Outbreaks of Spruce Aphid Damage Coastal Washington Sitka Spruce

Glenn Kohler, Forest Entomologist, Washington State Department of Natural Resources, glenn.kohler@dnr.wa.gov

Dave Houk, Forester, Grays Harbor Conservation District, dave.houk@graysharborcd.org

Sitka spruce in coastal areas of Washington and Oregon began showing widespread foliage loss and discoloration caused primarily by outbreaks of spruce aphid (Elatobium abietinum) this spring.

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Spruce aphid damage on needles of Sitka spruce. (Photo by Elizabeth Willhite, U.S. Forest Service)

By early summer, many affected trees had lost significant amounts of older foliage as warmer weather dried the damaged needles. Fortunately, spruce aphid rarely damages buds at the tips of branches, so many of the damaged trees were able to flush new growth by summer. However, a few trees have also lost much of their new foliage, possibly due to drought stress and other pests that feed on new needles such as other conifer aphids and bud moths.

Past outbreaks of this insect have been short-lived due to natural controls; including cold winter temperatures, late spring frosts, starvation from lack of nutritious foliage, and attack by natural enemies.

Spruce aphid is a non-native insect that was likely introduced from Europe and became established on the west coast in the early 1900s. The tiny yellow-green aphids are less than 1/16-inch-long and feed on sap in conifer needles, primarily on Sitka spruce and ornamental spruces. Periodic outbreaks have occurred on Sitka spruce trees from northern California to Alaska.

Spruce aphids actively feed and can produce multiple generations during the winter months, a time when few natural enemies are around to control them. Spruce aphid populations are primarily kept in check by cold winter temperatures, so development of outbreaks is most likely related to higher aphid survival during unusually mild winters.

In 2019, the annual insect and disease aerial survey recorded approximately 11,000 acres with Sitka spruce damaged by spruce aphid within 20 miles of the coast in Washington (estimate based on draft data and subject to change).

Damage ranged from the Columbia River north to Neah Bay on the Strait of Juan de Fuca, but the majority of damage (approximately 9,000 acres) was recorded in areas around Willapa Bay and Grays Harbor, including the Long Beach Peninsula and communities from Ocean Shores north to Taholah. The severity of damage was highly variable, even within a single stand. Some spruce appeared largely unaffected while neighboring spruce were nearly completely defoliated.

Typical symptoms of spruce aphid include loss of old needles, most severely affecting the base and shady portions of the tree. Less heavy infestations result in sparse older needles that may be spotty yellow or brown. Predicting the long-term effects to individual spruce trees in the first year of damage can be challenging because branches and trees that appear dead may actually have intact, healthy buds that will flush in spring 2020. If you are unsure whether a tree should be removed, either wait until spring bud break or if possible, examine new buds to see if they are normally formed and succulent.

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Heavy foliar damage is seen on Sitka spruce in Grays Harbor County. (Photo by Glenn Kohler, Department of Natural Resources)

The percentage of crown defoliation is a less reliable predictor of survival but can provide a faster assessment of a larger area. Trees with less than 50 percent defoliation are likely to survive and recover as aphid populations are reduced. Trees with up to 75 percent defoliation are also likely to survive unless they are weakened by other factors like drought stress or pests damaging new foliage. Branch or whole tree mortality may occur in those trees with more than 75 percent defoliation.

The most recent widespread outbreak of spruce aphid near the Washington coast occurred in 1998, when 12,400 acres with damage were recorded in some of the same areas affected in 2019. The damage was less noticeable the following year, and the majority of affected spruce recovered after aphid populations collapsed.

Management of spruce aphid in forest stands is usually not needed because outbreaks usually collapse after the first year and moderately damaged trees recover. Assessments of severely damaged trees for identification of potential salvage or hazard trees are more reliable following bud break. Monitoring of aphids on needles is best done in late winter (February to early March) to determine if populations are still high. Tapping branches over a sheet of paper or white cloth will make it easier to see these tiny aphids.

If high aphid populations are present in winter 2020, ornamental spruce or high-value trees can be protected using insecticides that target sucking insects. To be effective, insecticides need to be applied in late winter or early spring (March to early April), before needle drop occurs. Always read and follow the label when using pesticides. Avoid fertilizing trees during aphid outbreaks, because this can actually benefit sap-feeding insects.