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How to identify it
- It is a large inchworm, up to 40 to 45 mm (about 1.75 inches). The color is brown, gray or black-brown, variable, with bark-like mottling and many warts and ridges.[1][2][3][4]
- The second thoracic segment, just behind the head, is noticeably thickened, with broad, angular swellings on each side. The fourth and fifth abdominal segments each carry a pair of knobs on the back, sometimes joined by a ridge.[1][2][3][4]
- The head is small and flattened and held forward. A reliable clue is its coloring: the face and the line down the middle are pale and sharply bordered by a diffuse black area that darkens toward the top of the head.[1][2][4]
- A black or black-edged line runs down the middle of the back toward the rear, and a wavy black line often shows above the first pair of prolegs.[1][2][3]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Yellow slant-line (Tetracis crocallata) | Its caterpillar is similar but has a U-shaped mark that extends up around the top of the head.[2] |
| Variable antepione and large maple spanworm (Antepione thisoaria, Prochoerodes lineola) | The USDA Forest Service geometroid guide notes that the caterpillars of both can resemble the white slant-line.[1] |
Where it lives
It occurs in forests and woodlands across Canada, from southern British Columbia east to Nova Scotia, and in the eastern United States south to northern Florida and Arkansas.[1][4]
Biology and life cycle
The caterpillars feed on many trees and shrubs, including alder, ash, basswood, birch, black gum, cherry, elm, maple, oak, poplar, viburnum and willow, and conifers such as fir, hemlock, larch, pine, spruce and Douglas-fir.[1][2][3][4]
The pupa, usually in soil or debris, is the main overwintering stage. In southern New Jersey the moths fly in May and June.[1][3]
There is essentially one generation a year, with grown caterpillars from late July into October in Connecticut and Maryland. A small share of pupae produce moths in late summer, and the fate of that second brood in Connecticut is unknown; the eastern forest guide counts two generations in the North.[1][2][3]
At rest it holds all three pairs of legs tight under the thorax, and once disturbed it goes rigid like a stick.[1][3]
Damage and signs
- Each caterpillar feeds alone on the leaves or needles. Natural Resources Canada rates it a relatively uncommon and innocuous defoliator in Canada.[4]
Health risk
The forest sources describe the white slant-line only as a leaf and needle feeder and mention no hazard to people.[1][2][3][4]
Control
Prevention
Treatment isn't needed. The USDA Forest Service guides offer no control advice, and Natural Resources Canada considers it innocuous.[1][3][4]
Common questions
- What is the big warty brown inchworm that went stiff when I touched it?
- The white slant-line is a good candidate. It is a large, warty twig mimic with a swollen segment behind the head, and it becomes rigid after being disturbed.[1][3]
- When will I see white slant-line caterpillars?
- Mostly in late summer: grown caterpillars are found from late July to October in Connecticut and Maryland, and mainly from July to September in the northeastern conifer guide.[1][3]
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)
- USDA Forest Service, Forest Health Technology Enterprise Team, “Caterpillars of Eastern Forests, FHTET-96-34 (D. L. Wagner, V. Giles, R. C. Reardon and M. L. McManus, November 1997; scanned copy).” archive.org/details/caterpillarsofea001997 (accessed 2026-11-09)
- USDA Forest Service, Forest Health Technology Enterprise Team, “White slant (Tetracis cachexiata), from Caterpillars on the Foliage of Conifers in the Northeastern United States, FHTET-2004-01 (C. T. Maier, C. R. Lemmon, J. M. Fengler, D. F. Schweitzer and R. C. Reardon, 2004), Bugwood wiki archive.” wiki.bugwood.org/Archive:Caterpillars/Tetracis_cachexiata (accessed 2026-11-09)
- Natural Resources Canada, Canadian Forest Service, “White slant-line, Trees, insects and diseases of Canada's forests.” tidcf.nrcan.gc.ca/en/insects/factsheet/9118 (accessed 2026-11-09)
- iNaturalist, “Tetracis cachexiata taxon page (order and family).” www.inaturalist.org/taxa/131673 (accessed 2026-11-09)