Author: Scott M. Salom, Virginia Tech
Distribution and Hosts
The balsam woolly adelgid was introduced from Europe to North America around 1900. In the west, they have been found only in northern Idaho, but may occur in Montana. In the east, it has become an important pest of true firs and is established throughout the Fraser fir type in the southern Appalachians. The balsam woolly adelgid attacks all true fir Abies species, the most susceptible being Balsam Fir Abies balsamea and Fraser Fir A. fraseri. European fir species such as Silver Fir A. alba, having evolved alongside the insect, show greater resistance, but are not immune. It also causes considerable damage to the Fraser fir Christmas tree industry.
Description of Damage
The balsam woolly adelgid is considered a serious pest of forests, seed production, landscapes, 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.
During the adelgid's feeding process, the host tree is stimulated to produce abnormal wood cells. This reduces the tree's ability to translocate food and water. Initial symptoms of adelgid attack may include gouting of buds or twig nodes and some twig and branch dieback. This is very evident on seedlings, young understory trees, and Christmas tree plantings. Other damage may be stunted shoot and needle growth and loss of apical dominance in natural stands. A heavily infested tree may die within 2 to 7 years. As the tree, dies, portions of the crown or the entire crown will turn red.
Nymphs feed on the bark of all parts of the tree injecting a chemical which causes abnormal cell division. This produces annual rings composed of thick cells ("compression wood") in the stem, and stunting of terminal growth with distinct swellings around the buds and branch nodes ("gouting"). All sizes of trees are attacked, but infestations may be concentrated on the stems or in the crowns. Stem-attacked trees may be killed after 2-3 years of heavy feeding.
Adult adelgids are blackish purple, roughly spherical in shape, less than 1/32 inch (1 mm) long, and almost invisible to the naked eye. The adelgid produces a covering of white wax threads and appears as white, woolly dots about the size of pin heads on the surface of the tree's bole, limbs, and buds. Eggs are produced under the adults and are orange in color. The immature stage of the adelgid, known as a "crawler," is also orange, with legs and black eyes. Eggs and crawlers can be identified with the aid of a hand lens. 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 over 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.
The most obvious indicator of the aphids' presence is the white "Wool"-covered females on the bark of stems or branches during summer months. Without the wool, adults are about one-sixteenth of an inch long and dark purple to black in color. Overwintering nymphs are about one thirty-secondth of an inch long, amber colored, flattened, and fringed with whitish wax. "Gouts" can be on outer branch nodes and terminal buds, and can stop production of new shoots. Dying or dead branches and crowns are other symptoms.
May be mistaken for damage caused by scale insects, however, the "Wool-" covered females and "gouts" are distinctive.
The aphid has two generations per year, and occasionally three in the southern Appalachians. Eggs of the first generation hatch in late June and July, followed by a second generation in September and October. The "crawler" is the only mobile stage in the adelgid's life cycle. When a crawler begins feeding, it transforms into a first instar nymph and becomes stationary. Reproduction is parthenogenic each female lays up to 200 eggs. The adelgid overwinters as a first instar nymph and continues its development in the spring when the host tree starts its annual growth cycle.
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 over 40% Balsam Fir, and over 45 years old, are most susceptible. Trees over 25 and under 45 years old are moderately susceptible, and trees under 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. Other control measures include removal and destruction of infested trees.
Chemical control is effective, but extremely costly. Thus it's usually limited to high value resources such as recreation areas, seed sources, and shade, ornamental, and Christmas tree plantings. For chemical recommendations, please refer to the latest edition of the pest management guidelines for your region.
- 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 Lyon, H. H. 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. Canad J. For. Res. 5: 195-209.
- Insects and Diseases of Trees in the South. 1989. USDA Forest Service - Forest Health Protection. R8-PR16. 98 pp. Taken from http://fhpr8.srs.fs.fed.us/forstpst.html
- Furniss, R. L., and V. M. Carolin. 1977. Western For. Insects. USDA Forest Serv., Misc. pub. 1339, 654 p., illus
- Mitchell, R. G., G. D. Amman, and W. E. Waters. 1970. Balsam woolly aphid. USDA For. Serv., For. Pest Leaflet 118, 10 p., illus. Adelges piceae (Ratzeburg)
- Field Guide to Diseases and Insect Pests of Idaho and Montana Forests, USDA Forest Service Northern Region, Publication Number R1-89-54
- Salom, S. Balsam Woolly Adelgid. Publication 444-233, August 1996. http://www.ext.vt.edu/departments/entomology/factsheets/balwoade.html