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How to identify it
- The caterpillar is green or blue-green with whitish stripes along the back and lower sides; the lower stripe turns yellow near the breathing holes. A pair of thin, wavy black lines often runs just below the upper white stripe.[1][2]
- The head is yellowish green, with broad reddish black bands or a herringbone pattern of dark spots on each side.[1][2]
- Full-grown caterpillars reach 22 to 25 mm (just under an inch).[1][2]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Hemlock angle (Macaria fissinotata) | A very common hemlock feeder that may be impossible to tell from the spruce-fir looper as a caterpillar, since both eat hemlock. The USDA Forest Service guide advises identifying these two species from the adult moths, which can also be difficult.[1] |
| Green larch looper (Macaria sexmaculata) | A larch-feeding relative. The spruce-fir looper, which occasionally feeds on larch, is more blue-green, with proportionately thinner stripes.[1] |
Where it lives
It occurs across North America and Eurasia. In Canada it is found from British Columbia east to Newfoundland and Labrador, and in the eastern United States it reaches south to the northern Great Lakes region and the mountains of Georgia and Alabama.[1][2]
Biology and life cycle
Eastern hosts are fir, hemlock, larch and spruce. Pines and other conifers are evidently hosts too, though many published host records for this species may be misidentifications. In British Columbia the main hosts are Douglas-fir, western hemlock and Engelmann spruce, with other spruces, true firs, larches and western redcedar also used.[1][2]
The pupa overwinters. In Canada the moths fly in May and June, and each female lays up to 140 eggs on foliage or on lichens. The caterpillars feed in July and August and pupate in late August.[2]
In Connecticut and Maryland there are at least two generations, with full-grown caterpillars from July to November.[1]
Male moths of this species (listed as Semiothisa signaria dispuncta) were caught in a 1990 field test of synthetic sex attractants, in traps baited with 17-carbon epoxide compounds.[3]
Damage and signs
- Each caterpillar feeds alone on the needles, and although it is sometimes abundant, Natural Resources Canada rates it as innocuous.[2]
Health risk
Natural Resources Canada and the USDA Forest Service guide treat it only as a conifer defoliator, with no mention of risk to people.[1][2]
Control
Prevention
Natural Resources Canada classes this looper as innocuous, and neither it nor the USDA guide recommends control.[2][1]
Common questions
- Is the spruce-fir looper a threat to my spruce or fir trees?
- Not usually. It can be locally common or occasionally abundant, but each caterpillar feeds alone and Natural Resources Canada considers it innocuous.[1][2]
- How can I tell it from other green inchworms on hemlock?
- Often you can't from the caterpillar alone. It may be indistinguishable from the hemlock angle, and the USDA Forest Service guide recommends identifying the adult moth.[1]
Sources
- USDA Forest Service, Forest Health Technology Enterprise Team, “Geometroid Caterpillars of Northeastern and Appalachian Forests, FHTET-2001-10 (D. L. Wagner, D. C. Ferguson, T. L. McCabe and R. C. Reardon, September 2001; scanned copy).” archive.org/details/geometroidcaterp00wagn (accessed 2026-11-09)
- Natural Resources Canada, Canadian Forest Service, “Spruce fir looper, Trees, insects and diseases of Canada's forests.” tidcf.nrcan.gc.ca/en/insects/factsheet/8890 (accessed 2026-11-09)
- Journal of Chemical Ecology (via NCBI PubMed), “Synthesis and field testing of enantiomers of 6Z,9Z-cis-3,4-epoxydienes as sex attractants for geometrid moths (J. G. Millar et al., Journal of Chemical Ecology, 1990; abstract).” pubmed.ncbi.nlm.nih.gov/24264097/ (accessed 2026-11-09)
- iNaturalist, “Macaria signaria taxon page (order and family).” www.inaturalist.org/taxa/224262 (accessed 2026-11-09)