Cooley Spruce Gall Adelgid on Douglas-Fir

The life cycle of Cooley adelgid is complex and involves distinctly different forms on Colorado spruce and Douglas fir. Damage includes needle discoloration and injury.
Cooley Spruce Gall Adelgid on Douglas-Fir - Articles

Updated: November 21, 2017

Cooley Spruce Gall Adelgid on Douglas-Fir

White, waxy tufting of Cooley spruce gall adelgid. Courtesy of Sandy Gardosik, PDA

Adelges cooleyi (Gillette)

Hosts

  • Douglas-fir
  • Alternate hosts: Colorado blue spruce and occasionally other spruces (see Shoot and Branch Injury section for Cooley spruce gall adelgid on spruce)

Damage Potential

  • Moderate

Symptoms and Signs

  • Yellow spots on the needles
  • Needles with bends or crooks
  • Small, white, cottony balls on the underside of needles
  • Premature needle drop

Causes of Similar Symptoms

  • Rhabdocline needle cast
  • Douglas-fir needle midge

Identification

The Cooley adelgid is a small, soft-bodied insect with sucking mouthparts. Both winged and wingless forms exist, but the winged insect is not often seen. The easiest method of identification is the characteristic damage. On Douglas-fir, Cooley adelgid does not cause galls, as it does on spruce. Instead, it causes pale or yellow spots on the top of the needle. Frequently, the needle will have a bend or crook at this site. These spots or bends indicate feeding sites for the nymphs, which will be found on the underside of the needle. The crawlers and early nymphs are dark brown to black, flat, and have ridges across their bodies. They can be seen using a hand lens with at least 15X magnification. White, cottony masses may also be found on the underside of needles at certain times of year.

In early spring, examine the underside of needles on Douglas-fir. The egg masses will be beneath a waxy mass on needles of outer growth. At bud break, look at the base of elongating shoots for minute, dark nymphs feeding on the new needles.

Calendar of Activities

Biology and Life Cycle

The life cycle of Cooley adelgid is complex and involves distinctly different forms on Colorado spruce and Douglas fir. The pest can alternate between the two hosts or produce continual generations on either. Both winged and wingless forms are found; all adults on Douglas-fir are female.

On Douglas-fir, exposed nymphs overwinter on the underside of the needles (Figure 1). In early spring, these nymphs begin to feed and produce their waxy protective threads. Initially, the white wax is just a fringe around the dark body (Figure 2), but by the time they reach maturity, the entire body is engulfed (Figures 3 and 4).

Figure 1. Overwintering nymphs. Courtesy of Sandy Gardosik, PDA

Figure 2. Nymph with waxy fringe beginning to form around the body. Courtesy of Sandy Gardosik, PDA

Figure 3. Nymph covered with white, waxy fringe. Courtesy of Sandy Gardosik, PDA

Figure 4. Nymphs covered with white, waxy fringe. Courtesy of Brian Schildt, PDA

Two types of females result from these overwintering forms: wingless females that will remain on Douglas-fir and winged females that can fly to spruce and complete a non-damaging generation. The wingless females produce a cluster of up to 100 eggs (Figure 5), which begin to hatch as trees break bud. Nymphs crawl into the opening buds and feed on the elongating needles (Figure 6). Chlorotic spots and bending of the needles result from the nymphs feeding, but no gall results. (Figure 7). Subsequent generations occur on Douglas-fir through the season, but the first generation is the most damaging.

Figure 5. Cooley eggs laid under the waxy fringe of the mature female. Courtesy of Sandy Gardosik, PDA

Figure 6. Cooley nymphs feeding on new growth of Douglas-fir, resulting in needle kinking. Courtesy of PDA

Figure 7. Kinked needles and chlorosis caused by nymph feeding. Courtesy of Rayanne D. Lehman, PDA

Monitoring and Management Strategies

Plantation Establishment

  • When planting, separate Colorado blue spruce and Douglas-fir trees. This will not eliminate problems, but it may help lessen the severity.
  • Plant resistant/tolerant Douglas-fir varieties.
  • Remove any mature Colorado blue spruce or Douglas-fir that may be sources of infestation. Preseason
  • Scout for overwintering nymphs on the underside of needles.
  • Examine the undersides of needles on inner branches as well as last year’s growth.

Growing Season

  • Growing degree days: The recommended control period against nymphs is 22–81 GDDs in the spring (before nymphs wax over) and 2,800–3,000 GDDs in the fall.
  • Threshold level: No threshold has been established. Control for 2 years before harvest to have damage-free needles.
  • At the end of the season, evaluate results and update records.

Control Options

Biological

  • Encourage natural predators such as lacewings, assassin bugs, and lady beetles.

Mechanical

  • No recommendations are available at this time.

Biorational

  • No recommendations are available at this time.

Chemical

  • Dormant oil: Overwintering nymphs can be controlled by applying dormant oil before new growth starts in early spring or in late fall after first frost when trees are not actively growing.
  • Spring insecticide: The first application should occur after nymphs/immature females begin to swell but before they produce white, waxy threads to cover themselves. A second application in 7–10 days may be needed but must occur before bud break.
  • Fall insecticide: A single spray should be applied in late September or October to control the exposed nymphs and immature females before overwintering.
  • Insecticide applications are not recommended during the growing season since most stages will be protected under waxy threads or inside galls.

Next Crop/Prevention

  • Purchase and plant pest-free nursery stock from a reputable company.