Two species of small, soft-bodied insects infest spruce species during the course of rather complicated life cycles. These aphid-like insects called adelgids, produce galls that can disfigure and even kill their hosts. The two adelgids that can be found in the Chicago region are the eastern spruce gall adelgid (Adelges abietis) and the Cooley spruce gall adelgid (Adelges cooleyi).
Eastern Spruce Gall Adelgid: The adult female is a small, bluish-green, sucking insect covered by cottony, waxy strands. The eggs are dark olive-green to black and oval shaped. Nymphs range in color from yellow to bluegreen and can grow to about 1/8 inch long.
Cooley Spruce Gall Adelgid: Adults are dark reddish-brown with two pairs of wings and threadlike mouthparts used for sucking the plant juices from host trees. Nymphs are small, yellowish ovals, fringed with white waxy threads.
Eastern Spruce Gall Adelgid: Norway and white spruces are most frequently bothered, but they are also known to attack red, black and Engelmann spruces. Colorado blue spruce is occasionally damaged.
Cooley Spruce Gall Adelgid: This insect most frequently infests blue spruce (Colorado and Koster), as well as Sitka, Engelmann, and some oriental species. A secondary host is the Douglas-fir, used as a migratory feeding site.
Eastern Spruce Gall Adelgid: Two generations of female adelgids are produced each year on spruce hosts. Immature females will overwinter on branches near the terminal (end) buds. Before the buds open in early spring, eggs will be laid at the base of the buds. When they hatch 7-10 days later, the emergent young nymphs crawl to the base of the needles of tender new shoots bursting from bud sheaths and begin feeding. As they feed, they form their characteristic gall, about 1" long, which will completely enclose and protect them from weather, disease, and predators as they mature. While the gall is forming, the spruce shoot is growing through it, leaving the gall behind at the base of the stem. In August, the galls dry out and crack open, exposing nymphs that will crawl onto the needles and molt to become winged adults. These adults will fly to spruce branches where about 100 eggs are laid under webs of white, cottony wax on the unprotected new growth. Within a week, the eggs will hatch and the second generation of nymphs feed for a short time on new growth before settling near the buds to overwinter.
Cooley Spruce Gall Adelgid: In effect, it takes two years for the Cooley spruce gall adelgid to complete its life cycle. Eggs are laid in spring under white, cottony wax masses near the terminal bud. Nymphs will hatch to feed at the base of needles of the expanding buds and form the multi-chambered galls that enclose and protect them. The galls will dry up by midsummer and burst open, exposing the nymphs. The newly emerged winged adults will either continue their life cycle on the primary host spruce or fly to its secondary host, Douglas-fir, to lay the next generation of eggs. Because this generation will overwinter on the Douglas-fir, no galls are formed but feeding damage may occur. When this generation emerges the following spring, usually by mid-June, winged forms are produced that will fly back to the primary host spruce. At this time, they will lay the eggs that emerge to produce the gall-forming nymphs. If the adult stays on the host spruce, it will overwinter on twigs, making it possible to complete its life cycle on either one or two hosts. Injury will be more serious when the insect alternates between the two.
The adelgid gall formed by the feeding at the nymph stage of both insects inflicts the most damage. These cone-like growths will stunt the growth of the twigs on which they formed and distort the symmetrical beauty of the tree. Severe infestations can cause a branch to be killed. Because the galls of the eastern spruce gall adelgid develop at the base of shoots, the stems are weakened, making them more likely to break under the weight of snow or other physical stress. This basal location makes it impossible to remove galls without removing the entire twig, again detracting from the tree’s form and beauty.
The eastern spruce gall is pineapple-shaped, greenish-purple, and about 1/2 to 1" long at the base of seasonal twig growth. The cooley spruce gall adelgid produces a 1-3" long gall on the tips of new growth of host spruces. If they have settled on Douglas-fir to feed and overwinter, the feeding will cause prominent yellow spots on needles that can become bent and distorted. If an infestation is heavy, the tree can become so covered with the characteristic white egg masses that it can look snow-covered.
Use resistant plants where possible. When shopping for spruce, look for trees that are not already infested with galls. Removing galls which are still green is possible and usually can be done before mid- July. If the infestation is light, the galls can be hand-picked, thereby avoiding pruning, which will further disfigure the tree.
Look for signs of infestations in late March to early April. Since adelgids become active before the trees begin to show signs of breaking dormancy, it is important to start looking for them early. If you wait too long to spray chemical controls, the white cottony egg masses will be laid and will be impermeable to spray applications.
Note that any dormant oil sprayed on bluishhued trees will cause the foliage to turn dark green. The original blue color will not return until new growth is formed. If Cooley spruce gall adelgid needs to be controlled, make sure to include any nearby Douglas-fir in your coverage. Douglas-firs should be treated during their dormancy or when crawlers are first visible.
Spring sprays can be applied in mid-April, just after bud break, before females mature to lay eggs.
Fall sprays can be applied in September and October to control the winged females and overwintering immature females.
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