In light of the spruce aphid outbreak this year, local land owners and community stakeholders came together to gather insight on the pesky insect and how to defend against the damage it is causing.
“This is something that’s happening in our own backyards, in our own neighborhoods,” said OSU Extension Forester Dan Stark, who organized an informational meeting held at the Bob Chisholm Community Center on Sept. 16.
Dr. Christine Buhl, a forest entomologist for the Oregon Department of Forestry, gave a presentation at the meeting, which drew about 35 property owners, foresters, Master Gardeners, and other individuals from across the Oregon North Coast, including Hamlet, Olney, and Tillamook County.
Buhl’s purpose was to both clear up misconceptions about the methods for dealing with the insect pest, which is most commonly hosted by Sitka spruce on the coast, and put the epidemic into perspective.
According to Buhl, the department started receiving calls early in the season from a variety of concerned citizens, park rangers, and others who reported that a significant number of spruce trees along the coast seemed to be in poor condition. Upon further investigation, she identified the culprit of the noticeable defoliation to be the spruce aphid.
While the insect has a continual presence along the west coast, from Canada to California, large-scale infestation — or outbreaks — only occur periodically, Buhl said. The most recent spruce aphid outbreaks occurred in Alaska from 2015 to 2016 and in Oregon in 1998 and 2005.
The good news is they typically run their course in two to three years. Other extensive epidemics have not wiped out all spruce trees, which Buhl said gives her “hope we will make it through this one.”
A single event of the intensity experienced this season, however, can create damage that lingers for several years. According to information from the Department of Forestry, aphids feed on the sap in needles, causing them to turn yellow, then brown, and finally fall from the tree prematurely.
In many ways, spruce trees are resilient. As a type of conifer, they possess several years’ worth of needles at a given time. Spruce aphids only feed on the foliage produced in past years, not the current year foliage because concentration of terpenes is too high in new growth.
They tend to feed near the start of the growth season — or late winter and early spring — before the nitrogen from older needles has been allocated to cultivate other growth.
An outbreak that lasts for a single seasons is unlikely to lead to high tree mortality rates, even if devastating defoliation occurs, because buds are unaffected by aphid infestations and new growth flushes normally. A couple consecutive years of high-intensity infestation, however, exponentially increases the probability of issues such as reduced shoot growth, radial and height growth, and root mass, as well as, potentially, tree mortality.
The situation on the coast is further complicated, Buhl said, because of the statewide drought that has persisted since 2012. While drought stress does not make spruce trees more desirable for consumption, it does hinder their ability to rebound after an aphid attack.
In addition to sharing ways to identify the signs and symptoms of a spruce aphid infestation, Buhl also discussed a few methods for controlling the pest insect.
The best controls are natural ones, including predators such as ladybugs and patristic wasps. Additionally, a frost that drops temperatures to 14 degrees Fahrenheit or below can wipe out a large portion of the aphid population. Property owners can also boost a tree’s natural line of defense through irrigation, but it is a measure that, once taken, must be sustained long term, Buhl said.
Insecticides and other chemical controls are expensive alternatives that must be used in a certain way to be effective. Common controls for large spruce trees, such as a soil drench or stem injection, must be applied in the early spring. According to Buhl, sprays are often ineffective as they easily drift and hit non-target insects and natural predators.
She encouraged property owners with heritage trees or other spruce trees they are concerned about to speak with someone from ODF’s Forest Health program or an ISA-certified arborist. She also invited attendees at the meeting to become citizen scientists and help collect data that the state can use to monitor the aphid outbreak and the damage to the trees.
“We do need to know how extensive of damage we actually can expect trees to bounce back from,” Buhl said. “If it’s a little bit lower than we thought, we need to take some added steps for preventative management, which is going to be really costly.”