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How to identify it
- Mature larvae are 15 to 20 mm (about 0.6 to 0.8 inch) long, green to yellow-green (occasionally orange) with lengthwise stripes in shades of gray, green and yellow, and a black head at every stage. Newly hatched larvae are nearly black and turn dark green as they grow.[1][2]
- Larvae have eight pairs of fleshy abdominal prolegs, which separates them from caterpillars (five or fewer pairs). Young larvae feed in aggregations, often two or more on one needle, eating from the tip toward the base and frequently leaving the center rib.[1][2]
- The pupal case is a tough, papery, cylindrical cocoon with bluntly rounded ends, light brown to dark golden brown and 6 to 10 mm long, spun on needles, twigs, understory plants or the forest floor.[2][1]
- Adults are 5 to 8 mm long. Females are yellowish or brownish with serrated antennae; males are black with feathery (plumose) antennae. Eggs are pale yellow and sit inside slits cut in the edges of current-year needles.[1][2]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Western blackheaded budworm (Acleris gloverana) | A budworm that often outbreaks alongside the hemlock sawfly on the same trees. It prefers current-year needles, while young hemlock sawfly larvae feed on older needles. Sawfly larvae also have eight pairs of abdominal prolegs, where caterpillars have five or fewer.[1][2] |
| Hemlock looper | Another defoliator that can feed with the hemlock sawfly on western hemlock; the combined feeding of the two insects makes the damage more severe than either alone.[2] |
| Pine sawflies | Related Neodiprion sawflies. The hemlock sawfly is the species whose primary host is western hemlock, with mountain hemlock a lesser host.[1][2] |
Where it lives
Native to western North America west of the Rocky Mountains. It is most common in Pacific coastal forests from Oregon through British Columbia to southeast Alaska, and also occurs in interior hemlock forests of British Columbia, northeast Washington, Idaho and Montana. Significant outbreaks occurred in Oregon in the 1930s, in coastal British Columbia from 1996 to 2001, and in southeast Alaska in 2018 and 2019.[1][2]
Biology and life cycle
Western hemlock is the primary host and mountain hemlock a lesser one. During outbreaks, nearby Sitka spruce and Pacific silver (amabilis) fir can also be defoliated.[1][2]
There is usually one generation a year. Most of the population overwinters as eggs in the needles, which swell and darken before hatching in June. Larvae of various sizes can be found from late June through August; males have four feeding instars and females five.[1][2]
Young larvae feed in colonies on older needles and disperse through the crown as they mature, some feeding singly. Once feeding ends in late July, the prepupa spins a cocoon wherever it rests. Some prepupae stay dormant through a second winter or longer before becoming adults.[1][2]
Adults emerge from August through October, males first. The female uses her saw-like ovipositor to cut slits along needle edges, usually laying one egg per needle and preferring the current year's foliage. Fecundity ranges from fewer than 10 to over 100 eggs, with robust females averaging 60 to 70. Unfertilized eggs become males.[2][1]
Damage and signs
- Larvae are wasteful feeders that leave parts of needles uneaten, giving heavily fed trees a yellow or orange cast. High populations remove all the older foliage, so crowns look thin and gray.[2][1]
- Because the sawfly prefers old needles and feeds relatively late in the season, its impact on healthy trees is usually a temporary loss of radial growth, and most outbreaks collapse with little or no tree death.[1][2]
- When hemlock sawfly outbreaks coincide with western blackheaded budworm or hemlock looper, which eat the new foliage, trees can be completely defoliated. Two or more successive years of such defoliation weakens trees and raises the likelihood of top-kill and death, especially in smaller trees in exposed sites.[1][2]
- The 2018 to 2019 outbreak in southeast Alaska, during drier-than-normal conditions, produced aerial survey records of more than 530,000 acres of defoliation, over 100,000 acres of mortality and 190,000 acres of topkill, concentrated on Admiralty, Mitkof, Kuiu and Etolin islands.[2]
Health risk
Both sources treat the hemlock sawfly only as a forest defoliator; neither mentions any bite, sting or other risk to people or pets.[1][2]
Control
Prevention
In most situations no action is taken. Natural enemies keep hemlock sawfly populations below damaging levels, outbreaks are short-lived, and western hemlock recovers from defoliation, so there has been little interest in direct intervention since control programs of the early 1970s, which mainly targeted the western blackheaded budworm.[1]
Biological control
Cool summer and fall weather delays larval development, reduces egg laying and limits adults. Frequent summer rain spreads the fungus Entomophthora sphaerosperma through larval populations, and dry weather favors parasitism of pupae by wasps such as Delomerista japonica diprionis. In southeast Alaska between 1952 and 1974, most population declines followed two consecutive wetter-than-normal summers.[2][1]
Starvation and poor nutrition on heavily defoliated trees also lower sawfly fecundity and help outbreaks collapse.[2]
Pesticides
The Forest Service leaflet contains no pesticide recommendations. Natural Resources Canada notes that registered products change from year to year and advises checking the current registration database and consulting a local tree-care professional before treating.[2][1]
Common questions
- Why do my hemlocks look yellow and thin in late summer?
- Hemlock sawfly larvae eat older needles from midsummer on and leave partly chewed needles behind, which gives trees a yellow or orange tint. Heavy feeding removes the old foliage and leaves crowns thin and gray.[2][1]
- Will hemlock sawfly kill my trees?
- Usually not on its own. Most outbreaks end with little or no mortality because the sawfly spares new foliage. Trees are at risk when it feeds alongside western blackheaded budworm or hemlock looper for two or more years running.[2][1]
- Is it a caterpillar?
- No. It is a sawfly larva, which has eight pairs of abdominal prolegs where caterpillars have five or fewer. The adult is a small wasp-like insect, 5 to 8 mm long.[1][2]
Sources
- Natural Resources Canada, Canadian Forest Service, “Hemlock sawfly (V.G. Nealis, 2025), Trees, insects, mites, and diseases of Canada's forests.” tidcf.nrcan.gc.ca/en/insects/factsheet/1000010 (accessed 2026-11-10)
- USDA Forest Service, Forest Health Protection, Alaska Region, “Hemlock Sawfly, Forest Insect and Disease Leaflet 31 (E. Graham, revised October 2021).” www.fs.usda.gov/foresthealth/docs/fidls/FIDL-31-HemlockSawfly.pdf (accessed 2026-11-10)
- iNaturalist, “Neodiprion tsugae taxon page (order and family).” www.inaturalist.org/taxa/934593 (accessed 2026-11-10)