Insect Profile: Eastern Spruce Gall Adelgid (Adelgis abietis)
Eastern spruce gall adelgid (Adelgis abietis) is an insect that inflicts considerable damage to ornamental spruce trees. The larvae induce the formation of cone-like galls on developing twigs, which deforms and girdles them. Severe infestations can disfigure, and weaken trees, rendering them more susceptible to invasion from tree pathogens, and other insects.
Distribution & Habitat
Eastern spruce gall adelgid was introduced to North America from Europe sometime before 1900. The insect has since become widespread throughout the northeastern and midwestern United States, and southern Canada.
Eastern spruce gall adelgid most commonly infests Norway and white spruce. Infestations may also occur on blue and red spruce, albeit with less frequency.
Description & Life Cycle
Eastern spruce gall adelgid produces one generation each year. The insect overwinters as an immature female. Adult females are often referred to as stem mothers. Adult females become active in mid-April, reaching maturity soon thereafter. Once mature, the females lay one hundred to two hundred olive green eggs on host trees. They cover the eggs in a waxy, thread-like material. Yellow nymphs hatch from the eggs within seven to ten days, emerging as bud expansion begins. The newly hatched nymphs navigate to the needle bases to feed. As their feeding persists, the bases of the lower needles in the bud become slightly swollen, and discolored. Within a few weeks, the needle bases swell into tightly closed chambers. The insects’ feeding soon induces the formation of galls, which envelop the nymphs. Galls are green when they first appear. They turn tan or brown as they age. The galls serve as protection for the nymphs, concealing them from predators, parasites, and inclement weather. The nymphs progress through three instars while in the galls. In mid to late summer, the galls open, and the nymphs reemerge. The nymphs transform into winged females. Some remain on the host, while others fly to nearby spruce trees where they lay up to sixty eggs near the tips of needles. The adults expire soon after they have laid their eggs. The eggs hatch, revealing masses of nymphs, which attach themselves to a terminal twig at or near the dormant bud to overwinter.
Effects on Trees
Eastern spruce gall adelgid causes small pineapple-shaped galls to form at the base of new shoots. These galls stunt the growth of the shoots. Severe infestations can disfigure, and deform trees. Galls are an aesthetic nuisance as well. They reduce the ornamental value of host trees.
- When planting, select resistant varieties of spruce. Black, red, and Englemann spruces exhibit an increased resistance to eastern spruce gall adelgid.
- Prune, and dispose of green galls to reduce adelgid populations.
- Applications of dormant oil in early spring will effectively control overwintering nymphs, and prevent gall formation. Oil should not be used on blue spruce, as it can alter the tree’s coloration.
- Contact insecticides applied at budbreak will limit nymph populations. Additional applications can be performed in late September or early October to eliminate overwintering nymphs, and prevent gall formation the following spring.
Photo courtesy of Ohio State University.