By Whitney Cranshaw, professor and extension specialist in entomology, Colorado State University, 1998
Where you will find it: The Cooley spruce gall aphid distorts the terminal growth of spruce. This abnormal growth, or "gall", appears cone-like and is a light brown color after weathering. Galls are found in greatest number on the north and east sides of the tree. Incidence of galling has been unusually abundant over the past couple of years along much of the Front Range.
Habits of the Cooley spruce gall aphid: The insect is a type of "woolly aphid" that covers itself with white waxy threads. During the winter and early spring it is on the underside of spruce twigs, usually within a couple of inches of the buds. Just pinhead size through the winter, it resumes feeding and swells rapidly during late April and May. Ultimately the females produce a large mass of eggs, which hatch in synchrony with bud break.
The young aphids move to emerging needles and start to feed. They suck a bit of sap but also inject substances in their saliva that induce the new growth to swell and overgrow them. Protected by the plant tissues, they continue to grow within sells of the gall, becoming full grown in late spring. At this time the galls begin to dry out and crack open, and the Cooley spruce gall aphids crawl out on the needles. With one more molt they transform to a winged stage that leaves the tree permanently. A Douglas fir is their ultimate destination, their "alternate host". However, no galls are produced on Douglas fir and damage is slight. At the end of the growing season a few new winged forms will leave the plant and colonize spruce to establish new infestations.
Control of Cooley spruce gall aphid: This is almost entirely a cosmetic problem, so there is little need for controls to protect the health of the plant. However, many find the galls unattractive. The insect is easily managed by spraying the underside of twigs to kill the overwintering stages in spring before bud break Carbaryl, permethrin and horticultural oils are effective for control of Cooley spruce gall.
There is considerable variation in susceptibility of individual spruce trees to this insect. Some appear to have fairly chronic problems. Other trees, particularly many with a deep blue coloration, seem to have fair resistance.
Removing old galls has no effect on Cooley spruce gall management, as the insect has already migrated from the galls and they are never again occupied. Such removal may be useful if one is bothered by their appearance. Left alone they are buried by subsequent new growth and are little visible after a year or two.
For more information, see CSU Fact Sheet 5.534.
Photos: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010