Galls in general are not normally harmful to trees and shrubs. However, several galls occur in Illinois that not only are an aesthetic concern but also may have an economic impact on plants. These include horned oak gall (Callirhytis cornigera) and gouty oak gall (Callirhytis quercuspunctata), which are caused by cynipid wasps, and the cooley spruce gall adelgid (Adelges cooleyi) and eastern spruce gall adelgid (Adelges abietus). These are caused by adelgids that resemble aphids, except they don't possess cornicles.

Cooley spruce gall adelgid has a complex developmental sequence that uses alternate hosts to complete its life cycle. In some situations, this insect can continue to infest one host without the other being nearby. It takes 2 years to complete development on Douglas fir and spruce. Galls are formed on spruce but not on Douglas fir. This insect causes spruce trees to create conelike growth 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. They eventually turn brown. Also, adelgid feeding causes needle distortion and yellowing.

Winged adults migrate from Douglas fir to spruce around July. On twig terminals, females lay eggs that hatch into nymphs that overwinter on spruce trees. Just before budbreak in spring, the nymphs become active. They eventually develop into females that can each lay over 100 eggs on spruce terminals.

Eggs hatch into nymphs that crawl to the base of needles and feed. When the adelgids feed, they inject saliva that causes spruce trees to produce a succulent, cell-like compartment, or gall, about 2 to 3 inches long surrounding each individual adelgid. Generally, 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 lay eggs. This produces another generation of adelgids. Sometimes Douglas fir is so heavily infested that the needles appear to be sprinkled with snow. These eggs hatch into nymphs that overwinter on Douglas fir. No gall is formed on Douglas fir.

Just before budbreak, nymphs become active; their feeding causes needle distortion or yellowing. These nymphs develop into adults (in spring) and then pro-duce offspring (in summer) that may be either winged or wingless. Wingless adults remain on Douglas fir; winged adults migrate to spruce to complete the cycle. Two or more parthenogenic generations may occur each year on the needles of Douglas fir.

Management of this adelgid involves dealing with the pest on both spruce and Douglas fir trees. Pest-control materials recommended include carbaryl (Sevin), chlorpyrifos (Dursban), imidacloprid (Merit), insecticidal soap, and summer oil. Spruce: Apply sprays in spring just before budbreak to kill nymphs overwintering on trees as eggs. In addition, this prevents adults from laying eggs on young needles. Removing galls by pruning does not control the problem. Douglas fir: Apply pest-control materials such as insecticidal soap or summer 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. Adelgid feeding causes spruce trees to create pineapple-shaped galls, 1/2 to 1-1/2 inches long, at the base of new growth. Their feeding also distorts shoot growth. Galls 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, before budbreak, adelgids mature into adults, and each female deposits 100 to 200 eggs under a mass of cottony wax. Egg hatch normally occurs when buds open (in spring) and when new needles are exposed.

Feeding by the nymphs on new needles and then the base produces abnormal twig growth and then galls, which develop at the basal portion of shoots. The galls contain many cells filled with immature adelgids. Adelgids inside the gall are protected from natural enemies and environmental conditions. Galls detract from the beauty and symmetry of a spruce tree. Also, if galls are abundant in large numbers, this can decrease a tree's vitality or weaken it.

The galls eventually crack open in summer, and mature nymphs crawl out. The nymphs settle on a needle, and then change or transform into winged, egg-laying females. Females generally lay eggs near the tips of needles. The eggs hatch into young nymphs on terminal twigs, where they overwinter.

Pest-control materials advised for cooley spruce gall adelgid may also be used for managing eastern spruce gall adelgid. Apply sprays to trees just before budbreak to kill nymphs that hatch from the cottony egg masses at the base of needles.