Mountain-ash sawfly

Written by V.G. Nealis Revision 2024

• French common name: Tenthrède du sorbier
• Scientific name: Pristiphora geniculata (Hartig.)
• Kingdom: Animalia
• Phylum: Arthropoda
• Class: Insecta
• Order: Hymenoptera
• Family: Tenthredinidae
• Partial list of synonyms:
  • Nematus cheilon Zaddach   
  • Nematus geniculatus Hartig

General information and importance

The mountain-ash sawfly is an invasive species in North America. It was first found in the state of New York in 1926 and in Ontario in 1934. It has since become established throughout eastern North America. A separate introduction occurred in the state of Washington in 2009. Mountain-ash sawfly is now widely distributed in Washington, close to the Canadian border.

Mountain-ash sawfly can cause severe defoliation of mountain-ash (Sorbus) but rarely kills trees. In eastern Canada, the successful introduction of a European parasitoid has greatly reduced the significance of mountain-ash sawfly in many areas.

Distribution and hosts

Mountain-ash sawfly is native to Europe and possibly eastward to northern Asia. It is now established in North America from Ontario and Minnesota eastward to the Atlantic coast, and from the island of Newfoundland in the north to Virginia in the south. A recent introduction was detected in the western United States in the state of Washington. Mountain-ash sawfly feeds exclusively on European (Sorbus aucuparia) and American mountain-ash, (S. americana), with recent observations of feeding on native hawthorns (Crataegus) in western United States.

Host parts affected

Leaves

Symptoms and signs

Eggs may be apparent as enlarged “blisters” on leaf margins in spring. Small larvae are pale yellow or greenish with black dots on their sides, and a black head and legs. They feed openly in groups in early summer. They raise their back ends into an “S” shape when disturbed, creating a “fringe” with their bodies at the edge of the leaf. Final-stage larvae are 15 to 20 millimetres long, yellow-orange in colour, with black flecks on their sides, and orange heads and legs.

Damage can appear suddenly. Larvae avoid feeding on major veins of the leaf. Oftentimes, only the thick central vein and petiole remain, producing a skeletonized leaf.

Life cycle

There are two generations per year in North America. The second generation is partial and usually smaller than the first.

Mountain-ash sawfly overwinters as larvae in cocoons in the soil beneath infested trees. Larvae pupate within the cocoon in spring and adults emerge and mate from late May to early July. As in many related Hymenoptera, such as ants and bees, mountain-ash sawfly females produce only males from unfertilized eggs. The adult female sawfly uses her saw-shaped ovipositor to cut slits in leaf margins and insert eggs between the epidermal layers. This gives the impression of “blisters” along leaf edges.

Eggs hatch in about 1 week. Larvae feed gregariously at first but disperse throughout the tree crown as defoliation intensifies during the 2 to 3 week feeding period. Males have four instars, while females have five. Once larval development is complete, larvae drop to the ground and form a cocoon. A small proportion of this generation gives rise to the second generation in August. Some sawflies remain inactive in their cocoons for up to 5 years. Adults emerging in the spring may come from portions of several previous generations.

Damage

Mountain-ash sawfly may cause rapid, severe defoliation repeatedly over several years but rarely kills the host tree. This has been attributed to the relatively late season feeding of the large larvae. Most defoliation does not occur until after foliage production is complete and trees have already realized significant annual growth.

Initial feeding occurs at margins of leaves, concentrating on soft tissues and leaving a network of fine veins. Eventually entire leaves (except the petioles and large midribs) are consumed. Damage may be concentrated at the tops of trees. At higher population densities, however, the entire tree crown may be defoliated.

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.

Hand-picking of larvae and leaves (on which oviposition has been observed) is effective on smaller, ornamental trees.

Several species of generalist natural enemies attack mountain-ash sawfly but do not significantly reduce numbers. A successful classical biological control program began with the introduction of a European parasitoid, Olesicampe geniculatae, near Quebec City in the late 1970s. The parasitoid spread naturally 50 kilometres per year. By 1984, it occupied most of southern Quebec. Mountain-ash sawfly populations plummeted, and defoliation became rare. Successful introductions were repeated in Newfoundland in the late 1980s. Insecticides based on Bacillus thuringiensis var. kurstaki (Btk) do not work against this insect as they are sawflies (Hymenoptera), not caterpillars (Lepidoptera).

Pesticides registered for use against mountain-ash 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.

Selected references

Forbes, R.S.; Daviault, L. 1964. The biology of mountain-ash sawfly, Pristiphora geniculata (Htg.) (Hymenoptera: Tenthredinidae), in eastern Canada. The Canadian Entomologist 96(8): 1117–1133. https://doi.org/10.4039/Ent961117-8.

Hara, H.; Ibuki, S.; Shinohara, A. 2023. Taxonomic notes and new distribution and host plant records for sawflies and woodwasps (Hymenoptera, Symphyta) of Japan VIII. Bulletin of the National Museum of Nature and Science (Japan), Seres A (Zoology) 49(2): 57–74.

Looney, C.; Smith, D.R.; Collman, S.J.; Langor, D.W.; Peterson, M.A. 2016. Sawflies (Hymenoptera, Symphyta) newly recorded from Washington State. Journal of Hymenoptera Research 49: 129–159. https://doi.org/10.3897/JHR.49.7104

Quednau, F.W. 1990. Introduction, permanent establishment, and dispersal in eastern Canada of Olesicampe geniculatae Quednau and Lim (Hymenoptera: Ichneumonidae), an important biological control agent of the mountain ash sawfly, Pristiphora geniculate (Hartig) (Hymenoptera: Tenthredinidae). The Canadian Entomologist 122(5): 921–934. https://doi.org/10.4039/Ent122921-9

West, R.J.; Dixon, P.L.; Quednau, F.W.; Lim, K.P.; Hiscock, K. 1994. Establishment of Olesicampe geniculatae Quednau and Lim (Hymenoptera: Ichneumonidae) to control the mountain ash sawfly, Pristiphora geniculate (Hartig) (Hymenoptera: Tenthredinidae), in Newfoundland. The Canadian Entomologist 126(1): 7–11. https://doi.org/10.4039/Ent1267-1

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Photos

Mature mountain-ash sawfly larvae feeding on mountain-ash leaflets.

E. Bradford Walker, Vermont Department of Forests, Parks and Recreation, www.forestryimages.org

Mountain-ash sawfly larvae feeding on mountain-ash leaflets. When larvae are disturbed, their tail-ends often rear up into an S-shaped posture.

E. Bradford Walker, Vermont Department of Forests, Parks and Recreation, www.forestryimages.org