The Cooley spruce gall adelgid, Adelges cooleyi (Gillette), is often incorrectly referred to as an aphid. This close relative of aphids is originally from North America, but is now worldwide in its distribution, wherever its hosts are grown.
This adelgid makes a pine cone shaped gall on Colorado spruce but lives openly on Douglas-fir needles and branches. It also makes galls occasionally on Englemann and sitka spruces.
The young nymphs, called sexuparae and progredientes, feed on tender, expanding needles causing them to twist, curl and discolor. Certain trees seem to react more than others. The summer generation attach to the under surface of fully hardened needles and apparently cause little damage other than forming small waxy balls.
Though this pest is often called an aphid, it is actually a member of a closely related group, the adelgids. Aphids have a pair of cornicles, tubes on the upper tip of the abdomen, while adelgids do not. Adelgids also have very short antennae. The Cooley spruce gall adelgid has four distinct forms which can be found on Douglas-fir during a season. This pest also infests Colorado spruce where it forms a gall. On Douglas-fir, all the forms inhabit the needles and do not form a gall.
In nature, this insect alternates between spruce and Douglas-fir by undergoing a complex two-year life cycle. However, the adelgid can continue generations on Douglas-fir, year after year, without having to go to Colorado spruce. On Douglas-fir, the adelgid overwinters as an immature female (sistentes) attached to the underside of needles. In the spring, March to mid-April, these small dark brown adelgids begin to suck sap and molt into wingless females covered with long, white, waxy threads. The sistentes lay 10 to 30 eggs in the wax and these eggs produce two types of offspring - winged asexual females (sexuparae) and wingless asexual females (progredientes). Both of these forms move to the new needles, insert their mouthparts for feeding and maturation. The sexuparae fly in search of Colorado blue spruce while the progredientes lay eggs which hatch into new sistentes. By this time, the adults from spruce galls are arriving to produce additional sistentes which overwinter.
The most evident damage caused by this insect is by the first spring generation. Thus, controls are generally directed towards killing the overwintering sistentes before they lay eggs. In the past, we recommended that Douglas-fir and Colorado spruce not be planted together. As can be seen by the life cycle description, the adelgid can easily continue breeding on Douglas-fir. Insecticides used for control of this pest must cover the needles thoroughly. Because of this requirement, mist blowers have not been generally effective.
Strategy 1: Use Resistent Varieties - Certain varieties and provenances of Douglas-fir seem to be resistent or tolerant to adelgid attack. Usually the green-colored Douglas-firs are more resistant.
Strategy 2: Use Natural Controls - Though this adelgid may discolor the needles, the yellowing tends to disappear by the. This will allow for natural predators to build up. However, the spring generation often cause twisting of the needles and this distortion on some trees may not be acceptable.
Strategy 3: Summer Insecticide Sprays - Insecticide sprays will usually kill the active adelgids anytime during the growing season. However, summer sprays will not kill the migrating forms which arrive in the fall, and needle damage may have already occurred in the spring. See Bulletin 504 for currently registered insecticides.
Strategy 4: Fall Insecticide Sprays - Sprays applied in late-September through October kill the overwintering adelgids which cause the spring needle damage. Thorough coverage is needed on the inner branches where most of the adelgids attach for the winter. See Bulletin 504 for currently registered insecticides.
Strategy 5: Degree-Day Timing of Sprays - This insect becomes active in the spring at about 25-30 DD50 degrees F. Start accumulating degree-days in January or February and observe tree buds for the beginning of wax production after 25-30 DD50 have been accumulated. Eggs will probably be laid by 100 DD50. The time between first wax production and egg laying is ideal for insecticidal control.
Strategy 6: Delay Spraying Until Harvest - Since this pest does not kill Douglas-fir or kill needles, sprays may not be needed until two years before harvest in Christmas tree plantations. During the last two years of growth, sprays can be applied so as to produce a good looking product for sale. The older damaged needles will not be evident at cutting time. See Bulletin 504 for currently registered insecticides.
NOTE: Disclaimer - This publication may contain pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registrations, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author and Ohio State University Extension assume no liability resulting from the use of these recommendations.
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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