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How to identify it
- Newly hatched larvae are about 2.5 mm long and light green with no markings. From the third instar on they are dark green with three darker lengthwise stripes, and full-grown larvae reach 20 mm (about 0.8 inch).[1]
- Larvae feed communally in groups of 30 to 100 and at first eat only the edges of needles, leaving the uneaten middle filament attached to the twig. Disturbed larvae rear back and dab a sticky droplet of regurgitated resin on whatever threatens them.[1]
- Cocoons are tan, oval and about 6 by 3 mm, attached to remaining foliage and along branches.[1]
- Adults look like small wasps with four transparent wings and do not sting. Females are 6 to 8 mm long and brown with bead-like antennae of 18 to 20 segments; males are 4 to 5 mm, black, with feathery antennae of 21 to 23 segments.[1]
- During an outbreak a strong smell of balsam fir resin hangs over the stand as needles are chewed.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Balsam woolly adelgid | Another balsam fir pest, but a sap-feeder. Balsam fir sawfly damage is chewing: needles eaten from the edges with the middle filament left attached, turning yellow and then red behind green branch tips, with tan oval cocoons on the branches.[1] |
| Eastern spruce budworm | If larvae are chewing your fir, check for the sawfly's signs: it is told by its colonies of 30 to 100 dark green, three-striped larvae feeding together on needle edges, the strong resin smell during outbreaks, and the 6 by 3 mm tan cocoons.[1] |
| European spruce sawfly | An introduced sawfly on spruce whose larvae have five white stripes. Balsam fir sawfly larvae have three darker stripes on a dark green body and feed mainly on balsam fir.[1][5] |
Where it lives
Native to North America and found across Canada and the United States south to northern Mexico. The first recorded Canadian outbreak was in Saskatchewan in 1927; recent outbreaks have been in eastern Canada, especially New Brunswick and Newfoundland and Labrador.[1][2]
Biology and life cycle
The main host is young and semi-mature balsam fir. Larvae will also defoliate white spruce, black spruce and tamarack.[1]
There is one generation a year. Eggs overwinter in slots cut along needle edges and hatch from June to mid-July. Females pass through five or six instars in about 35 days; males have five instars and finish in about 30 days.[1]
Mature larvae spin cocoons in August, pupate for three to four weeks, and adults emerge in late August or September. After mating, the female cuts slots along the edges of current-year needles with her saw-like ovipositor and lays one egg per slot, typically about 35 eggs in all. Unfertilized eggs become males.[1]
Precommercial thinning changes the stand in ways that favor the sawfly: a field study in New Brunswick found that thinning reduced both plant-related and natural-enemy mortality of larvae, raising larval survival by 58 percent on average compared with untreated stands.[3][1]
Damage and signs
- Early signs are defoliation and yellow, partly eaten needles that turn red as they dry. Defoliated trees show green branch tips with red needles toward the interior; after heavy feeding the red needles fall, leaving only the current year's foliage.[1]
- Trees can die after three to five consecutive years of severe defoliation, though populations usually collapse first. Heavily defoliated trees grow more slowly for 13 to 18 years after an outbreak ends, are more open to secondary pests, and the dead needles on the ground raise the fire hazard.[1]
- Since 1990, eastern Canada has seen several severe, unusually long outbreaks in precommercially thinned stands of open-grown balsam fir, where larval mortality from predators, pathogens and parasites is lower. At epidemic levels, dense unthinned stands are defoliated too.[1][3]
Health risk
Adults resemble small wasps but do not sting. Larvae defend themselves with a sticky droplet of regurgitated resin; the sources mention no other effect on people or pets.[1]
Control
Prevention
Outbreaks were historically short and ended on their own through pathogens, parasites and predators. The recent persistence of outbreaks in thinned stands is what has created the incentive to intervene.[1][3]
Biological control
A host-specific nuclear polyhedrosis virus occurs naturally in balsam fir sawfly populations and multiplies as an outbreak grows. Spraying foliage with this virus while larvae are young exposes them early, lets the virus multiply inside them, and speeds the collapse. Aerial trials near Corner Brook, Newfoundland, in 2002 tracked how the applied virus spread into neighboring sawfly populations.[1][2]
Pesticides
In laboratory tests, a commercial neem (azadirachtin) product killed larvae in a concentration-dependent way, cut their feeding, slowed development and reduced pupal weight and adult emergence at sublethal doses.[4]
Registered products change from year to year; Natural Resources Canada advises checking the current pesticide registration database, treating only when population size warrants it and against the approved life stage, and consulting a local tree-care professional.[1]
Common questions
- Why are the inner needles of my balsam fir red while the tips are still green?
- That pattern is typical of balsam fir sawfly. Colonies of larvae eat the previous year's needles, which turn yellow and then red, while the current year's growth at the branch tips is left alone.[1]
- Will the sawfly kill my trees?
- Death usually takes three to five straight years of severe defoliation, and populations normally collapse before that. Mortality is more likely when drought or other insects add stress.[1]
- Do the adults sting?
- No. They look like small wasps with four clear wings but do not sting.[1]
Sources
- Natural Resources Canada, Canadian Forest Service, “Balsam fir sawfly (A.C. Skinner, 2025), Trees, insects, mites, and diseases of Canada's forests.” tidcf.nrcan.gc.ca/en/insects/factsheet/6564 (accessed 2026-11-10)
- MDPI (Insects), “Spread of a Gammabaculovirus within larval populations of its natural balsam fir sawfly (Neodiprion abietis) host following its aerial application (Graves, Lucarotti and Quiring 2012, Insects 3:912-929).” doi.org/10.3390/insects3040912 (accessed 2026-11-10)
- Wiley (Journal of Animal Ecology), “Ecosystem alteration modifies the relative strengths of bottom-up and top-down forces in a herbivore population (Moreau et al. 2006, Journal of Animal Ecology 75:853-861).” doi.org/10.1111/j.1365-2656.2006.01109.x (accessed 2026-11-10)
- Entomological Society of America (Journal of Economic Entomology), “Lethal and sublethal effects of a neem-based insecticide on balsam fir sawfly (Li et al. 2003, Journal of Economic Entomology 96:35-42).” doi.org/10.1093/jee/96.1.35 (accessed 2026-11-10)
- Natural Resources Canada, Canadian Forest Service, “European spruce sawfly (C.J.K. MacQuarrie, 2025), Trees, insects, mites, and diseases of Canada's forests.” tidcf.nrcan.gc.ca/en/insects/factsheet/6555 (accessed 2026-11-10)
- iNaturalist, “Neodiprion abietis taxon page (order and family).” www.inaturalist.org/taxa/542313 (accessed 2026-11-10)