Insect Pests
Bronze birch borer
Here in WA State: Yes
Infests: some types of birch trees
Symptoms: Their infestation is evident by the D-shaped holes left when the adult emerges from the tree’s trunk. Tunnels under the bark from the larvae cause swelling in the trunk, too. The most obvious symptom is the dieback seen at the top of birch trees. From June to September, leaves at the top of infected trees will turn brown and drop, leaving visibly bare branches.
Description: True to its name, the bronze birch borer is a bronze-colored, slender insect about a half of an inch long. It is native to warmer parts of North America, but outbreaks have been exacerbated by changing climate and hotter, drier summers. Its targets numerous kinds of birches found in Seattle such as the European White Birch, White-barked Himalayan Birch, and the Paper Birch.
Treatment: includes pesticides as a preventative measure. However, an important step includes planting birches resistant to bronze birch borer such as River Birch.
Winter moth
Here in WA State: Yes 
Infests: Oak, maple, and fruit trees are the most susceptible to this pest.
Symptoms: Its presence is seen by defoliation (fewer leaves on the tree) and holes in leaves which are visible starting in March into summer.
Description: The winter moth is an unassuming insect with greyish wings with brown bands measuring about an inch wingspan. The lifecycle begins when females attach eggs to the bark of trees. These eggs then mature into green caterpillars that then begin to feed on tree buds and leaves.
Treatment: Several management strategies exist. Some aim to directly target the insect, such as using horticultural oils or insecticides when moths are in early stages of the lifecycle. “Tree banding”, a sticky substance placed on the trunk, can help monitor for winter moth egg hatch.
Emerald ash borer
Here in WA State: Not yet
Infests: Numerous species of ash trees including white, blue, green, and black ash.
Symptoms: Like other invasive insects, signs of infestation include “galleries” under the bark, which are tunnels created by the insect in their early life stage, as well as D-shaped holes in the bark. This prevents the trees from getting enough food and nutrients to survive. Trees with Emerald Ash Borer die back from the top-down, loose their leaves, and grow small branches (shoots) out of the base of the tree/trunk.
Description: The emerald ash borer is a bright green, slender insect from Asia about a half of an inch long. They were first identified in the United States in the early 2000’s. They are known to kill more than 90% of susceptible ash trees when they arrive in an area.
Treatment: Preventative measures include removing ash trees in susceptible areas and replacing them with pest-resistant tree species. Insecticides can also be used by licensed professionals to kill the harmful larvae. In 2022 the Emerald Ash Borer was confirmed to be in Portland, Oregon initiating a statewide response for Washington in preparation for the pest’s arrival.
Citrus & Asian longhorn beetle
Here in WA State: Not yet
Infests: ash, elm, raintree, planetree, maple, buckeye, katsura, mimosa, mountain ash, birch, poplar, and willow
Symptoms: Adult beetles lay eggs in the bark, leaving perfectly round “exit holes” about a quarter of an inch wide. Symptoms of infestation include unusually yellowing leaves and dying branches that may drop.
Description: this insect is characterized by a black body between 0.5 and 1.5 inches long with white spots and long antennae but there are native beetles that look a lot like this pest. Asian Longhorn Beetle has a very distinct spot pattern as well as blue feet (they look like they are wearing blue slippers). It is native to Eastern Asia but has made its way to Eastern North America.
Treatment: Removal of infected trees or use of insecticides may prevent the infection of other trees.
Sirex woodwasp
Here in WA State: Not yet
Infests: It targets pine trees (the Pinus genus) including native trees like Ponderosa Pine and Lodgepole Pine.
Symptoms: The damage is done when females lay eggs in the bark with a mucus substance that carries a fungus. The eggs then hatch into larvae that tunnel into different layers of the bark, making them difficult to find. The needles on the pine trees will wilt and change color to yellow and red when infected, usually in August to December.
Description: The Sirex woodwasp is a small, slender brown insect native to Eurasia. It is not here in Washington state yet, but it is established in the Eastern US.
Treatment: Because this species spreads as it is transported via firewood, a good preventative measure is to “buy it where you burn it.”