Larch sawfly

Written by V.G. Nealis Revision 2024

• French common name: Tenthrède du mélèze
• Scientific name: Pristiphora erichsonii (Hartig)
• Kingdom: Animalia
• Phylum: Arthropoda
• Class: Insecta
• Order: Hymenoptera
• Family: Tenthredinidae
• Partial list of synonyms:
  • Holocneme erichsoni (Hartig)  
  • Lygaeonematus erichsoni Konow   
  • Nematus erichsonii Dahlbom   
  • Nematus leachei
  • Nematus notablis (Konow)  
  • Tenthredo erichsonii Hartig 

General information and importance

The native range and emergence of larch sawfly as a forest pest in Canada is uncertain and complex. The alternative synonyms listed above reflect this confusion among taxonomists. The earliest scientific records are from northern Europe in 1835 with limited, sporadic outbreaks occurring during the next 50 years. Subsequent collections in Siberia and Japan suggest the insect was distributed throughout the larch forests of Eurasia. It is believed that larch sawfly was introduced to Great Britain in the late 19^th century because larch is a not a native tree in Great Britain. A prolonged outbreak early in the 20^th century caused serious damage at several locations in England, Scotland, and Wales.

The first confirmed record of larch sawfly in North America was from Massachusetts in 1880, followed by discovery of the insect throughout northeastern United States and Canada within a few years. By the early 20^th century, significant outbreaks were occurring west of the Great Lakes. Within 40 years, larch sawfly was found wherever larch (Larix) occurred from coast to coast. In Canada, the most serious and sustained damage occurred in the Prairie and Maritime Provinces.

The sudden appearance and apparent western spread of larch sawfly infestations from their original discovery in North America, as well as the uncommon severity of outbreaks, led forest entomologists to conclude that larch sawfly was introduced to North America. The absence of specific natural enemies in North America compared to Europe was considered evidence of its origins elsewhere. However, analysis of tree ring records indicates repeated growth reductions in larch at several locations in North America dating back to the 18^th century. These records are indirect evidence that larch sawfly was the cause. Identification of distinct strains in Canada suggest that both native and accidentally introduced strains comprised outbreaks recorded in the 20^th century.

Distribution and hosts

Whether larch sawfly was introduced to North America at some point earlier in the colonial period or has always been here, the insect is now Holarctic. It is distributed throughout the northern circumpolar larch forests of Eurasia and North America. In Canada, it has been found on tamarack (Larix laricina), western larch (L. occidentalis), subalpine larch (L. lyallii), and exotic species of larch of Eurasian origin.

Tree parts affected

The stem and needles of developing shoots are affected during egg lay, whereas developing larvae tend to feed predominantly on the distal “tuft” foliage.

Symptoms and signs

The adult female larch sawfly cuts slits on the underside of developing shoots to lay eggs, inhibiting growth on one side, and causing characteristic curling of the damaged shoots. The newly hatched larvae are creamy white but become bright green as they feed. As larvae develop, their colour darkens to greyish green on top and white beneath, with no distinctive markings. Their head is black. The final larval stage reaches 16 millimetres in length.

The main damage is caused by sawfly larvae feeding openly in groups on the tuft foliage growing from previous year’s shoots. The entire crown of the tree can be defoliated in one season.

Life cycle

Larch sawfly has one generation per year and overwinters in a cocoon in the soil. Adults emerge during a protracted period from late May to mid-summer, so larvae of various stages can be found throughout most of the summer. Reproduction occurs through thelytokous parthenogenesis, in which females are produced from unfertilized eggs. Males are rare. Female sawflies cut a double row of slits on developing shoots using a saw-like ovipositor and lay eggs within these slits. Eggs hatch in about 10 days. Larvae are highly gregarious, feeding in groups throughout the season. They first bite notches in the needles and then move to the basal portion of the bud to feed. When threatened, larvae may exude a small droplet of resin and lift their tail end into the characteristic S-shaped defensive posture. Larvae feed for about 3 weeks, completing four or five instars. Once feeding is complete, larvae drop to the ground, burrow into the soil or duff, and form a cocoon. Some may remain in the cocoon stage for two or more years.

Damage

Larch is a deciduous conifer, losing and replacing all its foliage every year. For this reason, it is resilient to occasional defoliation by larch sawfly, as damage occurs later in the season after some growth has already occurred. Larch sawfly is uncommon and severe damage is rare in Eurasia. However, in North America, repeated and prolonged outbreaks have caused significant reduction in growth and tree mortality in the most severe cases.

Prevention and management

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.

The protracted feeding period of larch sawfly and relative tolerance of larch to occasional defoliation makes pesticide use impractical. Most severe outbreaks of larch sawfly in Canada were associated with forests in which natural enemies, parasitoids, predators, and disease were uncommon compared to Eurasia. A biological control program began early in the 20^th century to introduce natural enemies from Eurasia to North America.

The successful introduction of a parasitic wasp, Mesoleius tenthredinis, from England to Ontario occurred early in the 20^th century. Subsequent introductions to Manitoba, British Columbia, and the Maritime Provinces led to widespread distribution of the parasitoid, high parasitism rates, and commensurate decreases in larch sawfly populations. However, the larch sawfly eventually developed the ability to encapsulate the eggs of the parasitoid and became immune to attacks. Populations of larch sawfly rebounded to high densities. A different strain of this parasitoid was introduced from Germany in the late 1950s. It has since become the most common strain.

A second species of parasitic wasp from Europe, Olesicampe benefactor, was released in Manitoba and Saskatchewan in the 1960s. It increased rapidly but dispersed slowly, so parasitoids were collected in Manitoba and shipped to other parts of Canada and the United States where successful introductions were carried out. This parasitoid has become the dominant parasitoid controlling larch sawfly populations.

Pesticides registered for use against larch 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

Larch sawfly cocoon attached to needles of tamarack.

Thérèse Arcand

Female adult larch sawfly on tamarack.

Thérèse Arcand

Female adult of larch sawfly on tamarack.

Thérèse Arcand

Female adult of larch sawfly on tamarack.

Thérèse Arcand

Freshly laid eggs of larch sawfly on a tamarack twig.

Jean-Paul Laplante

Mature larch sawfly larva on tamarack.

Thérèse Arcand

A current-year tamarack shoot curled because of wounds inflicted during egg-laying by the female adult larch sawfly.

Paul Benoît

A tamarack stand partially defoliated by larch sawfly.

Pierre Therrien

A colony of larch sawfly larvae feeding on tamarack needles.

Jean-François Mouton

A close-up view of a tamarack twig damaged by larch sawfly during egg laying. The eggs cut into the slits on the twig have hatched and larvae have emerged.

Thérèse Arcand

A current-year tamarack shoot curled into a question mark because of wounds inflicted during egg-laying by the female adult larch sawfly.

Christian Hébert

Tamarack defoliated by the larch sawfly.

Luc Côté

A tamarack tree defoliated by larch sawfly.

Jean-François Mouton

A close-up view of a tamarack branch almost completely defoliated by larch sawfly.

Luc Jobin

Mature larch sawfly larva (lateral view) on western larch.

Tamarack defoliated by the larch sawfly.

Christian Hébert

Mature larch sawfly larvae on western larch.

Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia

Selected references

Coppel, H.C.; Leius, L. 1955. History of the larch sawfly, with notes on origin and biology. The  Canadian Entomologist 87(3): 103–111. https://doi.org/10.4039/Ent87103-3

Embree, D.G.; Underwood, G.R. 1972. Establishment in Maine, Nova Scotia, and New Brunswick of Olesicampe benefactor (Hymenoptera: Ichneumonidae), an introduced ichneumonid parasite of the larch sawfly, Pristiphora erichsonii (Hymenoptera: Tenthredinidae). The Canadian Entomologist 104(1): 89–96. https://doi.org/10.4039/Ent10489-1

Girardin, M.-P.; Berglund, E.; Tardif, J.C.; Monson, K. 2005. Radial growth of tamarack (Larix  laricina) in the Churchill Area, Manitoba, Canada, in relation to climate and larch sawfly (Pristiphora erichsonii) herbivory. Arctic, Antarctic and Alpine Research 37(2): 206–217. https://doi.org/10.1657/1523-0430(2005)037[0206:RGOTLL]2.0.CO;2

Girardin, M.-P.; Tardif, J.; Bergeron, Y. 2002. Dynamics of eastern larch stands and its relationship with larch sawfly outbreaks in the northern Clay Belt of Quebec. Canadian Journal of Forest Research 32(2): 206–216. https://doi.org/10.1139/x01-185

Ives, W.G.H. 1976. The dynamics of larch sawfly (Hymenoptera: Tenthredinidae) populations  in southeastern Manitoba. The Canadian Entomologist 108(7): 701–730. https://doi.org/10.4039/Ent108701-7

Jardon, Y.; Filion, L.; Cloutier, C. 1994. Tree-ring evidence for endemicity of the larch sawfly in North America. Canadian Journal of Forest Research 24(4): 742–747. https://doi.org/10.1139/x94-098

Nairn, L.D.; Reeks, W.A.; Webb, F.E.; Hildahl, V. 1962. History of larch sawfly outbreaks and their effect on tamarack stands in Manitoba and Saskatchewan. The Canadian Entomologist 94(3): 242–255. https://doi.org/10.4039/Ent94242-3

Nishimura, P.H.; Laroque, C.P.  2010. Tree-ring evidence of larch sawfly outbreaks in western Labrador, Canada. Canadian Journal of Forest Research 40(8): 1542–1549. https://doi.org/10.1139/X10-091

Tailleux, I.; Cloutier, C. 1993. Defoliation of tamarack by outbreak populations of larch sawfly in subarctic Quebec: measuring the impact on tree growth. Canadian Journal of Forest Research 23(7): 1444–1452. https://doi.org/10.1139/x93-182

Turnock, W.J. 1960. Ecological life-history of the larch sawfly, Pristiphora erichsonii (Htg.) (Hymenoptera: Tenthredinidae), in Manitoba and Saskatchewan. The Canadian Entomologist 92(7): 500–516. https://doi.org/10.4039/Ent92500-7

Wong, H.R. 1974. The identification and origin of the strains of the larch sawfly, Pristiphora erichsonii (Hymenoptera: Tenthredinidae), in North America. The Canadian Entomologist 106(11): 1121–1131. https://doi.org/10.4039/Ent1061121-11

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