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Adelges tsugae

Hemlock woolly adelgid. Photo: Tawny Simisky
Scientific Name: 
Adelges tsugae
Common Name: 
Hemlock Woolly Adelgid
Growing Degree Days (GDD's): 
Approx. 26-1000 GDD's (eggs), 1001-2200 GDD's (nymphs), Base 50F, March 1st Start Date (Source: National Phenology Network.)
Host Plant(s) Common Name (Scientific Name): 
Carolina hemlock (Tsuga caroliniana)
Eastern hemlock (Tsuga canadensis)
Insect Description: 

The hemlock woolly adelgid (HWA) was first detected in the eastern US in the 1950's. This pest was first reported in Connecticut in 1985 and Massachusetts in 1988. HWA has four forms known as progrediens (April and May), sistens (July - March; overwintering generation), sexuparae, and sexuales. Each of these forms goes through the following life stages: egg, nymph (4 instars), and adult. Adult females are approximately 1/16th of an inch in length, black, and circular in shape. They can be found feeding at the base of the needles, covered with a white, fluffy wax that may look like mini cotton balls at the base of the needles. Crawlers and nymphs are smaller, black in color, and sometimes ringed with a fringe of white waxy material. In the northeast, all hemlock woolly adelgids are female and reproduce parthenogenetically. HWA does most of its feeding during the cooler parts of the year, and completes most of its development from October through June. During the summer months, it spends its time as a first instar nymph (which is very difficult to see) at the base of hemlock needles in a summer dormancy known as aestivation. This generation (sistens) overwinters, and the females produce viable eggs in February and March the next year. Approximately half of those eggs develop into winged, migratory sexuparae who mature into adults and migrate to a spruce host (tiger tail spruce, Picea torano; not present in the US), where sexuales develop and a gall is formed. (Recently, in the US, massive migrations of the sexuparae have been reported on beaches in the Northeast. See Havill et. al 2022: "Mass deposition of hemlock woolly adelgid sexuparae on New England beaches" in the Journal of the Acadian Entomological Society.) The remaining sistens eggs become progrediens which develop into adults which remain on trees into June and July and produce eggs. These eggs hatch and become the crawlers (nymphs) of the sistens generation which settle at the base of needles and aestivate until October (Johnson and Lyon, 1991). The hemlock woolly adelgid is considered non-native to the eastern US (native to Japan and China), but native to the western US where western hemlock is mostly resistant. 

Damage to Host: 

Hemlock woolly adelgid can kill previously stressed trees in 3 to 5 years. The twigs and base of the needles of heavily infested trees appear coated with snow, due to the presence of the ovisacs created by the females. Needles turn yellow and drop prematurely, leading to sparse canopies. Large trees may be killed without intervention. Mature trees and trees planted on stressed sites may be the first to succumb to the hemlock woolly adelgid. Trees on healthy sites in forests in Massachusetts have in some cases persisted with adelgid infestations for decades, but often with reduced vigor and impacts to the ecological functioning (pre-HWA infestation) of the forest.


Monitor for cottony egg sacs on twigs at base of foliage starting in late February through June and again in the late summer into the fall. Check the lower branches first. Use a hand lens to observe the insects under magnification. At least 10X magnification is required to observe the presence of eggs and egg hatch beneath the waxy ovisacs in the spring. 

Cultural Management: 

Hemlocks that show resistance include Japanese hemlock (Tsuga diversifolia), mountain hemlock (Tsuga mertensiana), and western hemlock (Tsuga heterophylla). This insect is more active in cold weather. Avoid nitrogen fertilizer for infested trees, as the added nitrogen may help the adelgids.

Natural Enemies & Biological Control: 

Because there are currently no known parasites of adelgids, the biological control of hemlock woolly adelgid has focussed on national and international searches for predators and pathogens that may reduce HWA populations. Historically, three beetle predators (Sasajiscymnus tsugae, Laricobius nigrinus, and Scymnus sinuanodulus) have been released widely in the eastern United States for the biological control of the invasive hemlock woolly adelgid. Additional predators from China, Japan, and western North America continue to be researched.

Two species of silver flies from the Pacific Northwest may be potential biological control agents of hemlock woolly adelgid in the eastern United States (Leucotaraxis (formerly Leucopis) argenticollis and L. piniperda). Both species are predators of hemlock woolly adelgids that are native to the western United States and it is hoped that they could help reduce the introduced population of HWA in the eastern US.

Additional biological control agents of hemlock woolly adelgid includes but is not limited to: Laricobius osakensis.

Chemical Management: 

Abamectin (NL)

Acephate (NL)

Acetamiprid (L)

Azadirachtin (NL)

Bifenthrin (NL)

Chlorpyrifos (N)

Chromobacterium subtsugae (NL)

Clothianidin (NL)

Cyantraniliprole (NL)

Dinotefuran (NL)

Flonicamid+cyclaniliprole (N)

Horticultural oil (L)

Imidacloprid (L)

Insecticidal soap (NL)

Pyrethrins+sulfur (NL)

Tau-fluvalinate (NL)


Horticultural oils applied in the fall and spring, when temperatures and weather conditions allow, have been useful in managing this insect. 

Active ingredients that may be applied systemically include: abamectin (injection), acephate (injection), acetamiprid (injection), clothianidin (soil drench), cyantraniliprole (soil drench, soil injection), dinotefuran (soil drench), and imidacloprid (soil drench).

Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.

Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.

Read and follow all label instructions for safety and proper use. If this guide contradicts language on the label, follow the most up-to-date instructions on the product label. Always confirm that the site you wish to treat and the pest you wish to manage are on the label before using any pesticide. Read the full disclaimer. Active ingredients labeled "L" indicate some products containing the active ingredient are labeled for landscape uses on trees or shrubs. Active ingredients labeled "N" indicate some products containing the active ingredient are labeled for use in nurseries. Always confirm allowable uses on product labels. This active ingredient list is based on what was registered for use in Massachusetts at the time of publication. This information changes rapidly and may not be up to date. If you are viewing this information from another state, check with your local Extension Service and State Pesticide Program for local uses and regulations. Active ingredient lists were last updated: January 2024. To check current product registrations in Massachusetts, please visit: .