What Are Wooly Adelgids: Learn About Hemlock Woolly Adelgid Treatment

By: Jackie Carroll

Hemlock woolly adelgids are small insects that can seriously damage or even kill hemlock trees. Is your tree at risk? Find out about hemlock woolly adelgid treatment and prevention in this article.

What are Woolly Adelgids?

Only about a sixteenth of an inch (1.6 mm) long, woolly adelgids (Adelges tsugae) have a huge impact on hemlock trees in the eastern part of North America. Their feeding practices cause needles and branches to brown and die, and if the infestation is left untreated, the tree starves to death. Here are some interesting facts about these tiny pests:

  • All woolly adelgids are female. They reproduce asexually.
  • As they feed, they secrete waxy filaments that eventually cover their bodies. These filaments give them their “woolly” appearance. The woolly coat protects the insects and their eggs from predators.
  • Woolly adelgids sleep through the summer and are active when temperatures cool.

Hemlock Woolly Adelgid Damage

The woolly adelgid is an aphid-like insect that can grow and reproduce on all types of hemlocks, but only the eastern and Carolina hemlocks decline and die from an infestation. Watch closely for hemlock woolly adelgid damage. Early detection gives your tree a much better chance of survival.

The insects feed by sucking sap from hemlock needles, and the needles die one by one. If nothing is done to stop the infestation, the entire branch may die. Here is a season-by-season list of danger signs:

  • In spring, you may see orange-brown eggs when you look closely at the base of the needles.
  • In early summer, the eggs hatch and upon close inspection you may be able to see tiny, reddish-brown, crawling insects.
  • Summer is the easiest time to spot the insects. They go dormant during the heat of summer, but first they spin little white nests of a waxy, wooly-looking substance. The nests are much easier to see than the insects themselves.
  • Woolly adelgids come out and begin feeding again in fall and winter.

Woolly Adelgid Control

The best treatment of woolly adelgids on a small tree is to spray the tree with horticultural oils. Spray in spring after the eggs hatch but while the insects are still crawling, and follow the label instructions. This method won’t work on large trees. They should be treated with a systemic insecticide by injection or soil treatment. These are short-term solutions.

The treatment must be repeated every year. There are no good organic treatment methods, but scientists are working with some of the woolly adelgid’s natural enemies to see if they can be used to protect hemlock trees.

This article was last updated on

MSU Extension

December 8, 2016 - Author: Deborah G. McCullough

Hemlock woolly adelgid (Adelges tsugae Annad) has been on Michigan’s “most unwanted” list for years. This invasive forest insect has killed hundreds of thousands of hemlocks (Tsuga canadensis) in eastern states. It threatens more than 170 million hemlock trees in Michigan forests, and if not controlled, it will also kill hemlock trees in landscapes.

Small, localized infestations of hemlock woolly adelgid (HWA) were recently discovered in western lower Michigan. Evidence suggests that some of these infestations are at least 10 years old and probably originated when infested hemlock trees from other states were planted in landscapes. Surveys are continuing, and additional HWA infestations may yet be found. This bulletin is designed to help you learn to recognize HWA and understand the potential impacts of this invader in Michigan.

Hemlock shoot with hemlock woolly adelgid.

The Hemlock Woolly Adelgid programs at Cornell are managed through the NYS Hemlock Initiative. Please visit for more information on HWA in New York, HWA management and hemlock conservation strategies around the state.


The hemlock woolly adelgid (HWA, Adelges tsugae) is an aphid-like, invasive insect that poses a serious threat to forest and ornamental hemlock trees (Tsuga spp.) in eastern North America. HWA are most easily recognized by the white “woolly” masses of wax, about half the size of a cotton swab, produced by females in late winter. These fuzzy white masses are readily visible at the base of hemlock needles attached to twigs and persist throughout the year, even long after the adults are dead.

Hemlock woolly adelgid has been in New York for at least the past 20 years. Originally confined to the lower Hudson Valley, it has since moved north to near Albany and west of Buffalo. It was detected in the Finger Lakes region in 2008 subsequent investigation revealed that it was largely confined to the southern parts of Seneca and Cayuga Lakes. In the intervening years HWA has spread, and is now found at higher elevations south of Cayuga Lake and nearly to the north ends of Seneca and Cayuga Lakes it’s also found in the Rochester area

Cottony balls produced by HWA on eastern hemlock twig. Photo by Mark Whitmore, Cornell University.

Hemlock Woolly Adelgid (HWA)

HWA looks like a tiny ball of cotton attached to twigs at the base of needles on hemlock trees. This tiny, aphid-like insect produces a dense mass of waxy hairs to protect it during cold weather, which is the cottony ball that’s visible on the twig. An interesting aspect of its biology is that it actually grows during the winter and through to early summer. During summer and fall is very tiny, without the waxy fluff it finds a place at the base of a needle and waits for the end of fall, when it will begin to grow and produce waxy fibers covering its body. The best time for HWA detection is between February and May. You can find it by looking at the undersides of hemlock branches.

HWA damage causes thinning in hemlock canopy. Photo by Robert L Anderason of the USDA Forest Service, courtesy of bugwood.org.

Managing HWA

So what can you do if you have hemlocks on your property? The most important thing is to see if your hemlocks have HWA on them if you don’t have the bugs, you don’t need to treat. Examine the branches and compare them with the photo included here and others on our website. Once you’ve determined that you have HWA, don’t panic! You have time to act. In many cases even trees whose canopies have thinned can be brought back to full health. We recommend consulting with a licensed arborist and registered pesticide applicator to save your trees, but there is an effective product that homeowners can purchase at the local garden store and use themselves. Always read and follow carefully instructions on the label of any pesticide you are using.

Hemlock Woolly Adelgid

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

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

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

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

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

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

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

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.

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.

Pest Alert – Treating Hemlock Woolly Adelgid

Unfortunately, an insect is killing our hemlock trees. The only option is an insecticide. Treating trees for hemlock trees for woolly adelgid is fairly straightforward.

You simply apply an insecticide to the soil near the tree and the tree takes up the product. The sap of the tree then becomes poisonous to the adelgid which drinks hemlock sap as its food.

You can use liquids that you mix up in a bucket and pour on the soil:

Or you can use pellets that you bury near the tree:

Trees that are not more than 50% defoliated have a good chance to recover.


  • The hemlock woolly adelgid is a tiny, aphid-like insect that is a serious pest of hemlock in Maryland. It was originally introduced from Japan but is not a pest there. The adelgid ranges on the east coast from North Carolina to southern New England.
  • In the eastern US, the adelgid has killed thousands of Eastern and Carolina hemlock in the past 50 years.
  • Adelgids are found primarily on the young branches of mature hemlocks at the bases of the needles.
  • They suck the sap from the branches and may inject a toxin into the tree during feeding.
  • In heavy infestations, the feeding damage results in rapid desiccation and discoloration of the foliage.
  • A heavily infested tree may die within four years.
  • Management
  • Adelgids overwinter as adults. In March and April they begin to lay brownish orange eggs underneath the body of the female which is covered with woolly white wax. In Maryland, the eggs hatch in April and May.
  • The next life stage is the crawlers or immatures. They are reddish brown and are present throughout the summer. The crawler is the dispersal stage and is spread primarily on the wind but also by birds, forest animals, and humans during nursery, logging, and recreational activities.
  • Upon finding a suitable place to feed the crawlers settle, generally on branch terminals at the bases of needles. The settled crawler is about 0.3 mm long, and is black with a white filamentous fringe around its outer perimeter and down the center of its back.

Closeup of Hemlock woolly adelgid crawler
Photo: Kelly Oten, North Carolina Forest Service, Bugwood.org

  • As the young adelgids grow, they cover themselves with a white woolly wax. If the wool is removed the black color of the insect can be seen.
  • They are all females and are about 2 mm when fully grown.
  • There are two generations a year on hemlock in Maryland.
  • All of the adults of the overwintering generation are wingless and remain on hemlock where they lay eggs the following spring.
  • The subsequent spring generation produces mostly wingless adults that remain on hemlock. Some winged individuals may be produced.

Plan now for spring treatment of hemlock woolly adelgid

As we’ve previously reported, the hemlock woolly adelgid has been making its way north along the Lake Michigan shore. But, according to the just complete CAKE CISMA survey, it has (thankfully) not made its way to the island. Still, it is important to keep an eye out both here and on the mainland if you, relatives or friends have property there. The following article from the Michigan DNR provides tips on spotting the HWA, treatment and more. If you believe you have HWA on your island property, please contact the CAKE CISMA office: Phone: 231-533-8363. Email: [email protected]

For more information, see our prior article about the CAKE CISMA survey visit.

If hemlock trees on your property show signs of hemlock woolly adelgid infestation, now is a good time to plan for spring treatment of this invasive species. Hemlock woolly adelgid (pronounced -ə-ˈdel-jəd ), native to Asia, has been detected in Allegan, Ottawa, Muskegon Oceana, Mason and Benzie counties in Michigan. These small insects suck sap from hemlock twigs and ultimately can cause tree death.

HWA ovisacs are found on branch undersides.

Insecticides are available to control the insect, and in many cases, landowners easily can apply them by carefully following label instructions and application rate guidance. Due to certain restrictions on the use of these insecticides, you may need the services of a licensed pesticide application business. If one or more trees are infested, make plans to act this year. Without treatment, trees with hemlock woolly adelgid are likely to die within four to 10 years. Weakened trees on a home landscape could spell disaster during high winds or storms, and eventually they will have to be removed. Loss of hemlocks in forested areas can reduce shade, winter cover, food and habitat for birds, fish and mammals.

Products containing either imidacloprid or dinotefuran as the active ingredient and labeled for use on adelgids are effective in combatting the insect.

  • Imidacloprid moves slowly through trees, taking at least a year to reach the top of a large tree. However, one application will protect the tree for at least four years.
  • Dinotefuran moves through hemlock trees more quickly, making it an ideal choice for large, old trees or those showing decline due to infestation. Dinotefuran protects trees for one to two years.

No matter which treatment you select, be sure your treatment plan will include all hemlocks on your property over the next few years. If hemlock woolly adelgid is on your site, hemlocks without symptoms are very likely to be infested over time. This includes trees on your property as well as neighboring properties. It’s a good idea to discuss treatment plans with neighbors and coordinate efforts when possible.

Can I treat trees myself?

Application of imidacloprid or dinotefuran is simple enough for many landowners to do themselves. Products containing these chemicals are available at garden supply stores, packaged under various trade names in liquid or granular form. Check the label or ask for assistance in selecting the right product.

Hemlock trees have feathery branches.

Imidacloprid and dinotefuran products available at garden supply stores generally are applied to the soil close to the tree trunk, where they are absorbed through the root system. Plan your application for a time between early April and late October when the ground has thawed and soil moisture is moderate – not too dry or saturated. Follow all label directions, wear appropriate safety gear and determine the right application rate to ensure positive results. To protect the environment, do not allow pesticide to enter or runoff into storm drains, drainage ditches, gutters or surface waters. Some products have restrictions on the amount that can be applied to an area per year. Be sure to read the label carefully to determine if the amount you need falls within these limits. If not, you may need to adopt a multiyear plan or hire a professional.

More information on do-it-yourself treatment can be found in the MSU Extension bulletin: Guidelines for homeowner treatments of hemlock trees infested with hemlock woolly adelgid, available at Michigan.gov/HWA.

When should I call a professional?

Pesticide injection by certified applicator.

Licensed pesticide application businesses have a broader range of options for applying treatments than consumers, and their professional skills are recommended in certain situations. A county-by-county list of businesses holding pesticide application licenses can be found on the Michigan Department of Agriculture and Rural Development’s website, Michigan.gov/MDARD. Look for one that is licensed in the “ornamental” category (3B). If your hemlock trees are within 75 feet of a body of water or in areas with a high water table, or if flowering plants or shrubs are growing around the hemlocks you wish to protect, a trunk injection or bark treatment may be necessary to avoid affecting the environment, groundwater or other insects. Professional applicators can provide these types of treatments.

What should I expect after treatment?

Hemlock woolly adelgid’s cottony, white ovisacs will linger for a time following treatment. If trees are treated in the spring, check new growth in late fall or winter for any fresh signs of infestation.

After treatment, trees should be checked every year. If the insect has returned after dinotefuran was used, reapplication may be needed after one to two years. For imidacloprid, consider retreatment every four to five years.

Do my trees have hemlock woolly adelgid?

If you have hemlock trees on your property, it is important to check them for signs of hemlock woolly adelgid, which infests only hemlock trees. If you are not sure whether your trees are hemlocks, use the Michigan Invasive Species Program’s eastern hemlock identification guide. The adelgid’s round, white, cottony ovisacs are most visible in the winter and are located on the undersides of hemlock branches at the base of the needles. The publication Hemlock Woolly Adelgid Look-Alikes, available at Michigan.gov/HWA, provides images and information on identifying this and other pests commonly mistaken for it.

If you have hemlock trees on your property, it is important to check them for signs of hemlock woolly adelgid, which infests only hemlock trees. If you are not sure whether your trees are hemlocks, use the Michigan Invasive Species Program’s eastern hemlock identification guide. The adelgid’s round, white, cottony ovisacs are most visible in the winter and are located on the undersides of hemlock branches at the base of the needles. The publication Hemlock Woolly Adelgid Look-Alikes, available at Michigan.gov/HWA, provides images and information on identifying this and other pests commonly mistaken for it.

HWAs are hard to see. Best to look
for their ovisacs as shown above.

How do I report an infestation?

If you suspect trees on your property have hemlock woolly adelgid, report it using the Midwest Invasive Species Information Network at MISIN.MSU.edu. You can report from the field using the MISIN smartphone app, which will log the location and allow you to upload photos of the suspect signs of the insect.

You also can take pictures, note the tree’s location and email the information to the Michigan Department of Agriculture and Rural Development at [email protected] or report by calling 800-292-3939. Someone will respond to let you know if hemlock woolly adelgid is present or not.

Please do not clip infested branch samples and transport or mail them. This could accidentally spread the insect to new areas. A state interior quarantine makes it illegal to move hemlock anywhere within or out of Allegan, Ottawa, Muskegon, Oceana, or Mason counties. Currently there is no known hemlock woolly adelgid in Benzie County, as the single-tree detection was destroyed. Waste hemlock material in the quarantined counties may be moved to approved disposal sites within the quarantine zone.

For more information on identifying and managing hemlock woolly adelgid, visit Michigan.gov/HWA.

Michigan’s Invasive Species Program is cooperatively implemented by the Michigan Departments of Agriculture and Rural Development, Environment, Great Lakes, and Energy and Natural Resources.

Contact: Rob Miller 517-614-0454, or Joanne Foreman 517-284-5814

Watch the video: Hemlock Wooly Adelgid Insect: Killer of Hemlocks!

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