CONRAD CLOUTIER and LOUISE FILION Abstract Introduction In
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RECENT OUTBREAK OF THE LARCH SAWFLY, 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, Larix laricina (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 Pristiphora 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 insect 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.