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How to identify it
- Adults are robust, oval beetles about 1/6 inch (2 to 3 mm) long. Utah State describes them as dark with white to yellow scales, and the Canadian Forest Service as mottled white and brown; the antennae are clubbed.[1][2]
- Larvae are white, legless grubs with a brown head, about 4 mm long when full grown.[1][2]
- Fresh attacks show as pinholes about 2 mm across in the bark, especially in cracks, with boring dust below them and sap bleeding from the wounds. Later the bark over each gallery sinks, cracks and turns reddish, with a row of small ventilation holes 8 to 10 mm apart along it.[2]
- Under the bark, the egg galleries run across the branch, 3 to 8 cm (about 1 to 3 inches) long, and encircle and girdle it; the larval tunnels run out from them at right angles.[2][1]
Where it lives
The western ash bark beetle ranges through the western half of North America, from Manitoba to British Columbia and south to Texas and Chihuahua, Mexico. In Canada it is known only from the southern Prairie Provinces, but the Canadian Forest Service says it may occur wherever ash is grown.[2]
Biology and life cycle
Ash is the host. On the Canadian Prairies all ash species are susceptible, and green ash is the one attacked most often.[2][1]
The beetles are usually present in low numbers and colonize damaged or recently dead wood, such as limbs broken by wind, snow or ice; Utah State notes they prefer highly stressed trees.[2][1]
Adults spend the winter in small chambers they cut in the bark of the lowest 6 inches (15 cm) or so of the trunk; Utah State also reports late-stage larvae overwintering under the bark.[2][1]
From mid-April to May the adults walk or fly up to the crown and bore into the branches. Each female cuts a gallery across the grain and lays her eggs in small pockets along both sides of it.[2]
Larvae hatch in May, feed in the inner bark until early July, and pupate in mid-July. New adults appear in late July and August, move to uninfested parts of the branches, often a crotch, and feed under the bark for several weeks.[2]
From mid-September to early November the adults leave the branches and walk, drop or fly to the base of ash trees to overwinter, and dust from their boring collects on the ground around the trunk.[2]
Utah State reports typically one generation a year.[1]
Damage and signs
- Feeding in the sapwood and inner bark girdles branches, and the leaves above the damage turn color and die; on girdled branches the leaves start to yellow by late June or early July.[1][2]
- Attacks are usually on smaller branches, but the beetles can attack the trunk, and if the main stem is attacked the tree may die.[1][2]
- The beetles can introduce fungal pathogens.[1]
- Very old ash trees are especially vulnerable and can become starting points for outbreaks that then spread to healthy trees.[2]
Health risk
Neither Utah State nor the Canadian Forest Service mentions any risk to people or pets; both describe it only as a bark beetle of ash trees.[1][2]
Control
Prevention
Reduce stress on ash trees. Regular watering, and fertilizing, especially during drought, helps them resist attack.[1][2]
After drought, wind or ice storms weaken ash, watch for rising beetle numbers so problem areas are caught early, and consider removing weakened older trees.[2]
Sanitation
Prune out dead, weakened and crisscrossing branches, which are potential breeding sites, and remove infested branches before the beetles emerge. The Canadian Forest Service advises removing infested branches and trees before April and burning or burying them, since chipping may not destroy all the brood.[2][1]
Pesticides
Control is generally not warranted. Where ash bark beetles are active and a susceptible tree needs protection, Utah State lists carbamate or pyrethroid insecticides applied to the bark and branches to prevent damage.[1]
Common questions
- Why are branches dying on my ash, with tiny holes and sap oozing from the bark?
- Western ash bark beetles are one cause. They bore holes about 2 mm across, often in bark cracks, sap often bleeds from the wounds, and their egg galleries girdle the branch so the leaves above yellow and die.[2][1]
- Will ash bark beetles kill my tree?
- Usually they stay in stressed or injured branches, and Utah State says control is generally not warranted. An attack on the main stem can kill a tree, and outbreaks after drought or storms can kill many branches.[1][2]
- Can I just chip the infested branches?
- The Canadian Forest Service warns that chipping may not destroy all the brood and advises burning or burying infested wood, removed before April.[2]
Sources
- Utah State University Extension, Integrated Pest Management, “Ash Bark Beetle (Ornamental Pest Guide).” extension.usu.edu/planthealth/ipm/ornamental-pest-guide/arthopods/bark-beetles/ash-bark-beetle.php (accessed 2026-11-08)
- Natural Resources Canada, Canadian Forest Service, “Western ash bark beetle (Trees, insects and diseases of Canada's forests).” tidcf.nrcan.gc.ca/en/insects/factsheet/1000142 (accessed 2026-11-08)
- iNaturalist, “Hylesinus californicus taxon page (order and family).” www.inaturalist.org/taxa/221931 (accessed 2026-11-08)