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How to identify it
- Adults are shiny dark reddish-brown, slender and elongate, with no stripes or markings on the wing covers or thorax, and about 3.7 mm long by 1.3 mm wide. The front of the head looks the same in both sexes, unlike the striped ambrosia beetle, where the female's is convex and the male's deeply concave.[1][3]
- Entrance holes are very small, perfectly round and about 1.3 mm in diameter, smaller than those of the striped ambrosia beetle, and a dark fungal stain shows up around them a few days after attack. The white sawdust pushed out of the hole separates ambrosia beetles from bark beetles, which stay in the bark and make brown dust.[2]
- The main gallery runs into the sapwood across the grain, then branches, up to six times, into brood galleries that curve along annual growth rings, all in one horizontal plane, with larvae developing in short cradles above and below the parent tunnels. Gnathotrichus galleries are generally more extensive and deeper than those of the striped ambrosia beetle and sometimes reach the heartwood.[3][1]
- The two species are told apart under a lens by the underside of the head: G. retusus has a small median projection at the edge of the mouth cavity and G. sulcatus does not. The Idaho guide gives G. retusus as 3.3 to 3.7 mm and G. sulcatus as 2.8 to 3.5 mm, with G. retusus much the commoner of the two in Idaho.[3]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Striped ambrosia beetle (Trypodendron lineatum) | Stockier, with two pale lengthwise stripes on each wing cover, larger entrance holes, a strong spring flight, adults that overwinter in forest litter, and a preference for logs cut the previous fall or winter.[1][2] |
| Granulate ambrosia beetle (Xylosandrus crassiusculus) | An introduced ambrosia beetle of eastern hardwoods and nursery trees; Gnathotrichus species are native western conifer beetles of logs, stumps and dying trees.[1] |
| Pine bark beetles | Bark beetles work in the bark and inner bark and produce brown dust; these beetles penetrate the sapwood, produce white sawdust and do not usually attack living trees.[2] |
Where it lives
Western coniferous forests. Natural Resources Canada records G. sulcatus from British Columbia and from Newfoundland and Labrador, and the Idaho guide lists it from British Columbia and Mexico south to Guatemala and Honduras and in US states from Arizona and California to Oregon, Utah and Washington, with G. retusus recorded from British Columbia, Mexico, California and Washington and from counties across northern and central Idaho. Infestation levels are highest in the coastal forests of British Columbia and southeast Alaska, with damaging populations in coastal Washington, Oregon and northern California.[2][3][1]
Biology and life cycle
Unlike the striped ambrosia beetle, every Gnathotrichus life stage overwinters inside the log, stump or dead tree, and new adults stay in the brood galleries until the following spring. The beetles need slightly warmer weather to fly, with peak flights in May and June and some flight activity continuing through the warm season; Natural Resources Canada describes overlapping generations, a first flight slightly later in spring than Trypodendron and a second major flight in late summer.[1][2]
Males begin the boring in this genus. During the initial boring the beetles release an aggregation pheromone that quickly brings in many more, mating takes place on the bark near the entrance hole, and the pair then extends the gallery and keeps it clear of boring dust.[1]
Eggs go into niches along the brood galleries and the larvae feed on the fungus the parents carried in, enlarging the niche into a pupal chamber; development to young adult takes 6 to 10 weeks. There is one generation a year, but parent beetles may keep extending galleries and producing offspring through the warmer months and may fly to a second host in mid or late summer.[1][3]
These are fungus feeders, not wood feeders. The fungi associated with Gnathotrichus have been identified as Ambrosiella sulcati, Raphaella sulcati and Graphium species, and their growth produces the stain around the tunnels. Ambrosia beetles favor the moist basal portions of stems of trees that are dying or were recently killed by bark beetles or broken by wind or snow, and many are attracted to the odor of ethyl alcohol produced in dying trees, so they are not primary tree killers.[2][3]
Damage and signs
- The loss is in wood value. The beetles bore into the sapwood, where the clearest, most valuable lumber is, and the resulting dark-stained pinholes downgrade lumber and veneer; mills also lose yield to heavier slabbing. Dark-stained galleries with branching pupal niches show up on plywood veneer and on the tangential face of lumber.[2][1]
- Gnathotrichus beetles are less fussy about the age of the wood than the striped ambrosia beetle: they infest trees felled at any time of year, including February through late summer, will reinfest the same log or tree for more than one year, can colonize logs felled two or three years earlier, and have been known to attack debarked logs. G. sulcatus has also been reported attacking the sapwood of freshly sawn green hemlock lumber, and Natural Resources Canada notes these beetles will attack trees felled as recently as two weeks before flight.[1][2]
- Hosts span most commercial conifers. In the Pacific Northwest true firs, Douglas-fir and hemlock are preferred over cedar; in California G. sulcatus turns up more in true firs and G. retusus in pines; in Idaho G. retusus infests most conifers and paper birch, in the bases of standing trees killed by bark beetles, in freshly felled merchantable trees and in stumps.[1][3]
Health risk
A pest of logs and sawn wood with no reported effect on people.[1]
Control
Prevention
At yards and mills, maintain rapid turnover of high-value logs, store logs in ponds or on watermist decks, keep green lumber away from log storage, debris and tree lines, and locate sort yards away from forested areas. Pond storage reduces infestation but exposed log surfaces are still attacked, so misting the surface during flight season helps.[1]
Wood moisture matters because the ambrosia fungi need it: downed trees left with limbs and foliage dry out through transpiration, which in some cases reduces how readily ambrosia beetles can colonize them.[1]
Sanitation
Get logs out of the woods and through the yard quickly, and keep potential breeding and overwintering material cleared from around sort yards. Because Gnathotrichus can attack wood cut in any season and reinfest older logs, slow turnover rather than a single seasonal window is the risk.[1][2]
Traps and physical control
Pheromone-baited funnel traps are used to monitor these beetles and reduce local numbers around mills and sort yards, alongside the storage and turnover practices rather than instead of them.[1]
Common questions
- How do I tell these from the striped ambrosia beetle?
- By the beetle and the hole. Gnathotrichus adults are slender, plain dark reddish-brown with no stripes, and their entrance holes are about 1.3 mm, smaller than Trypodendron's. The striped ambrosia beetle is stockier with two pale stripes on each wing cover.[1][3][2]
- Will summer-felled logs be safe?
- Not from these beetles. Gnathotrichus species infest timber felled at any season, including trees down only two weeks, and can colonize logs felled two or three years before; the striped ambrosia beetle is the one largely limited to fall- and winter-cut wood.[1][2]
- Do they attack live trees?
- Not usually. They go for dying trees, trees recently killed by bark beetles, wind- or snow-broken stems, stumps and felled logs, drawn in part by the ethanol odor of stressed wood.[3][2]
Sources
- USDA Forest Service, Pacific Northwest Research Station, “Forest Insect & Disease Leaflet 170: Ambrosia Beetles of Western Conifers (Daterman and Overhulser).” www.fs.usda.gov/foresthealth/docs/fidls/FIDL-170-AmbrosiaBeetles.pdf (accessed 2026-11-10)
- Natural Resources Canada, Canadian Forest Service, “Ambrosia beetle (Gnathotrichus sulcatus), Insects and Diseases of Canada's Forests factsheet 2839.” tidcf.nrcan.gc.ca/en/insects/factsheet/2839 (accessed 2026-11-10)
- University of Idaho, Idaho Forest, Wildlife and Range Experiment Station, “Furniss and Johnson. Field Guide to the Bark Beetles of Idaho and Adjacent Regions, Station Bulletin 74 (2002).” objects.lib.uidaho.edu/fwres/fwres201.pdf (accessed 2026-11-10)
- iNaturalist, “Gnathotrichus sulcatus taxon page (order and family).” www.inaturalist.org/taxa/48731 (accessed 2026-11-10)