Hemlock sawfly
- French common name: Diprion de la pruche
- Scientific name: Neodiprion tsugae Middleton
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Hymenoptera
- Family: Diprionidae
General information and importance
Hemlock sawfly is a native forest insect in North America, west of the Rocky Mountains. It is most common in Pacific coastal forests from Oregon through British Columbia to southeast Alaska, and in the moist interior forests on the windward side of the continental divide in British Columbia, Idaho, and Montana. Populations have reached damaging levels on several occasions, especially in the northern portions of its coastal range. Defoliation may be significant in both mature and juvenile hemlock (Tsuga). Increases in hemlock sawfly populations frequently co-occur with outbreaks of western blackheaded budworm (Acleris gloverana). In these cases, damage to trees is compounded.
The common name “sawfly” refers to the female adult’s saw-like ovipositor, which she uses to cut slits in the foliage of the host tree and deposit her eggs. Unfertilized eggs become males. Adult sawflies resemble their winged bee and wasp relatives with membranous wings. They differ with a broader “waist” between the thorax and abdomen.
Distribution and hosts
This sawfly can be found in Pacific coastal forests in Oregon, Washington, British Columbia, and southeast Alaska. It can also be found in interior hemlock forests west of the Rocky Mountains, south of 54° N in British Columbia to Idaho and Montana. Primary hosts are western hemlock (T. heterophylla) and, to a lesser extent, mountain hemlock (T. mertensiana). During outbreaks, nearby Sitka spruce (Picea sitchensis) and amabilis fir (Abies amabilis) may be damaged.
Tree parts affected
Feeds preferentially on older foliage of tree hosts.
Symptoms and signs
Sawfly larvae are superficially similar in appearance to the larvae of moths and butterflies (caterpillars). Both groups have three pairs of segmented legs on their thorax and a variable number of unsegmented prolegs (fleshy leg-like protuberances) on the bottom of their abdominal segments. Sawfly larvae have either no prolegs, or more commonly, six or more pairs of prolegs. Caterpillars have five or fewer pairs of prolegs. Significant variation and overlap in the appearance of related sawfly species makes identification challenging unless the characteristics of several life stages can be observed.
In autumn, pale yellow eggs are laid singly in slits cut into the edges of current-year needles. Larvae emerge in June the following year. They are black at first but then turn to dark green. Larval stages feed in colonies, preferentially on old needles. This distinguishes them early in the season from the co-occurring western blackheaded budworm, which prefers current-year needles. As hemlock sawflies mature, they may consume current-year foliage if older foliage is depleted. Mature larvae are 15 to 20 millimetres long, with eight abdominal prolegs. Their dominant body colour is green to yellow green. They have longitudinal stripes of various shades of grey, green, and yellow. All stages of hemlock sawfly larvae have a black head. Adults are 5 to 8 millimetres long. Adult females are yellowish brown with serrated antennae. Adult males are black with plumose (feather-like) antennae.
In severe infestations, the tree crown appears thin and grey. Damaging levels of hemlock sawfly are often associated with outbreaks of western blackheaded budworm (which prefer new foliage), resulting in a greater degree of damage and making the identification of the specific agent of greatest observed damage uncertain.
Life cycle
Hemlock sawfly has one generation per year. Eggs are deposited in slits cut along the edge of current-year needles by females in autumn. The insect overwinters in the egg stage. The following June, eggs darken and swell. Newly emerged larvae feed in colonies beginning at the tips of the needles and eat toward the base. As they mature, sawfly larvae disperse throughout the tree crown and may continue to feed in groups or singly on old foliage. Once feeding is complete in late July, hemlock sawflies enter a prepupal stage. They spin a cocoon wherever they rest on needles, twigs, understory plants or in the soil. A small proportion of this prepupal population may enter physiological dormancy (diapause) through a second winter. Adults emerge from August through to October to mate and lay eggs.
Cool summer and autumn conditions typical of northern, coastal forests can affect hemlock sawfly survival directly by delaying maturation of larvae. This consequently inhibits the activity of adults as autumn temperatures cool. Wet summers promote the spread of a pathogenic fungus that kills sawfly larvae. Many species of native parasitoids have been recorded attacking hemlock sawfly.
Damage
The preference of hemlock sawflies for older foliage and a relatively late feeding period reduces the impact of the insect on host trees. A temporary decrease in radial increment can be expected. However, if populations are very high and concurrent with outbreaks of western blackheaded budworm, current-year foliage will also be consumed. When this situation occurs, it makes the impact on the host more severe. If severe defoliation from both insects occurs two seasons in a row, the likelihood of top-kill and tree mortality increases, especially on smaller juvenile trees in exposed situations.
Prevention and management
Damage to coastal forests by concurrent outbreaks of hemlock sawfly and western blackheaded budworm have been waning since the 1970s. Except for a recent outbreak of hemlock sawfly in Alaska, the most significant damage from these joint outbreaks has been attributed to western blackheaded budworm, especially in juvenile forest stands. The last of the control programs in the early 1970s primarily targeted western blackheaded budworm. The current consensus is that many natural enemies respond to maintain hemlock sawfly populations below damaging levels. When outbreaks do occur, they are short-lived and have little effect on the host. The evident recovery of western hemlock from defoliation also has contributed to reduced interest in direct intervention during outbreaks.
Pest management strategies for a particular pest vary depending on several factors. These include:
- the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
- the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
- an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
- how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
- the value of the host(s) versus the costs of pest management approaches; and
- consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.
Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.
Pesticides registered for use against hemlock sawfly under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered pesticides and product information for use against this insect. The application of any registered product should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree-care professional. Pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.
Photos
Dorso-lateral view of mature larva, on western hemlock.
Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia
Prepupal (left) and mature (right) larvae on western hemlock.
Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia
Colony of larvae feeding on mountain hemlock, various color morphs.
Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia
Selected references
Duncan, R.W. 2006. Conifer defoliators of British Columbia. Natural Resources Canada, Canadian Forest Service. Pacific Forestry Centre. Victoria, British Columbia. 359 p.
Graham, E. 2022. Hemlock sawfly. United States Department of Agriculture, Forest Service, Alaska Region 10. https://www.fs.usda.gov/detailfull/r10/forest-grasslandhealth/?cid=FSEPRD568804&width=full [Accessed March 2024]
Furniss, R.L.; Carolin, V.M. 1977. Western forest insects. United States Department of Agriculture, Forest Service. Washington, D.C. Miscellaneous Publication 1339. 654 p.
Hard, J.S. 1976. Natural control of hemlock sawfly, Neodiprion tsugae (Hymenoptera: Diprionidae), populations in southeast Alaska. The Canadian Entomologist 108(5): 485–498. https://doi.org/10.4039/Ent108485-5
Hard, J.S.; Torgersen, T.R.; Schmiege, D.C. 1976. Hemlock sawfly. United States Department of Agriculture, Forest Service. Washington, D.C. Forest Insect & Disease Leaflet 31. 7 p.
Ross, H.H. 1955. The taxonomy and evolution of the sawfly genus Neodiprion. Forest Science 1(3): 196–209.
Torgersen, T.R. 1969. Hymenopterous parasites of the hemlock sawfly, Neodiprion tsugae Middleton, in southeast Alaska, with a key to larval remains. Journal of the Entomological Society of British Columbia 66: 53–62.
Wallace, D.R.; Cunningham, J.C. 1995. Diprionid sawflies. Pages 193–232 in J.A. Armstrong and W.G.H. Ives, editors. Forest insect pests in Canada. Natural Resources Canada, Canadian Forest Service, Science and Sustainable Development Directorate. Ottawa, Ontario.