Balsam Woolly Adelgid

Contact: Eric Day, Manager, Insect Identification Laboratory

Publication 444-233, August 1996

Homomptera: Adelgidae, Adelges piceae (Ratzeburg)

Distribution and Hosts

Native to central Europe, the balsam woolly adelgid is now distributed throughout eastern and western North America. It attacks all true firs, Abies spp., including balsam and Fraser fir.

Description of Damage

Introduced from Europe around 1900, the balsam woolly adelgid is considered a serious pest of forest, seed production, landscape, and Christmas trees. It generally concentrates either on the outer portions of tree crowns or on the main stem and large branches. Stem infestations are usually more serious, causing greater levels of damage and mortality.

Crown infestations are characterized by abnormal drooping of the current shoots and gouting of the outer twigs. The crown becomes increasingly thin and dieback may occur. Persistent crown infestation can kill a tree over a number of years. Stem attacks are characterized by the conspicuous presence of white woolly masses that, under heavy attack, give the lower bole a whitewashed appearance.

The wood responds to adelgid feeding in an "allergic" manner that causes swelling of the sapwood, and results in gouting of the twigs and increased heartwood formation in the sapwood, called "rotholz or redwood." This abnormal growth of sapwood tissue inhibits water flow within the tree.

Identification

The life stages include the egg, three nymphal stages, and the adult. Adult females (there are no males) are wingless, oval, purplish-black insects, about 0.8 mm in length, and are covered with secretions of waxy threads that appear as a dense white wool mass. A female is capable of laying > 200 eggs in a cluster near her body. The first instar "crawlers," reddish-brown and about 4 mm in length, are the only stage of the insect capable of moving and dispersing. Once the crawler finds a suitable feeding location, it inserts its tube-like mouthparts into the bark of the host and remains there for the rest of its life. The second and third instars are about 0.5 to 0.65 mm in length, respectively, and closely resemble the adult.

Life History

The winter is passed as a dormant first instar nymph. Development is completed in the spring, with adult numbers peaking in late May or early June. Second generation adults peak in mid August and produce the overwintering generation.

In forest situations, silvicultural and management techniques can be used to reduce adelgid populations and damage. Tree stress may be minimized by thinning overstocked stands, by fertilizing sites of poor nutrient quality (although some nitrogenous fertilizers such as urea can increase insect survival), and by replanting or encouraging more tolerant tree species and varieties. A damage-hazard rating system based on site and stand characteristics associated with severe adelgid damage can be used to aid in management decisions (Page, 1975). The main variables used in the system are site elevation, soil moisture regime, percent balsam fir by basal area, total basal area of balsam fir, and stand age. In general, lower elevation, dry sites with > 40 percent balsam fir, and > 45 years old are most susceptible. Trees > 25 and < 45 years old are moderately susceptible, and trees < 25 years old are least susceptible. In Christmas tree plantations, if only a few trees are infested, rogueing or burning those trees should be sufficient for managing the pest.

Chemical control can be used effectively on ornamental, seed production, and Christmas trees. For chemical recommendations, please refer to the latest edition of the Pest Management Guide for horticultural and forest crops, Virginia Cooperative Extension Publication 456-017.

Prepared by S.M. Salom, Department of Entomology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0319.

References

Amman, G.D. 1962. Seasonal biology of the balsam woolly aphid on Mt. Mitchell, North Carolina. J. Econ. Entomol. 55:96-98.

Anonymous. 1989. Insects and Diseases of Trees in the South. USDA For. Serv. Protection Report R8-PR16.

Drooz, A.T. 1985. Insects of Eastern Forests. USDA For. Serv. Misc. Publ. No. 1426.

Hain, F.P. 1988. The balsam woolly adelgid in North America. pp. 87-109. In A.A. Berryman (ed.). Dynamics of Forest Insect Populations. Plenum Publ. Corp.

Johnson, W.T. and H.H. Lyon. 1988. Insects That Feed on Trees and Shrubs. Cornell Univ. Press. Ithaca, N.Y. 556 pp.

Page, G. 1975. The impact of balsam woolly aphid damage on balsam fir stands in Newfoundland. Can. J. For. Res. 5:195-209.

Visit Virginia Cooperative Extension