Spruce trees are highly susceptible to insect infestations that cause symptomatic swelling problems on plant surfaces. This problem has the potential to cause severe damage to spruces. Familiarize yourself with the cause and symptoms associated with these insects and subsequent effects. Knowledge of effective control methods will keep spruce trees healthy in the home garden.
Extremely small insect pests that resemble aphids infest spruce trees and lead to the development of galls. The two species of insects responsible for this problem are the eastern spruce gall adelgids (Adelges abietis) and Cooley spruce gall adelgids (Adelges cooleyi). These insects attack nearly every species of spruce as well as Douglas firs, according to the University of Wisconsin Extension.
Adelgid nymphs feed on buds, releasing a poisonous saliva as they eat. The saliva results in swelling, referred to as a gall. Eastern spruce gall adelgids cause the formation of galls at the base of newer shoots; galls reach a length of 1 1/2 inches and take on a shape that resembles a pineapple, according to the University of Wisconsin Extension. Cooley spruce gall adelgids result in the formation of galls at the tips of stems. Purple or green galls reach a length of up to 3 inches with a diameter of up to 3/4 inch.
Though eastern spruce gall adelgids do not kill the stems on which they grow, Cooley spruce gall adelgids do. By the middle of summer, fully developed galls change to a brown color and die, killing any attached foliage. Spruce branches often die and in extreme cases, stunted tree growth occurs, according to the University of Wisconsin Extension. Trees become cosmetically marred as galls remain on the tree.
For natural control of galls on spruce trees, remove and destroy plant parts affected by galls during spring. Galls are still developing during the spring season, so damage and future infestation is minimized with removal. The University of Wisconsin Extension suggests against planting spruce trees near Douglas firs as they both act as host plants to gall adelgids. Additionally, sanitize pruning tools between each cut and between uses from one plant to another to inhibit the spread of insects and disease.
For chemical control, apply an insecticide for stronger control during any season but winter, particularly during cases of severe gall formation. Completely cover all foliage for effective management. For preventive control, spray during spring before bud break to control egg-laying adults or spray during autumn from September through October to prevent overwintering of adelgids that will emerge and lay eggs in spring. Sprays during summer are diagnostic in nature, controlling active adelgids; summer sprays do not remain active into the fall. Choose horticultural oil or insecticidal soap as your chemical control measure, according to the Cornell University Cooperative Extension.