Western Balsam Bark Beetle
Western balsam bark beetle, Dryocoetes confusus, adult, in lateral view.
(USDA Forest Service photo.)
Dryocoetes confusus
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.
General Distribution in Alaska: Southeast Alaska.
Current Status & Distribution in Alaska (2024)
The roughly 50 acres of western balsam bark beetle (WBBB) mapped in 2024 was comprised of several scattered small pockets in the Skagway River Valley and on the mountainside east of the Taiya River near its confluence with Taiya Inlet. Additionally, the WBBB activity mapped in Haines along the Chilkat River in 2023 was ground checked in 2024 and determined to have been mapped in error. The 2023 WBBB polygons that could be assessed were determined to be porcupine-killed lodgepole pine and will be corrected in the 2023 aerial detection survey dataset.
Ground Detection Survey Observations: None.
Aerial Detection Survey Observations: Western balsam bark beetle was observed on nearly 50 acres in 2024, all in scattered small pockets in the Skagway River Valley and on the mountainside east of the Taiya River near its confluence with Taiya Inlet.
iNaturalist Observations: None.
Historic Activity
Western balsam bark beetle is a pest of subalpine fir. Rarely is it observed causing extensive mortality of subalpine fir, but due to the limited range of that host species in Alaska, even small amounts of affected acreage can be notable.
Western balsam bark beetle damage was observed on less than 15 acres in 2023. Much of the observed activity was concentrated in the Skagway River drainage near its junction with the White Pass Fork. A few scattered additional affected subalpine fir were observed near Skagway and on the west side of Taiya Inlet. Surveyors also observed scattered western balsam bark beetle damage along the north side of the Chilkat River northwest of Haines (~30 individual trees plus 9 acres).
This presumably native beetle was initially identified as the cause of subalpine fir mortality two years into an outbreak in the Skagway area roughly twenty years ago. Since that time, the river valleys north of Skagway and the Taiya Inlet have been the only areas in Alaska where western balsam bark beetle damage has commonly been observed.
While the 2023 western balsam bark beetle damage mapped along the Chilkat River is not a dramatic distance from the Skagway area as the crow flies, this nonetheless marks a geographic departure from the typically impacted areas. As such, this area will be a priority for ground-based assessments in 2024.
No discernable western balsam bark beetle damage was observed in 2022. Western balsam bark beetle damage was observed on roughly 90 acres in 2021, with the majority of the observed activity concentrated in the Skagway River drainage near its junction with the White Pass Fork. 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 2018. Activity occurred in the Skagway area, along the Taiya and Skagway River drainages, from the river mouths up to Mt. Cleveland and White Pass, respectively.
Distribution of Host Trees, 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.
Western balsam bark beetles, Dryocoetes confusus Adult male (left-most specimens) and female (right-most specimens).
(USDA Forest Service photo.)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.
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 Dr. Elizabeth Graham, Entomologist, U.S. Forest Service, at elizabeth.e.graham@usda.gov.