Balsam Woolly Adelgid

Adelges piceae

Invader Images

    • The adelgids' protective woolly coating of wax
    • BWA
    • Swelling around buds and branch nodes
    • Stunted terminal growth
    • Foliage turns yellow to red to brown.
    • Fiddle-shape to the tree crown

Identification

Balsam woolly adelgid is an insect that affects true fir trees such as the balsam fir. The insect is tiny, only visible on its host as a white, wooly spot. 

Signs and Symptoms

  • Stunting of the terminal growth
  • Swelling around buds and branch nodes
  • Retarded growth
  • “Fiddle-shape” to the tree crown
  • Foliage yellowing to red to brown

See image slideshow above for signs and symptoms.

Biology

Origin

Balsam woolly adelgid is native to central Europe and was brought to the northeastern US around 1900, most likely on infested nursery stock. The insect was found on the West Coast in 1929 and in the southeastern US in the 1950’s. The insect is now common throughout the native range of true firs in eastern North America and along the Pacific West Coast.

Habitat

True firs are the only known hosts of the balsam woolly adelgid (BWA). BWA occurs on balsam fir in the northeastern Unites States and Maritime Provinces of Canada,  bracted balsam fir and Fraser fir in the Appalachian mountains, subalpine fir in the western United States, and Pacific silver fir and grand fir in the Pacific Northwest.

Life Cycle

BWA populations in North America are composed entirely of females. The males that are needed to complete the life cycle are not produced because the alternate host isn't present. 

BWA overwinters as an immature nymph. Nymphs mature into adults in March or April. As the nymphs mature, they get plump and produce a wooly coating of wax to protect themselves from predators. This covering completely hides the adults from view. Eggs are laid in a clutch behind the female. In the lab, females have produced up to 200 eggs. This number is presumably much smaller in nature. 

Crawlers hatch from the eggs within a month. Crawlers seek out a suitable site to feed (if they don't they will die) and they never leave this site again. Crawlers molt into nymphs and sink their feeding tube into the bark of the tree. Once the insect is back in its adult stage, it will lay eggs in that same spot. 

During the growing season, it takes about one month for nymphs to molt into adults. BWA may have two to the generations per year, depending on temperature. At any given time during the growing season, you may find all of the life stages of this insect: egg, crawler, nymph, and adult. 

Ecological threat

The introduction of balsam woolly adelgid has caused a reduction in tree species diversity and age structure in forests across the United States. This has had a cascading effect on species that depend on firs, such as moose, snowshoe hares, and songbirds for food and habitat.

Several species of insect predators have been introduced into North America from other parts of the world, primarily Europe. These include 3 beetles - Laricobius erichsonii (Rosenhauer) (Derondontidae); Pullus impexus (Mulsant) (Coccinellidae), and Aphidecta obliterata (L.) (Coccinellidae); and 3 flies - Aphidoletes thompsoni Mohn (Cecidomyiidae), Cremifania nigrocellulata (Czerny) (Chamaemyiidae), and Leucopis obscura Haliday (Chamaemyiidae). 

Trees themselves can respond to BWA attack by forming patches of a wound layer in the bark that is impermeable to insect attack. Cold weather has also proven to be effective in managing BWA populations. 

Management Options

  • It is possible to grow balsam fir in a state infested with balsam woolly adelgid. We’ve been doing so for eighty-plus years.
  • Look for an increase in BWA following mild winters.
  • Where fir mortality is occurring, especially on upland sites and where larger-crowned trees are dying first, consider that BWA could be the cause, even if the signs of BWA are inconspicuous. If signs of BWA are difficult to find, the infestation might have collapsed.
  • The presence of BWA may justify a shorter rotation age, as stand vulnerability increases with age.
  • Even where BWA has killed trees in the overstory, fir can be regenerated if desired.
  • Parts of North America don’t have BWA. Shipping live fir trees or freshly cut woody material to these regions could spread the insect. Currently, within North America, only the State of Michigan has a quarantine regulating the importation of fir.
  • For more information, consult Vermont Forest Health: Balsam Woolly Adelgid

Vermont Distribution

Balsam woolly adelgid has spread throughout Vermont.

Citations

Photo Credit

Bruce Watt, University of Maine, Bugwood.org

Ladd Livingston, Idaho Department of Lands, Bugwood.org

Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

North Carolina Forest Service, Bugwood.org

USDA Forest Service Region 8, Southern USDA Forest Service, Bugwood.org

information credit

North Carolina State University

USDA Forest Service Forest Insect and Disease Leaflet 118

Vermont Forest Health Balsam Woolly Adelgid Leaflet