RECENT OUTBREAK OF THE , Pl?ZSTZPHORA ERICHSONII (HARTIG), IN SUBARCTIC QUEBEC CONRADCLOUTIER and LOUISEFILION Centre d'ktudes nordiques, Universitk Laval, Qukbec, Qu6bec, Canada G1K 7P4

Abstract Can. Ent. 123: 611-619 (1991) Larch sawfly oviposition activity as revealed by scarring of long shoots of eastern larch, (DuRoi) K. Koch, was measured at various locations in the high boreal forest and forest tundra in Quebec in 1988 and 1989. The data show that larch sawfly is established up to the tree line, even on isolated larch growing under climatic con- ditions that are extreme for this tree. Frequency distributions of scarred shoots as a function of time suggest that larch sawfly populations reached outbreak levels in the 1980s, with peak numbers in 1981 for the high boreal, and in 1985 for the subarctic regions sampled. Trends in long shoot production by larch trees started to Ructuate simultaneously with the sudden increase in larch sawfly populations along the Grande Rivitre de la Baleine in 1984. In this region in 1985, the proportion of long shoots used by erichsonii averaged 20-35%, which may have limited further pop- ulation increase. Although foliage reduction and branch mortality were observed, mor- tality of whole trees was not a general characteristic of this outbreak.

Cloutier, C., et L. Filion. 1991. Cycle 6pidkmique &ent de la tenmedu rnklkze, Pristiphora erichsonii (Hartig), en milieu subarctique qukbecois. Can. Ent. 123: 611-619.

L'activitk de la tenthrkde du mClkze, signalee par la presence de cicatrices de ponte sur les pousses longues du mCltze laricin, Larix laricina (DuRoi) K. Koch a Ct6 ivalu6e h plusieurs sites de la haute for& boreale et de la toundra forestikre du Quibec en 1988 et 1989. Les donnCes indiquent que la tentWe du mClkze est Ctablie jusqu'h la limite des arbres, meme sur des mClkzes isolCs poussant sous des conditions climatiques exa&mes pour cette eseed'arbre. La fmuence des pusses scarifites en fonction du temps indique que les populations r6gionales de la tenthde ont atteint des niveaux epidtmiques au cours des annCes 1980, ayant culminC vers 1981 pour la haute fokt boreale et 1985 pour les sites subarctiques 6chantillonnCs. La production de pousses longues par le mtlkze laricin a cesg de croitre r5gulikrement lors de I'ti6vation sou- daine de la population de tenthfie le long de la Grande RivEre de la Baleine en 1984. Dans ce secteur en 1985, la proportion des pousses longues scarifiCes par Pristiphora erichsonii atteignait 20-35%, un niveau d'utilisation qui a possiblement limit6 la pour- suite de l'augmentation de la population de tenthrkde. Malgri que la duction de I'ampleur du feuillage et la mort de branches aient it6 observ6es, la mortalid d'arbres entiers n'est pas apparue comme une caractCristique ginkrale de cette 6pidCmie de la tenthkde du mClkze.

Introduction In northern regions of eastern North America, large-scale perturbations play an important role in determining the composition and dynamics of forest communities (West et al. 1981). Wildfires limit the extent of forest community development, and favour spe- cies adapted to fire (Payette and Gagnon 1979; Rowe 1983; Payette et al. 1989). Pertur- bations resulting from herbivore outbreaks can affect communities even more selec- tively, because of the generally high level of specificity of herbivore-plant relationships. Long-term effects of herbivore outbreaks on the structure and development of coniferous forest communities have been hypothesized (e.g. Dansereau 1958) but very little data are available. Herbivore epidemics are well documented for the southern boreal forest (e-g. Morris 1%3; Blais 1983), but their occurrence in the high boreal and subarctic regions has only occasionally been investigated (Niemela et al. 1980). Insect outbreaks in these 612 THE CANADIAN ENTOMOLOGIST MayIJune 1991 regions may be relatively common, as observed in northern Europe (Tenow 1972; Bal- tensweiler et al. 1977). The larch sawfly, Pristiphora erichsonii (Hartig), is the foremost defoliator of eastern larch, Larix laricina (DuRoi) K. Koch, in North America. In the absence of conclusive evidence, the origin of P. erichsonii in North America has been the subject of debate (e.g. Coppel and Leius 1955; Nairn et al. 1962). The hypothesis most frequently put forward has been the accidental introduction of the insect from Europe (Provancher 1885; Fyles 1906; Turnock and Muldrew 1972). Hagen (1881) first reported P. erichsonii in North America from European larch at the Harvard Arboretum near Boston, MA. Defoliation by the sawfly first attracted the attention of entomologists in Canada in 1883 in the eastern townships and Quebec City, P.Q. (Fyles 1884; Fletcher 1885; Provancher 1885). During the decades following its presumed introduction, P. erichsonii rapidly spread throughout the North American range of Lark spp., frequently reaching outbreak levels (McGugan and Coppel 1962; Turnock and Muldrew 1971; Ives and Muldrew 1984). Larch sawfly populations have been monitored regularly in the southern part of its Canadian range since 1936, especially through the Forest Insect and Disease Survey, and studied intensively in Manitoba and Saskatchewan (Ives 1976, and references therein). However, for the northern part of the range of larch, and particularly in the forest tundra, population data apparently were collected only once for the Northwest Temtories (Turnock and McLeod 1966). For the regions located on the east side of Hudson Bay, only casual (Wong 1974) or indirect (Hustich 1950; Arquillihe et al. 1990) observations of its presence in the forest tundra are available. The objective of this study was to survey the distribution and measure the activity of the larch sawfly in the high boreal forest and the forest tundra in the Hudson Bay area, Quebec. We collected data on oviposition activity of P. erichsonii reflecting population trends during the 1980s, and its distribution in subarctic Qutbec up to the tree line, where direct evidence of its activity was collected even on remotely isolated larch trees and groves. In an attempt to measure the potential impact of this herbivore on northern larch stands, we also present data on recent fluctuations of larch shoot growth in relation to their use by the larch sawfly for oviposition, as revealed by shoot scarring. Methods Data on recent fluctuations in larch sawfly activity were obtained by dating larch shoots scarred by ovipositing females. Larch trees grow short and long shoots (Clausen and Kozlowski 1967), but egg laying by P. erichsonii is restricted to the long shoots; general information on P. erichsonii is available in Drooz (1960), Martineau (1985), and Ives and Wong (1988). Scars and curvature of shoots used by larch sawfly can persist for years. For normal larch shoots, dating of scarred shoots is possible by counting growth units on live branches and branchlets, starting from the tip (Drooz and Meyer 1955). Sampling sites (Fig. 1) were selected to cover a range of ecological conditions, from the high boreal forest up to the tree line along a south-north axis, and from the coastal area to 100-200 km inland. In four of the subarctic sites eastern larch forms mixed pop- ulations with white spruce, Picea glauca (Moench) Voss, or black spruce, Picea mariana (Mill.) B.S.P. At the Lac Kakiattuq site, larch is present as a small isolated grove of only about 200 trees marking the present tree line in this part of the Ungava peninsula (Cayouette 1987). The Rivikre Boniface site is characterized by extensive old black spruce stands and one larch tree, about 75 years old, was located in the area. All sites were visited during the summer of 1989, but data also were collected in 1988 at some sites. At each site except Boniface, from 10 to 30 trees were inspected for scarred shoots. Trees bearing scarred shoots were sampled by cutting two to six branches (depending on foliage density) 1-2 m above the ground surface. In dry or windy sites, mortality of shoots scarred in recent years was suggested by the presence of dried or broken shoots bearing oviposition scars. On Volume 123 THE CANADIAN ENIOMOLOCIfl 61 3

FIG. 1. Study area and locations of sites where eastern larch was sampled for evidence of oviposition by Pris- tiphora erichsonii in 1988-1989. such trees, some dead scarred shoots may have broken off completely, which would bias our larch sawfly activity estimates. This possibility was minimized by rejecting the few trees examined that had dead or broken scarred shoots. Branches were examined in the laboratory by counting and dating scarred shoots. Counts and dates were used to establish a frequency distribution of sawfly attack as a function of the year of shoot elongation for each site. Data on yearly variations in P. erichsonii abundance in relation to long shoot density were collected in 1988 at Kuujjuarapik, and at another site 100 km to the east, along the Grande Rivike de la Baleine (GRB) (Fig. 1). The percentage of scarred shoots was esti- mated on seven trees (four at Kuujjuarapik, and three at the GRB site) approximately 6 m high. Trees were selected for uniformity of crown development and growing conditions, 614 THE CANADIAN ENTOMOLOGIST MayIJune 1991 and were sampled by cutting six branches 70 cm in length from each tree. Four branches were cut at 2 m above ground and two at 4 m, to include a large portion of the crown. To estimate yearly variations in the percentage of long shoot attack at each site, all long shoots produced since 1980 on sampled trees were counted, dated, and classified as normal or scarred. Data on shoot density and percentage shoots scarred were expressed on a per branch basis for each tree and then averaged over the number of trees sampled at each site. Results and Discussion Evidence of larch sawfly oviposition during the 1980s was found at all sites visited, including the isolated grove at Lac Kakiattuq at the tree line, and the isolated tree at the Boniface site. At all sites, the incidence of attack of trees inspected was close to 100%. Shoots scarred 20-25 years ago were found, but their small numbers probably represent only fractions of the initial densities of these old attacks. Few shoots scarred before 1978 were found; this may have been because scars disappear rapidly after 10 years or more through secondary growth, or because of low activity of P. erichsonii during the 1960s and most of the 1970s. However, we found large numbers of scarred shoots at the beginning of the 1980s at the southmost site in the high boreal forest, where growth conditions are possibly the best and most favourable to scar healing. Therefore, we can assume th'at the data accurately represent trends in sawfly activity for at least 10 years. Fluctuations in Scarred Shoot Abundance. Frequency distributions of attack as a func- tion of time (Fig. 2) reflect yearly variations in P. erichsonii density, and possibly also in the number of eggs laid per shoot. Numbers of eggs per shoot vary as a function of shoot length, which would decrease as trees become stressed during a larch sawfly outbreak (Wallace 1954; Turnock 1960; Maltais et al. 1980). However, for the present outbreak, evidence of shoot length decline was found only at one dry continental site of the four sites investigated (unpublished data). Furthermore, the decline became significant only in 1988, 4 years after the beginning of the outbreak. This suggests that yearly variations in relative frequency of scarred shoots resulted mostly from fluctuations in sawfly abundance, with little if any effect from changes in the size of egg clusters. The patterns of attack indicated that P. erichsonii became very active at the beginning of the 1980s in the high boreal zone (Radisson-LG2), and starting in 1984 or 1985 at the subarctic sites sampled (Fig. 2). The observed trends suggest that sawfly populations went through a population cycle lasting several years. At the Radisson-LG2 site, the outbreak seems to have started in 1979, peaked in 1981, and recessed in 1984. At five other sites (Kuujjuarapik up to Lac a L'Eau Claire) where eastern larch is distributed more-or-less continuously in the forest tundra, the sawfly population cycle appears to have been syn- chronous (Fig. 2). At these sites, the outbreak seems to have started in 1984, peaked in 1985 (1986 at Lac Fagnant and Kuujjuarapik), recessed in 1987, increased slightly in 1988 (except at Lac Fagnant and Lac Bienville), and recessed again in 1989. The 1989 data probably underestimate the insect's activity, because sites were visited in July, before egg laying was complete. The isolated larch of Lac Kakiattuq and Rivikre Boniface show a similar increase in 1985 and 1986, a clear recession in 1987, and a higher increase in intensity in 1988 (Fig. 2). This suggests that the outbreak may last longer here than else- where in the subarctic. Minor site-specific variations, such as the presence of only 2 years of significant attack since 1984 on the Boniface tree, are difficult to interpret, but possibly reflect the isolated character of these small populations. Shoot Density in Relation to Larch Sawfly Activity. Data collected in 1988 on the use of long shoots in relation to their availability are shown in Figure 3. At the peak of the outbreak in 1985, the average incidence of attack of available long shoots was estimated at 20% in Kuujjuarapik, and 34% at the GRB site, indicating relatively high densities of P. erichsonii adults. Heavy defoliation of larch trees was observed at the GRB site by Volume 123 THE CANADIAN ENTOMOLOGIST

75 1 Lac Kakiattuq - Isolated grove (N-63 shoals)

75 Lac I'EBU Claire (N-229 shoots) % 4

751 Lac Gulllaume Delisle (N=1054shods)

75 1 Lac Blenville (N=1739 shoots)

75 1 Lac Fagnant (~=307shoots)

Year FIG.2. Frequency distributions of long shoots of eastern larch used for oviposition by Pristiphora erichsonii (1975-1989) at various locations in subarctic Qutbec.

collaborators working in that area in 1985 and 1986. Although no or branches were collected at that time, P. erichsonii was most likely responsible. Year-to-year variations of long shoot density were very similar for the two regions during the 1980s, except 1987-1988 (Fig. 3). In both regions, shoot numbers increased slightly and regularly during 1980-1983, dropped in 1984, increased again in 1985 and 1986, and dropped again in 1987. Variations in the density of long shoots fluctuated with greater amplitude after 1983, i.e. after populations of P. erichsonii had reached outbreak levels, in the Kuujjuarapik- GRB region. This suggests the larch sawfly's population increase starting in 1984 may 616 THE CANADIAN ENTOMOLOGIST

a shoots produced % shoots scarred

200

150 loo

50 g rO -0 0 200 CU) -o Grande Rivibre de la Baleine 150

100

50

0 1980 1981 1982 1983 1984 1985 1986 1987 1988 Year

FIG. 3. Average number of long shoots per branch of eastern larch, and percentage long shoot use for oviposition by Prisriphora erichsonii (198C1988) at Kuujjuarapik and Grande Rivihre de la Baleine, QuCbec.

have significantly affected the production of long shoots on these trees, which in turn may have affected subsequent dynamics of the sawfly which is limited to these shoots for oviposition. During the population peak, long shoot use by P. erichsonii could be inversely related to shoot density if the number of shoots available for oviposition was limiting. This is confirmed by a significant negative correlation between shoot availability and percentage used for oviposition (Fig. 4; N = 35; r = 0.393; p = 0.02) for the 5-year period 1984-1988 (data for 198&1983 are not included because the insect was too rare before 1984 for shoots to be a limiting factor). Shoot use for oviposition and shoot availability also may be correlated if we assume that trees or branches defoliated by the insect reacted by reducing production of long shoots in the year following attack. Based again on the data for 1984-1988, no significant cor- relation was found between shoot use in year x, and shoots produced in year x + 1 (N = 35; r= 0.167; p= 0.34). The absence of a significant correlation also could result from the use of long shoots by females being poorly related to defoliation by mature larvae, due to fluctuations in egg and early larval mortality. A delayed response of long shoot production to the recent population increase of P, erichsonii seems to be apparent in the GRB data. Here, the high rate of shoot use at the peak of the outbreak (average 35% for 1985, Fig. 3) was followed by long shoot production decline in 1987 and 1988. In contrast, there was a sharp rise in long shoot production in Kuujjuarapik in 1988, and shoot use during the outbreak cycle was spread more throughout the cycle and peaked at a significantly lower level (20%). Obviously, the relationship between larch sawfly activity and shoot growth of larch is not simple. The generally increasing trend in long shoot production (Fig. 3) is expected on trees expanding their crown under favourable conditions. Minor changes in this trend, such as the small drop of 1984 and the larger one of 1987, might be related to climatic stress. A possible effect of a cold growing season is not supported by the absence of a significant correlation between the mean temperature of the late spring-early summer period Volume 123 THE CANADIAN ENTOMOLOGIST 60 3 y = 22.2 - 0.07~ 50 - R square = 0.1 54

. Q 3 40- 5 30 - I V)

El I 0 100 200 300 400 Shoots produced

FIG. 4. Regression between percentage use of long shoots for oviposition by Pristiphora erichsonii, and number of long shoots produced per branch, Kuujjuarapik and Grande Rivikre de la Baleine (198k1988).

(May, June, July) and long shoot production at Kuujjuarapik (N = 8 years; r = 0.12; p =0.77), the only site for which temperature data are available. Other climatic factors may have played a role, as extensive wildfires during recent years along the Grande Rivikre de la Baleine suggest that conditions have been generally dry. Biological factors other than insect defoliation also may have affected crown expansion. Factors related to long-term growth, reproductive, and defense strategies of larch trees should be considered (Morris 1951; Niemela et al. 1980; Rhoades 1985). Tree mortality following P. erichsonii outbreaks often has been reported in southern parts of the range of eastern larch (e.g. Turnock 1954; Beckwitz and Drooz 1956). The outbreak reported here may not have produced all its potential impact yet, but mature tree mortality was rarely observed. Larch mortality would likely result only after several years of moderate to severe defoliation by P. erichsonii (Beckwitz and Drooz 1956; Ives and Nairn 1966; Kulman 1971). Although we did not observe heavy defoliation in 1988 and 1989, it was probably widespread during the mid-1980s, particularly at continental sites of lower latitudes. The effects that are now apparent include reduced foliage and terminal growth, short shoots switched to long shoots, dead or dying branches, and small trees dying, all known responses of larch trees to prolonged defoliation by larch sawfly larvae (Heron 1952; Nairn 1958; Turnock 1960; Carroll 1964; Ives and Nairn 1966). Such effects were not analysed in detail and therefore cannot be attributed to P. erichsonii with cer- tainty, because other defoliators of larch including the larch budmoth, Zeiraphera impro- bana Walker, are present in the region. However, most of the damage probably results from defoliation by the larch sawfly. There are indications that P. erichsonii has been present on larch in northern QuCbec prior to the mid-1960s, the age of the oldest scars collected in this study. A dendrochron- ological analysis showed that severe larch growth reductions, not parallelled in white spruce and black spruce, occurred at least twice in this century, culminating around 1905 and 1940 (Arquillibre et al. 1990). This suggests that P. erichsonii has been a component of the forest communities in subarctic QuCbec for a period possibly extending back to just a few years after it first defoliated the stands of mature larch on the south shore of the St. Lawrence River during the early 1880s (Fletcher 1885; Provancher 1885; Fyles 1892). 618 THE CANADIAN ENTOMOLOGIST MayIJune 1991 Although its impact on southern larch stands at the end of the last century is generally said to have been devastating (e.g. Jarvis 1904; Fyles 1906), it is not clear that it was so severe in northern forest communities. It is now established that the insect is adapted to exploit and breed on L. laricina under the most extreme climatic conditions that eastern populations of this tree can tolerate in North America. Our study shows that this defol- iator's effects on trees are apparent throughout the larch stands of the forest tundra. How- ever, we need to know more to understand its long-term impact on northern forest communities. Acknowledgments We thank I. Tailleux, Y. Jardon, F. Quinty, and R. Fortier for assistance with sample collection and sorting. Thanks also to Y. Begin and S. Payette for help with logistics and hospitality at camp sites. This study was supported by grants from the Fonds pour la Formation de Chercheurs et 1'Aide a la Recherche du QuCbec, and the Natural Sciences and Engineering Research Council of Canada. References Arquilliere, S., L. Filion, K. Gajewski, and C. Cloutier. 1990. A dendroecological analysis of eastern larch (Larix laricina) in subarctic Qutbec. Can. J. For. 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Part 111. Biological Con- trol of Forest Insect Pests in Canada, 1959-1968. Technical Communication No. 4. Commonwealth Insti- tute of Biological Control, Trinidad. Commonwealth Agricultural Bureaux, Farnham Royal. 1972. Characteristics of Bessa harveyi (Diptera: Tachinidae) suggesting the historic introduction of the larch sawfly to North America. Man. Ent. 6: 49-53. Wallace, D.R. 1954. Some reactions of larch trees to larch sawfly defoliation. Can. Dept. Agric. For. Biol. Div. Prog. Rep. 10: 2. West, D.C., H.H. Shugart, and D.B. Botkin (Eds.). 1981. Forest Sucession Concepts and Application. Springer Verlag, New York, NY. 5 17 pp. Wong, H.R. 1974. Identification and origin of the strains of the larch sawfly, Pristiphora erichsonii (Hymenop- tera: Tenthredinidae), in North America. Can. Ent. 106: 1121-1 131. (Date received: 7 August 1990; date accepted: 29 January 1991)