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How to identify it
- The grown caterpillar is a large inchworm, 30 to 37 mm long, apple green with broad whitish-green stripes along the back and a narrow white stripe low on each side. Each spiracle is marked by a prominent yellow spot, and the head is plain green.[1][2]
- The moth is mottled gray brown with a wingspread of 25 to 39 mm, and it is on the wing early, from mid-March on the south coast to mid-June in the interior.[1][2]
- Feeding is concentrated in the upper crown, on the previous year's needles.[1][2]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Phantom hemlock looper | The phantom hemlock looper (Nepytia phantasmaria) is a commonly associated looper on the same coastal conifers. Its caterpillar is lime green with a black-spotted head, and its moth is white with black markings rather than mottled gray brown.[2] |
| Hemlock looper | The western hemlock looper (Lambdina fiscellaria lugubrosa) feeds in the same forests and is listed by the USDA manual as a commonly associated geometrid.[2] |
Where it lives
It is distributed along the Pacific coast from Alaska to southern California and east to Alberta, with a preference for the more humid areas. Natural Resources Canada's records are from British Columbia.[2][1]
Biology and life cycle
The caterpillars are solitary feeders, mainly on conifers. Principal hosts are Douglas-fir, western hemlock, western redcedar, true firs and spruce; Natural Resources Canada also lists mountain hemlock, western larch and western white pine.[2][1]
There is one generation a year. The insect overwinters as a pupa buried in the duff, and the moths emerge from mid-March on the south coast to mid-June in the central interior. Each female lays up to 80 eggs singly on branches and trunks.[1][2]
The young caterpillars feed first on the undersides of needles; later stages feed openly, taking foliage of all ages but preferring the previous year's needles, mostly in the upper crown.[1][2]
Feeding is finished by early August. The caterpillars then drop to the ground, burrow into the litter and pupate.[1][2]
Damage and signs
- Severe defoliation can kill tree tops and occasionally whole trees. The species was not considered destructive until 1960 and 1961, when an epidemic killed some 2,400,000 cubic feet of western hemlock on the west coast of Vancouver Island.[1][2]
- Further outbreaks followed on the Queen Charlotte Islands in 1963 and 1964 and on Vancouver Island in 1968 and 1969, and Natural Resources Canada also records one near Revelstoke in 1952. All subsided abruptly from natural causes.[2][1]
Health risk
Both sources treat it only as a conifer defoliator and report no harm to people.[1][2]
Control
Prevention
No control is suggested by either source. Outbreaks have ended abruptly on their own, and on a single landscape conifer the solitary caterpillars are unlikely to need treatment.[2][1]
Common questions
- What is the big green inchworm with white stripes on my hemlock or Douglas-fir?
- On the Pacific coast, a 30 to 37 mm apple-green looper with whitish stripes down the back and a yellow spot at each spiracle is the greenstriped forest looper.[1][2]
- Is it a serious pest?
- Usually not. It is normally abundant without doing noticeable harm, but a 1960 to 1961 outbreak killed a large volume of western hemlock on Vancouver Island, and severe defoliation can kill tree tops.[2][1]
Sources
- Natural Resources Canada, Canadian Forest Service, “Greenstriped forest looper, Trees, insects and diseases of Canada's forests.” tidcf.nrcan.gc.ca/en/insects/factsheet/1000022 (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, “Melanolophia imitata taxon page (order and family).” www.inaturalist.org/taxa/224588 (accessed 2026-11-10)