Swaine jack pine sawfly
- French common name: Diprion de Swaine
- Other common names: Swaine jack-pine sawfly
- Scientific name: Neodiprion swainei Middleton
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Hymenoptera
- Family: Diprionidae
General information and importance
Swaine jack pine sawfly is native to North America, east of the Rocky Mountains. A series of damaging outbreaks occurred between 1945 and 1970 at several locations in Ontario and Quebec. The most intense infestations occurred in mature, open-grown jack pine stands on poor soils. The last large-scale outbreak was in Quebec in 1994. Since then, the few damaging infestations that have been recorded have been localized.
Distribution and hosts
Swaine jack pine sawfly occurs east of the Rocky Mountains in Canada from Alberta to Nova Scotia and in the Great Lake states of the United States. It is most common in the eastern portion of the range of its primary host, jack pine (Pinus banksiana). The eastern portion ranges west of Lake Superior in Ontario and Minnesota through the Great Lakes and extends south of the 49th parallel to the Lac St-Jean region of Quebec. The cool climates of the northern limit of its range are insufficient for the development of feeding stages. Warming trends in northern jack pine forests favour the survival and range expansion of Swaine jack pine sawfly.
Jack pine is by far the most common host of Swaine jack pine sawfly. Other pine species may be slightly damaged if they are close to an infestation. However, successful completion of the life cycle on other pine species is less likely.
Tree parts affected
Swaine jack pine sawflies preferentially consume older foliage of jack pine early in the season because of a chemical deterrent in fresh, current-year foliage. As the new foliage matures over the growing season, it becomes more acceptable to older sawfly larvae.
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, or fleshy leg-like protuberances, on the bottom of their abdominal segments. Sawfly larvae have a pair of prolegs on six or more abdominal segments. Caterpillars have only 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.
Adult Swaine jack pine sawflies are active from June to late July. Eggs are inserted into the base of new needles of jack pine trees in mid-summer, leaving visible scars for several weeks. Larvae feed gregariously, so colonies and feeding damage are concentrated and readily observed at the tips of branches below expanding new shoots in mid- to late summer. Mature larvae are 18 millimetres long. The head is orange to brown in colour. The body is pale, greenish yellow, with faint, grey-green stripes on either side. Swaine jack pine sawflies prefer older foliage, so damaged trees may have only new foliage remaining at the shoot tips.
Life cycle
Swaine jack pine sawfly has one generation per year. Adults emerge in June to late July. Eggs are laid singly in pockets cut by the adult female on the outer edges of one, or both, of the paired needles of jack pine. The female’s full complement of eggs tends to be laid on needles of a single shoot. Larvae hatching from this cluster of eggs form a colony and feed gregariously. Feeding ends with the onset of cooler weather in September and October. Larvae drop to the ground and spin a thin cocoon in the upper layer of soil, moult to a prepupal stage, and enter a state of physiological dormancy (diapause) for the winter. Development to reach the pupal and adult stages resumes the following spring. A portion of the population may remain in diapause for another year.
Successful completion of Swaine jack pine sawfly’s life cycle in a single year requires relatively warm autumn weather. If temperatures fall too low before maturation is complete, larval survival is greatly reduced. Consequently, only minor infestations have occurred north of 49o N. For this same reason, open-grown stands with more exposure to direct sunlight favour the survival of Swaine jack pine sawfly.
Damage
The late feeding season of Swaine jack pine sawfly reduces its impact on trees, as little damage occurs during the period of maximum growth of new needles. By the time the largest, most damaging sawfly larval stages occur, the elongation of shoots is almost complete for the season. New foliage sustains little damage because larvae prefer old foliage. This further reduces the impact on the tree. Nonetheless, during the period of widespread outbreaks prior to 1970, tree mortality was reported in Quebec.
Prevention and management
The relationship between ambient seasonal temperature and survival of Swaine jack pine sawfly has resulted in reliable indicators for risk-rating jack pine stands. Open forest stands more exposed to warm sunlight permit earlier completion of the feeding cycle, favouring overwinter survival. Similarly, locations south of 49o N, or near the moderating influence of large bodies of water, typically have warmer autumn temperatures and are at greater risk of experiencing outbreaks.
A naturally occurring nuclear polyhedrosis virus was found in outbreak areas in Ontario and Quebec during the 1950s. The virulence of the virus was increased in laboratory tests by selecting strains that produced the most rapid infection and death of sawfly larvae. Experimental aerial spray trials in the early 1960s gave excellent control results. However, no further development of the method has occurred since 1966, as outbreaks have become less frequent and widespread. The virus remains a naturally occurring, significant source of mortality in high-density populations of Swaine jack pine sawfly.
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 Swaine jack 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.
Photos
Lateral view of the adult female (length: 6-8 mm)
Thérèse Arcand
Three larvae feeding on the same jack pine needle
Thérèse Arcand
Close-up of egg-laying scars
Thérèse Arcand
Colony of young larvae on a jack pine shoot
Thérèse Arcand
Colony of young larvae on a jack pine shoot
Thérèse Arcand
Section of a severely defoliated young jack pine
Claude Monnier
Mature jack pine forest destroyed after a severe attack
Robert Gagnon
Service canadien des Forêts
Service canadien des Forêts
Selected references
Becker, G.C., Jr.; Benjamin, D.M. 1964. Biology of the Swaine jack-pine sawfly in Wisconsin. The Canadian Entomologist 96(4): 589–599. https://doi.org/10.4039/Ent96589-4
Ikeda, T.; Matsumura, F.; Benjamin, D.M. 1977. Chemical basis for feeding adaptation of pine sawflies Neodiprion rugifrons and Neodiprion swainei. Science 197(4302): 497–499. https://doi.org/10.1126/science.197.4302.497
McLeod, J.M. 1970. The epidemiology of the Swaine jack-pine sawfly, Neodiprion swaineii Midd. The Forestry Chronicle 46(2):126–133. https://doi.org/10.5558/tfc46126-2
Price, P.W.; Tripp, H.A. 1972. Activity patterns of parasitoids on the Swaine jack pine sawfly, Neopdiprion swainei (Hymenoptera: Diprionidae), and parasitoid impact on the host. The Canadian Entomologist 104(7): 1003–1016. https://doi.org/10.4039/Ent1041003-7
Ross, H.H. 1955. The taxonomy and evolution of the sawfly genus Neodiprion. Forest Science 1(3): 196–209.
Tripp, H.A. 1965. The development of Neodiprion swainei Middleton (Hymenoptera: Diprionidae) in the province of Quebec. The Canadian Entomologist 97(1): 92–107. https://doi.org/10.4039/Ent9792-1
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.