Balsam fir sawfly
- French common name: Diprion du sapin
- Scientific name: Neodiprion abietis (Harris)
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Hymenoptera
- Family: Diprionidae
General information and importance
Balsam fir sawfly is a native pest of coniferous forests across Canada. This insect can cause severe, localized damage but tree mortality usually occurs only in combination with feeding damage by other insects or other stressful factors such as drought. Trees defoliated by the insect’s larval stage have reduced tree vigour and growth. Outbreaks of the insect typically reoccur every 10 years and last for three to five years. Recent trends are towards longer, more damaging outbreaks, especially in managed forests and plantations, which is of concern to the forest industry and Christmas tree growers.
The common name “sawfly” refers to the female adult’s saw-like ovipositor, which she uses to cut slits in the foliage of the host tree and deposit her eggs. Unfertilized eggs become males. Adult sawflies resemble their winged bee and wasp relatives with membranous wings but a broader “waist” between the thorax and abdomen.
Distribution and hosts
Balsam fir sawfly is native to North America and is found across Canada, the United States, and south to northern Mexico. The first recorded Canadian outbreak of this insect occurred in 1927 in Saskatchewan. Recent outbreaks have occurred in eastern Canada, especially New Brunswick and Newfoundland and Labrador.
Balsam fir sawfly feeds on needles of young and semi-mature balsam fir (Abies balsamea). The larvae will also defoliate white spruce (Picea glauca), black spruce (P. mariana), and tamarack (Larix laricina).
Tree parts affected
Symptoms and signs
Balsam fir 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, more commonly, six or more prolegs on abdominal segments whereas 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.
Young larvae hatch from eggs deposited in the previous year’s new foliage and begin feeding in communal groups of 30 to 100 individuals. These groups initially consume only the outer perimeters of needles and leave distinctive uneaten middle filaments attached to twigs.
In early summer, initial obvious signs of infestation are defoliation and yellow, partially consumed, dying needles. The yellow colour gradually turns to red as needles dry. Defoliated trees have characteristic green branch tips and red-coloured needles toward the interior of the tree. After heavy defoliation, the red needles fall to the ground, leaving trees bare of all but current foliage. During an outbreak, there is a characteristic strong odour of balsam fir resin as needles are eaten. Balsam fir sawfly cocoons can be found among remaining foliage and along branches of attacked trees. The tan-coloured cocoons are oval and about 6 × 3 millimetres.
The first-instar larvae are about 2.5 millimetres long whereas full-grown larvae can reach 20 millimetres prior to pupation. First and second instars are light green in colour and lack any other markings. Third to fifth (or sixth) instars are dark green with three darker longitudinal stripes down the length of their body. Adults emerge from ends of cocoons. Adults somewhat resemble a small wasp with four transparent wings; they do not, however, sting. Adult females are 6 to 8 millimetres long, with a brown body and bead-like antenna containing 18 to 20 segments. Adult males are slightly smaller at 4 to 5 millimetres long, with a black body and black feather-like antenna with 21 to 23 segments. Larvae of balsam fir sawfly have a distinctive defence mechanism when provoked: they will rear back their heads, regurgitate a droplet of terpenoids (resinous plant material), and dab this sticky material on whatever threatens them.
Life cycle
Balsam fir sawfly has one generation per year. The insect overwinters as eggs in slots cut along the edges of needles. First-instar larvae hatch in June to mid-July. The development of male and female balsam fir sawfly differs. Females have five or six instars during the larval developmental stage and this period typically takes 35 days. Males have five instars and complete development in about 30 days. Throughout summer, larvae feed on host trees and in August, when they are mature, they spin a cocoon. Pupation inside cocoons takes about three to four weeks. Adults emerge from cocoons in late August or September, mate, and fertilized females use their saw-shaped ovipositor to cut slots along the edges of current needles and then deposit a single egg into each slot. Adult females typically lay 35 eggs.
Damage
Tree death can occur after three to five years of consecutive, severe defoliation. Sawfly populations, however, typically collapse before this occurs. Heavily defoliated trees show a reduction in incremental growth rates and productivity for 13 to 18 years following the collapse of an outbreak. Weakened trees are more vulnerable to secondary pest attacks. Additionally, stands that have experienced heavy defoliation have a potentially higher fire hazard due to the dead, dry needle stockpile that has accumulated on the forest floor.
Since 1990, eastern Canada has seen several severe outbreaks in areas where the forest industry has made a significant investment in silvicultural practices such as pre-commercial thinning. Thinning generally removes weakened, suppressed, or damaged trees to allow greater growing space and sunlight for future higher-valued crop trees. These managed stands of young to middle-aged trees of more open-growing balsam fir are particularly vulnerable to balsam fir sawfly defoliation. Insect defoliation cycles are uncharacteristically long and insect densities are higher in these managed stands than typically found in unmanaged stands. There is generally lower larval mortality due to natural predators, pathogens, and parasites in thinned stands that likely contribute to lengthy defoliation cycles. When sawfly populations reach epidemic levels, unthinned stands of densely growing balsam fir are also attacked and 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 (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.
Prior to the large, persistent outbreaks in the 1990s in eastern Canada, balsam fir sawfly outbreaks were short in duration and controlled naturally by pathogens, parasites, and predators. Thinned forest stands provide for a lower probability of crossover among balsam fir sawfly populations and therefore a reduced spread of natural biological control agents. The recent persistence of outbreaks and the loss of valuable stands has provided incentive to control balsam fir sawfly populations quickly.
One natural pathogen that occurs in balsam fir sawfly populations is a host-specific virus called nuclear polyhedrosis virus. As an outbreak grows, this virus multiplies in balsam fir sawfly population and acts as a natural control. Spraying individual trees, plantations, and stands with this virus will quickly expose the insects to the virus, accelerating a collapse. Larvae consume the virus from sprayed foliage. Applying the virus when larvae are young will allow time for it to multiply inside its body and cause mortality, saving valuable trees and stands.
Pesticides registered for use against balsam fir 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
Close-up of egg outside of egg-slit
Thérèse Arcand
Mature larva on fir twig (length: 20 mm)
Thérèse Arcand
Larva
Thérèse Arcand
New fir shoot, showing many needles bearing egg-slits
Thérèse Arcand
Female and male adults near their respective cocoons (length: 6-8 mm and 4-5 mm, respectively)
Thérèse Arcand
Fir twig defoliated: 1) current year's foliage is intact; 2) 1-year-old needles are chewed and reddened; and 3) 2-year-old needles have disappeared
Thérèse Arcand
Close-up of a reddened fir crown following an outbreak
René Martineau
Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia
Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia
Selected references
Anstey, L.J.; Quiring, D.T.; Ostaff, D.P. 2002. Seasonal changes in intra-tree distribution ofimmature balsam fir sawfly (Hymenoptera: Diprionidae). The Canadian Entomologist134(4): 529–538. https://doi.org/10.4039/Ent134529-4
Johns, R.C.; Fidgen, J.; Ostaff, D.P. 2013. Host-tree oviposition preference of balsam fir sawfly, Neodiprion abietis (Hymenoptera: Diprionidae), in New Brunswick, Canada. The Canadian Entomologist 145(4): 430–434. https://doi.org/10.4039/tce.2013.24
Li, S.Y. 2003. Notes on larval instars and adult antennae of Neodiprion abietis (Hymenoptera: Diprionidae). The Canadian Entomologist 135(5): 745–748. https://doi.org/10.4039/n03-044
Li, S.Y. 2005. Virulence of a nucleopolyhedrovirus to Neodiprion abietis (Hymenoptera: Diprionidae). Journal of Economic Entomology 98(6): 1870–1875. https://doi.org/10.1093/jee/98.6.1870
Li, S.Y.; Skinner, A.C. 2005. Influence of larval stage and virus inoculum on virus yield in insect host Neodiprion abietis (Hymenoptera: Diprionidae). Journal of Economic Entomology 98(6): 1876–1879. https://doi.org/10.1093/jee/98.6.1876
Lucarotti, C.J.; Beatrixe, H.W.W.; Lapointe, R.; Morin, B.; Levin, D.B. 2012. Pathology of a Gammabaculovirus in its natural balsam fir sawfly (Neodiprion abietis) host. Psyche: A Journal of Entomology. 2012: Article ID 646524. https://doi.org/10.1155/2012/646524
Lucarotti, C.J.; Kettela, E.G.; Mudryj, G. 2006. The registration of Abietiv™: a biological control product based on Neodiprion abietis nucleopolyhedrovirus for use against its natural host, the balsam fir sawfly. SERG International Report. 47 p.
Lucarotti, C.J.; Morin, B.; Graham, R.I.; Lapointe, R. 2007. Production, application, and field performance of Abietiv™, the balsam fir sawfly nucleopolyhedrovirus. Virologica Sinica. 22: 163–172. https://doi.org/10.1007/s12250-007-0018-z
Moreau, G. 2006. Past and present outbreaks of the balsam fir sawfly in western Newfoundland: An analytical review. Forest Ecology and Management 221(1–3): 215–219. https://doi.org/10.1016/j.foreco.2005.09.020
Moreau, G.; Lucarotti, C.J.; Kettela, E.G.; Thurston, G.S.; Holmes, S.; Weaver, C.; Levin, D.B.; Morin, B. 2005. Aerial application of nucleopolyhedrovirus induces decline in increasing and peaking populations of Neodiprion abietis. Biological Control 33(1): 65–73. https://doi.org/10.1016/j.biocontrol.2005.01.008
Ostaff, D.P.; Piene, H.; Quiring, D.T.; Moreau, G.; Farrell, J.C.G.; Scarr, T. 2006. Influence of pre-commercial thinning of balsam fir on defoliation by the balsam fir sawfly. Forest Ecology and Management 223(1–3): 342–348. https://doi.org/10.1016/j.foreco.2005.11.012
Parsons, K.; Quiring, D.; Piene, H.; Farrell, J. 2003. Temporal patterns of balsam fir sawfly defoliation and growth loss in young balsam fir. Forest Ecology and Management 184(1–3): 33–46. https://doi.org/10.1016/S0378-1127(03)00145-2
Parsons, K.; Quiring, D.; Piene, H.; Moreau, G. 2005. Relationship between balsam fir sawfly density and defoliation in balsam fir. Forest Ecology and Management 205(1–3): 325–331. https://doi.org/10.1016/j.foreco.2004.10.033
Piene, H.; Ostaff, D.P.; Eveleigh, E.S. 2001. Growth loss and recovery following defoliation by the balsam fir sawfly in young, spaced balsam fir stands. The Canadian Entomologist 133(5): 675–686. https://doi.org/10.4039/Ent133675-5
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.