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How to identify it
- The full-grown caterpillar is up to 15 mm long and brown, with a rusty-brown back, a narrow creamy-white stripe along each upper side and another along the spiracles, and a broad dark stripe between them. The head is brown and unmarked.[1]
- It feeds in the open on the foliage, and sometimes the cones, of Douglas-fir, one caterpillar to a site rather than in colonies.[1][2]
- Caterpillars are present from May to July, pupae in July, and the moths fly from July to August.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Spruce coneworm | The spruce coneworm (Dioryctria reniculelloides) is the closest match. Natural Resources Canada says its back and lower side stripe are much duller in colour, and it prefers different hosts: its overwintered caterpillar mines needles and then enters a bud, cone or pollen cone of spruce or fir.[1][2] |
| Fir coneworm | The fir coneworm (Dioryctria abietivorella) is a destructive cone feeder in the West, leaving a round clear hole and pitch-free webbed castings on fir and Douglas-fir cones. Dioryctria pseudotsugella is mainly a foliage feeder and is not treated as a pest.[2][1] |
| Western spruce budworm | Western spruce budworm caterpillars feed on the same Douglas-fir foliage, and the two insects' populations go up and down together. The budworm is a far more damaging defoliator; this moth is described as innocuous.[1] |
Where it lives
It is generally distributed throughout British Columbia from the Skeena and upper Fraser drainages southward, and the USDA manual records it in Washington, Oregon, Idaho, Montana, Utah, Colorado and New Mexico; Natural Resources Canada says it extends south to California.[1][2]
Biology and life cycle
Douglas-fir is the principal host. Western hemlock and grand fir are secondary, and amabilis fir, subalpine fir, Engelmann and white spruce, Rocky Mountain juniper and western redcedar are also listed. The USDA manual says it feeds commonly on the foliage or cones of Douglas-fir and only rarely on grand fir, spruce and hemlock.[1][2]
The moth appears to overwinter in the egg stage. Caterpillars feed from May to July, pupate in July, and the adults emerge from July to August.[1]
The caterpillar is a free-living, solitary defoliator: it feeds on and moves about the foliage in the open rather than boring inside the tree, which sets it apart from the trunk- and shoot-mining members of its genus.[1][2]
Its population fluctuations are synchronous with those of the western spruce budworm.[1]
Damage and signs
Health risk
The sources treat it only as a feeder on conifer foliage and cones and mention no effect on people.[1][2]
Control
Prevention
No control is called for: Natural Resources Canada describes the insect as innocuous. A striped brown caterpillar on Douglas-fir foliage in early summer can be left alone.[1]
Biological control
Because its numbers track the western spruce budworm, expect to see more of this moth in budworm outbreak years and fewer when the budworm declines.[1]
Common questions
- Is Dioryctria pseudotsugella a pest of Douglas-fir?
- Not in any practical sense. It is common and sometimes abundant, but Natural Resources Canada calls it an innocuous defoliator, and the USDA manual records it feeding on foliage or cones without noting damage.[1][2]
- How do I tell it from the spruce coneworm?
- The two are similar, but on Dioryctria pseudotsugella the rusty-brown back and the lower pale stripe are much brighter, and it lives mostly on Douglas-fir foliage rather than in spruce and fir cones.[1]
- When would I see the caterpillar?
- From May to July. It pupates in July and the moths fly from July into August.[1]
Sources
- Natural Resources Canada, Canadian Forest Service, “Dioryctria moth (Dioryctria pseudotsugella), Trees, insects and diseases of Canada's forests.” tidcf.nrcan.gc.ca/en/insects/factsheet/11444 (accessed 2026-11-10)
- USDA Forest Service, “Western Forest Insects, Miscellaneous Publication 1339 (R. L. Furniss and V. M. Carolin, November 1977; scanned copy).” archive.org/details/CAT78702369 (accessed 2026-11-10)
- iNaturalist, “Dioryctria pseudotsugella taxon page (order and family).” www.inaturalist.org/taxa/218486 (accessed 2026-11-10)