Cooley spruce gall adelgid
Cooley spruce gall adelgids (Adelges cooleyi) are gall forming hemipterans. During the spring and summer, adults appear as white, cottony spots on affected trees. When infestations are heavy, entire trees can be covered in a white cottony substance, which is produced by females to protect the eggs. In early stages, individuals are around 1.3 mm (1/20 inch) in length and have black, flat, oval bodies with a protective coating of white cotton-like wax. The egg masses are white, cotton-like, and deposited on spruce twigs.
- Cooley spruce gall adelgids are small insects often covered with a white, cotton-like wax for protection.
- Feeding of these insects introduces a compound to plant tissues that promotes the formation of galls. Galls begin to form on spruce trees in the spring or beginning of summer. Young galls are green but gradually turn brown by mid-summer. The galls are often mistaken for pinecones.
- Cooley spruce gall adelgids typically complete their life on both spruce and Douglas fir, but tend to form galls only on spruce trees.
- Feeding injuries on Douglas fir appear as yellow spots and bent needles, while the injury on spruce is usually only aesthetic. If occurring in large numbers, the adelgids can affect seed production in Douglas fir.
Adults of the Cooley spruce gall adelgid. Note the white, cotton-like substance covering the insects. Image credit: William M. Ciesla, Forest Health Management International, Bugwood.org
Blue spruce with galls produced by the Cooley spruce gall adelgid. Plant fluids can be seen protruding from the gall, indicating that adults will soon emerge. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Broken gall showing the developing Cooley spruce gall adelgids on a blue spruce. Image credits: Whitney Cranshaw, Colorado State University, Bugwood.org
Beginning stages of a gall on a Sitka spruce. Image credits: Christine Buhl, Oregon Department of Forestry, Bugwood.org
Gall formed by the Cooley spruce gall adelgid. Notice the browning of the gall, indicating that the winged form has emerged. Image credits: William M. Ciesla, Forest Health Management International, Bugwood.org
Life history and habits
Cooley spruce gall adelgids alternate between two hosts, Douglas fir and spruce, and take one year to complete their life cycle. Female nymphs overwinter underneath young spruce branches. When spring begins, the overwintered female nymphs mature into adults before laying eggs near developing buds. The eggs hatch during bud break and the newly emerged nymphs migrate to new growth where they feed at the base of developing needles, resulting in the formation of galls. Immatures actively feed and develop in chambers inside the gall. The galls are green and inconspicuous at this stage. In mid-summer, winged adults emerge from galls which causes the chambers to open and the gall to dry out. After emerging, most of the adults migrate to Douglas fir to lay eggs and the abandoned galls become increasingly noticeable after a few weeks.
No galls are produced on Douglas fir. In late summer, some of the females develop wings and migrate to spruce to deposit eggs. Overwintering nymphs emerge in the fall. Females that do not migrate to spruce produce overwintering nymphs on Douglas fir.
The saliva of Cooley spruce gall adelgids induces physiological changes in the host tree that result in the formation of galls. Often, the presence of galls on a tree is an indicator of infestation. On spruce, galls are formed on the tips of branches and typically kill new growth. However, injury to spruce trees is aesthetic and causes the tree little to no harm. The cucumber-shaped galls are around 5-10 cm (2-4 inches) in length. They are found on the new growth of spruce trees and are often mistaken for pinecones. In the spring, the galls are light green but will dry out and turn brown in the summer. Most galls are formed on the shaded sides of trees, and individual trees seem to vary in their susceptibility to the adelgids and likelihood of developing galls.
On Douglas fir, immatures of the Cooley spruce gall adelgid feed on the needles which can cause discoloration, distortion, and premature needle drop. In large numbers, these insects can affect seed production. They also excrete honeydew which can lead to the growth of sooty mold on the tree.
Planting spruce and Douglas fir close together should be avoided. Since both trees are usually needed to complete development, maintaining a large distance between them will disrupt the life cycle of this pest.
Galls can be pruned from small spruce trees before adults emerge in July. Removing old, brown galls will only improve the aesthetic appearance of the tree since adults will have already emerged.
Control is generally not necessary, and densities of the Cooley spruce gall adelgid vary from one season to the next. To prevent the formation of galls for aesthetic purposes, insecticides can be applied in the fall or spring before eggs are laid.
Pesticides are most effective when applied before the formation of galls. This can be done in the fall or in the spring before females lay eggs in late April. Horticultural oils can also provide some protection but tend to cause temporary discoloration on spruce needles. Foliar applications of insecticidal soaps are most effective on Douglas fir when applied to the underside of spruce terminals to target overwintering insects.
Childs, R., 2011. University of Massachusetts Amherst – Center for Agriculture, Food, and the Environment. Available https://ag.umass.edu/landscape/fact-sheets/cooley-spruce-gall-adelgid
Government of Canada. 2015. Cooley spruc gall adelgid. Government of Canada. Available https://tidcf.nrcan.gc.ca/en/insects/factsheet/5307#:~:text=The%20presence%20of%20the%20Cooley,in%20the%20spring%20and%20summer
Munson, S., 2010. Cooly Spruce Gall Aphid. United States Department of Agriculture – Forest Service. Available https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5187372.pdf
Maine Forest Service. 2000. Cooley Spruce Gall Adelgid. Maine Department of Agriculture, Conservation, and Forestry. Available https://www.maine.gov/dacf/mfs/forest_health/insects/cooley_spruce_gall_adelgid.htm
Swier, S., 2016. Spruce Gall Adelgid. University of New Hampshire – Extension. Available https://extension.unh.edu/sites/default/files/migrated_unmanaged_files/Resource002819_Rep4174.pdf
USU. (n.d.). Cooly Spruce Gall Adelgid. Utah State University – Extension. Available https://extension.usu.edu/pests/ipm/ornamental-pest-guide/arthopods/aphids-adelgids/cooley-spruce-gall-adelgid