European spruce sawfly
- French common name: Diprion européen de l'épinette
- Scientific name: Gilpinia hercyniae (Hartig)
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Hymenoptera
- Family: Diprionidae
General information and importance
European spruce sawfly belongs to a family of insects that all use their saw-like ovipositor to deposit eggs into the tissues of plants. The larvae that hatch from these eggs then feed on the needles of the host plant. European spruce sawfly is an introduced pest of forests and spruce (Picea) trees in general and was first detected near Ottawa, Ontario, in 1922. How the sawfly was introduced to Canada is not known, but the most likely source was importation on horticulture trees.
The insect is a primary pest of black spruce (Picea mariana) and white spruce (P. glauca) but will feed on all spruce species. All tree ages are susceptible to damage by European spruce sawfly, but younger trees may succumb faster. Outbreaks of the insect were first noted in 1930 in the Gaspe region of Quebec and spread through the Maritimes, eastern Quebec, and the adjacent portions of the United States before collapsing in the late 1930s through the combination of an introduced viral pathogen and biological control agents. There have been no significant outbreaks recorded since the late 1940s.
Distribution and hosts
European spruce sawfly is recorded in Canada from Manitoba eastward through the Atlantic provinces, including Newfoundland and Labrador, and may also occur in British Columbia and Alberta. In the United States, the insect is recorded from Maine to South Carolina and Georgia as well as in Michigan, Minnesota, and Washington. The native range is likely central and northern Europe and northern Asia, extending as far east as Korea and Japan. It is also recorded as introduced to the British Isles.
European spruce sawfly feeds on most species of spruce. In Canada, black spruce and white spruce are the most impacted species. However, white spruce is a preferred host and experienced more decline and mortality during outbreaks. The insect has also been recorded feeding on red spruce (P. rubens), Colorado spruce (P. pungens), and Sitka spruce (P. sitchensis). Norway spruce (P. abies) is the main host in the native range.
Tree parts affected
Symptoms and signs
European spruce sawfly eggs are pale green, 0.5 to 1.9 millimetres long and are laid in one-year-old needles. Larvae of European spruce sawfly are the most visible stage, beginning as pale yellow changing to dark green with five longitudinal white stripes as they mature. The head of the larva is black in younger instars becoming brown in older larvae. The larvae of the insect are 15 to 20 millimetres long at maturity. The adult stage of the insect is black with yellow markings and 4.0 to 9.6 millimetres long.
Larvae of European spruce sawfly eat the needles of infested spruce trees. The insect prefers one- to three-year-old foliage but may eat new foliage—particularly when it infests white spruce - when trees are heavily defoliated.
European spruce sawfly damage often begins in the lower crown, and at the edge of stands. Younger larvae may be difficult to see but become more apparent as the larvae mature. The insect produces large, bright green frass that may accumulate in the foliage or on the forest floor.
Life cycle
European spruce sawfly has one to three generations per year in North America with two generations being more common in Canada. The first generation emerges from May to June and females lay their eggs singly in needles. These eggs hatch in about 10 days and require about 40 days to complete development. After the larva completes feeding, it drops to the ground and creates a pupa in the soil beneath the tree. The second generation emerges from July to August and there may be overlap in generations on the same plants. The insect overwinters in the litter or underground at the base of trees as a pupa and can withstand temperatures as low as -27oC. Some individuals may spend a second summer underground as a pupa, emerging in the third summer to lay eggs. The insect is mostly parthenogenic; however, males are produced at very low rates (1 male for every 1,200 females) but rarely observed.
European spruce sawfly populations are regulated by the combined action of a viral pathogen and two or three introduced parasitoids. Outbreaks of the species were common in the 1930s but are rarely observed now.
Damage
Feeding by European spruce sawfly results in the defoliation of old needles. This reduces the photosynthetic capacity of the tree and can eventually cause mortality because spruce trees are unable to replace this foliage. When old needles are not available, the larvae may also consume young needles. Young trees may be more susceptible to damage from European spruce sawfly because of the lack of old needles. Older trees may be able to sustain significant defoliation every year for many years before succumbing. Trees that have been defoliated by European spruce sawfly may see a reduction in growth in tree height that may persist after defoliation has ended. Secondary pests, like bark beetles, can sometimes attack trees that are weakened by European spruce sawfly defoliation, resulting in tree mortality.
European spruce sawfly outbreaks in the 1930s was the dominant forest health issue at the time. The response included the importation and release of 26 different predatory and parasitic insects in Canada to control the pest. Control was eventually achieved by the accidental discovery and introduction of a viral pathogen that rapidly suppressed outbreaks in the late 1930s. Populations of the pest have since been regulated at low levels by the combined action of a parasitic fly and parasitic wasp that were introduced to Canada during the biological control program. This effect was demonstrated in the 1970s when aerial spray programs for the spruce budworm (Choristoneura fumiferana) also killed the two parasitic species, resulting in a temporary resurgence of European spruce sawfly in New Brunswick. The virus, however, reasserted itself and ended the outbreak.
Prevention and management
Pest management strategies for a particular pest vary depending on several factors. These include:
- the population level of the pest (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.
European spruce sawfly is an introduced pest. The last significant outbreaks were recorded in the 1930s though occasional defoliation may be noticed where local populations experience short-lived outbreaks. These outbreaks are regulated by introduced natural enemies and a viral disease. Birds and small mammals may also prey on larvae and cocoons, as will other invertebrate predators.
European spruce sawfly outbreaks are rare and short-lived. Most trees can sustain defoliation for several years without mortality. On small trees, the larvae can be removed by hand.
Photos
Adult female European spruce sawfly (dorsal view) on a white spruce needle.
Thérèse Arcand
Male adult European spruce sawfly pinned along with its cocoon from which it emerged following pupation. Males have similar colouration to females but have plumose (feather-like) antennae.
Thérèse Arcand
Female adult European spruce sawfly laying eggs on a white spruce needle.
Thérèse Arcand
Mature European spruce sawfly larva on white spruce.
Thérèse Arcand
European spruce sawfly larvae feeding on white spruce needles.
Thérèse Arcand
European spruce sawfly cocoon lying in the exposed mineral soil beneath a black spruce forest.
Thérèse Arcand
European spruce sawfly eggs deposited singly in slits cut into white spruce needles.
Thérèse Arcand
Young white spruce growing in the open and severely defoliated by European spruce sawfly during the 1930s outbreak in Quebec.
René Martineau
Mature spruce forest defoliated by European spruce sawfly during the 1930s outbreak in Quebec.
René Martineau
Selected references
Balch, R.E. 1936. The European spruce sawfly outbreak in 1935. The Canadian Entomologist 68(2): 23–31. https://doi.org/10.4039/Ent6823-2
Balch, R.E.; Simpson, L.J. 1932. A European Sawfly (Diprion polytomum (Hartig) attacking spruce in the Gaspe Peninsula, Que. The Canadian Entomologist 64(7): 162–163. https://doi.org/10.4039/Ent64162-7
Bird, F.T.; Burk, J.M. 1961. Artificially disseminated virus as a factor controlling the European spruce sawfly, Diprion hercyniae (Htg.) in the absence of introduced parasites. The Canadian Entomologist 93(3): 228–238. https://doi.org/10.4039/Ent93228-3
Bird, F.T.; Elgee, D.E. 1957. A virus disease and introduced parasites as factors controlling the European spruce sawfly, Diprion hercyniae (Htg.), in central New Brunswick. The Canadian Entomologist 89(8): 371–378. https://doi.org/10.4039/Ent89371-8
MacQuarrie, C.J.K.; Lyons, D.B.; Seehausen, M.L.; Smith, S.M. 2016. A history of biological control in Canadian forests, 1882–2014. The Canadian Entomologist 148(S1): S239–S269. https://doi.org/10.4039/tce.2015.66
Magasi, L.P., Syme, P.D. 1984. Gilpinia hercyinae (Hartig), European spruce sawfly (Hymenoptera: Diprionidae). Pages 295–297 in J.S. Kelleher and M.A. Hulme, editors. Biological control programmes against insects and weeds in Canada 1969-1980. Commonwealth Agricultural Bureaux, Slough, England.
McGugan, B.M.; Coppel, H.C. 1962. Part II. Biological control of forest insects—1910–1958. Pages 35–115 in A review of the biological control attempts against insects and weeds in Canada. Commonwealth Institute of Biological Control Trinidad, Commonwealth Agricultural Bureaux, Technical Communications No. 2 Farnham Royal, England.
Neilson, M.M.; Morris, R.F. 1964. The regulation of European spruce sawfly numbers in the Maritime provinces of Canada from 1937 to 1963. The Canadian Entomologist 96(5): 773–784. https://doi.org/10.4039/Ent96773-5
Reeks, W.A.; Barter, G.W. 1951. Growth reduction and mortality of spruce caused by the European spruce sawfly, Gilpinia hercyniae (Htg.) (Hymenoptera: Diprionidae). The Forestry Chronicle 27(2): 140–156. https://doi.org/10.5558/tfc27140-2