Introduced pine sawfly

Written by V.G. Nealis Revision 2024

French common name: Diprion importé du pin
Other common names: White pine sawfly, imported pine sawfly
Scientific name: Diprion similis (Hartig)
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Diprionidae
Partial list of synonyms:
- Neodiprion similis

General information and importance

Introduced pine sawfly is native to northern Eurasia and was introduced to North America early in the 20^th century, likely on nursery stock. It was originally discovered in a nursery in Connecticut, United States, in 1914 and was first recorded in Canada in southern Ontario in 1931. The adult females are weak fliers, so it has dispersed slowly. The main pathway for spread has been the movement of nursery stock. Introduced pine sawfly can feed on most pines (Pinus) within its range but causes limited damage in forest situations.

The common name ‘sawfly’ refers to the female adult’s saw-like ovipositor, which is used to cut slits in the foliage of the host tree and deposit eggs. Adult sawflies resemble their winged bee and wasp relatives, with membranous wings but a broader “waist” between the thorax and abdomen. The introduced pine sawfly is the only representative of the genus Diprion in North America.

Distribution and hosts

This sawfly is native to Eurasia from the United Kingdom, through Fennoscandia to Siberia and China. It was introduced to North America and is now resident roughly throughout the range of eastern white pine (Pinus strobus). Its range is west of the Great Lakes (Manitoba, Canada and Minnesota, United States), to the northeast Atlantic coast (Newfoundland and Labrador, Canada), to the southern United States (North Carolina). Within this introduced range, it feeds on most native and introduced pines, especially eastern white pine, jack pine (P. banksiana), and Scots pine (P. sylvestris). It is also recorded on several other pine species on nursery stock in Canada, including western white pine (P. monticola), lodgepole pine (P. contorta), limber pine (P. flexilis), ponderosa pine (P. ponderosa), pitch pine (P. rigida), red pine (P. resinosa), Austrian or black pine (P. nigra), Mugo pine (P. mugo), and other pine species in the United States and Eurasia.

Host parts affected

The first generation of introduced pine sawfly feeds on old foliage in early summer before new foliage has fully developed. The second-generation feeds on all foliage.

Symptoms and signs

Introduced pine 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, or fleshy leg-like protuberances, on the bottom of their abdominal segments. The difference is that sawfly larvae have either no prolegs or six or more pairs on their abdominal segments, while 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.

Eggs of this sawfly are laid in a series of slits cut in the edge of mature pine needles and covered in a frothy substance. Unlike other pine sawflies, the eggs have no spaces between them on the needle. Eggs swell considerably within the needles before hatching. Newly hatched larvae are 2.5 millimetres long, uniform yellowish green, with black thoracic legs. These small larvae feed as a colony and quickly retreat to the base of needles if disturbed. As they develop, larvae disperse throughout the tree crown and become solitary feeders. When the larger larvae are disturbed, they exhibit characteristic sawfly defence responses, such as curling their body and regurgitating fluids. Mature larvae are 18 to 28 millimetres long. They are distinctively coloured, with a mottled base colour and a double black dorsal line running the length of their body. On each side of this line is a yellowish stripe punctuated with transverse, brown markings. The lateral body is a brown to black colour, with many rounded yellow and white spots. The underside is pale yellow. There are eight pairs of abdominal prolegs. The body of larvae are sparsely covered with minute spines. Cocoons are brown, papery, and lozenged-shaped. They are usually found near feeding sites, in bark crevices or in the soil beneath the tree.

Life cycle

There are two overlapping generations per year, and a partial third generation in southern locations. Adults emerge from cocoons at irregular intervals throughout the season. Adults initiating the first generation in Canada are active in late May to early June. They mate immediately and lay their eggs in a series of slits cut along the edges of previous years’ needles. The number of eggs per needle varies, depending on the length of the pine needle. Newly hatched larvae initially feed as a group but soon disperse throughout the tree and forage individually. Most larvae of this first generation complete their development by mid-July and spin cocoons where they pupate. Adults are active again by late-July and may include both the new generation, as well as adults emerging from prolonged dormancy from the previous winter, creating overlapping generations. This summer generation feeds on both old and new foliage, completing larval development by early autumn. They also spin cocoons, often clustered at the bases of smaller twigs, in bark crevices, on nearby trees, or in the soil where mature larvae drop from the tree. These late-season sawfly larvae progress to the eonymph (non-feeding larval or prepupal stage) and pass the winter within the cocoons. Pupation and adult emergence occur in the spring.

The introduced pine sawfly is attacked by several native natural enemies, including birds, small mammals, and insect predators. Several insect parasitoids also have been introduced from Europe as biocontrol agents.

Damage

Despite its wide distribution and host range, introduced pine sawfly has been infrequently reported as a significant pest in natural forest situations. Severe defoliation resulting in branch dieback in the upper canopy of the tree and occasional tree mortality have been limited to eastern white pine in a few locations. The close association of introduced pine sawfly with nurseries, however, may result in greater economic damage and increases the risk of inadvertent movement of this introduced insect to new locations.

Prevention and management

Native birds and insects eat introduced pine sawfly. In the 1950s, a program to introduce European insect parasitoids of pine-feeding sawflies to Canada resulted in the establishment of several biocontrol agents that attack introduced pine sawfly in North America. The greatest success was seen in the northern portions of the range.

Feeding larvae in nurseries and on ornamental plantings can be removed by hand. However, this is more laborious than the processes for other pine-feeding sawflies because of the introduced pine sawflies’ solitary feeding habit.

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 introduced pine 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

Coppel, H.C.; Benjamin, D.M. 1965. Bionomics of the Nearctic pine-feeding diprionids. Annual Review of Entomology 10: 69–96. https://doi.org/10.1146/annurev.en.10.010165.000441

MacQuarrie, C.J.K.; Lyons, D.B.; Seehausen, M.L.; Smith, S.S. 2016. A history of biological control in Canadian forests, 1882–2014. The Canadian Entomologist 148(Supplement S1): S239–S269. https://doi.org/10.4039/tce.2015.66

Middleton, W. 1923. The imported pine sawfly. United States Department of Agriculture. Wahington, D.C. Department Bulletin No. 1182. 22 p.

Monro, H.A.U. 1935. Observations on the habits of an introduced pine sawfly Diprion simile Htg. The Canadian Entomologist 67(7): 137–140. https://doi.org/10.4039/Ent67137-7

Van Driesche, R.G.; LaForest, J.H.; Bargeron, C.T.; Reardon, R.C.; Herlihy, M. 2013. Forest pest insects in North America: a photographic guide. United States Department of Agriculture, Forest Service. Washington, D.C. FHTET-2012-02. 715 p.

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. https://publications.gc.ca/site/eng/9.889231/publication.html

Cite this fact sheet

Nealis, V.G. 2024. Introduced pine sawfly. In J.P. Brandt, B.I. Daigle, J.-L. St-Germain, A.C. Skinner, B.C. Callan, and V.G. Nealis, editors. Trees, insects, mites, and diseases of Canada’s forests. Natural Resources Canada, Canadian Forest Service, Headquarters. Ottawa, Ontario.