments if HWA has been reported in their county. There is no preemptive treatment for the hemlock adelgid.

There are three methods for applying the systemic insecticide:

1. Soil drenching. The appropriate amount of insecticide, based on the product label, is diluted with water and applied evenly to the soil surface or into a series of small holes spaced evenly around the base of the tree at a distance of 6 to 12 inches. This is followed by an equal volume of water to help move the insecticide down into the soil. Surface litter should be raked away prior to treatment to ensure good penetration. Soil should be moist at the time of treatment so the area may need to be watered in advance if it is dry.

Treating Hemlock Trees for HWA:
Soil Injection

2. Soil injection. The insecticide solution is injected several inches beneath the moistened soil surface with a hand-powered Kioritz soil injector. This injector allows good placement of the insecticide that allows for efficient uptake by the roots, and reduces the chances of runoff of the insecticide from the application site.

3. Trunk injection. Measured doses of imidacloprid are delivered through a small tube that is inserted into holes drilled into the base of the tree. The number of doses required is determined by the circumference of the tree. Trunk injection is not always effective, much more expensive than the soil drench or injection, causes extensive tree wounding that contributes to overall tree stress, and provides a shorter period of protection. However, this approach provides less chance for environmental contamination in rocky soils near open water. Most trunk injections must be made by professional arborists.

Research has shown dramatic recovery of HWA infested trees following applications of imidacloprid in residential settings. In the study, trees with little new growth but no dieback recovered most quickly; trees in poorest condition showed impressive recovery but it occurred more slowly.

Other Practices

Maintain tree health by irrigating trees during prolonged dry periods and remove dead or dying limbs. Adelgid-infested trees should not be fertilized because adelgids thrive on fertilized trees. HWA can be moved from tree to tree by songbirds, so bird feeders should not be placed near hemlocks.

Life Cycle

An understanding of the HWA life cycle is useful because foliar sprays are most effective when the newly hatched “crawler” stage is active. Egg laying begins early, before the first wildflowers bloom in early spring. Small reddish brown nymphs (also called crawlers) that hatch from groups of up to 300 eggs in March either disperse from the tree or settle near the base of needles, sink their sucking mouthparts into twigs, and begin to feed on sap. The crawlers have a distinctive white fringe around their sides, their white protective covering is secreted later. Adelgids without the covering are very susceptible to contact with insecticidal soap or horticultural oil. These adelgids mature over the next few weeks and lay 50 to 200 eggs that will hatch in mid-April to early May. These crawlers are present during the spring flush of hemlock foliage. They settle on new growth and are inactive during the summer. Development resumes in September and partially-grown HWA spend the winter on tree branches and twigs covered by the white woolly material that they secrete. Development resumes in late winter and continues until March when these adelgids mature and begin the two generation per year life cycle again. In one year a single adelgid female could produce 90,000 offspring.

Movement and Dispersal

HWA was first reported in the eastern US in Virginia in the mid 1950’s. Since then, it has spread north to Connecticut and south through North Carolina. It reached eastern Tennessee in 2002, and was first reported in eastern Kentucky in spring 2006. The infested area expands at the rate of about 15 miles per year. HWA can be blown by winds, carried by birds and other wildlife, and moved on infested nursery stock. It produces several generations per year and has an extremely high reproductive potential (up to 300 eggs per female) so numbers can increase rapidly in a short time.

Impact on Forests and Landscapes

The HWA is established in about half of the hemlock range in the eastern US and has killed about 90% of the hemlocks in Shenandoah National Park in Virginia. Tree mortality has caused a drastic shift in forest species composition. Because hemlock provides extensive cover and shade and is so prevalent in watersheds and riparian zones, their loss has drastic effects on ecological processes such as nutrient cycling, decomposition, leaching, stream temperatures, and stream organisms. Extensive hemlock mortality leads to a loss of hemlock-dependent wildlife. Song birds are especially at risk. Kentucky has a significant hemlock component throughout the east, all of which is in danger of infestation by HWA. Ornamental plantings in urban settings are equally susceptible.

Biological Control

The HWA came from Asia without any of the natural enemies that keep populations in check in its native range. In addition, it feeds during the cooler parts of the year when our general predators are still dormant for the winter. Several very small lady beetles have been imported and released in an attempt to provide some control of expanding HWA populations.


Research on chemical and biological control techniques and resistant hemlock varieties is on-going. Proactive monitoring is essential for early detection of new infestations. Isolated infestations should be treated immediately. Public education and coordinated information dissemination is needed to minimize the impacts of HWA in Kentucky.


Rusty Rhea, USDA Forest Service, reviewed this publication and provided valuable input. We thank Dr. Tom Coleman for allowing the use of his pictures.

Issued: 5/06
Revised: 5/06

CAUTION! Pesticide recommendations in this publication are registered for use in Kentucky, USA ONLY! The use of some products may not be legal in your state or country. Please check with your local county agent or regulatory official before using any pesticide mentioned in this publication.


Photos: Tom Coleman, University of Kentucky Entomology

Hemlock trees showing woolly adelgid damage.

The hemlock woolly adelgid is a tiny, aphid-like insect that is ravaging hemlocks from Maine to Georgia. It attacks large native hemlock forests as well as hemlock landscape trees in your yard. The woolly adelgid is considered a serious threat to species of eastern hemlocks as well as the overall health of forest and river ecosystems.

Hemlocks are one of the most popular trees planted in parks and backyards, so this problem can hit close to home. If the hemlocks in your yard are infested with the woolly adelgid, here’s what you need to do.

About the Woolly Adelgid

It’s hard to pronounce, and sometimes it’s even harder to see. The hemlock woolly adelgid (HWA) is a fluid feeding insect that snuck into the United States from Asia, where it’s a harmless forest resident. It was first found in the western part of the U.S. back in the 1920s, but the woolly adelgid didn’t pose a threat there either. However, as the woolly adelgid made its way east, it soon became apparent that the tiny insect was a serious threat to eastern and Carolina hemlocks.

The woolly adelgid is spreading faster than it can be controlled, resulting in extensive damage to native forests and threatening the survival of the species. If you have eastern or Carolina hemlocks in your yard, keep a close eye on your trees – the sooner you can spot and treat the problem, the better the chances of your tree recovering.

Look for the waxy nodules on the stems of hemlock trees.

How to Identify the Woolly Adelgid

The woolly adelgid is easiest to spot in spring and early summer. It targets soft new growth, setting up camp right where the needles meet the stem. The hatching insects feed on the sap at the base of the needles, eventually causing those needles (and soon, the entire branch) to die. As the infestation grows, the tree eventually starves to death. Here’s what to look for, depending on the season:

  • Spring: Orangey-brown eggs.
  • Early summer: Tiny reddish-brown crawling insects. They almost look like pepper sprinkled on the stems.
  • Summer: The young insects spin a little white nest, made of a waxy, woolly-looking substance. The small white nodules should be visible at the base of the needles along the stems. This is the easiest way to identify the woolly adelgid.
  • Fall: During the heat of summer, the woolly adelgid goes dormant. They come back out and start feeding in fall and over the winter.

How to Control the Woolly Adelgid

If you’ve spotted a woolly adelgid infestation in your trees, there are options available for treatment. Unfortunately, the most effective options aren’t the most organic ones, so exercise caution when treating your trees.

Begin by treating the trees that are the healthiest, the most integral to your landscape design, and farthest from streams and water sources. It’ll take a few months to see a change, and you’ll need to continue monitoring your trees every year. If the treatment is effective, you should see the insects disappear and new needles start to grow.

Examples of chemical treatment options for woolly adelgid on hemlock trees.

Soil Treatment

Soil treatment, or soil drenching, is considered the most effective method of treating the woolly adelgid in home landscapes. Follow these steps:

  1. Look for an insecticide with the active ingredient Imidacloprid. It’s sold under a variety of names, including Bayer Advanced Tree & Shrub Insect Control and Ortho Max Tree & Shrub Insect Control. You’ll need the large bottle of concentrate, rather than the ready-to-use spray bottle.
  2. Dig a circular trench around the tree about one-foot from the trunk of the tree and approximately 3” deep.
  3. Mix the concentrate according to package instructions, and pour it into the trench.

Soil drenching can take several months to take effect. If your trees are seriously threatened, you may want to pair this treatment with one of the spraying options below.

Spraying Trees

If your hemlocks are small, you can use a foliar spray to spray the needles and stems with Imidacloprid.

  1. Mix the concentrate yourself in a handheld sprayer, or buy ready-to-use spray bottles.
  2. Spray the entire tree, making sure that every branch is covered both top and bottom.
  3. Spray until you see excess dripping off – any missed branches can still harbor the pest!

Dormant Oil and Insecticidal Soap

You can also obtain decent results using sprays of dormant oil or insecticidal soap. These options are less toxic than the chemicals but should still not be used around water sources.

  1. Apply dormant oil or insecticidal soap in the fall, making sure to cover the entire tree.
  2. Since oils and soaps are not long-lasting, you’ll need to repeat the treatment every year.

Treating hemlock trees with oil or soap is a good companion treatment to go along with soil drenching.

If left untreated, branches infected by woolly adelgids die.

Natural Predators

Since large forests obviously can’t be treated with insecticides, researchers are working to develop natural predators to release for large-scale control of the woolly adelgid. One predatory beetle is looking promising, and they’re commercially available from Conservation Concepts, Inc, in North Carolina. Contact your county extension office for more information.

Professional Options

If your trees are near water, or if the soil is too rocky to dig trenches, professional pesticide applicators have a couple more tricks up their sleeve, including high-pressure sprayers and water-safe trunk injections.

Further Information

  • Hemlock Woolly Adelgid Control (North Carolina Extension)
  • Hemlocks at Risk (Ohio Division of Forestry)
  • Save Our Hemlocks (saveourhemlocks.org)

Hemlock Woolly Adelgid Treatment and Facts

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    Briarcliff Manor, NY

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    Historic Hudson Valley, NY

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How to treat hemlock trees for hemlock woolly adelgid


June 22, 2017 – Author: David Smitley

Hemlock woolly adelgid is established in isolated locations in the western Lower Peninsula of Michigan. As of May 2017, hemlock woolly adelgid (also referred to as HWA) had been found in localized areas of Allegan, Muskegon, Oceana and Ottawa counties, and additional surveys are underway. To see current infestation maps, visit the HWA page on the Michigan Department of Agricultural and Rural Development website.

Be sure your tree is a hemlock: Identifying hemlock trees

Before looking for hemlock woolly adelgid, make sure your tree is a hemlock and not a fir, pine or spruce tree. Hemlocks can be distinguished from pine and spruce by their short, flat needles. Hemlock needles are only about a half-inch long, which is much shorter than most pine needles. You can roll pine and spruce needles between your fingers, but not the flat hemlock needles. The underside of hemlock needles have pale white stripes, which will distinguish them from pines and spruces. However, the undersides of Douglas fir and balsam fir needles may also have pale white stripes, so you will need to look at the shape of the needles to distinguish hemlocks.

The only other conifers with short, flat needles are firs and Douglas fir, which is technically not a fir. Eastern hemlock needles can be distinguished from fir needles by their shape: they are slightly wider at the base and taper to a rounded tip with a dull point, while fir and Douglas fir needles have parallel sides for nearly the entire length of the needle.

White spruce. Photo by T. Davis Sydnor, The Ohio State University, Bugwood.org

Austrian pine. Photo by Tom DeGomez, University of Arizona, Bugwood.org

Eastern hemlock. Photo by Bill Cook, Michigan State University; Bugwood.org

Eastern hemlock. Photo by Rob Routledge, Sault College, Bugwood.org

Balsam fir. Photo by Becca MacDonald, Sault College, Bugwood.org

Balsam fir. Photo by Virginia Tech

Douglas fir. Photo by Paul Wray, Iowa State University, Bugwood.org

Identifying hemlock woolly adelgid

It is critical to be sure you correctly identify whether or not your tree is a hemlock because hemlock woolly adelgid is only found on hemlocks. Once you are sure your tree is a hemlock, you will need to be able to recognize hemlock woolly adelgid.

A few other insects can be confused with hemlock woolly adelgid. The easiest way to identify hemlock woolly adelgid is to look at hemlock shoots for the white “wool” the adelgid produces while feeding. Each little white ball of wool, called an egg sac, is actually wax secreted by an adelgid. The adelgids feed at the base of the needles, where the needles attach to the woody portion of the shoot. It is often easier to see hemlock woolly adelgid wool on the undersides of shoots, but you can also find them on the upper side as well.

Check hemlock shoots for the hemlock woolly adelgid’s white “wool” egg sac. Photo by Robert L. Anderson, USDA Forest Service, Bugwood.org.

Hemlock woolly adelgid’s white “wool” egg sac. Photo by Chris Evans, University of Illinois, Bugwood.org

Hemlock woolly adelgid has only been found in a few locations in Michigan at this time. Many people who check their hemlock trees may notice small, white spots that look a bit like hemlock woolly adelgid. Other insects, including the elongate hemlock scale, oak skeletonizer cocoons, caterpillar cocoons or spider egg sacs may be present on hemlock needles or shoots. Compare the pictures of hemlock woolly adelgid above with the look-alike images below to help correct identification.

Elongate hemlock scale. Photo by Eric R. Day, Virginia Polytechnic Institute and State University, Bugwood.org

Oak skeletonizer cocoons. Photo by James Solomon, USDA Forest Service, Bugwood.org

If it appears your hemlock tree is infested with hemlock woolly adelgid and you would like to confirm your identification, take photos, note the location of the tree and report it to the Michigan Department of Agriculture and Rural Development (MDARD) by emailing [email protected] You can also call MDARD at 800-292-3939 to report a suspected hemlock woolly adelgid infestation. It will be especially helpful to notify MDARD if the infestation is outside of the current infestation areas indicated on their HWA website.

Please do not clip infested branch samples and transport them or mail them to another location for identification. You could accidentally spread hemlock woolly adelgid to areas that are not yet infested.

Treatments for hemlock woolly adelgid

If you have confirmed that one or more of your hemlock trees is infested with hemlock woolly adelgid, consider treating them with a systemic insecticide. If the adelgid is not controlled, infested trees will decline and eventually die, usually within four to 10 years.

A simple treatment method for homeowners entails applying a systemic insecticide to the soil around the tree trunk base. The insecticide is absorbed by tree roots then transported within the tree to the shoots and needles. The active ingredient (i.e., the toxic compound) in products available to homeowners will be either imidacloprid or dinotefuran (Table 1). Imidacloprid is absorbed by roots and moves through trees relatively slowly, sometimes taking a year to reach the shoots in the top of a large tree. The advantage of using imidacloprid, however, is that one application will protect the tree for at least four years.

In contrast, dinotefuran moves into roots and through the tree quicker than imidacloprid, but does not last as long. Dinotefuran should protect trees for one to two years after an application.

All of the systemic insecticide products listed in Table 1 should be applied as a soil drench or a granular application to the soil at the base of the trunk. Follow the label instructions to determine how much product to use for each tree, as the amount of insecticide is based on the size of the tree trunk (circumference).

To apply the insecticide as a soil drench, carefully mix the correct amount of product in a bucket of water. Rake or brush away old needles, leaves and other debris from the soil around the tree, then pour the solution around the base of the trunk. You may need to go around the trunk several times until all of the solution has been poured as close to the base of the trunk as possible. Let the solution soak in and the soil surface dry before allowing people or pets to enter the treatment area.

For granular products, remove debris from the soil then spread the correct amount of granules around the base of the tree, as close to the trunk as possible. Protect people, pets and wildlife from the granules by using a watering can or sprinkler to help the insecticide soak into the soil, then cover the treated area with a thin layer of mulch.

The products listed in Table 1 can be applied any time between early April and late October.

When making pesticide applications, carefully read the label and follow all directions. Do not exceed the label rate as there is a maximum amount of imidacloprid that can be applied per acre in a given year, usually 0.4 pound of active ingredient. The per-acre limit will only be of concern if you plan to treat very large hemlock trees or many small trees. For example, the maximum amount of imidacloprid per acre per year will be reached when you treat 16 trees with a 24-inch trunk circumference, or 8 trees with a 48-inch circumference, or 4 trees with a 96-inch circumference.

You may prefer to hire a landscaper or arborist who is has pesticide applicator certification to treat your trees. Tree care professionals have access to a wider range of products and application methods for hemlock woolly adelgid. (See “Options for Protecting Hemlock Trees from Hemlock Woolly Adelgid.”)

The hemlock woolly adelgid wool (egg sacs) will not disappear immediately after treatment, but will eventually weather away. Examine the new needles one or two years after applying the insecticide to determine if the treatment was effective.

Hemlock trees will need to be protected from hemlock woolly adelgid in the future by treating them once every one to two years with products containing dinotefuran, or once every four to five years with products containing imidacloprid. Toxicity to pollinators following the uptake of imidacloprid or dinotefuran is not critical because bees are not attracted to hemlock pollen and few flowering plants can grow in the shade under a hemlock.

Table 1. Examples of homeowner products that can be used as a soil drench or granular application to protect hemlock trees from hemlock woolly adelgid. Many more products containing imidacloprid or dinotefuran are available.

Type of application

Active ingredient

Example of product

Soil drench

Imidacloprid (1.47%)

Bonide Annual Tree and Shrub Insect Control with Systemaxx

Soil drench

Imidacloprid (2.94%)

Bayer Advanced 12 Month Tree and Shrub Insect Control


Dinotefuran (2%)

Ortho Tree and Shrub Insect Control Granules

Note: Reference to commercial products or trade names does not imply endorsement or bias against those not mentioned.

The authors wish to thank John M. Bedford, MDARD, for review of this article.

Updated July 2017


Related Topic Areas

Natural Resources, Forestry, Invasive Species, Landscaping, Home Trees & Shrubs, Lawn & Garden, Gardening in Michigan, Integrated Pest Management


David Smitley
[email protected]

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Hemlock Woolly Adelgid

Pest: Hemlock Woolly Adelgid (Adelges tsugae Annand)
Order: Homoptera
Family: Adelgidae


The hemlock woolly adelgid was introduced into Massachusetts in 1988 from an already existing infestation in Connecticut. It attacks both the Carolina and Eastern hemlock and is capable of severely weakening and killing its host plants. Healthier plants, prior to infestation, may endure longer, but previously stressed plants may die in 3-5 years. The key to its management is to recognize it early and implement the proper management strategies.

The Pest:

The hemlock woolly adelgid (HWA) is a tiny insect that is closely related to the aphids. Adelgids, in general, are associated with conifers and produce galls and or woolly masses. The HWA only produces woolly masses.

This insect is atypical of most insect species in the Northeast in that it is inactive for much of the growing season and very active throughout the winter. From mid-July until mid-October, they are immatures (nymphs) and are flat, black, oval, and ringed with a fringe of white waxy strands. These very tiny nymphs can be found at this time settled on the stems of the host at the base of the needles. They neither feed nor develop during this time period. In mid-October, they resume feeding by using their piercing-sucking mouthparts to remove plant sap from the twigs of the host plant. While they feed, they develop into adults with new egg-masses appearing beginning in March. These are the most noticeable stage of the pest and have the appearance of small, white cotton balls lined up at the base of the needles. From then until June there can be many life stages present at one time and much feeding and injury can occur. There are two generations per year. Magnification is required to see all of the individual life stages.

Plants growing in stressful sites (compacted soils, ledgy soils, poor drainage, drought prone, etc.) are much more apt to succumb within 3-5 years from this pest’s invasion. Hemlocks in more natural (healthier) settings, such as the forest, have been shown to withstand infestations longer. Once found in the landscape or nursery, management is usually necessary. Forest trees may present a management challenge, although options do exist. For more information regarding management of HWA in New England Forests, visit:

Management Strategies:

The HWA, in the landscape and nursery, is very manageable if the problem is addressed early and aggressively. One option that poses reduced risk to non-target insects is the use of horticultural oils. These can be applied as either a dormant oil spray and/or as a summer spray. In order for these to be effective, the application must be thorough throughout the entire plant and may need to be reapplied later in the growing season. It is essential to kill virtually all of the HWA on a plant or the population will soon reappear at unacceptable levels.

Other active ingredients that have been shown to effectively manage the hemlock woolly adelgid include imidacloprid and dinotefuran. Imidacloprid has been shown to take several months to take effect after application, but research suggests it may protect trees from hemlock woolly adelgid for up to 5 years. Imidacloprid has not been shown to effectively manage the elongate hemlock scale (Fiorinia externa). In cases where a hemlock is infested with both the elongate hemlock scale and hemlock woolly adelgid, dinotefuran is an active ingredient that can manage both insects. Dinotefuran may be effective within weeks of application. Dinotefuran may protect trees from HWA for approximately 2 years, according to research. Products containing dinotefuran are currently listed as state restricted use products in Massachusetts. For more information, visit: https://www.mass.gov/pesticide-product-registration.

Both imidacloprid and dinotefuran work best as a systemic application and can be introduced into the plant via soil or trunk injection. Other application methods include soil drenches and basal sprays. The basal spray application is made when the product is applied to the bark of the tree from the root flare up to approximately 4 ft. high on the trunk. These active ingredients can also be used in a management program with horticultural oils; if the time of year is not conducive for an application of imidacloprid, then an oil can be applied with a follow-up application of the other. Special care must be taken when using imidacloprid or dinotefuran due to toxicity to bees and other pollinators. However, with some of the aforementioned application methods (such as injections or basal sprays), risks to pollinators can be greatly reduced as these application methods greatly reduce the risk of drift during application onto plants in bloom. (Which is to be avoided anyway.)

Once this pest is in any given area, it will constantly pose a threat to all hemlock, even those that have been treated. Therefore, all hemlocks in these areas should be monitored carefully several times a year and treatments applied when it is found, as necessary. Wind and birds are the primary means for moving this pest from one area to another but humans can also move this pest on plants if care is not taken. Certain species of Asian hemlocks are resistant to this pest and nurseries are beginning to grow and sell those in the United states. The western hemlock (Tsuga heterophylla) is a native species and is highly resistant to this pest but it does not grow well at all outside of its native range of the northwestern USA.

Also see our Hemlock Woolly Adelgid FAQ

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.

Download Pest Sheet

  • 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 MIN-jet Iron 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

Adelges tsugae (Annand)

  • On this page:
    • Description
    • Life Cycle
    • Spread
    • Quarantine
    • Monitoring
    • Control

pdf version

The hemlock woolly adelgid (HWA) is a small aphid-like insect that feeds on hemlock (Tsuga spp.). This insect was introduced from Japan to Virginia in the early 1950’s and has since spread north to Maine and south to Georgia. Known populations in Maine are confined to coastal regions of the state and are scattered. If you think you have found HWA please report it to our office to help us target survey and biological and chemical control.

Hemlocks growing in landscapes can be managed through an integrated approach including monitoring for HWA, cultural practices to enhance tree vigor and limit pest invasion, mechanical and chemical measures to reduce HWA populations and support tree health, and support of natural enemies. Control measures for forest trees are limited, but detection in the forests is important to help limit spread and increase management opportunities.


This insect can be recognized by the presence of a dry, white woolly substance on the young twigs of hemlock. This “wool” can generally be found year round, but it is most abundant and conspicuous in the spring when egg masses are present. The wool covers the insect in all but its earliest life stages. As they feed their woolly covering expands—the “wool” is a waxy material that comes out of pores on the insect’s body. It resembles the tip of a cotton swab, but is up to1/8th inch diameter on average when the insect is mature.

Injury occurs as a result of the insects sucking sap and probably injecting toxic saliva while feeding. Damage from accumulated injuries causes the needles on infested branches to dry, turn a grayish-green color, and then drop from the tree. Buds are also killed, so little new growth is produced on infested branches. Dieback of major limbs progresses from the bottom of the tree upwards, even though the infestation may be found throughout the tree. Trees weakened by HWA often succumb to diseases and wood-boring insects and are readily broken and thrown by wind.

Life Cycle

Hemlock woolly adelgid in our region completes two overlapping generations a year. A general timeline follows. During March and April, adults of the overwintering generation each lay up to 300 eggs within their woolly covering. Crawlers hatch from April through May, and then settle on the twigs near the bases of the needles where they insert their piercing and sucking mouthparts. There they feed throughout their development. This spring generation matures by the middle of June, and deposit an average of 75 eggs each. The crawlers hatch in early July and settle on the new growth. They are generally settled and dormant by the beginning of August. In mid-October feeding resumes and the characteristic woolly covering begins to develop. Nymphs feed during the winter and mature by spring.


Even though it spends most of its life firmly attached to hemlock twigs HWA has been spreading relatively rapidly in North America. Eggs and crawlers, the only stages that are unattached, are present from March through July when they are readily dispersed by wind, birds, deer and other mammals, including people. Moving infested plants any time of the year can result in spread of this pest.


This insect is subject to a State Quarantine. Movement of hemlock material from quarantined areas is restricted.


It is important to detect HWA infestations early to maximize management options. Visual inspections of the undersides of branches are the best way to tell if a hemlock is infested. Because of the HWA lifecycle, hemlocks should be inspected twice a year—at the beginning and end of Daylight Savings Time. The insects will have little wool in November, but should stand out against the dark green foliage of the hemlock By March the wool will be well developed. This may seem practical only for ornamental trees, but a simple sampling system is available for forests.


Cultural Control

A number of cultural practices may reduce the risk of hemlocks becoming infested by HWA. They all work by reducing the risk of exposure to eggs and crawlers of HWA and should be practiced from March through July. Because birds, squirrels and deer are important dispersal agents, any effort to discourage these animals from visiting hemlocks—such as removing bird feeders in the spring and summer—will reduce the risk of those trees becoming infested. Care should also be taken when moving any material from infested areas onto uninfested property. Clean vehicles, clothing, etc., after visiting forests, recreational areas, parks or other properties with infested hemlocks.

Plan any hemlock cutting in and around infested areas for August through February to limit risk of spreading this insect. Prune hemlock branches, both infested and uninfested, likely to come in contact with carriers of HWA such as hikers, campers or delivery trucks. Never move live hemlock from infested areas.

Maintaining good growing conditions can play an important role in the survival of hemlock. Because hemlock is often shallow rooted, it is particularly prone to stress in dry periods. Therefore, during periods of drought, important ornamental hemlocks should be watered to ensure that they receive 1 inch of water per week (including rainfall) over the area beneath the dripline of the crown. Apply water slowly to allow uptake by the tree. Pruning and reducing crowding of target trees may encourage new shoot growth and help support vigor. Although fertilizer may improve the growth and vigor of uninfested trees, the added nitrogen also enhances adelgid survival and reproduction—do not fertilize hemlocks in or near adelgid infested areas.

Mechanical Control

Clipping heavily-infested twigs from branches will reduce adelgid populations. However, extensive clipping may harm appearance and health of the tree. Eggs and crawlers of HWA are often dislodged from hemlock twigs by wind and rain. Most are unable to find their way back onto a host and die. Therefore, directing a strong stream of water at infested branches periodically during April through July may help reduce HWA populations.

Infestations of HWA often start on large hemlocks that intercept the prevailing wind or that are especially attractive to birds and other wildlife. When such a tree becomes heavily infested, it can serve as an effective “launch pad” for adelgid eggs and crawlers. Selective removal of these heavily infested reservoir trees in the fall or winter may reduce local and long distance spread of the pest.

Biological Control

A number of insects feed on the HWA in eastern North America, but overall they have not been able to keep up with adelgid reproduction. Several predator beetles that specialize in adelgid have been introduced in the northeast in the hope that natural controls can be established in the forest. These insects undergo rigorous screening before approval for release. They are not generally recommended for use on ornamental trees because of the time needed for their control to be realized, incompatibility with insecticides, high cost and tendency to disperse. A permit from the Department of Inland Fisheries and Wildlife is necessary before release of these and other animals in Maine.

Chemical Control

Chemical control is an important part of managing the health of HWA infested ornamental hemlocks. Other control measures can reduce adelgid numbers on hemlock, but infested trees usually decline rapidly in the absence of chemical control. It is important to understand that periodic treatments will be necessary over the life of the infested tree to maintain its health and value as an ornamental. The initial decision of whether to use chemicals should weigh the value of the trees relative to the anticipated cost of long term treatments. Consider identifying individual trees or groups of trees that have special value or significance on the property and concentrating control efforts on those trees.

Several pesticides are registered for control of HWA. Some are available for homeowner use, while others are available for commercial use only by a licensed pesticide applicator (list of applicators). An effective method for controlling HWA on ornamentals is to thoroughly drench infested trees with horticultural oil or insecticidal soap. Thorough coverage is necessary for control. This treatment may be needed one or two times a year to protect foliage quality; it has a low impact on beneficial insects.

Systemic insecticides with the active ingredient imidacloprid (eg. Merit 75 WSP, Xytect 75 WSP) can be applied as a soil drench or soil injection. Research from the Connecticut Agricultural Experiment Station demonstrates that the lowest labeled rate is effective on trees up to two feet in diameter at breast height; larger trees require the higher labeled rates. For trees under three feet in diameter, a single application can keep the pest below significantly damaging levels for more than four years. The active ingredient is harmful to a broad array of invertebrates; special care should be taken near water.

Caution: For your own protection and that of the environment, apply the pesticide only in strict accordance with label directions and precautions.

Adapted from University of Rhode Island GreenShare Fact Sheet: Hemlock Woolly Adelgid.

For more information: Maine Forest Service HWA information page, US Forest Service HWA information page

Hemlock Woolly Adelgid Frequently Asked Questions

What is this insect?

The hemlock woolly adelgid (HWA) (Adelges tsugae Annand) is a tiny insect that is closely related to the aphids. It has a piercing-sucking mouth type and feeds on plant sap. It is found on the twigs at the base of the needles on the host plant. As an immature, it is only about a millimeter in size, flat, oval, black in color with a fine ring of white wax around its perimeter. It is a problem with our native Eastern (Canada) hemlock (Tsuga canadensis) and the ornamental Carolina hemlock (Tsuga caroliniana).

Is this a serious problem?

The feeding activity of this insect injures the host plant by withdrawing plant sap. In addition, it is believed that the adelgid introduces a toxin into the plant that eventually damages the vascular system, thus incurring plant death. Hemlocks that are suffering from previous stresses, such as drought, may succumb to this damage in 3-5 years. Hemlocks that are healthy prior to attack, and which are on good growing sites, may tolerate the adelgids’ presence for 7-10 years before showing visible signs of decline. Once found on a hemlock, they should be treated.

What is the life cycle of the HWA?

The HWA is different than most of the insects found in the Northeast in that it is dormant through much of the growing season and active throughout much of the winter. The HWA settles onto the twigs as immatures (crawlers) by mid-July and becomes dormant; they neither feed nor develop during this period. The immatures remain dormant until mid-October when they come out of dormancy and resume feeding. They continue feeding and developing until approximately early March when, as adults, they begin to produce eggs. This is when most people become aware of their presence. The eggs are surrounded by ribbons of a white, waxy material that appears as small cotton balls lined up at the base of the needles. These eggs will hatch and a new generation will begin feeding. These mature in late May – early June and then another batch of eggs is produced. These immatures feed until mid-July and then become dormant. All HWA are females and the vast majority of these are wingless. A very small percentage develop wings and fly away to seek a spruce host. However, there are apparently no appropriate spruce species in this country that support this adelgid and this limited migration of the HWA becomes a dead end for them. Given the cold-tolerant nature of this pest, the HWA has the potential to move throughout the native range of eastern hemlock in all of New England.

How does the HWA get from one tree to another or one geographic region to another if it doesn’t fly?

The adelgid is mostly moved by wind, birds, and mammals, including humans. In fact, it is often moved over long distances on nursery plants that are shipped from areas of infestation to uninfested areas. Migrating birds that roost in infested trees can pick up the eggs in their feathers and then move them to a new area. The HWA originally came into southern New England from more southern states on the winds of a hurricane in the mid-1980’s.

Can this pest be moved on tree-care equipment like chain saws and chippers or in bark mulch?

It is possible to move this pest on equipment. However, the time of year is important. The eggs are most likely the easiest to move from the original host to a new un-infested tree. Research in this area indicates that once nymphs are settled and feeding on a hemlock, moving them to a new host is difficult. Given that bark mulch is made from the bark of the trunk and major branches, there should be limited or no adelgids present in those areas. However, it is not impossible. Bark mulch should not be used as soon as it is made, for a variety of reasons other than the HWA. If six months has passed from the time of manufacture, and the pile has been turned during this period, then very limited movement of the adelgid would be expected onto another host. Also, the mulch is at the base of the plant and this tiny insect would find it a difficult journey to get up onto the twigs in order to feed. It is important to note that New Hampshire, Maine and Vermont do have specific quarantine restrictions on all hemlock material, including live plants, that originates from areas of known infestations of this insect.

Are there any hemlock species that are resistant to this pest?

Yes. The mountain hemlock (Tsuga mertensiana) and the western hemlock (Tsuga heterophylla) are both native plants in this country and are highly resistant to HWA. However, when these plants are under previous stress, they can succumb to this pest. The unfortunate note is that neither of these plants grows very well outside of their native range of the Pacific Northwest in the Cascade Range. A few Asian species of hemlock are being looked at that are also resistant and these may have future potential for New England.

What about natural predators of HWA?

There are a few ladybug beetles that have been researched for their potential in controlling the HWA. The most common one is Pseudoscymnus tsugae. It is about the size of a poppy seed and is all black in color. Some success has been noted in trials but large-scale availability has been hindered by high production costs: the cost of producing these for research purposes has been as high as $1-$2 per beetle and many thousands of beetles may be needed per tree. Other similar species are also being examined.

Are my hemlocks doomed? What should I do?

This insect is manageable in the landscape and nursery if found early and treated. Forest trees pose a different challenge and thus are almost impossible to treat effectively or economically. Many of those may be lost in time. The best overall strategy is to be aware of its signs and to monitor for it on a regular basis. Once found, there are a couple of options.

  • Horticultural oil sprays work extremely well when and where they can be properly utilized. It is important to thoroughly apply the oil throughout the tree. Homeowners can treat their own hemlock shrubs and hedges with this product. Large trees will need to be sprayed by a professional arborist with the proper spray application equipment. A dormant oil spray can be applied while the plant is dormant and the correct weather conditions prevail, generally late March into April. Dormant oils must be applied before the buds open and when there will not be freezing temperatures for 24 – 48 hours after application in order to reduce injury to the plants. Do not spray on cold days. Temperatures of at least 45 degrees F. are recommended. However, avoid overcast or wet days which greatly slows the drying time of the oil.
  • Horticultural oils can also be applied at the “summer” rate during much of the growing season. Again, avoid cool, overcast days or hot and humid days. Oil sprays are used for established populations of the HWA but offer no preventative benefits if the adelgid isn’t present. Hemlocks infested with the HWA should be treated with oil sprays both at the beginning of the growing season and then once again towards the end of summer to insure proper control. Smaller hemlocks (shrubs) may only require one application. Once under control, continue to monitor for future re-infestations and then treat once found.
  • Imidacloprid (e.g. Merit™) is also effective against the HWA. It works best as a systemically applied pesticide either through the soil or by injection into the trunk. For soil application, there is a homeowner version of this product or it may be applied by a professional. When soil applied, it may require 8-12 weeks before the insecticide is up in the tree and coming into contact with the adelgid. When trunk injected (by a professional) the translocation time may only be 3-5 weeks. However, more than one application may be needed in a growing season. If a tree has been infested for several years, imidacloprid may not work well since the host’s vascular system may have been greatly compromised and the product won’t move through the tree to where it is needed. Some general use chemical pesticides are also listed for the HWA and can be applied as a foliar spray. However, many of these will not be effective against the egg stage, while horticultural oil kills all life stages of the HWA, including the eggs.

What’s the best strategy for dealing with the HWA? Should I just cut all my hemlocks down now and plant something else?

The best strategy is to be acutely aware of this pest, its signs and damage, and to monitor for it regularly. Once found, it needs to be treated. The HWA is very manageable but hemlocks need to be thought of as high maintenance plants in those areas where the adelgid already exists. The worst thing to do is to not deal with it when it arrives. Such populations will only lead to the demise of the tree and act as a reservoir for the pest for your neighbor’s trees. Also, hemlock tends to be a rather shallow-rooted tree and prone to drought stress which can exacerbate the problem with HWA. Be sure to water hemlocks deeply (1 inch of water once a week) during times of high temperatures and limited rainfall.

Should I feed my trees with fertilizer to make them stronger in combating the HWA?

By adding fertilizer to the soil around plants, one is not “feeding” the tree. This is a common misunderstanding. Fertilizers offer the tree additional tools to enhance the natural production of their own food. In most cases, the occasional application of small amounts of fertilizer to trees can result in a more vigorously growing tree. However, it is extremely important to note that the addition of nitrogen (N) to hemlocks that are infested with the HWA only leads to a quicker demise of those plants. Insects seek certain building blocks of life from nature in their food sources. In the case of HWA, a tree that is very succulent and high in these particular building blocks offers to greatly enhance the success of the resident populations of HWA on such trees. Research strongly shows that hemlocks that are infested with HWA should not be fertilized with nitrogen.

Written by: Robert Childs
Revised: 10/2011

Hemlock Woolly Adelgid

Adelges tsugae Annand

The hemlock woolly adelgid has been recorded as a pest in Oregon, California, Virginia, Delaware, Maryland, New York, Pennsylvania, New Jersey, Connecticut, Rhode Island, West Virginia, North Carolina, South Carolina, Georgia, Maine, Vermont, New Hampshire, Tennessee, and Kentucky. Generally, this pest has not caused severe damage in the western United States. However, in much of Pennsylvania it has caused significant damage to eastern hemlock, Tsuga canadensis in ornamental plantings and the forest.

Figure 1. Hemlock woolly adelgid “cottony” masses.


The most obvious sign of a hemlock woolly adelgid infestation is the copious masses of white filaments of wax produced by females (Fig. 1). These “cottony” masses normally persist throughout the season and into the following year, even after the insects are dead.

The overwintering females are black, oval, soft-bodied, and about 2 mm long. They are concealed under their characteristic white waxy mass.

Life History

The overwintering adult females begin laying eggs in large clusters in the cottony masses during warm weather in late winter and early spring. They continue to lay eggs into June. The eggs are oblong, 0.25 mm long by 0.15 mm wide, and brownish-orange. Eggs start to hatch in early April, and depending on spring temperatures, hatching is completed by late June.

The newly hatched nymphs or “crawlers” are reddish-brown with a small white fringe near the front. The settled crawler is about 0.3 mm long, black with a white fringe around the body and down the back. The developing nymphs are dark reddish-brown. They continue to increase in size with active feeding. They become mature by late September and spend the winter on trees as mature females.


Hemlock woolly adelgid populations usually are located near the bark at the base of the needles. Host plants are injured by the adelgids inserting their piercing-sucking mouthparts into the base of the needles and removing plant fluids. Moderate hemlock woolly adelgid populations may cause a reduction in tree health. Severe infestations may result in premature needle drop, reduced twig growth, dieback, or death of trees.


The best time to effectively manage this pest is late September through October. Registered insecticides applied according to label directions during this period target overwintering females. A mid- to late June spray may help reduce the number of developing nymphs.

Soil injection of systemic insecticides labeled for management of this pest may be applied by commercial applicators around large trees. This management strategy is appropriate when thorough coverage is difficult to achieve using ground application equipment. Early spring soil injections usually work best against this pest when sufficient soil moisture exists. Prior to soil injecting a registered material, applicators may need to irrigate around an infested tree to provide adequate soil moisture.


Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.

Authored by: Gregory A. Hoover, Sr. Extension Associate

October 2009

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