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Pine weevil (Hylobius abietis) damage to cuttings and seedlings of Norway spruce
Forest Ecology and Management 160 (2002) 11–17
Pine weevil (Hylobius abietis) damage to cuttings and seedlings of Norway spruce ˚ ke Thorse´na, Staffan Mattssonb, Jan Wesliena Mats Hannerza,*, A a
The Forestry Research Institute of Sweden (SkogForsk), Uppsala Science Park, SE-751 83 Uppsala, Sweden b Skogssa¨llskapet Fo¨rvaltnings AB, Kungsa¨ngsva¨gen 21, SE-753 23 Uppsala, Sweden Received 4 May 2000; received in revised form 30 November 2000
Abstract Damage caused by pine weevil (Hylobius abietus L.) to planted seedlings and cuttings of Norway spruce (Picea abies (L.) Karst.) was studied at five clearcut sites in south-eastern Sweden. The main objective was to compare the two types of stock in terms of attack frequency and mortality due to pine weevil feeding. Cuttings and seedlings with the same initial stem-base diameter (4 mm) were compared. Two sites were harvested and scarified shortly before planting, two were harvested shortly before planting, but were not scarified, and one was harvested 2 years before and scarified the autumn before planting. The total mortality 5 years after planting was highest, greater than 90%, at the new, non-scarified sites, and lowest, 23%, at the old, scarified site. More than 90% of the mortality was caused by pine weevil feeding. Attack frequency and pine weevil induced mortality were significantly higher among seedlings than among cuttings. Mortality due to pine weevil damage was 4–43% higher in seedlings than in cuttings after the fifth year. Of the cuttings and seedlings that were attacked in the first year, a significantly higher frequency of the seedlings were girdled. The higher resistance of cuttings to pine weevil damage may partly explain the more rapid growth of cuttings reported in other studies. However, the causes of their higher resistance need to be further investigated. The thicker bark and needles on the stem base of the cuttings could be important in this respect. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Cutting; Hylobius abietis; Insect herbivory; Picea abies; Seedling; Sweden
1. Introduction Damage by insects, mainly caused by the large pine weevil, Hylobius abietis (L.), is a significant hindrance in the establishment of new pine (Pinus sylvestris L.) and spruce (Picea abies (L.) Karst.) forests through planting. Other species that cause significant damage include H. pinastri (Gyllenhal) and the scolytid Hylastes cunicularius (Erichson). According to Ollas *
Corresponding author. Tel.: þ46-18-18-85-00; fax: þ46-18-18-86-00. E-mail address: [email protected] (M. Hannerz).
(1992), 2-year survival was less than 75% in about half of the plantations established in 1989 in Sweden, largely due to these insects. All three species breed in conifer stumps and are attracted to the fellings by volatiles released from the newly cut stumps and slash. In southern and central Scandinavia, the flight period of H. abietis starts in mid or late May and is essentially over within a month. Adult weevils oviposit throughout the summer (Lekander et al., 1985). Eggs are laid on the roots and in the soil nearby (Nordenhem and Nordlander, 1994). Weevils from both the parent generation and the new generation may be present in large numbers at a site for several
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consecutive years following felling (von Sydow, 1997, ¨ rlander et al., 1997). O Feeding by pine weevils scars the phloem of the seedlings, and girdling at the stem base rapidly kills them. Seedlings can be treated with an insecticide (permethrin) to reduce feeding and the risk of seedling mortality. Soil scarification also reduces insect damage (Ha¨ ggstro¨ m, 1958; So¨ derstro¨ m, 1974; von ¨ rlander and Nilsson, 1999). However, Sydow, 1997; O it seems likely that the use of permethrin will be banned in the future due to concerns about workers’ health, and related environmental issues. Furthermore, scarification alone is seldom sufficient to limit damage to acceptable levels. A fallow period combined with intensive scarification, to reduce grass vegetation, is another alternative (although expensive), but such scarification is rarely acceptable for environmental reasons. Physical protective systems are being developed as alternatives to insecticide treatment. Some of these show promising results, but need further improvement to be biologically and economically justified (von Hofsten et al., 1999). The risk of girdling by pine weevils depends not only on feeding activity but also on seedling size, as documented in many studies (Lekander and So¨ derstro¨ m, 1969; Eidmann, 1969; Selander, 1993; ¨ rlander and Nilsson, 1999; Thorse´ n Kohmann, 1995; O et al., 2001). The vigour of the planting stock also influences its susceptibility to pine weevil damage, and water stress increases susceptibility to insect damage after planting out Scots pine seedlings (Selander and Immonen, 1992). Planted stocks of Norway spruce comprised of rooted cuttings generally perform better than seedlings of comparable genetic origin in terms of height growth and frost resistance (Gemmel et al., 1991; Hannerz, 1994; Hannerz and Wilhelmsson, 1998). The usual method of cutting propagation is to cut off currentyear shoots from a mother plant and insert them in a suitable substrate, where they form adventitious roots. Gemmel et al. (1991) suggested that the generally higher survival rates and faster initial growth of cuttings in the field may be partly due to them having higher resistance to pine weevil damage. However, no data were presented that could be used to test the hypothesis. If there were differences in pine weevil damage between the stock types, they might have been explained by differences in size, as resistance to
pine weevil damage increases with stem-base diameter (Thorse´ n et al., 2001), and young cuttings have a larger stem base diameter than seedlings of comparable age (Dekker-Robertson and Kleinschmit, 1991). In this study, we compare pine weevil damage, survival and growth of containerised Norway spruce cuttings and seedlings at five sites in south-eastern Sweden. Type and severity of feeding, survival, vigour and growth development were registered for individual cuttings and seedlings during the first 3 years after planting. Survival was also registered 5 years after planting. Further on, initial stem base diameter was recorded, so it was possible to compare pine weevil damage for cuttings and seedlings with the same initial stem thickness.
2. Material and methods 2.1. Sites and planting stock The experiments were planted in 1988 on five clearcut sites, located at latitudes 578110 –578350 , 95– 225 m a.s.l. in the province of Sma˚ land in southeastern Sweden (Table 1). On two of the sites planting was done soon after harvesting without previous scarification (new, non-scarified: NN1 and NN2). Three of the sites were scarified with a harrow. Two of these were harvested and scarified in the same spring as the planting was done (new, scarified: NS1 and NS2), while the third had a fallow period of 2 years after harvesting, and was scarified in the autumn of 1987 (old, scarified: OS). The moisture, texture and fertility of the sites were close to the average for forested sites in the region. The diversity of treatments among the sites was chosen to cover a broad range in risk of pine weevil damage. The NN clearcuts were expected to get the highest load of pine weevil attacks, and the OS clearcut the lowest. The main purpose of the experiment was to compare survival and growth of bare-root seedlings of various ages and sizes, and the results from this part of the study were reported by Mattsson and Thorse´ n (1992). This article is restricted to a comparison of 2-year-old container-grown cuttings and seedlings (Table 2). In terms of genetic origin, the seedlings were of Belorussian provenance (Vitebsk), and the
M. Hannerz et al. / Forest Ecology and Management 160 (2002) 11–17
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Table 1 Description of the experimental sites and dates of forestry measurements used to determine the impacts of pine weevils on Norway spruce seedlings and cuttings
Clearcut harvesting Scarification Planting Soil moisture Soil texture Vegetation typeb a b
NS1a
NS2a
OSa
NN1a
NN2a
Spring 1988 Spring 1988 28 April 1988 Mesic Glacial till Grass
Spring 1988 Spring 1988 5 May 1988 Mesic Glacial till Grass
Winter 1985/86 Autumn 1987 27 April 1988 Mesic Glacial till Low herb
Autumn 1987 – 29 April 1988 Mesic Glacial till Grass
Autumn 1987 – 2 May 1988 Mesic Glacial till Grass
Description of sites: (NS1 and NS2) new clearcut, scarified; (OS) old clearcut, scarified; (NN1 and NN2) new clearcut, no scarification. Terminology after Ha¨ gglund and Lundmark (1977).
and after autumn feeding in the first year, and in mid summer and after the autumn feeding in the second and third years. A last inventory of survival was made after the fifth year. Feeding scars on the main stem were grouped into six classes: 0–5, where 0 was no feeding and 1–5 were based on proportion of the stem surface damaged, in 20% classes. Whether or not each plant had been girdled by the feeding was also recorded. Mortality and cause of death were registered at each inventory. The damage-causing agents noted were drought, browsing, flooding, pine weevil, Hylastes spp., and other insects. In this study, we analysed the following variables: frequency of cuttings and seedlings attacked by pine weevils in the first growing season, frequency of girdling among attacked cuttings and seedlings in the first growing season, mortality due to pine weevil damage after the first, third and fifth growing seasons, and total mortality after the fifth growing season. The growth of surviving, non-attacked cuttings and seedlings was also analysed at one site (OS). The volume of each cutting and seedling was estimated as a cone.
cuttings were derived from bulk-propagated openpollinated progenies from the south-Swedish seed orchard Maglehem. Differences between the stock types were evaluated in an analysis encompassing all the cuttings and seedlings. Also, to eliminate the possible effects of variations in stem thickness, differences between seedlings and cuttings of the same initial diameter (3.5–4.5 mm, the most frequently represented diameter class) were also assessed. The experimental design at each of the sites was randomised blocks, with treatment plots being planted in rows within the blocks. At each site NS1, NS2, OS and NN2 there were five blocks with 40 seedlings or cuttings per block for each of the treatments, and at site NN1 there were nine blocks with 20 seedlings or cuttings per block. 2.2. Assessments and analyses Initial height and diameter were measured immediately after planting. Height, stem base diameter, and type and severity of damage were also measured soon after the spring feeding (early June), in mid summer
Table 2 Stock types and average initial height and diameter in Norway spruce cuttings and seedlings Stock type
Cutting Seedling a
Age (year)
2 2
Container system
Height average (cm)
Diameter average (mm)
Cellpot IIIb Combicell 47c
18.6 20.4
4.2 3.4
Classes 1.5–2.5, 2.5–3.5 mm, etc. Cell volume 130 cm3, 400 plants per m2. c Cell volume 165 cm3, 446 plants per m2. b
Frequency of diameter classes (mm)a 2
3
4
5
6
8 89
142 520
500 310
295 55
30 5
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One of the sites (NN1) was harrowed by mistake after the first growing season, hence, this site was only analysed for survival and attacks in the first year. Statistical analyses of mortality and damage were based on arcsine square-root transformed plot frequencies. No significant deviations from normality or homoscedasticity were found among the residuals. Analyses of variance were performed both for all seedlings and cuttings, and for those only in the 4 mm diameter class, with the following linear model, using the SAS procedure MIXED: yijk ¼ m þ Pi þ Sj þ pi sj þ eijk
ðmodel 1Þ
where yijk is the observation of plot ijk, m Pi the fixed effect of stock type i (i ¼ 1; 2), Sj the fixed effect of site j (j ¼ 1; . . . ; 5), pisj the random effect of interaction between stock type and site (between-site-error) and eijk the random residual effect of plot ijk (withinsite-error). Site OS was excluded from the analysis of girdled seedlings and cuttings due to the low number of pine weevil attacks at this site. Site NN1 was excluded from the analysis of mortality after the first year, since the site was harrowed by mistake after the first growing season. The growth development of surviving and nonattacked seedlings and cuttings was analysed only for the OS site. Survival was too low at the other sites to allow any reliable comparisons. The purpose was to determine if there were growth differences between
the two stock types that could be attributed to vigour at the time of planting. Analysis of variance was performed with the following linear model using the SAS procedure MIXED: yijk ¼ m þ Pi þ bj þ pbij þ eijk
ðmodel 2Þ
where yijk is the observation of seedling/cutting ijk, m pi the fixed effect of stock type i (i ¼ 1; 2), bj the random effect of block j (j ¼ 1; . . . ; 5), and eijk the random within-plot error. Differences between cuttings and seedlings were evaluated using Student’s t-tests with the above models.
3. Results Damage caused by pine weevils was most severe on the non-scarified sites NN1 and NN2, where over 90% of the cuttings and seedlings were attacked, and 80% died in the first year (Table 3). Pine weevil induced mortality was lowest at the scarified site with a fallow period (OS). The newly clearcut but scarified sites (NS1 and NS2) were intermediate in terms of attack frequency and mortality. The major source of damage was the pine weevil. The second most important agents of damage were other insects, mainly Hylastes sp., which caused 2% of the mortality. Most of the mortality took place in the first growing season. The effects of stock type and site were significant at the 5% probability level for all of the tested variables,
Table 3 Percentage of Norway spruce damage and mortality at five study sites as combined averages for cuttings and seedlings in all diameter classes Sitea
NS1 NS2 OS NN1f NN2 a
Total, dead year 5e
Pine weevil induced damage and mortality Attacksb
Girdledc
Dead year 1d
Dead year 3d
Dead year 5d
61.6 77.5 26.5 90.5 90.4
46.7 60.8 10.0 81.9 83.1
44.7 58.0 5.8 80.5 81.9
48.7 71.5 17.5 – 86.6
49.2 73.0 17.8 – 87.9
53.8 76.2 23.5 – 91.2
Description of sites: (NS1 and NS2) new clearcut, scarified; (OS) old clearcut, scarified; (NN1 and NN2) new clearcut, no scarification. Percent plants with feeding scars, total in the first season. c Percent plants that were girdled by pine weevil, total in the first season. d Percent plants killed by pine weevil damage after the first, third and fifth seasons. e Percent dead plants, all causes, after five seasons. f This site was harrowed by mistake after the first growing season. b
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Table 4 Results of analyses of variance of mortality and damage frequency, according to model 1, of data for cuttings and seedlings with initial diameter of 4 mm, following arcsine square-root transformation of plot frequenciesa Fixed effects
Stock type Site d.f. of denominator
d.f.
1 4 4
Attacks year 1b
Girdled first yearc
Dead year 1d
Dead year 5d
F-value
P-value
d.f.
F-value
P-value
d.f.
F-value
P-value
d.f.
F-value
P-value
29.50 35.09
0.006 0.002
1 3 3
14.92 4.32
0.03 0.13
1 4 4
17.76 29.98
0.014 0.003
1 3 3
16.23 27.14
0.028 0.011
a
The random effect stock type site was used as denominator. Percent cuttings or seedlings with feeding scars, total in the first season. c Percent pine weevil attacked cuttings or seedlings that were girdled in the first season. The OS site excluded. d Percent cuttings or seedlings killed by pine weevil after the first season (all sites) and the fifth season (all sites except NN1, new clearcut, no scarification). b
regardless of whether all cuttings and seedlings were tested (results not shown), or just those with an initial diameter of 4 mm (Table 4). Seedlings had a significantly higher frequency of feeding attacks in the first year and pine weevil induced mortality than cuttings (Figs. 1 and 2). Of the cuttings and seedlings that were attacked in the first year, the frequency of girdling was significantly higher among seedlings than among cuttings (Fig. 3). The initial height and volume of surviving, nonattacked seedlings at the OS site were higher than those of the surviving cuttings (calculated for initial diameter of 4 mm) (Table 5). After three growing seasons, the seedlings had significantly larger height, diameter and volume than the cuttings.
Fig. 1. Percentage of Norway spruce cuttings and seedlings (with initial diameter of 4 mm) attacked by pine weevil in the first year, and S.E. Description of sites: (NS1 and NS2) new clearcut, scarified; (OS) old clearcut, scarified; (NN1 and NN2) new clearcut, no scarification.
Fig. 2. Percentage of Norway spruce cuttings and seedlings (with initial diameter of 4 mm) killed by pine weevil after 5 years in the field and S.E. Description of sites: (NS1 and NS2) new clearcut, scarified; (OS) old clearcut, scarified; (NN1 and NN2) new clearcut, no scarification.
Fig. 3. Percentage of pine weevil attacked Norway spruce seedlings and cuttings with an initial diameter of 4 mm that were girdled in year 1, and S.E. The OS site was excluded from the analyses of girdling due to the low number of attacked seedlings and cuttings at this site. Description of sites: (NS1 and NS2) new clearcut, scarified; (OS) old clearcut, scarified; (NN1 and NN2) new clearcut, no scarification.
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Table 5 Height, diameter and volume (mean S.E.) of Norway spruce cuttings and seedlings (with initial diameter of 4 mm) at the OS site (old clearcut, scarification) that were not attacked by pine weevil and that survived over the third growing season Variablea
Cuttings (N ¼ 88)
Seedlings (N ¼ 51)
Result of t-testb
Height, year 0 (cm) Height, year 3 (cm) Diameter, year 3 (mm) Volume, year 0 (cm3) Volume, year 3 (cm3)
18.02 44.34 8.34 0.75 9.77
20.84 55.16 10.01 0.87 16.08
***
(0.55) (0.91) (0.37) (0.02) (0.74)
(0.79) (1.30) (0.52) (0.03) (1.06)
*** *** *** ***
a
Year 0 is at planting, and year 3 after the third growing season. Student’s t-test according to model 2; ns: not significant. *** P < 0:001. b
4. Discussion The differences among the sites in pine weevil induced mortality followed the expected pattern. Planting on fresh clearcuts with undisturbed soil and without chemical or physical protection of the seedlings is doomed to fail in southern Sweden ¨ rlander and Nilsson, 1999). Scarification can help to (O reduce the damage on fresh clearcuts, but the mortality levels are still unacceptably high from a practical forestry perspective. Acceptable levels of survival were found in this study only at the old clearcut, in combination with fresh scarification. The positive effect of using cuttings rather than seedlings was shown at all sites. The cuttings had 4–43% higher survival than the seedlings at the various sites. This higher rate of survival is significant from an economic viewpoint, but optimising the choice of stock type will not solve the pine weevil problem alone. The effects of using different types of stock, in combination with other measures, such as physical protection, scarification, fallow-periods and shelter trees ought to be further studied. The general results of the study support the hypothesis of Gemmel et al. (1991), that pine weevil damage might explain some of the growth rate superiority of cuttings over seedlings. Cuttings and seedlings differ in various properties which might help to explain the difference in pine weevil resistance. The thicker bark and needles on the cutting’s root collar might inhibit the pine weevil from feeding. Cuttings are ontogenetically older than seedlings: in this case, for instance, they were cut from 3-year-old seedlings. Growth rhythm is altered towards later bud burst
and earlier growth cessation in ontogenetically older planting stock (Ununger et al., 1988), and this may explain why cuttings can tolerate frost better than comparable seedlings (Hannerz, 1994; Hannerz and Wilhelmsson, 1998). Other properties, such as the chemical composition of the wood and bark, might also change with age of the planting material, and these possibilities need to be further investigated in order to explain fully the results of the present study. Cuttings and seedlings are very difficult to compare rigorously and unambiguously. In this study, the two types of stock had different genetic origins, with seedlings from a Belorussian provenance and cuttings from the seed orchard Maglehem. Thus, possible genetic effects cannot be excluded. However, experience from genetic field trials with Norway spruce in southern Sweden suggests that genetic origin generally has a small or negligible effect on mortality after planting, including pine weevil induced mortality (Karlsson and Ho¨ gberg, 1998; Hannerz et al., 1999). The vigour of the material at the time of planting could also be important. Selander and Immonen (1992) demonstrated that seedlings under water stress were more heavily attacked by pine weevil than nonstressed seedlings, and water stress could be a factor in the present study. However, all seedlings and cuttings were treated carefully and equally to maintain their vigour and water potential before planting. Furthermore, the seedlings would be expected to have lower growth rates than the cuttings following planting, if they were more stressed, but the study of non-attacked cuttings and seedlings at the OS site suggested that the growth rates of the seedlings were at least as strong, or stronger, than those of the cuttings.
M. Hannerz et al. / Forest Ecology and Management 160 (2002) 11–17
Norway spruce cuttings are mainly used in clonal forestry as a tool for duplicating genetically superior, selected clones (Ritchie, 1991). Several studies have demonstrated that cuttings have significant physiological advantages over seedlings, for stocks of similar genetic origin, including faster initial growth (Gemmel et al., 1991), reduced risk of winter damage (Hannerz, 1994) and greater resistance to spring and autumn frost damage (Hannerz and Wilhelmsson, 1998). This study demonstrates that resistance to pine weevil damage can be added to this list of advantages.
Acknowledgements The establishment of the experiment was financially supported by the Swedish Forestry Research Foundation (SSFf). Many persons gave technical assistance ˚ ke in the field. We are especially grateful to Per-A Arvidsson. We also thank the landowner, AssiDoma¨ n AB, for hosting the experiments. John Blackwell corrected the English. References Dekker-Robertson, D.I., Kleinschmit, J., 1991. Serial propagation in Norway spruce (Picea abies (L.) Karst.): results from later propagation cycles. Silvae Genet. 40, 202–214. Eidmann, H.H., 1969. Ru¨ sselka¨ ferscha¨ den an verschiedenen Nahrungspflanzen. Anzeiger f. Scha¨ dlingskunde u. Pflanzenschutz 42, 22–26. ¨ rlander, G., Ho¨ gberg, K.-A., 1991. Norway spruce Gemmel, P., O cuttings perform better than seedlings of the same genetic origin. Silvae Genet. 40, 198–202. Ha¨ gglund, B., Lundmark, J.-E., 1977. Site index estimation by means of site properties. Stud. For. Suec. 138, 1–38. Ha¨ ggstro¨ m, B., 1958. Resultat av na˚ gra fo¨ rso¨ ksplanteringar. Norrl. ˚ rg. 1958, 162–178 (in Swedish). Skogsva˚ rdsfo¨ rb. Tidskr. A Hannerz, M., 1994. Winter injuries to Norway spruce observed in plantations and in a seed orchard. SkogForsk, Report No. 6. p. 22. Hannerz, M., Wilhelmsson, L., 1998. Field performance during 14 years growth of Picea abies cuttings and seedlings propagated in containers of varying size. Forestry 71, 373–380. Hannerz, M., Sonesson, J., Ekberg, I., 1999. Genetic correlations between growth and growth rhythm observed in a short-term test and performance in long-term field trials of Norway spruce. Can. J. For. Res. 29, 768–778.
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¨ rlander, G., 1999. Mekaniska von Hofsten, H., Pettersson, M., O snytbaggeskydd–en la¨ gesrapport. SkogForsk, Resultat nr 24, p. 6 (in Swedish with English abstract). Karlsson, B., Ho¨ gberg, K.-A., 1998. Genotypic parameters and clone site interaction in an 11-year-old series of five clone tests with Norway spruce (Picea abies (L.) Karst) in south Sweden. For. Genet. 5, 21–30. Kohmann, K., 1995. Hight growth and survival of different spruce seedling types. Rapport fra Skogforsk 6/95, p. 14 (in Norwegian, with English summary and legends). Lekander, B., So¨ derstro¨ m, V., 1969. Studie o¨ ver snytbaggeangrepp pa˚ barrtra¨ dsplantor. Sv. Skogsv. Fo¨ rb. Tidskr. 67, 351–383 (in Swedish with English summary and legends). Lekander, B., Eidmann, H.H., Bejer, B., Kangas, E., 1985. Time of oviposition and its influence on the development of Hylobius abietis (L.) (Col., Curculionidae). Z. Ang. Entomol. 100, 417–421. ˚ ., 1992. Fo¨ ryngring med fo¨ rhinder. In: Mattsson, S., Thorse´ n, A Frumerie, G. (Ed.), The rationalisation conference 1992. The Forest Operations Institute of Sweden, Redogo¨ relse No. 1, pp. 35–48 (in Swedish with English summary). Nordenhem, H., Nordlander, G., 1994. Olfactory oriented migration through soil by root living Hylobius abietis (L.) larvae (Col., Curculionidae). J. Appl. Ent. 117, 457–462. Ollas, R., 1992. Avga˚ ngar och skador i 1989 a˚ rsplanteringar. Snabbinformation fra˚ n skogsva˚ rdsenheten, Skogsstyrelsen, Jo¨ nko¨ ping, Skogsva˚ rd 4/1992 (in Swedish). ¨ rlander, G., Nilsson, U., 1999. Effect of reforestation methods on O pine weevil (Hylobius abietis) damage and seedlings survival. Scand. J. For. Res. 14, 341–354. ¨ rlander, G., Nilsson, U., Nordlander, G., 1997. Pine weevil O abundance on clearcuttings of different ages: a 6-year study using pitfall traps. Scand. J. For. Res. 12, 225–240. Ritchie, G.A., 1991. The commercial use of conifer rooted cuttings in forestry: a world overview. New For. 5, 247–275. Selander, J., 1993. Survival model for Pinus sylvestris seedlings at risk from Hylobius abietis. Scand. J. For. Res. 8, 66–72. Selander, J., Immonen, A., 1992. Effect of fertilization and watering of Scots pine seedlings on the feeding preference of the pine weevil (Hylobius abietis L.). Silva Fennica 26, 75–84. So¨ derstro¨ m, V., 1974. Soil treatment. Sv. Skogv. Fo¨ rb. Tidskr. 72(1), 157–165 (in Swedish with English summary and legends). ˚ ., Mattsson, S., Weslien, J.-O., 2001. Influence of stem Thorse´ n, A diameter on the survival and growth of containerised Norway spruce seedlings attacked by pine weevils (Hylobius spp.). Scand. J. For. Res. 16, 21–29. Ununger, J., Ekberg, I., Kang, H., 1988. Genetic control and agerelated changes of juvenile growth characters in Picea abies. Scand. J. For. Res. 3, 55–66. von Sydow, F., 1997. Abundance of pine weevils (Hylobius abietis) and damage to conifer seedlings in relation to silvicultural practices. Scand. J. For. Res. 12, 157–167.
Pine weevil (Hylobius abietis) damage to cuttings and seedlings of Norway spruce ˚ ke Thorse´na, Staffan Mattssonb, Jan Wesliena Mats Hannerza,*, A a
The Forestry Research Institute of Sweden (SkogForsk), Uppsala Science Park, SE-751 83 Uppsala, Sweden b Skogssa¨llskapet Fo¨rvaltnings AB, Kungsa¨ngsva¨gen 21, SE-753 23 Uppsala, Sweden Received 4 May 2000; received in revised form 30 November 2000
Abstract Damage caused by pine weevil (Hylobius abietus L.) to planted seedlings and cuttings of Norway spruce (Picea abies (L.) Karst.) was studied at five clearcut sites in south-eastern Sweden. The main objective was to compare the two types of stock in terms of attack frequency and mortality due to pine weevil feeding. Cuttings and seedlings with the same initial stem-base diameter (4 mm) were compared. Two sites were harvested and scarified shortly before planting, two were harvested shortly before planting, but were not scarified, and one was harvested 2 years before and scarified the autumn before planting. The total mortality 5 years after planting was highest, greater than 90%, at the new, non-scarified sites, and lowest, 23%, at the old, scarified site. More than 90% of the mortality was caused by pine weevil feeding. Attack frequency and pine weevil induced mortality were significantly higher among seedlings than among cuttings. Mortality due to pine weevil damage was 4–43% higher in seedlings than in cuttings after the fifth year. Of the cuttings and seedlings that were attacked in the first year, a significantly higher frequency of the seedlings were girdled. The higher resistance of cuttings to pine weevil damage may partly explain the more rapid growth of cuttings reported in other studies. However, the causes of their higher resistance need to be further investigated. The thicker bark and needles on the stem base of the cuttings could be important in this respect. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Cutting; Hylobius abietis; Insect herbivory; Picea abies; Seedling; Sweden
1. Introduction Damage by insects, mainly caused by the large pine weevil, Hylobius abietis (L.), is a significant hindrance in the establishment of new pine (Pinus sylvestris L.) and spruce (Picea abies (L.) Karst.) forests through planting. Other species that cause significant damage include H. pinastri (Gyllenhal) and the scolytid Hylastes cunicularius (Erichson). According to Ollas *
Corresponding author. Tel.: þ46-18-18-85-00; fax: þ46-18-18-86-00. E-mail address: [email protected] (M. Hannerz).
(1992), 2-year survival was less than 75% in about half of the plantations established in 1989 in Sweden, largely due to these insects. All three species breed in conifer stumps and are attracted to the fellings by volatiles released from the newly cut stumps and slash. In southern and central Scandinavia, the flight period of H. abietis starts in mid or late May and is essentially over within a month. Adult weevils oviposit throughout the summer (Lekander et al., 1985). Eggs are laid on the roots and in the soil nearby (Nordenhem and Nordlander, 1994). Weevils from both the parent generation and the new generation may be present in large numbers at a site for several
0378-1127/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 1 ) 0 0 4 6 7 - 4
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consecutive years following felling (von Sydow, 1997, ¨ rlander et al., 1997). O Feeding by pine weevils scars the phloem of the seedlings, and girdling at the stem base rapidly kills them. Seedlings can be treated with an insecticide (permethrin) to reduce feeding and the risk of seedling mortality. Soil scarification also reduces insect damage (Ha¨ ggstro¨ m, 1958; So¨ derstro¨ m, 1974; von ¨ rlander and Nilsson, 1999). However, Sydow, 1997; O it seems likely that the use of permethrin will be banned in the future due to concerns about workers’ health, and related environmental issues. Furthermore, scarification alone is seldom sufficient to limit damage to acceptable levels. A fallow period combined with intensive scarification, to reduce grass vegetation, is another alternative (although expensive), but such scarification is rarely acceptable for environmental reasons. Physical protective systems are being developed as alternatives to insecticide treatment. Some of these show promising results, but need further improvement to be biologically and economically justified (von Hofsten et al., 1999). The risk of girdling by pine weevils depends not only on feeding activity but also on seedling size, as documented in many studies (Lekander and So¨ derstro¨ m, 1969; Eidmann, 1969; Selander, 1993; ¨ rlander and Nilsson, 1999; Thorse´ n Kohmann, 1995; O et al., 2001). The vigour of the planting stock also influences its susceptibility to pine weevil damage, and water stress increases susceptibility to insect damage after planting out Scots pine seedlings (Selander and Immonen, 1992). Planted stocks of Norway spruce comprised of rooted cuttings generally perform better than seedlings of comparable genetic origin in terms of height growth and frost resistance (Gemmel et al., 1991; Hannerz, 1994; Hannerz and Wilhelmsson, 1998). The usual method of cutting propagation is to cut off currentyear shoots from a mother plant and insert them in a suitable substrate, where they form adventitious roots. Gemmel et al. (1991) suggested that the generally higher survival rates and faster initial growth of cuttings in the field may be partly due to them having higher resistance to pine weevil damage. However, no data were presented that could be used to test the hypothesis. If there were differences in pine weevil damage between the stock types, they might have been explained by differences in size, as resistance to
pine weevil damage increases with stem-base diameter (Thorse´ n et al., 2001), and young cuttings have a larger stem base diameter than seedlings of comparable age (Dekker-Robertson and Kleinschmit, 1991). In this study, we compare pine weevil damage, survival and growth of containerised Norway spruce cuttings and seedlings at five sites in south-eastern Sweden. Type and severity of feeding, survival, vigour and growth development were registered for individual cuttings and seedlings during the first 3 years after planting. Survival was also registered 5 years after planting. Further on, initial stem base diameter was recorded, so it was possible to compare pine weevil damage for cuttings and seedlings with the same initial stem thickness.
2. Material and methods 2.1. Sites and planting stock The experiments were planted in 1988 on five clearcut sites, located at latitudes 578110 –578350 , 95– 225 m a.s.l. in the province of Sma˚ land in southeastern Sweden (Table 1). On two of the sites planting was done soon after harvesting without previous scarification (new, non-scarified: NN1 and NN2). Three of the sites were scarified with a harrow. Two of these were harvested and scarified in the same spring as the planting was done (new, scarified: NS1 and NS2), while the third had a fallow period of 2 years after harvesting, and was scarified in the autumn of 1987 (old, scarified: OS). The moisture, texture and fertility of the sites were close to the average for forested sites in the region. The diversity of treatments among the sites was chosen to cover a broad range in risk of pine weevil damage. The NN clearcuts were expected to get the highest load of pine weevil attacks, and the OS clearcut the lowest. The main purpose of the experiment was to compare survival and growth of bare-root seedlings of various ages and sizes, and the results from this part of the study were reported by Mattsson and Thorse´ n (1992). This article is restricted to a comparison of 2-year-old container-grown cuttings and seedlings (Table 2). In terms of genetic origin, the seedlings were of Belorussian provenance (Vitebsk), and the
M. Hannerz et al. / Forest Ecology and Management 160 (2002) 11–17
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Table 1 Description of the experimental sites and dates of forestry measurements used to determine the impacts of pine weevils on Norway spruce seedlings and cuttings
Clearcut harvesting Scarification Planting Soil moisture Soil texture Vegetation typeb a b
NS1a
NS2a
OSa
NN1a
NN2a
Spring 1988 Spring 1988 28 April 1988 Mesic Glacial till Grass
Spring 1988 Spring 1988 5 May 1988 Mesic Glacial till Grass
Winter 1985/86 Autumn 1987 27 April 1988 Mesic Glacial till Low herb
Autumn 1987 – 29 April 1988 Mesic Glacial till Grass
Autumn 1987 – 2 May 1988 Mesic Glacial till Grass
Description of sites: (NS1 and NS2) new clearcut, scarified; (OS) old clearcut, scarified; (NN1 and NN2) new clearcut, no scarification. Terminology after Ha¨ gglund and Lundmark (1977).
and after autumn feeding in the first year, and in mid summer and after the autumn feeding in the second and third years. A last inventory of survival was made after the fifth year. Feeding scars on the main stem were grouped into six classes: 0–5, where 0 was no feeding and 1–5 were based on proportion of the stem surface damaged, in 20% classes. Whether or not each plant had been girdled by the feeding was also recorded. Mortality and cause of death were registered at each inventory. The damage-causing agents noted were drought, browsing, flooding, pine weevil, Hylastes spp., and other insects. In this study, we analysed the following variables: frequency of cuttings and seedlings attacked by pine weevils in the first growing season, frequency of girdling among attacked cuttings and seedlings in the first growing season, mortality due to pine weevil damage after the first, third and fifth growing seasons, and total mortality after the fifth growing season. The growth of surviving, non-attacked cuttings and seedlings was also analysed at one site (OS). The volume of each cutting and seedling was estimated as a cone.
cuttings were derived from bulk-propagated openpollinated progenies from the south-Swedish seed orchard Maglehem. Differences between the stock types were evaluated in an analysis encompassing all the cuttings and seedlings. Also, to eliminate the possible effects of variations in stem thickness, differences between seedlings and cuttings of the same initial diameter (3.5–4.5 mm, the most frequently represented diameter class) were also assessed. The experimental design at each of the sites was randomised blocks, with treatment plots being planted in rows within the blocks. At each site NS1, NS2, OS and NN2 there were five blocks with 40 seedlings or cuttings per block for each of the treatments, and at site NN1 there were nine blocks with 20 seedlings or cuttings per block. 2.2. Assessments and analyses Initial height and diameter were measured immediately after planting. Height, stem base diameter, and type and severity of damage were also measured soon after the spring feeding (early June), in mid summer
Table 2 Stock types and average initial height and diameter in Norway spruce cuttings and seedlings Stock type
Cutting Seedling a
Age (year)
2 2
Container system
Height average (cm)
Diameter average (mm)
Cellpot IIIb Combicell 47c
18.6 20.4
4.2 3.4
Classes 1.5–2.5, 2.5–3.5 mm, etc. Cell volume 130 cm3, 400 plants per m2. c Cell volume 165 cm3, 446 plants per m2. b
Frequency of diameter classes (mm)a 2
3
4
5
6
8 89
142 520
500 310
295 55
30 5
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One of the sites (NN1) was harrowed by mistake after the first growing season, hence, this site was only analysed for survival and attacks in the first year. Statistical analyses of mortality and damage were based on arcsine square-root transformed plot frequencies. No significant deviations from normality or homoscedasticity were found among the residuals. Analyses of variance were performed both for all seedlings and cuttings, and for those only in the 4 mm diameter class, with the following linear model, using the SAS procedure MIXED: yijk ¼ m þ Pi þ Sj þ pi sj þ eijk
ðmodel 1Þ
where yijk is the observation of plot ijk, m Pi the fixed effect of stock type i (i ¼ 1; 2), Sj the fixed effect of site j (j ¼ 1; . . . ; 5), pisj the random effect of interaction between stock type and site (between-site-error) and eijk the random residual effect of plot ijk (withinsite-error). Site OS was excluded from the analysis of girdled seedlings and cuttings due to the low number of pine weevil attacks at this site. Site NN1 was excluded from the analysis of mortality after the first year, since the site was harrowed by mistake after the first growing season. The growth development of surviving and nonattacked seedlings and cuttings was analysed only for the OS site. Survival was too low at the other sites to allow any reliable comparisons. The purpose was to determine if there were growth differences between
the two stock types that could be attributed to vigour at the time of planting. Analysis of variance was performed with the following linear model using the SAS procedure MIXED: yijk ¼ m þ Pi þ bj þ pbij þ eijk
ðmodel 2Þ
where yijk is the observation of seedling/cutting ijk, m pi the fixed effect of stock type i (i ¼ 1; 2), bj the random effect of block j (j ¼ 1; . . . ; 5), and eijk the random within-plot error. Differences between cuttings and seedlings were evaluated using Student’s t-tests with the above models.
3. Results Damage caused by pine weevils was most severe on the non-scarified sites NN1 and NN2, where over 90% of the cuttings and seedlings were attacked, and 80% died in the first year (Table 3). Pine weevil induced mortality was lowest at the scarified site with a fallow period (OS). The newly clearcut but scarified sites (NS1 and NS2) were intermediate in terms of attack frequency and mortality. The major source of damage was the pine weevil. The second most important agents of damage were other insects, mainly Hylastes sp., which caused 2% of the mortality. Most of the mortality took place in the first growing season. The effects of stock type and site were significant at the 5% probability level for all of the tested variables,
Table 3 Percentage of Norway spruce damage and mortality at five study sites as combined averages for cuttings and seedlings in all diameter classes Sitea
NS1 NS2 OS NN1f NN2 a
Total, dead year 5e
Pine weevil induced damage and mortality Attacksb
Girdledc
Dead year 1d
Dead year 3d
Dead year 5d
61.6 77.5 26.5 90.5 90.4
46.7 60.8 10.0 81.9 83.1
44.7 58.0 5.8 80.5 81.9
48.7 71.5 17.5 – 86.6
49.2 73.0 17.8 – 87.9
53.8 76.2 23.5 – 91.2
Description of sites: (NS1 and NS2) new clearcut, scarified; (OS) old clearcut, scarified; (NN1 and NN2) new clearcut, no scarification. Percent plants with feeding scars, total in the first season. c Percent plants that were girdled by pine weevil, total in the first season. d Percent plants killed by pine weevil damage after the first, third and fifth seasons. e Percent dead plants, all causes, after five seasons. f This site was harrowed by mistake after the first growing season. b
M. Hannerz et al. / Forest Ecology and Management 160 (2002) 11–17
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Table 4 Results of analyses of variance of mortality and damage frequency, according to model 1, of data for cuttings and seedlings with initial diameter of 4 mm, following arcsine square-root transformation of plot frequenciesa Fixed effects
Stock type Site d.f. of denominator
d.f.
1 4 4
Attacks year 1b
Girdled first yearc
Dead year 1d
Dead year 5d
F-value
P-value
d.f.
F-value
P-value
d.f.
F-value
P-value
d.f.
F-value
P-value
29.50 35.09
0.006 0.002
1 3 3
14.92 4.32
0.03 0.13
1 4 4
17.76 29.98
0.014 0.003
1 3 3
16.23 27.14
0.028 0.011
a
The random effect stock type site was used as denominator. Percent cuttings or seedlings with feeding scars, total in the first season. c Percent pine weevil attacked cuttings or seedlings that were girdled in the first season. The OS site excluded. d Percent cuttings or seedlings killed by pine weevil after the first season (all sites) and the fifth season (all sites except NN1, new clearcut, no scarification). b
regardless of whether all cuttings and seedlings were tested (results not shown), or just those with an initial diameter of 4 mm (Table 4). Seedlings had a significantly higher frequency of feeding attacks in the first year and pine weevil induced mortality than cuttings (Figs. 1 and 2). Of the cuttings and seedlings that were attacked in the first year, the frequency of girdling was significantly higher among seedlings than among cuttings (Fig. 3). The initial height and volume of surviving, nonattacked seedlings at the OS site were higher than those of the surviving cuttings (calculated for initial diameter of 4 mm) (Table 5). After three growing seasons, the seedlings had significantly larger height, diameter and volume than the cuttings.
Fig. 1. Percentage of Norway spruce cuttings and seedlings (with initial diameter of 4 mm) attacked by pine weevil in the first year, and S.E. Description of sites: (NS1 and NS2) new clearcut, scarified; (OS) old clearcut, scarified; (NN1 and NN2) new clearcut, no scarification.
Fig. 2. Percentage of Norway spruce cuttings and seedlings (with initial diameter of 4 mm) killed by pine weevil after 5 years in the field and S.E. Description of sites: (NS1 and NS2) new clearcut, scarified; (OS) old clearcut, scarified; (NN1 and NN2) new clearcut, no scarification.
Fig. 3. Percentage of pine weevil attacked Norway spruce seedlings and cuttings with an initial diameter of 4 mm that were girdled in year 1, and S.E. The OS site was excluded from the analyses of girdling due to the low number of attacked seedlings and cuttings at this site. Description of sites: (NS1 and NS2) new clearcut, scarified; (OS) old clearcut, scarified; (NN1 and NN2) new clearcut, no scarification.
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M. Hannerz et al. / Forest Ecology and Management 160 (2002) 11–17
Table 5 Height, diameter and volume (mean S.E.) of Norway spruce cuttings and seedlings (with initial diameter of 4 mm) at the OS site (old clearcut, scarification) that were not attacked by pine weevil and that survived over the third growing season Variablea
Cuttings (N ¼ 88)
Seedlings (N ¼ 51)
Result of t-testb
Height, year 0 (cm) Height, year 3 (cm) Diameter, year 3 (mm) Volume, year 0 (cm3) Volume, year 3 (cm3)
18.02 44.34 8.34 0.75 9.77
20.84 55.16 10.01 0.87 16.08
***
(0.55) (0.91) (0.37) (0.02) (0.74)
(0.79) (1.30) (0.52) (0.03) (1.06)
*** *** *** ***
a
Year 0 is at planting, and year 3 after the third growing season. Student’s t-test according to model 2; ns: not significant. *** P < 0:001. b
4. Discussion The differences among the sites in pine weevil induced mortality followed the expected pattern. Planting on fresh clearcuts with undisturbed soil and without chemical or physical protection of the seedlings is doomed to fail in southern Sweden ¨ rlander and Nilsson, 1999). Scarification can help to (O reduce the damage on fresh clearcuts, but the mortality levels are still unacceptably high from a practical forestry perspective. Acceptable levels of survival were found in this study only at the old clearcut, in combination with fresh scarification. The positive effect of using cuttings rather than seedlings was shown at all sites. The cuttings had 4–43% higher survival than the seedlings at the various sites. This higher rate of survival is significant from an economic viewpoint, but optimising the choice of stock type will not solve the pine weevil problem alone. The effects of using different types of stock, in combination with other measures, such as physical protection, scarification, fallow-periods and shelter trees ought to be further studied. The general results of the study support the hypothesis of Gemmel et al. (1991), that pine weevil damage might explain some of the growth rate superiority of cuttings over seedlings. Cuttings and seedlings differ in various properties which might help to explain the difference in pine weevil resistance. The thicker bark and needles on the cutting’s root collar might inhibit the pine weevil from feeding. Cuttings are ontogenetically older than seedlings: in this case, for instance, they were cut from 3-year-old seedlings. Growth rhythm is altered towards later bud burst
and earlier growth cessation in ontogenetically older planting stock (Ununger et al., 1988), and this may explain why cuttings can tolerate frost better than comparable seedlings (Hannerz, 1994; Hannerz and Wilhelmsson, 1998). Other properties, such as the chemical composition of the wood and bark, might also change with age of the planting material, and these possibilities need to be further investigated in order to explain fully the results of the present study. Cuttings and seedlings are very difficult to compare rigorously and unambiguously. In this study, the two types of stock had different genetic origins, with seedlings from a Belorussian provenance and cuttings from the seed orchard Maglehem. Thus, possible genetic effects cannot be excluded. However, experience from genetic field trials with Norway spruce in southern Sweden suggests that genetic origin generally has a small or negligible effect on mortality after planting, including pine weevil induced mortality (Karlsson and Ho¨ gberg, 1998; Hannerz et al., 1999). The vigour of the material at the time of planting could also be important. Selander and Immonen (1992) demonstrated that seedlings under water stress were more heavily attacked by pine weevil than nonstressed seedlings, and water stress could be a factor in the present study. However, all seedlings and cuttings were treated carefully and equally to maintain their vigour and water potential before planting. Furthermore, the seedlings would be expected to have lower growth rates than the cuttings following planting, if they were more stressed, but the study of non-attacked cuttings and seedlings at the OS site suggested that the growth rates of the seedlings were at least as strong, or stronger, than those of the cuttings.
M. Hannerz et al. / Forest Ecology and Management 160 (2002) 11–17
Norway spruce cuttings are mainly used in clonal forestry as a tool for duplicating genetically superior, selected clones (Ritchie, 1991). Several studies have demonstrated that cuttings have significant physiological advantages over seedlings, for stocks of similar genetic origin, including faster initial growth (Gemmel et al., 1991), reduced risk of winter damage (Hannerz, 1994) and greater resistance to spring and autumn frost damage (Hannerz and Wilhelmsson, 1998). This study demonstrates that resistance to pine weevil damage can be added to this list of advantages.
Acknowledgements The establishment of the experiment was financially supported by the Swedish Forestry Research Foundation (SSFf). Many persons gave technical assistance ˚ ke in the field. We are especially grateful to Per-A Arvidsson. We also thank the landowner, AssiDoma¨ n AB, for hosting the experiments. John Blackwell corrected the English. References Dekker-Robertson, D.I., Kleinschmit, J., 1991. Serial propagation in Norway spruce (Picea abies (L.) Karst.): results from later propagation cycles. Silvae Genet. 40, 202–214. Eidmann, H.H., 1969. Ru¨ sselka¨ ferscha¨ den an verschiedenen Nahrungspflanzen. Anzeiger f. Scha¨ dlingskunde u. Pflanzenschutz 42, 22–26. ¨ rlander, G., Ho¨ gberg, K.-A., 1991. Norway spruce Gemmel, P., O cuttings perform better than seedlings of the same genetic origin. Silvae Genet. 40, 198–202. Ha¨ gglund, B., Lundmark, J.-E., 1977. Site index estimation by means of site properties. Stud. For. Suec. 138, 1–38. Ha¨ ggstro¨ m, B., 1958. Resultat av na˚ gra fo¨ rso¨ ksplanteringar. Norrl. ˚ rg. 1958, 162–178 (in Swedish). Skogsva˚ rdsfo¨ rb. Tidskr. A Hannerz, M., 1994. Winter injuries to Norway spruce observed in plantations and in a seed orchard. SkogForsk, Report No. 6. p. 22. Hannerz, M., Wilhelmsson, L., 1998. Field performance during 14 years growth of Picea abies cuttings and seedlings propagated in containers of varying size. Forestry 71, 373–380. Hannerz, M., Sonesson, J., Ekberg, I., 1999. Genetic correlations between growth and growth rhythm observed in a short-term test and performance in long-term field trials of Norway spruce. Can. J. For. Res. 29, 768–778.
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¨ rlander, G., 1999. Mekaniska von Hofsten, H., Pettersson, M., O snytbaggeskydd–en la¨ gesrapport. SkogForsk, Resultat nr 24, p. 6 (in Swedish with English abstract). Karlsson, B., Ho¨ gberg, K.-A., 1998. Genotypic parameters and clone site interaction in an 11-year-old series of five clone tests with Norway spruce (Picea abies (L.) Karst) in south Sweden. For. Genet. 5, 21–30. Kohmann, K., 1995. Hight growth and survival of different spruce seedling types. Rapport fra Skogforsk 6/95, p. 14 (in Norwegian, with English summary and legends). Lekander, B., So¨ derstro¨ m, V., 1969. Studie o¨ ver snytbaggeangrepp pa˚ barrtra¨ dsplantor. Sv. Skogsv. Fo¨ rb. Tidskr. 67, 351–383 (in Swedish with English summary and legends). Lekander, B., Eidmann, H.H., Bejer, B., Kangas, E., 1985. Time of oviposition and its influence on the development of Hylobius abietis (L.) (Col., Curculionidae). Z. Ang. Entomol. 100, 417–421. ˚ ., 1992. Fo¨ ryngring med fo¨ rhinder. In: Mattsson, S., Thorse´ n, A Frumerie, G. (Ed.), The rationalisation conference 1992. The Forest Operations Institute of Sweden, Redogo¨ relse No. 1, pp. 35–48 (in Swedish with English summary). Nordenhem, H., Nordlander, G., 1994. Olfactory oriented migration through soil by root living Hylobius abietis (L.) larvae (Col., Curculionidae). J. Appl. Ent. 117, 457–462. Ollas, R., 1992. Avga˚ ngar och skador i 1989 a˚ rsplanteringar. Snabbinformation fra˚ n skogsva˚ rdsenheten, Skogsstyrelsen, Jo¨ nko¨ ping, Skogsva˚ rd 4/1992 (in Swedish). ¨ rlander, G., Nilsson, U., 1999. Effect of reforestation methods on O pine weevil (Hylobius abietis) damage and seedlings survival. Scand. J. For. Res. 14, 341–354. ¨ rlander, G., Nilsson, U., Nordlander, G., 1997. Pine weevil O abundance on clearcuttings of different ages: a 6-year study using pitfall traps. Scand. J. For. Res. 12, 225–240. Ritchie, G.A., 1991. The commercial use of conifer rooted cuttings in forestry: a world overview. New For. 5, 247–275. Selander, J., 1993. Survival model for Pinus sylvestris seedlings at risk from Hylobius abietis. Scand. J. For. Res. 8, 66–72. Selander, J., Immonen, A., 1992. Effect of fertilization and watering of Scots pine seedlings on the feeding preference of the pine weevil (Hylobius abietis L.). Silva Fennica 26, 75–84. So¨ derstro¨ m, V., 1974. Soil treatment. Sv. Skogv. Fo¨ rb. Tidskr. 72(1), 157–165 (in Swedish with English summary and legends). ˚ ., Mattsson, S., Weslien, J.-O., 2001. Influence of stem Thorse´ n, A diameter on the survival and growth of containerised Norway spruce seedlings attacked by pine weevils (Hylobius spp.). Scand. J. For. Res. 16, 21–29. Ununger, J., Ekberg, I., Kang, H., 1988. Genetic control and agerelated changes of juvenile growth characters in Picea abies. Scand. J. For. Res. 3, 55–66. von Sydow, F., 1997. Abundance of pine weevils (Hylobius abietis) and damage to conifer seedlings in relation to silvicultural practices. Scand. J. For. Res. 12, 157–167.