Forest & Grassland Health

Western Balsam Bark Beetle  

Dryocoetes confusus Swain

Host(s) in Alaska:

Subalpine fir (Abies lasiocarpa), other true firs, and very rarely, Engelmann spruce and lodgepole pine

Habitat(s): phloem, inner bark, and bark

Damage: Beetle larvae feeding in phloem tissue can girdle trees, resulting in tree mortality.

Click on the image for a larger version.

Dryocoetes confusus, adult, shown in lateral habitus

Dryocoetes confusus, adult, in lateral view.

Adult male and female Dryocoetes confusus specimens

Adult male (left-most specimens) and female (right-most specimens) Dryocoetes confusus, in lateral (top row) and dorsal (bottom row) views. 

The nuptial chamber and star-shaped larval galleries of Dryocoetes confusus

Star-shaped larval galleries radiating from the nuptial chamber of Dryocoetes confusus in subalpine fir sapwood.

Photograph of subalpine fir damage caused by the bark beetle, Dryocoetes confusus

Subalpine fir damage caused by a Dryocoetes confusus infestation. 

Current Status in Alaska (2020 Update)

Due to the pandemic we were unable to perform aerial surveys in 2020, as such we did not document any western balsam bark beetle activity. In 2019, Western balsam bark beetle damage was observed on 106 acres, remaining steady with the 110 acres observed in 2018Activity occurred in the Skagway area, along the Taiya and Skagway River drainages, from the river mouths up to Mt. Cleveland and White Pass, respectively. Western balsam bark beetle attacks subalpine fir, which has a limited distribution in Alaska. As such, even small amounts of affected acreage are notable. 

Symptoms, Biology, & Impacts 

Adult western balsam bark beetles are range from tawny to dark reddish-brown in color and are approximately 3.4–4.3 mm in length. The elytra (forewings) are rounded posteriorly and without spines. Both male and female beetles have a distinct patch of ochre setae (hairs) on their frons, which is on anterior surface of their head capsules. This setal brush is quite dense and prominent on adult females, resembling a high top haircut. Larvae are yellowish-white in color with tan head capsules and are characteristically c-shaped; late-instar larvae can reach 3–4 mm in length. 

Adult male beetles attack host trees and construct nuptial chambers beneath the bark in the middle of the summer. Female beetles alight to the chambers and mate with the males, after which the female beetles construct egg galleries that radiate from the nuptial chamber, forming a stellate (star-shaped) gallery pattern (fig. 2). Larvae feed away from the egg galleries, which can result in a confusing gallery pattern. Further, adult females can produce eggs during the initial attack in the summer and the following spring, resulting in a confusing progeny. Since the gallery pattern and progeny can be confusing, the species was given the fitting name Dryocoetes confusus.  

Distribution in Alaska

Distribution of true firs in Alaska.

Distribution of the hosts for western balsam bark beetle in Alaska, subalpine fir (Abies lasiocarpa) and Pacific silver fir (Abies amabilis). Western balsam bark beetle damage has only been confirmed on subalpine fir in Alaska. Tree host distribution models were produced by the Forest Health Assessment & Applied Sciences Team in 2011 based on stand density index (240m mapping resolution). Borders were enhanced for visibility in this map. 

Links & Resources

McMillin, J., Malesky, D., Kegley, S., Munson, A.S. 2017. Western Balsam Bark Beetle. Forest Insect Disease Leaflet. 184. Available here.

 

For more information about this agent, contact Entomologist Dr. Sydney Brannoch at sydney.brannoch@usda.gov.

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