Galls, in general, are not harmful to trees and shrubs. In fact, they can make plants look more attractive (in my opinion). However, there are several galls or gallmakers that occur in Illinois that are not only an aesthetic concern but also may have an economic impact on trees and shrubs. These include the horned oak gall (Callirhytis cornigera) and gouty oak gall (Callirhytis quercuspunctata), which are caused by cynipid wasps, and Cooley spruce gall adelgid (Adelges cooleyi) and eastern spruce gall adelgid (Adelges abietus). These last two galls are caused by adelgids, which are insects that resemble aphids—except they don’t possess the cornicles (tailpipes).
Cooley spruce gall adelgid has a complex development sequence that utilizes alternate hosts to complete its life cycle. In some situations, this adelgid can continue to infest one host without the other host being nearby. It typically takes 2 years to complete development on Douglas fir and spruce. Galls are formed on spruce—not on Douglas fir. Cooley spruce gall adelgid causes spruce trees to form conelike structures on the tips of new growth. Susceptible hosts include Colorado blue, Sitka, oriental, and Engelmann spruce. The galls are green to purple, 1 to 2 inches long, and resemble small pineapples (think Hawaii). These galls eventually turn brown. Adelgid feeding may also cause needle distortion and yellowing.
Winged adults migrate from Douglas fir to spruce around July. On the terminal growth, female adelgids lay eggs that hatch into nymphs, which overwinter on spruce trees. Prior to budbreak in spring, the nymphs become active. They eventually develop into adult females, with each female capable of depositing over 100 eggs on the terminal growth of spruce. The eggs hatch into nymphs that crawl to the base of needles and feed. As the adelgids feed, they inject saliva that contains compounds that cause spruce trees to produce a succulent, cell-like compartment, or gall, about 2 to 3 inches long, which surrounds each individual adelgid. During summer, the galls formed on the tips of twigs on spruce become woody and open, releasing winged adults that migrate to Douglas fir, where the females deposit eggs—producing another generation of adelgids. Sometimes Douglas fir may be so heavily infested that the needles appear to be sprinkled with snow. These eggs hatch into nymphs that overwinter on Douglas fir. Again, no gall is formed on Douglas fir.
Just before budbreak, the nymphs become active and initiate feeding, which causes needle distortion or yellowing. These nymphs develop into adults during spring and then produce offspring in the summer that may be either winged or wingless. Wingless adults remain on Douglas fir, whereas winged adults migrate to spruce to complete the life cycle. Two or more parthenogenetic (without mating) generations may occur each year on the needles of Douglas fir.
Management of Cooley spruce gall adelgid involves dealing with the insect on both spruce and Douglas fir trees. Insecticides recommended for control include carbaryl (Sevin), imidacloprid (Merit), insecticidal soap, or a summer (horticultural) oil. For spruce, apply sprays in spring just before budbreak to kill the nymphs overwintering on trees as eggs. This also prevents adult females from laying eggs on young needles. It should be noted that removing galls by pruning would not control the problem. For Douglas fir, apply recommended insecticides such as insecticidal soap or summer (horticultural) oil to control nymphs and prevent further infestations. When installing a new landscape, it is best to avoid interplanting spruce with Douglas fir.
Eastern spruce gall adelgid attacks Norway, white, and black spruce. Feeding by eastern spruce gall adelgid causes spruce trees to form pineappleshaped galls (again, think Hawaii) that are 1/2 to 1-1/2 inch in length, at the base of new growth. Adelgid feeding also distorts shoot growth. Galls that are produced in midsummer open and release adelgids. However, unlike Cooley spruce gall adelgid, these remain on the spruce. Eastern spruce gall adelgid overwinters as an immature female in cracks and crevices at the base of buds. In spring, prior to budbreak, adelgids mature into adults, and each female deposits 100 to 200 eggs under a mass of cottony wax. Eggs hatch, in general, when buds open in the spring and when new needles are exposed.
Feeding by the nymphs on new needles and then on the base produces abnormal twig growth and then galls, which develop at the base of the shoots. The galls contain many cells filled with immature adelgids, which are protected from natural enemies and environmental conditions. These galls detract from the aesthetics and symmetry of spruce trees. Additionally, if galls are abundant in large numbers, this can decrease tree vitality or weaken it to the point of increasing susceptibility to wood-boring insects.
The galls eventually open in summer, and mature nymphs crawl out. Nymphs settle on the needles and then change or transform into winged, egg-laying females. The females generally deposit eggs near the tips of needles. Eggs hatch into young nymphs that congregate on terminal growth, where they overwinter.
Insecticides recommended for control of Cooley spruce gall adelgid may also be used for eastern spruce gall adelgid. Be sure to apply sprays to trees just before budbreak to kill the nymphs that hatch from the cottony egg masses located at the base of needles.