Gall Aphids and Woolly Aphids on Spruce and Hemlock

Newly formed galls on white spruce.

Newly formed galls on white spruce.

U.S. Department of Agriculture
Forest Service - Alaska Region
March 1984

Printed by
Cooperative Extension Service
University of Alaska
and U.S Department of Agriculture Cooperating


by Edward H. Holsten and Wayne Vandre


Gall aphids and woolly aphids are commonly encountered by the homeowner on ornamental spruce and hemlock. These aphids seldom become abundant enough to cause serious damage to forest stands in Alaska. Adverse climatic conditions, such as cool temperatures, as well as parasites and predators usually check aphid reproduction and population growth. Several species can, however, become quite damaging to ornamentals and young forest trees. This brochure is intended to provide the homeowner with descriptions of aphid damage and aphid life history and guidelines to help reduce damage by aphids.



Gall aphids are common on white spruce in south central and interior Alaska. These aphids cause the tree to form conspicuous, cone-shaped galls on the twigs of spruce (Cover Photo). The aphids live within the protective galls for part of their life cycle. The galls are seldom abundant enough to cause serious damage to the tree. Repeated attacks by the insect, however, can result in dead twigs and a generally ragged appearance of foliage, which usually is of great concern to the homeowner. The galls are somewhat cone-like in appearance and range in length from one-half to two inches. They are light green to dark purple while developing in the first year, but become reddish brown the second year after the aphids have vacated the galls.

Woolly aphids are more commonly encountered than gall aphids. During the spring and summer, woolly tufts appear on the needles and twigs of both white spruce and western hemlock. At times the aphids are so abundant that the infested branch or tree appears to be covered with snow (Figure 1). Similar to the gall aphids, woolly aphids are of little consequence in the forest, but large populations can seriously weaken and sometimes kill ornamental trees. Aphid feeding causes a loss in plant vigor as a result of the removal of plant juices, and infested foliage becomes yellowish brown. After severe infestations, the foliage may turn brown and fall, from the branches.

Figure 1. Woolly aphids on white spruce.

Figure 1. Woolly aphids on white spruce.


The life history of these insects is unusual and complex. The identifications and biologies of Alaskan woolly aphids and gall aphids are not fully understood.

Woolly aphids (Adelges tsugae (Annand.)) on western hemlock are common throughout south central Alaska, but their biology in Alaska is unknown. In Canada and the Pacific Northwest, however, a one-year life cycle is common for this species, which feeds only on hemlock. The young aphids are oval, 1/32 to 1/16 of an inch long, and yellow-brown in color. In mature stages they are dark brown to black and are covered with a white, waxy wool. This woolly material is secreted by the aphids and serves as a protective covering. Populations of woolly aphids probably consist only of females which are able to reproduce asexually (without mating). Aphids are "cold-blooded" and are quite responsive to changes in temperature. When temperatures increase in the spring and summer, large aphid populations appear in a relatively short time. As temperatures are usually higher in residential areas than in forest settings, aphid populations are usually larger in residential areas.

The life cycle and identification of woolly aphids on spruce are not fully understood in Alaska, but the insects are believed to be either the ragged spruce gall aphid (Pineus similis (Gill.)); eastern spruce gall aphid (Adelges abietis (L.)); and/or an asexual population of the Cooley-Spruce gall aphid (Adelges cooleyi (Gill.)). The life history of the ragged spruce gall aphid and the eastern spruce gall aphid is described in the following paragraph. Certain stages of these aphids produce waxy, white filaments. The Cooley-Spruce gall aphid normally feeds on both spruce and Douglas-fir; it produces galls on spruce and wooly tufts on Douglas-fir. The aphid must complete part of its life cycle on Douglas-fir to produce galls on spruce; however, populations of the aphid that do not cause galls live on spruce year-around and produce woolly tufts. The latter appears to be the case in Alaska where Douglas-fir is not found except as a rare ornamental.

The life history of gall aphids on spruce in Alaska is likewise not well-known. Both the ragged spruce gall aphid and the eastern spruce gall aphid attack all spruces, especially white spruce, and occur across the continent. They have no alternate hosts, and repeated attacks by the insects often result in dead twigs and a generally ragged appearance of foliage. There can be two generations of these aphids per year. At certain life stages, they produce waxy, woolly tufts. Eggs are laid by the overwintering aphids in May. The eggs give rise to three types of aphids, two of which cause galls to form. The galls increase in size during the summer until they open, releasing the winged adults. In July these adults lay eggs which produce the new overwintering generation.


The suppression of woolly aphids and gall aphids is usually not warranted on forested lands. In landscape situations, where spruce and hemlock are grown as ornamentals, these aphids can be more damaging to the health of trees than in forest settings, and the homeowner may select one of the following alternatives for reducing damage by aphids.

Alternative I: If aphid feeding is low to moderate and spruce and hemlock are vigorous and show little needle discoloration and galling, damage is minimal, and the use of pesticides is usually not warranted. These aphids are preyed upon by numerous enemies, including lady bugs, lace-wings, syrphid flies, and mites. However, the following steps should be taken:

Figure 2. Location and depth of fertilization.
  • Care should be taken to avoid damaging the trunk, injuring the roots, altering the drainage patterns, or severely compacting the soil. Make sure that trees receive adequate water through-out the growing season. Excess soil should not be placed on top of or removed from the area over the root zone. Such actions can cause water stress and/or suffocation of the trees.
  • Spring fertilization helps promote tree vigor. The University of Alaska Cooperative Extension Service recommends that 1 to 2 pounds of fertilizer per inch of tree diameter be applied by making a concentric series of holes 8 to 10 inches deep around the tree starting 2 feet from the trunk and extending a few feet beyond the dripline (Figure 2). Any complete lawn or garden fertilizer high in phosphorus is adequate. Fertilization should begin in the spring and continue through the summer. Stop fertilization before the tree goes into fall dormancy. This feeding program may not be necessary every year. Fertilizer uptake, soil type, rainfall, weather, and grass cover all will determine the frequency of reapplication.
  • If the tree is lightly infested with gall aphids, control can be effected by pruning and burning the green, closed galls in spring and early summer. Removal of brown galls is ineffective because the aphids have left them.

Alternative 2: If spruce and/or hemlock have large populations of woolly and/or gall aphids, chemical control may be necessary. However, these aphids can be difficult to control, and strict attention to timing of spray applications is essential. In general, chemical control is more effective when directed at the immature stages, particularly in the spring. For control of gall aphids, a systemic insecticide, such as Orthene® (acephate) or Cygon® (dimethoate), can be applied as the buds begin to swell in the spring. The white woolly aphids can be sprayed anytime. The systemic insecticides named above provide adequate control. Because of the small size of the aphids and their protective waxy, woolly cover, thorough spraying is necessary to saturate the foliage if contact insecticides, such as Cythion® (malathion) and Diazinon® (diazinon), are used. A spreader-sticker may increase the effectiveness of contact insecticides. When using insecticides, all label instructions should be followed.

CAUTION: Pesticides can be injurious to humans, domestic animals, desirable plants, and fish or other wildlife, if they are not handled or applied properly. Use all pesticides selectively and carefully. Follow recommended practices for the disposal of surplus pesticides and pesticide containers. Mention of a pesticide in this publication does not constitute a recommendation for use by the USDA, nor does it imply registration of a product under Federal Insecticide, Fungicide, and Rodenticide Act, as amended. Mention of a proprietary product does not constitute an endorsement by the USDA.

Additional information on this insect can be obtained from your local University of Alaska Cooperative Extension Service office, Alaska State Forestry office, or from:

Forest Health Protection
State and Private Forestry
USDA Forest Service
3301 C. Street, Suite 202
Anchorage, AK 99503-3998.
Phone: (907) 743-9455

Forest Health Protection
State and Private Forestry
USDA Forest Service
2270 Sherwood Lane, Suite 2A
Juneau, AK 99801
Phone: (907) 586-8811

Institute of Northern Forestry
USDA Forest Service
Fairbanks, Alaska 99701

Gall Aphids and Woolly Aphids on Birch and Hemlock, by Edward H. Holsten, Entomologist, U.S. Forest Service, Alaska Region, State and Private Forestry, and Wayne Vandre, Horticulture Specialist, Cooperative Extension Service, University of Alaska, March 1984.

The University of Alaska's Cooperative Extension Service programs are available to all, without regard to race, color, age, sex, creed or national origin.

Issued in furtherance of Cooperative Extension work, acts of May 9 and June 30,1914, in cooperation with the U.S. Department of Agriculture, Dr. James W. Mat thews, Director, Cooperative Extension Service, University of Alaska.