Spruce aphid (Elatobium abietinum) is a non-native insect that was likely introduced from Europe and became established on the west coast sometime in the early 1900s.
These tiny, yellow-green aphids are less than 1/16 inch long and feed on sap in conifer needles, primarily those of Sitka spruce and other ornamental species of spruce.
Typical symptoms of spruce aphid include the oldest needles on the tree becoming sparse with yellow or brown colored spots.
Heavier infestations can lead to the loss of old needles throughout the shadiest portions of the tree, such as in the interior canopy near the trunk, among the lowest branches, or other areas that may be shaded by adjacent trees.
Sitka spruce in coastal areas of Washington and Oregon began showing widespread symptoms of foliage loss and discoloration caused by a spruce aphid outbreak in spring, 2019.
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.
In some instances, trees also lost much of their new foliage.This is possibly due to drought stress or other pests that feed on new needles such as other species of conifer aphids or bud moths.
Historical records cite periodic spruce aphid outbreaks from northern California to Alaska.
The last time there was a significant outbreak of spruce aphid near the Washington coast occurred in 1998 when 12,400 acres with damage was recorded in some of the same areas affected this year.
Records indicate that tree damage in 1998 was less noticeable the following year and the majority of affected spruce recovered after aphid populations collapsed.
Outbreaks of this insect are typically 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 aphids actively feed and reproduce multiple generations during the winter months when cold temperatures usually keep their populations in check. Development of outbreak-level populations is most likely related to higher aphid survival during unusually mild winters, such as coastal areas experienced in the winter of 2018/2019.
On July 1 of this year, the Washington DNR and USDA Forest Service’s annual insect and disease aerial survey recorded approximately 11,000 acres* of Sitka spruce damaged within 20 miles of the coast in Washington (*estimate based on draft data and subject to change).
This year’s damage ranges from the mouth of the Columbia River north to Neah Bay on the Strait of Juan de Fuca, with the majority of damage (approximately 9,000 acres) recorded in communities and forested areas around Willapa Bay and Grays Harbor, north to Taholah.
The severity of damage is highly variable; some spruce appear largely unaffected while neighboring spruce were nearly 100% defoliated.
Predicting long term effects to individual spruce trees in the first year of damage can be challenging because branches and trees which appear dead may actually have intact, healthy buds that will flush in spring, 2020.
Therefore, any salvage or hazard assessments of severely damaged trees will be more reliable following bud break next year.
If you are unsure whether a tree should be removed, either wait until spring bud break, or, if possible now, examine new buds to see if they are normally formed and succulent.
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% defoliation are likely to survive and recover as aphid populations are reduced. Trees with up to 75% 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% defoliation.
Management of spruce aphid in forest stands is usually not needed since outbreaks often collapse under natural circumstances while damaged trees recover. Monitoring of aphids on needles is best done in late winter (February to early March) to determine if populations are still high.
To check for aphids, hold a sheet of white paper or cloth under a branch, then tap or gently shake the branch. This will knock these tiny aphids onto the paper or cloth, making it easier to see them.
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.
Some insecticides result in some non-target or residual toxicity to other animals including bees, birds, or fish, so please be keenly aware of this when selecting any pesticide product. Always read and follow the label when using pesticides, or if hiring a contractor, request to see a copy of the Material Safety Data Sheet for any product they plan to use.
Avoid fertilizing trees during aphid outbreaks since this can actually benefit sap feeding insects.
For more information, the Oregon Department of Forestry has developed this factsheet on the spruce aphid.