Forest & Grassland Health

Hemlock Sawfly

Hemlock sawfly defoliation near Chaik Bay on western Admiralty Island in 2018.

Hemlock sawfly defoliation, Chaik Bay, Admiralty Is., 2018.

Neodiprion tsugae Middleton

Primary host(s) in Alaska:

Western hemlock (Tsuga heterophylla (Raf.) Sarg.) and occasionally other nearby conifers

Damage: Defoliation is usually restricted to older needles throughout the tree crown. All age classes can be affected.



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hemlock sawfly larvae

Hemlock sawfly larvae.

Branch with feeding damage from hemlock sawfly

Hemlock sawfly defoliation. Note the intact new growth.

Western hemlock defoliated by hemlock sawfly

Western hemlock heavily defoliated by hemlock sawfly.

Hemlock sawfly larva infected with fungus

Hemlock sawfly larva infected with an entomopathogenic fungus.

A golden-crowned kinglet feeding on a hemlock sawfly larva. Credit: Kerry Howard.

Golden-crowned kinglet feeding on a hemlock sawfly larva. Photo credit: Kerry Howard.

Current Status in Alaska (2020 Update)

Hemlock sawfly damage on North Admiralty Island, 2019.

Severe defoliation caused by hemlock sawfly on north Admiralty Island, July 2019.

Hemlock sawfly activity has decreased throughout Southeast Alaska and is not expected to persist in 2021. More than 530,000 acres of hemlock sawfly defoliation damage have been mapped across Southeast Alaska since 2018. Active defoliation was recorded on over 124,000 acres during scan and sketch surveys, and mortality attributed to hemlock sawfly feeding was recorded on over 80,000 acres. Often mortality and defoliation occurred in the same area, therefore the total area attributed to hemlock sawfly damage was over 143,000 acres. Ground observations have also shown a decrease in larval activity. In 2019 over 380,000 acres of defoliation was recorded.

The current outbreak is the first notable damage from hemlock sawfly recorded since 2013. Defoliation was heaviest in areas with southern or western facing aspects. Hemlock sawfly larvae preferentially feed on the older foliage of western hemlock, often leaving part of the needle uneaten, resulting in thin inner tree crowns. Typically, outbreaks last a couple years and may result in growth loss and topkill. Tree mortality is limited unless hemlock sawfly outbreaks co-occur with outbreaks of the western blackheaded budworm. 

Historically, activity has fluctuated from having little to no damage observed one year and thousands of acres of damage recorded the following year. Environmental conditions are indirectly tied to sawfly population levels; entomopathogenic fungi are more abundant during cool/wet summers. Southeast Alaska exhibited warmer and drier than average summer conditions in 2018 and 2019, which limited this fungal growth, allowing larval populations to build to outbreak status. Weather conditions in 2020 returned to the typical cool/wet summer and were not as conducive to larval development.  

The most common hemlock defoliators are hemlock sawfly, which feeds on older foliage, and western blackheaded budworm (Acleris gloverana), which feed on new buds and needles. Other damage agents of hemlock include hemlock canker and hemlock-blueberry needle rust.

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Hemlock Sawfly in Southeast Alaska: A Tiny Insect with an Insatiable Appetite (2019)

Symptoms, Biology & Impacts

The hemlock sawfly completes one generation per year, but some individuals remain in extended diapause as prepupae, taking 2 or more years to become adults. The sawfly overwinters in the egg stage, and eggs hatch in June. Young larvae feed initially in colonies on old foliage. As larvae mature, they disperse and many feed singly. Male sawfly larvae have four feeding instars and most females have five. Mature larvae often move about in the tree crowns and may also migrate up or down tree trunks before pupating. Although no more than one generation occurs annually, larvae of various sizes can be found from late June through August. After feeding is completed, the larvae enter a prepupal stage in which the body becomes shortened and the cocoon is spun. Common pupation sites are bare twigs in the lower crowns of hemlock trees, in the duff, or on understory shrubs. Adult sawflies emerge from the cocoons from August through October, males normally emerging first.Two or more larvae often feed on the same needle, starting from the tip and feeding to the base. Frequently the center rib is left by the early-instar larvae. Immature larvae feed only on old foliage, but mature larvae will also attack new foliage if all of the older foliage is depleted. High populations of the sawfly may remove all of the older foliage. This makes the tree crowns appear thin and gray. If defoliation occurs for two or more successive years, trees are weakened and may die or succumb later to diseases or other insects. During outbreaks, many cocoons can be seen on twigs, foliage, underbrush, and the forest floor.

The eggs are deposited in slits cut by the female along the edge of hemlock needles. Usually the female deposits only one egg in a needle, but occasionally she deposits two or more. Robust females produce an average of 60 to 70 eggs, but individual fecundity varies from less than 10 to over 100 eggs, depending on degree of host defoliation and resultant vigor of individual females. Current year's foliage is preferred for egg laying.

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Guidelines for Reducing Damage

Adverse weather, a fungal disease, and parasites control hemlock sawfly populations and damage in southeast Alaska. Cold summer and fall weather affect the sawfly directly by delaying larval maturity, reducing fecundity (successful reproduction), and inhibiting oviposition.

Between 1952 and 1974, most declining sawfly population trends in southeast Alaska occurred after two consecutive wetter than normal summers. Summer precipitation affects the sawfly indirectly through its effect on disease and parasites. Frequent rainfall promotes the spread of the fungus, Entomophthora sphaerosperma Fres., in larval populations; and dry weather results in increased parasitization of cocooned sawflies by Delomerista japonica diprionis Cush., Oresbius tsugae tsugae (Cush.), and Itoplectis quadricingulatus (Prov.). Starvation and poor nutrition caused by depletion of host foliage contribute to population collapses by reducing sawfly fecundity.

Survey Method

During outbreaks, moderate to severe hemock sawfly defoliation can be mapped by aerial survey. In 2019, ground-based plots are being used to assess defoliation damage, as well as the incidence of larvae. Results from this survey, in combination with assessments of natural population controls, will provide information about the likely trajectory of the outbreak. Additionally, Forest Health Protection is working with partners to develop remote sensing methods to better estimate the total outbreak area in Southeast Alaska. We are looking at changes in reflectance signatures from 30 meter resolution Landsat images captured before and after hemlock sawflies began to feed. In 2019, aerial surveys will be used to validate modeled versus actual outbreak areas.

Hemlock sawfly abundance by location in Southeast Alaska.

Hemlock sawfly ground survey observations by island: the proportion of plots with hemlock sawfly, the proportion of trees within plots with healthy vs. infected hemlock sawfly, the average abundance rating for sawflies (1= <10 sawflies/branch, 2=10-20 sawflies, 3= >20 sawflies), and the average defoliation rating (1= 0-20% defoliation, 2= 21-50% , 3= >50%).   

Distribution & Damage Maps

The hemlock sawfly is an important defoliator of western hemlock in southeastern Alaska and the coastal area of British Columbia, Washington, and Oregon. It also occurs in the interior of British Columbia and in Montana and Idaho.

LandTrendr analysis, showing low (yellow), medium (orange), and high (red) magnitude of change

Results from LandTrendr analysis, showing low (yellow), medium (orange), and high (red) magnitude of change related to hemlock sawfly defoliation  


Hemlock sawfly damage mapped during the 2018 and 2019 aerial detection survey in Alaska.

Hemlock sawfly damage mapped during the 2018 and 2019 aerial detection surveys.

Map of hemlock sawfly range highlighted in green

    Hemlock sawfly range, in green.  

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Links to Media Stories, Resources & Publications

Hemlock sawfly outbreak continues for a second summer - Petersburg Pilot 07/25/19

Why are Juneau’s hemlock trees turning brown? This one insect is to blame - Juneau Empire 07/21/19

Second summer sawfly outbreak browns hemlock trees around Southeast - 07/16/19

Furniss, R. L., and P. B. Dowden. 1941. Western hemlock sawfly, Neodiprion tsugae Middleton, and its parasites in Oregon. J. Econ. Entomol. 34:46-52. 

Hard, John S. 1971. Sequential sampling of hemlock sawfly eggs in southeast Alaska. USDA For. Serv. Res. Note PNW-142, 9 p., illus. Pac. Northwest For. and Range Exp. Stn., Portland, Oreg.

Hard, John S. 1976. Natural control of hemlock sawfly, Neodiprion tsugae Middleton (Hymenoptera: Diprionidae), populations in southeast Alaska. Can. Entomol. 108:485-498.

Hard, John S., and D. C. Schmiege. 1968. The hemlock sawfly in southeast Alaska. USDA For. Serv. Res. Pap. PNW-65, 11 p. Pac. Northwest For. and Range Exp. Stn., Portland, Oreg. Available here

Hard, John S., and Torolf R. Torgersen. 1975. Field and laboratory techniques for evaluating hemlock sawfly infestations. USDA For. Serv. Res. Note PNW-252, 23 p., illus. Pac. Northwest For. and Range Exp. Stn., Portland, Oreg. 

Hopping, G. R., and H. B. Leech. 1936. Sawfly biologies, Neodiprion tsugae Middleton. J. Can. Entomol. 68:71-79.

Torgersen, Torolf R. 1968. Parasites of the hemlock sawfly, Neodiprion tsugae, in coastal Alaska. Annu. Entomol. Soc. Am. 61(5):1155-1158. Available here

Content prepared by Elizabeth Graham, PhD.  Forest Entomologist, Forest Health Protection,

Content adapted from USFS Forest Insect Disease Leaflet 31, Hemlock Sawfly, by John S. Hard, Torolf R. Torgersen, and Donald C. Schmiege, Revised August 1976. Available here.

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