As Oregon gets hotter and drier due to climate change, tree-killing invasive pests that prefer warmer temperatures are thriving — endangering Sitka spruce along the coast, firs in the mountains and ash trees lining streams and waterways.

A mild winter may have led to a small outbreak of spruce aphids, Oregon forestry officials say, damaging Sitka spruce along the Central Oregon coast, mostly concentrated in Yachats.

“The last time we saw it this extensively was in 2019, where we had about 15,000 acres that we mapped from aerial survey that contained some damage,” said Oregon Department of Forestry entomologist Christine Buhl, adding that her team is still assessing the damage from this year’s outbreak.

Buhl said this year’s spruce aphid outbreak doesn’t appear to be as large as the one in 2019.

Spruce aphids are tiny, green insects that made their way to Oregon from Europe in the 1920s. They feast on the sap within tree needles during spring months, leaving branches patchy and brown. Infestation damage is usually visible in May and June.

Spruce can rebound after one aphid attack, but repeated annual attacks might kill a tree. Buhl worries about the future of coastal Sitka spruce if Oregon continues having mild winters, especially when these attacks are paired with worsening drought and wildfires.

“This could be just enough to push some trees that are close to the threshold of mortality,” Buhl said.

Another long-established, sap-sucking insect from Europe, the balsam woolly adelgid, also seems to be relishing increasingly warmer seasons in Oregon. This pest targets “true fir” species — that is, species belonging to the Abies genus, like noble and grand firs. Oregon’s ubiquitous Douglas-fir tree is safe from this insect, since it is not a true fir. Douglas-fir trees have their own genus, called Pseudotsuga.

Adelgids are attacking firs at higher elevations in the Cascades, mostly in public wildlands. Similar to aphids, these pests leave branches patchy and brown — potentially increasing wildfire risk in these forests.

“Red needles are very flammable,” Buhl said. “They’re more flammable than green needles. They’re more flammable than a bare wood skeleton of a tree.”

Both spruce aphids and adelgids have existed in Oregon for about a century. Annual cold snaps have largely kept their populations in check. Buhl said warmer temperatures may allow these populations to grow over time.

A changing climate might also allow new pests to establish in Oregon, including the emerald ash borer. Oregon foresters found the emerald ash borer in Forest Grove in 2022, the first time it was found on the West Coast.

Researchers expect this pest to eventually decimate nearly all ash trees in North America, leading ash trees to go the way of the American chestnut.

Invasive emerald ash borer could wipe out Oregon’s ash trees. Here’s how you can help

That would jeopardize Oregon’s streams, rivers and other waterways that rely on native Oregon ash trees for shade, keeping temperatures cool. Increased temperatures can grow cyanobacteria and other toxins, endangering aquatic species as well as Oregonians’ drinking water.

Climate change is fueled by carbon dioxide and other greenhouse gas emissions trapping heat in the atmosphere. The largest source of greenhouse gas emissions from human activities in the U.S. is from burning fossil fuels for electricity, heat, and transportation.

News Source : https://www.opb.org/article/2025/07/01/climate-change-tree-pest-outbreaks-oregon-spruce-aphids/