Figure 63. Galls on spruce caused by Cooley spruce gall agelgid.
Symptoms/Signs: Cone-like galls, 12-75 mm long, are formed on spruce branch
tips as a result of feeding by the nymph stage of this insect. In
late spring, galls are light green to purplish in color and the
nymphs may be found inside. In late summer and fall, galls dry up
and open and the nymphs emerge, flying to Douglas-fir to complete
the next phase of their life cycle. The galls may persist on the
branches for several years.
Figure 64. Cross section of gall showing nymphs of spruce gall adelgid on spruce.
On Douglas-fir the insect feeds on the sap of new needles, new shoots,
and developing cones. Nymphs are oval and black with a white waxy
fringe. Adults are dark brown and covered with white, wooly wax.
Feeding on Douglas-fir does not cause galls to form but rather yellowing
and twisting of the needles.
Biology: The entire life cycle requires
2 years. There are six stages in addition to the eggs and crawlers
when both host trees are present.
Figure 65. Waxy filaments ("cotton") created by Cooley spruce gall adelgid on Douglas-fir branch.
Effects: This insect has a very complex life cycle involving two hosts,
spruce and Douglas-fir. On spruce, galls are unimportant in forest
situations. However in nurseries, plantations, and on ornamentals,
galls are a concern since they kill branch tips and can stunt and
deform trees.
On Douglas-fir, infested needles turn yellow and become twisted.
Heavy infestations on poor sites result in defoliation.
Similar Insects and Diseases: Damage on
Douglas-fir is similar to that caused by needle midges or needle
cast.