Spruce aphids are at work beyond Oregon’s North Coast.
The small, non-native insects have caused Sitka spruce trees in neighborhoods and on commercial timberlands from Arch Cape to Ocean Park, Washington, to turn brown this spring. On Monday, Christine Buhl, a forest entomologist with the Oregon Department of Forestry investigated reports of infestations and found evidence of the pests from Pacific City to Cape Meares and inland from Tillamook.
The good news is she expects many of the trees she examined — even the ones that looked very bad — to survive. The aphids impacted last year’s needles and will not affect this year’s new growth.
There are no reports of spruce aphid infestations on Oregon’s South Coast, but the area experienced a cold snap that likely checked aphid populations. Farther north, the winter was mild, providing perfect conditions for an aphid invasion.
While pest outbreaks are one expected consequence of global climate change, Buhl is not ready to call this year’s spruce aphid infestation “the new normal.”
Aphids seem to come in cycles. Foresters have seen far worse spruce aphid infestations in Alaska and Canada and trees have rebounded, Buhl said.
Still, it is a concern and the state will keep a close eye on trees going in to next year. Buhl and others plan to monitor for aphid populations and set sticky traps on a mix of public and private lands beginning next February.
“It could be that we’re moving towards an increased frequency in these outbreaks just because conditions are better for some of these insects,” Buhl said. “That is something worth thinking about for long-term impacts.”
Spruce aphids feed on sap in needles, particularly spruce and cause trees to brown and prematurely drop their needles. If Oregon’s central and north coastal areas experience another mild winter, the spruce trees may have to weather yet another infestation with only one year’s supply of needles.
“And that is damaging,” Buhl said. “They could have a really hard time.”
In extreme cases, spruce aphid infestations can lead to the death of a tree though the insects are rarely associated with massive tree die-offs.
Luke Colvin, owner of Arbor Care Tree Specialists in Astoria, said his crews have had to take down a few trees this spring, but he cautioned people against pre-emptively cutting down infested trees.
“One or two years of spruce aphid damage is not enough to kill a tree,” Buhl agreed. “Even the most ragged looking trees will still produce needles this year and they should be given time to rebound.”
As Buhl traveled to sites, shaking tree branches and looking to see what dropped onto her sleeves, she found “pretty low numbers” of aphids compared to the amount of damage they had clearly caused.
Conclusion: “Their numbers are dropping,” she said.
Spruce aphid populations can boom during mild winters, but typically peak early in the season and begin to drop as resources dwindle, predators emerge and colonies become overcrowded.
Along with the aphids, Buhl also found natural enemies such as lacewings.
Experts had wondered if other factors besides aphids might be causing the trees to appear sickly and ragged.
Buhl found no sign of other diseases that could be causing issues for the trees on the Central Coast. Drought, which has been source of stress for trees across the state since 2012, is certainly a factor but spruce aphids are to blame for the bulk of the damage.
A forest health alert from the Oregon Department of Forestry gave people a list of management options for dealing with this year’s aphid outbreak. These ranged from simply waiting out the infestation to insecticide injections and sprays labeled for use against the spruce aphid.
“But chemical treatment is not advised, in most situations, due to prohibitive cost and impacts on non-targets,” the alert noted.
Trees severely impacted by aphids are not necessarily a fire danger. At least not yet. The most volatile trees are the ones holding their brown or “red” needles, which still contain oils that are very flammable.
When those needles drop, the tree is just wood.
“And until that tree falls down, the fuel risk is greatly reduced,” Buhl said.
For now, the lack of needles might help the trees if drought conditions persist, she added. Trees will often drop older needles to get through a period of drought.