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How to identify it
- The caterpillar is violet to gray brown (UMass: violet, pinkish-gray brown) with numerous fine, wavy longitudinal stripes edged with black, the stripes nearest the midline often orange; full-grown larvae are about 1.5 inches (37 mm).[1][2]
- The top of the first eight abdominal segments carries shiny black warts bearing prominent black bristles, longer and more conspicuous than those of the small phigalia, and the areas around the spiracles are often swollen, warted and flushed with orange. The head is tan with black mottling.[1]
- The male moth spans about 1.5 inches (37 mm), with a whitish thorax, two rows of black dots down the abdomen, and forewings dotted with dark brown specks, crossed by three blackish lines and edged with a row of black spots; an entirely blackish melanic form also occurs in western Massachusetts.[2]
- The female is ash gray, unmarked and virtually wingless, with vestigial, functionless wings about 3 mm long; she cannot fly and is seldom seen.[2]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Small phigalia (Phigalia strigataria) | A related spring looper with similar warting, but its warts and bristles are shorter and less conspicuous, and its color is usually some mix of yellow and brown rather than violet-gray with fine black-edged stripes.[1] |
| Cankerworms | Other spring inchworms of hardwoods covered on PestControlBasics; the half-wing larva is told by its rows of shiny black warts with long bristles and its fine wavy black-edged striping.[1] |
| Green and brown cranberry spanworms | The usual cranberry spanworms are green or plain brown; UMass counts the spiny looper in with them in sweep samples but notes its patchy aggregations and warty, striped larva.[2] |
Where it lives
Listed in the Forest Service's guide to eastern forest caterpillars as common on many hardwoods; the forest life history was worked out in Virginia and West Virginia, and UMass records it from western Massachusetts and from cranberry bogs, where it appears in dense, localized patches.[1][2]
Biology and life cycle
There is a single generation a year, and the caterpillars are found from May to July (reported active in May and June on cranberry, where larvae finished feeding and burrowed into the sand to pupate in the first week of June). Winter is spent as a pupa in the soil.[1][2]
Adults emerge in early spring and climb vertical surfaces. Males appear several days to a week before the females and rest on tree trunks, where mating takes place; the flightless females then climb upward to lay their eggs, usually on dead twigs. The light-colored male is cryptic on birch bark.[2]
First-instar larvae actively balloon, hanging on silk lines below the dead twigs where the eggs were laid and riding air currents to new sites. There are five larval instars.[2]
Because the females do not fly, a patch of larvae on a cranberry bog may be the progeny of a single female that ballooned in from the uplands as a first instar, completed development, overwintered as a pupa on the bog, attracted a male and laid her eggs there; UMass found a dense aggregation on a bog edge in May 1998 and concluded the pupa can survive on the bog.[2]
Damage and signs
- Common on many hardwoods, especially apples, oaks, maples, basswoods, hickories and elms.[1]
- On cranberry the larvae feed from the leaves of new growth down to the old growth and then move to adjacent new growth; some nip the stems of new shoots, which fall over, and browned tips are obvious in areas of older feeding. Patchy feeding and larval aggregation are highly characteristic.[2]
Health risk
Neither source mentions any effect on people; the larva's black bristles are described as identification marks, not as a hazard.[1][2]
Control
Prevention
In woodlands and yards no control is described; the Forest Service guide treats the half-wing as a common native hardwood caterpillar.[1]
Traps and physical control
On cranberry bogs, scout early and visually inspect the vine areas where spanworms have been a problem, since larger larvae hold onto the vines and may be missed by sweeping; UMass counts the other spanworm species in with green and brown spanworms against the 18-larvae-per-25-sweeps threshold and notes that localized circular patches can be quite damaging.[2]
Biological control
Bacillus thuringiensis products are effective against cranberry spanworms when they are small; larger caterpillars are much harder to control by any type of spray.[2]
Common questions
- Why is it called the half-wing?
- The female's wings are vestigial stubs about 3 mm long and functionless; she cannot fly, is ash gray and unmarked, and is seldom detected. Only the male has full wings.[2]
- How does a flightless moth reach a cranberry bog?
- As a caterpillar: newly hatched first instars balloon on silk lines from the dead twigs where the eggs were laid and ride air currents to new sites, and UMass considers that the likely route onto a bog.[2]
- What trees does the caterpillar feed on?
- Many hardwoods, especially apples, oaks, maples, basswoods, hickories and elms, with caterpillars present from May to July in a single generation.[1]
Sources
- 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 2026-11-10)
- UMass Amherst Cranberry Station, Center for Agriculture, Food, and the Environment, “Confusing Spring Caterpillars in Cranberry (1999) (UMass Cranberry Station fact sheet).” www.umass.edu/agriculture-food-environment/cranberry/fact-sheets/confusing-spring-caterpilars-in-cranberry-1999 (accessed 2026-11-10)
- iNaturalist, “Phigalia titea taxon page (order and family).” www.inaturalist.org/taxa/209713 (accessed 2026-11-10)