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How to identify it
- The moth is white with a wingspan of about 25 mm (1 inch). Natural Resources Canada describes 3 or 4 golden brown spots along the front edge of each forewing and 2 or 3 very thin, often faint, cross lines of the same color; the 1972 Eastern Forest Insects guide (under the older name Physostegania pustularia) calls it pure white except for four brownish spots on the forewing margin.[2][3]
- The caterpillar is an inchworm (looper), usually sea green but sometimes dull wine colored, with a cream to yellow stripe running the length of each side of the back and narrow pale lines. The head is green to tan and unmarked.[1][2]
- Full-grown size is given differently: Natural Resources Canada says an average of 20 mm, the Forest Service caterpillar guide says under 2.5 cm, and the 1972 guide says about 15 mm, with wrinkled skin and two whitish lines down the back bordered by yellowish-white lines.[2][1][3]
- Below the side stripe the body often has a yellow or orange cast, and caterpillars about to pupate turn pinkish.[1]
- Damage on shade trees shows as many holes in the leaves and large notches along the edges.[2]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Fall cankerworm | Another defoliator that has fed alongside lesser maple spanworm during outbreaks on red maple; check the moth and caterpillar markings described here to tell them apart.[2] |
Where it lives
It ranges from Canada, including the Maritime Provinces, southern Quebec and southern Ontario, south to northern Florida, Alabama and Texas, in forests, woodlands, swamps, parks and yards wherever red maple grows. The 1972 Forest Service guide knew it mainly from Maine, where it was occasionally abundant, and scattered places in southern Canada. GBIF holds about 13,400 U.S. records since 1990, most of them from New York, Massachusetts, Michigan and Vermont.[1][2][3][4]
Biology and life cycle
Red maple is the main host. Sugar maple is also eaten but seems less prone to injury.[2][1]
The eggs, gray to pink, are laid one at a time on maple branches and trunks and spend the winter there. They hatch in late May and early June.[2][1]
The caterpillars feed mostly in June, largely at night from the undersides of leaves, where they are easy to find with a flashlight. In Connecticut and Maryland there seems to be one drawn-out generation, with grown caterpillars mainly from late May to July and a few into August; the 1972 guide gives May and June.[2][1][3]
Pupation starts in late June or early July and takes about 10 days. Moths first appear about July 9 in eastern Canada and peak in mid-July, rest by day and fly in the evening.[2][1]
Damage and signs
- The caterpillars are heavy feeders and can eat all the green tissue of a leaf, leaving only the main veins.[2]
- Outbreaks are unusual. In the mid-1970s an eruption in central and southern New Brunswick, together with fall cankerworm and a leaf roller, caused moderate to severe defoliation of red maple, mostly in forest stands but also on ornamental trees.[2]
Health risk
The sources describe it only as a maple-feeding caterpillar and an abundant moth at lights; they mention no harm to people.[1][2]
Control
Prevention
Inspect red and sugar maples during the first 10 days of June for small green caterpillars feeding on the leaves.[2]
Traps and physical control
Because the caterpillars feed at night on leaf undersides, a flashlight check after dark is an easy way to find them on small trees.[1]
Food for birds
Abundant native caterpillars like this one feed wildlife. In a Pennsylvania study that used DNA from droppings to work out what Louisiana waterthrushes eat, the lesser maple spanworm (listed as Speranza pustularia) was among the moth prey detected.[5]
The waterthrush is a streamside songbird; the study found it ate a wider range of land insects along acidified streams, where its preferred aquatic prey was scarce.[5]
Common questions
- What are the small white moths with brown spots at my lights in August?
- In the Northeast, one likely candidate is the lesser maple spanworm: from late July through much of August it can be the most abundant moth at lights in southern New England. It is white with golden brown spots on the front edge of the forewings.[1][2]
- Do lesser maple spanworms hurt maple trees?
- Usually not, because numbers normally stay low. In rare outbreaks, like one in New Brunswick in the 1970s, the caterpillars have caused moderate to severe defoliation of red maple.[2]
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 2026-11-11)
- Natural Resources Canada, Canadian Forest Service, “Lesser maple spanworm, Trees, insects and diseases of Canada's forests.” tidcf.nrcan.gc.ca/en/insects/factsheet/8840 (accessed 2026-11-11)
- USDA Forest Service, “Eastern Forest Insects, Miscellaneous Publication 1175 (W. L. Baker, 1972; scanned copy).” archive.org/details/easternforestins1175bake (accessed 2026-11-11)
- Global Biodiversity Information Facility (GBIF), “Macaria pustularia: U.S. occurrence records since 1990 by state (GBIF occurrence search).” api.gbif.org/v1/occurrence/search?taxonKey=9368398&country=US&hasGeospatialIssue=false&occurrenceStatus=PRESENT&year=1990%2C2026&facet=stateProvince&facetLimit=400&limit=0 (accessed 2026-11-11)
- PeerJ, via PubMed Central, “Stream acidification and reduced aquatic prey availability are associated with dietary shifts in an obligate riparian Neotropical migratory songbird (Trevelline et al. 2018).” pmc.ncbi.nlm.nih.gov/articles/PMC6037135/ (accessed 2026-11-11). PMID 30002974
- iNaturalist, “Macaria pustularia taxon page (order and family).” www.inaturalist.org/taxa/736797 (accessed 2026-11-09)