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How to identify it
- The full-grown caterpillar is up to 42 mm long and comes in two distinct forms. The common form ranges from dark brown to pale grey, with a pale grey head marked dark on each lobe; the top of each abdominal segment carries a pair of dark spots in its front third (faint on light individuals) and a pair of dark D-shaped marks near its rear edge.[1]
- The second form is rusty brown with a broken creamy yellow stripe down the middle of the back and prominent yellow spots along the sides.[1]
- Caterpillars are present from June to September on Douglas-fir, lodgepole pine, ponderosa pine, western white pine or western hemlock, and the moths fly in September and October.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Gray spruce looper | The USDA manual says the western conifer looper resembles the gray forest (gray spruce) looper, Caripeta divisata, as a larva but is much less common in British Columbia. C. divisata's caterpillar is about 37 mm long with a pale brown, herringbone-marked head and a yellowish, gray or brown body with off-white or yellowish stripes, and it overwinters as a pupa, whereas C. aequaliaria overwinters as an egg.[2][1] |
Where it lives
Natural Resources Canada records it from the south coast and southern interior of British Columbia, mountainous western Alberta and south to Oregon; the USDA manual lists Oregon, British Columbia and Alberta.[1][2]
Biology and life cycle
The principal hosts are Douglas-fir, lodgepole pine and ponderosa pine; western white pine and western hemlock are also fed on. The caterpillar is a free-living defoliator that moves about on the foliage.[1][2]
The species overwinters as an egg. Caterpillars feed from June to September, pupate in September, and the moths emerge from September to October.[1]
Damage and signs
Health risk
The sources describe it only as a conifer needle feeder and mention no effect on people.[1][2]
Control
Prevention
No control is needed. The caterpillars feed singly, the species is relatively uncommon, and the USDA manual records no significant damage from this genus.[1][2]
Common questions
- What is the large grey or brown inchworm on my Douglas-fir?
- In British Columbia, Alberta or Oregon, a grey-to-brown inchworm up to 42 mm long with paired dark spots and D-shaped marks on its back, feeding on Douglas-fir or pine needles between June and September, matches the western conifer looper.[1]
- Does it harm the tree?
- No meaningful harm is recorded: Natural Resources Canada rates it a relatively uncommon solitary defoliator, and the USDA Forest Service says Caripeta larvae cause no significant damage.[1][2]
- When do the moths fly?
- Adults emerge from September to October, and the eggs they lay overwinter.[1]
Sources
- Natural Resources Canada, Canadian Forest Service, “Caripeta aequaliaria, Trees, insects and diseases of Canada's forests.” tidcf.nrcan.gc.ca/en/insects/factsheet/8494 (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, “Caripeta aequaliaria taxon page (order and family).” www.inaturalist.org/taxa/216122 (accessed 2026-11-10)