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The distribution of pine sawfly cocoons (Diprionidae) in Scots pine stands in relation to stand edge and tree base
Forest Ecology and Management, 54 ( 1992 ) 193-203
193
Elsevier Science Publishers B.V., Amsterdam
The distribution of pine sawfly cocoons (Diprionidae) in Scots pine stands in relation to stand edge and tree base Jii-i Simandl
Institute of Entomology, CzechoslovakAcademy of Sciences, Branigovskd 31, 370 05 ~esk~ Bud~jovice, Czechoslovakia (Accepted 7 November 1991 )
ABSTRACT Simandl, J., 1992. The distribution of pine sawfly cocoons (Diprionidae) in Scots pine stands in relation to stand edge and tree base. For. Ecol. Manage., 54:193-203. The distribution of cocoons of the sawfly community (Diprionidae) within Scots pine (Pinus silvestris L. ) stands was quantified for endemic populations in relation to stand edge and distance from tree trunks on the basis of old and living cocoons that had accumulated for several years in the litter. Pine stands were 40 and 80 years old and located in southern Bohemia (Czechoslovakia). The density of living sawfly cocoons overwintering in the litter was low, 0.25-0.48 m- 2, (average 0.37 ) and comprised 2.7% of 2346 cocoons in the samples. Numbers of cocoons decreased with increasing distance from stand edge with 40.5% of the cocoons near the stand edge, in contrast to 25.1% in the stand. Significant differences in cocoon densities also occurred in stands of both ages examined. The cocoon densities decreased nearly linearly with increasing distance from the host tree trunk. About 37% of the cocoons occurred less than 0.3 m from the tree base. The method of circular samples may be more effective at lower population densities.
INTRODUCTION D e t a i l e d d a t a o f e n d e m i c sawfly c o c o o n p o p u l a t i o n s are scarce in the litera t u r e a n d are n e e d e d for b e t t e r u n d e r s t a n d i n g o f t h e i r regulating factors ( H a n s k i , 1987 ). T h e d i s t r i b u t i o n o f c o c o o n s a n d s a m p l i n g m e t h o d s h a v e been i n v e s t i g a t e d m a i n l y d u r i n g o u t b r e a k s or high p o p u l a t i o n levels o f e c o n o m i cally i m p o r t a n t species, a n d t h o s e w h o s e l a r v a e feed in colonies ( M c L e o d , 1961, 1966; Stark a n d D a h l s t e n , 1961; L y o n s , 1964). T h e d i s t r i b u t i o n patt e r n o f a single species c a n be i n f l u e n c e d b y high larval densities. N i k l a s a n d F r a n z ( 1957 ) f o u n d c o c o o n s o f N e o d i p r i o n sertifer o n l y in the g r o u n d , in contrast M o r r i s a n d C a m e r o n ( 1 9 3 5 ) r e c o r d e d m o s t c o c o o n s o f this species o n
Correspondence to: J. Simandl, Institute of Entomology, Czechoslovak Academy of Sciences, Branigovsk~i 31,370 05 (~esk6 Budrjovice, Czechoslovakia.
© 1992 Elsevier Science Publishers B.V. All rights reserved 0378-1127/92/$05.00
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J. SIMANDL
the trunks or in the crowns of host trees. During the periods of high density of diprionid larvae, and at outbreaks, most of the summer cocoons of the outbreak species (N. sertifer) as well as autumn generations (Diprion pini) occurred on tree trunks or in the crowns (Escherich, 1942; Styles, 1959) - up to 90% was reported by Kurir (1977a,b) in Diprion pallidum and by Luterek et al. (1982) in the complex of pine sawflies. In general, incidence of cocoons is usually higher under infested trees in most sawfly species as reported, for example, by Ives and Turnock ( 1959 ) for Pristiphora erichsonii (Htg.) or for Neodiprion swainei (McLeod, 1966). However, some larvae crawl away from the trees before spinning (Lyons, 1964). Sawfly cocoons are exposed to natural enemies during prolonged diapause when the mortality rate partly depends on the duration of exposure. The aim of the present study was to quantify long-term trends in the sawfly cocoon (populations) distribution under circumstances of latency in managed pine woods. MATERIALS A N D M E T H O D S
Area of investigations Two plots were established in managed, pure Scots pine stands (Pinus silvestris L. ) in southern Bohemia ( Czechoslovakia ): (A) near ~ev~tin and (B) Blatmi. In each area plots were established in a mature (over 80 years old) and medium-aged (about 40 years) stands (see Table 1 ). Cocoons were sampled during winter periods (November-March) in 1984 and 1985. Ground vegetation in the mature stands was mainly mosses (Leucobryum glaucum, Dicranum spp. ) and lichens ( Cetraria islandica, Cladonia spp. ) with areas of Vaccinium myrtillus and Vaccinium vitis-idaeae, the vegetation not being continuous. There were litter layers 35-120 mm thick in the investigated stands. TABLE 1 Characteristics of the investigated Scots pine stands of Piceeto-Pinetum forest association in 1986 in southern Bohemia (CS) Plot t
Mean age (years)
Mean DBH ( mm )
Meantree height ( m )
Tree density h a - 1
A
48 81
110 190
12.5 17.5
2200 1300
B
44 90
120 250
12.0 22.0
2400 1100
JA, ~ev~tin plot, 500 m above sea level; B, Btatmi plot, 450 m above sea level.
PINE SAWFLY COCOONS IN SCOTS PINE STANDS
195
Methods Three zones were established in each stand from the south-southeastern edge, bordering a meadow, or a wide road in the stand: Zone I, less than 5 m; Zone II, 5.1-15 m; Zone III, greater than 15.1 m. The abundance (density) of cocoons was examined in relation to the stand edge and to trees selected at random in each zone. The distribution of cocoons was determined within each zone with two methods: ( 1 ) a circle of 0.3 m diameter was marked around the base of each of three trees in each zone; (2) squares of 0.25 m per side were selected in four directions and spaced at 0.25 m intervals from the base of each tree. In mature stands the squares (n = 80) extended to 1.75 m from the base of each of five trees in each zone, and to 1.25 m in medium-aged stands (n = 60) in each zone (five trees). Cocoons were extracted from the sample units with sieves in the field or by thorough examination of the samples cut out from the ground and transported to the laboratory. All the cocoons were counted: living, empty and those parasitized and preyed upon. Cocoons comprised eight species of pine sawflies in the region (Simandl, 1989) and, no outbreak of sawflies had been recorded during and prior to the year of sampling. The total area of pine litter sampled was 188 m 2.
Statistical analysis Statistical analysis of differences among sample sizes was performed using analysis of variance (three-way ANOVA) followed by the Student-NewmanKeul test. Multiple regression was used for evaluation of dependence of density of cocoons on zone and distance from tree trunk. RESULTS A total of 2346 cocoons occurred in the pine litter sampled. Of these 1893 cocoons were collected in square and 453 in circular units. Only 2.7% (density 0.25-0.48 cocoons m - 2, X= 0.37 ) of the sampled cocoons were living. I n the sample of old cocoons, sawflies had emerged from 23.1%, parasitoids from 39.3%, 34.8% had been destroyed by predators, and 2.9% could not be classed. The most frequent number of cocoons per square unit was one (n-- 118 ), but the highest proportion of cocoons was recorded in 41 sample units containing six cocoons each. The cumulation of cocoons was found in some sites such as around tree bases (in bark crevices as well) but the frequency of cocoons per square sample unit varied greatly. For example, no cocoons occurred in many samples and several contained more than 20. However the number of square units without cocoons (the average density of cocoons) decreased with increasing distance from the tree; the percent of sample units with and without cocoons related to the distance from the trees are shown in Fig. 1. Only 17.5% (n--441) of all square sampling units contained at least one cocoon.
196
J. SIMANDL I00 86.5
u) t-
82.2
8].1
78.4
..I
z nl ,,,"
I 50
Z
,~ ,7
10
6.2 . . . . .. .. . . . .
•" . " - " - -
5.1 .
.
.
.~. ; :.' ; ; ; ..~ . . ... ' : .
j
.
.
].1
. . . .2.4 ....
. . . ... . . . .
Fig. 1. Proportions of square sample units without sawfly cocoons (n = 2520, empty columns) and units containing cocoons ( n = 720, hatched columns) within each sampled distance in Scots pine stands (~ev~tin and Blatn~i, 1984-1985 ). a, only mature stands (over 80 years old, n = 360).
A. 30-
25-
v 15
;
z
j
r-
1
I
I
i
--.,:
i
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"" i
I. •
c II.m
c III.m
~,~
c i.m
c ii.m
i
c iii.m
zone
age
Fig. 2. Distribution of cocoon densities (n m -2, .~___SE) related to the stand edge and the comparison of samples from circular and square sample units (94 m 2 in each plot). Probes: circular, []; square, rt. Locality (P<0.001): (A) ~ev6tin; (B) Blatn~i. Age (ns): c, 40 years; m, greater than 80 years. - - - , trend in stands 40 years old; - - , trend in stands greater than 80 years old.
PINE SAWFLYCOCOONS IN SCOTS PINE STANDS
197
TABLE 2 Total cocoon densities (n m-2, ~ + SE) of pine sawflies sampled per plot, age and zone (square plus circular sample units) in 1984-1985 in southern Bohemia (three-way ANOVA) Plot
Age
Zone
~
SE
A
40
I II III I II III
16.4 13.0 8.9 10.8 11.1 8.4
2.10 1.32 2.10 1.40 2.16 1.90
I II III I II III
15.5 11.9 10.3 22.9 19.7 11.6
1.78 1.45 1.71 2.80 1.69 2.10
I*** II III
16.4b 13.9b 9.8a
1.13 0.90 0.98
80
B
40
80
Total All***
All ns
***P < 0. 001. "SNon-significant.
Density related to stand edge For endemic population levels, the edge effect was the main factor influencing the dispersion of sawflies overwintering in the litter. The densities of cocoons decreased in each zone towards the stand interior (Fig. 2, Table 2 ). ~EV~TtN
20
]LATNA
I0
0-0.25
0.5-0.75
1.0-1.25
1.5-1.75
DISTANCE FRON TREES ( N )
Fig. 3. Distribution of pine sawfiy cocoons (n m -2) in relation to distances from tree trunks (square sample units, 1984-1985 ).
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Total differences among individual zones were significant ( P < 0.001 ), Zone III (25.1% of the sampled cocoons) being distinctly separate from Zones I (40.5%) and II (34.4%). The densities of cocoons in each zone of the individual plots ranged from 8.4 to 22.9 cocoons m -2 and were sub-equal in the mature stand at ~ev~tin where the difference of the cocoon densities between Zones I and III was lowest (Table 2 ). Therefore cocoons were concentrated along the edges and in gaps under the most infested trees. It was distinct particularly in mature stands where the most distant sampled areas (1.5-1.75 m) showed a higher cocoon density in comparison with the previous two distances sampled where the decrease in densities was distinct (Fig. 3 ). Density in relation to the tree
The distribution pattern also was influenced by the aggregation of cocoons at the base of trees. Changes of the cocoon density in relation to distance from tree were distinct and diminished nearly linearly with the increasing distance from the tree trunk (Fig. 3 ). The highest cocoon density was recorded around the tree base as found by both sampling methods (15.0 cocoons m -2 in circular and 15.9 in square units (Table 3). The differences between samples near the tree bases at 0-0.25 m and those lying farthest at 1.0-1.25 m was about 16.3%. Sawfly larvae most frequently spin cocoons near the base of the TABLE 3 Mean density (n m-2) of sawfly cocoons in Scots pine stands in relation to distances from tree trunks (square sampling units, 1984-1985 ). Standard error within parentheses Distance from trees (m)
Zone
Localities gev~tin
Blatn~t
0 -0.25
I II III
20.7 (2.15) 13.9 (1.33) 9.2 (1.39)
20.3 (2.12) 20.9 (1.11) 10.5 (1.26)
0.5-0.75
I II III
11.2 (0.84) 6.8 (0.78) 6.4 (0.40)
18.5 (0.58) 14.3 (0.88) 10.1 (0.68)
1.0-1.25
I II III
8.5 (1.10) 4.9 (0.55) 4.3 (0.24)
11.7 (0.68) 13.7 (0.64) 11.0 (0.75)
1.5-1.751
I II III
15.7 (2.00) 15.5 (0.42) 8.0 (0.64)
9.3 (1.08) 12.5 (1.10) 9.6 (1.33)
lOnly mature stands.
PINE SAWFLY COCOONS IN SCOTS PINE STANDS
199
trees if the litter layer is thick enough. More cocoons per square metre were collected by the method of circular samples than by square samples in general (Fig. 2) as well as in comparison with samples taken within 25 cm of tree base. The differences in cocoon density between mature and medium-aged stands were not statistically significant ( P > 0.05 ) (Table 2). The numbers of old cocoons, accumulated in the litter, were higher ( P < 0.001 ) at Blatn~i than at ~ev~tin (see Table 2 ) differing by 15.7% (368 cocoons). The largest number of cocoons occurred in the mature stand at Blatn~i (density 18.1 cocoons m-2; SE= 1.41 ) with the southern-edge exposure. There the number of Gilpin& frutetorum (F.) was relatively high (J. Simandl, personal observations, 19851988). The regression equation of the number of cocoons per sample unit dependent on zone and distance from tree is as follows:
Cd=a-bxDe-cXDs where a = 21.386; b = 2.904; c = 1.441; Do is distance from the stand edge (m) expressed as values of 1, 2, or 3; Ds is distance from tree trunk expressed as values of 1, 2, 3 or 4. All regression coefficients (a, b, c) are significantly different from zero (for a, b: P<0.001; for c: P<0.05). The cocoon populations of the sawfly community at relatively low densities for a long time had a distinct tendency to aggregate towards sunny edges (see Fig. 2) of pine stands as well as within managed stands within 0.3 m of host tree trunks (see Fig. 3). The dependence discovered will facilitate a more effective sampling of sawfly species overwintering in the litter during the period of low population densities. DISCUSSION
Most of the sawflies defoliating coniferous trees exhibit the same way of overwintering, i.e. in the cocoon stage in forest litter (one exception are Neodiprion sawflies). At the same time a small part of the cocoon population of sawflies overwintering in the litter can be placed outside the litter (Luterek et al., 1982), especially at higher population density of the insects (see Kurir, 1977a). During the period of endemic population levels of sawflies most of larvae were traveling to the stand litter every autumn (J. Simandl, personal observations, 1985 ). A tendency for cocoon numbers to decrease with increasing distance from tree trunk was reported in Gilpinia hercyniae (Htg.) (Prebble, 1943 ) and P. erichsonii (Htg.) (Ives and Turnock, 1959). Ohnesorge and Thalenhorst ( 1956 ) and Pointing ( 1957 ) reported that cocoons of some spruce sawflies were concentrated under tree crowns. The distribution of cocoons found in
200
J. SIMANDL
the vicinity of trees indicates that the distribution of endemic sawfly populations in managed pine stands over 40 years of age corresponds to the dispersion of Neodiprion spp. cocoons found at higher population densities on insolated young pines (Stark and Dahlsten, 1961 ). However, Lyons (1964) concluded that "cocoon density within stands is not strongly related to the distance from the nearest tree" in two Neodiprion species. It seems then that at low population densities the dependence on distance from a tree can be distinct in a species complex overwintering in the litter (Gilpinia spp., Diprion spp., Macrodiprion ). The prespinning mobility of sawfly larvae is good and they may crawl on the ground up to a distance of 70 cm before spinning (Lyons, 1964). However, when the litter is sufficiently thick (about 1 cm and more) near the host tree base and neither extremely dry nor moist (Ives, 1955 ), most larvae travel (not fall) to the litter down the trunk on sunny days (J. Simandl, personal observations, 1985 ) and spin their cocoons in the vicinity of the tree. The cocoon distribution pattern depends not only on the habits of larvae, but also on environmental factors, including the character of the ground cover, the thickness of litter, the density of tree canopy, etc. Adult females are partly responsible for the distribution pattern as well, because they oviposit more frequently on insolated parts of tree crowns where their offspring will find optimal life conditions (Lyons, 1962, 1964). The pattern of spinning is very irregular in open stands (Lyons, 1964) in contrast to dense stands where the pattern can be clearly discernible. The incidence of cocoons was very low under some trees although it was relatively high under the adjoining ones; one explanation can be the dispersion of populations according to negative binomial series in an area (Wilson and Gerrard, 1971 ). The suitability of the negative binomial in describing frequency distribution of cocoons does not mean that aggregation is in any sense explained, since the negative binomial has been deduced from a number of widely contrasting hypotheses regarding the mechanism of dispersal. The high abundance of old cocoons, in particular, in the mature stand at Blatn~i (22.9 m - 2) indicated that the population level of sawflies had been relatively higher for some time. In managed, medium-aged stands where the tree canopy is maintained dense, with relatively small crown projections of interior trees, it was impossible in some cases to decide which tree the cocoons (larvae) originated from when many were found farther than 1 m from the trees. A predisposition to increased mortality caused by parasitoids or by predators existed in sites with cocoon aggregations (Hanski and Parviainen, 1985 ). Thus distribution affects the role of beneficials in the regulation of latent populations. Aggregation of cocoons at tree bases or in stand edges did not involve one or two species whose larvae live in colonies (e.g.D. pini), because the majority of species with solitary larvae formed the sawfly community of adjoining stands of similar character (Simandl, 1989 ). Many forest insects prefer forest edges, an ecotone that is very suitable for
PINE SAWFLY COCOONS IN SCOTS PINE STANDS
201
development of defoliators and characterized by large tree crowns with a large biomass of needles and optimal abiotic conditions. In general, similar aggregation patterns of endemic populations along forest edges was reported in other forest insect defoliators (see Campbell et al., 1976; Bellinger et al., 1989). Diprionids number among the most abundant insects among Scots pine defoliators (Larsson and Tenow, 1980) and with a relatively high representation among pine-dwelling arthropods (J. Simandl, unpublished data, 1988 ) therefore relatively vacant sites (niches) can be successfully occupied by the diprionid complex. The results concerning the stand edge effect were confirmed by the measurement of falling frass, most of which was usually recorded along the edges (J. Simandl, personal observations, 1985-1988 ). The number and configuration of sample units performed seemed to have been sufficient, as the same unit size was often used (see Ohnesorge and Thalenhorst, 1956) for prognosis of sawflies. For cocoon densities from 2 to 3 m -a, Borodin (1975) recommended the unit size (0.0625 m 2) used in the investigation. The circular samples around trunks seem to be a more effective method of sampling at low population densities of sawflies. Thus Scots pine stand management should be focused on maintaining stands below 80 years of age dense, which is better both for the tree quality (as generally recommended for P. silvestris by forest managers) and a certain limiting of development of sawfly populations. If the sampling of lower sawfly populations in large managed pine woods endangered by sawflies is to be effective the prognoses should be based on more frequent sampling near tree bases (if stand gaps are present) and along the edge areas than within the stands to avoid underestimating the populations. ACKNOWLEDGMENTS
I thank Prof. H.H. Eidmann, Dr. E. Olofsson (University of Uppsala, Sweden), Dr. V. Zumr (Institute of Entomology, Czechoslovakia) and two anonymous referees for their valuable comments and editorial assistance on the first version of the manuscript and Dr. J. Lep~ (Centre of Biomathematical Methods, Czechoslovak Academy of Sciences) for statistical assistance.
REFERENCES Bellinger, R.G., Ravlin, F.W. and McManus, M.L., 1989. Forest edge effects and their influence on Gypsy Moth (Lepidoptera Lymantriidae) egg mass distribution. Environ. Entomol., 18: 835-840. Borodin, A.L., 1975. Optimalization of a method for counting the cocoons of the rusty pine sawfly Neodiprion sertifer (Hymenoptera, Diprionidae). Ekologiya, 3:60-67 (in Russian). Campbell, R.W., Miller, M.G., Duda, E.J., Biazak, C.E. and Sloan, R.J., 1976. Man's activities
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and subsequent gypsy moth egg-mass density along the forest edge. Environ. Entomol., 5: 273-276. Escherich, K., 1942. Die Forstinsekten Mitteleuropas. Bd. 5, Berlin, pp. 51-243. Hanski, I., 1987. Pine sawfly population dynamics - - patterns, processes, problems. Oikos, 50: 327-336. Hanski, I. and Parviainen, P., 1985. Cocoon predation by small mammals, and pine sawfly population dynamics. Oikos, 45: 1 2 5 - 1 3 6 . Ives, W.G.H., 1955. Effect of moisture on the selection of cocooning sites by the Larch Sawfly, Pristiphora erichsonii (Hartig). Can. Entomol., 87:301-311. Ives, W.G.H. and Turnock, W.J., 1959. Estimation of cocoon populations of the larch sawfly, Pristiphora erichsonii (Htg.). Can. Entomol., 91:650-661. Kurir, A., 1977a. Einspinnen and Clberwinterung der 1. Generation der Gew6hnlichen Kiefernbuschhornblattwespe, Diprion pini Linnaeus (Hym., Diprionidae) in der Baumkrone der Weisskiefer (Pinus sylvestris Linnaeus). Z. Angew. Entomol., 84: 47-52. Kurir, A., 1977b. Beobachtungen zur Bionomie der Blassen Kiefernbuschhornblattwespe, Diprion pallidum Klug. (Hym., Diprionidae) wiihrend der Gradation in Kiirnten 1971/1972. Z. Angew. Entomol., 84:155-163. Larsson, S. and Tenow, O., 1980. Needle-eating insects and grazing dynamics in a mature Scots pine forest in Central Sweden. Ecol. Bull. (Stockholm), 32: 269-306. Luterek, R., Gralicki, L. and Krystek, J., 1982. Places of cocoons formation by pine sawflies (Hymenoptera, Tenthredinidae). Pr. Kom. Nauk Roln. Kom. Nauk Lesn., 54:61-66 (in Polish, English summary). Lyons, L.A., 1962. The effect of aggregation on egg and larval survival in Neodiprion swainei Midd. (Hymenoptera, Diprionidae). Can. Entomol., 94: 49-58. Lyons, L.A., 1964. The spatial distribution of two pine sawflies and method of sampling for the study of population dynamics. Can. Entomol., 96:1373-1407. McLeod, J.M., 1961. A technique for the extraction of cocoons from soil samples during population studies on the Swaine sawfly, Neodiprion swainei Midd. (Hymenoptera, Diprionidae). Can. Entomol., 93: 888-890. McLeod, J.M., 1966. The spatial distribution of cocoons of Neodiprion swainei Midd. in Jack pine stand. I. A cartographic analysis of cocoon distribution, with special references to predation by small mammals. Can. Entomol., 98: 430-447. Morris, R.S. and Cameron, E., 1935. The biology ofMicroplectronfuscipennis Zett. (Chalcididae), a parasite of the pine sawfly (Diprion sertifer Geoff. ). Bull. Entomol. Res., 26: 407419. Niklas, O.F. and Franz, J., 1957. Begrenzungsfaktoren einer Gradation der Roten Kiefernbuschhornblattwespe (Neodiprion sertifer (Geoff.)) in Stidwestdeutschland 1953 bis 1956. Mitt. Biol. Bundesanst. Berlin, 89:39 pp. Ohnesorge, B. and Thalenhorst, W., 1956. Zur Kenntnis der Fichten-Blattwespen. IV. Die Dispersion. Z. Pflanzenkr. Pflanzenschutz., 63:197-211. Pointing, P.J., 1957. Studies on the comparative ecology of two sawflies Piconema alaskensis Roh. and Pikonema dimmockii Cress. (Tenthredinidae, Hymenoptera). Ph.D. Thesis, Univ. Toronto. Prebble, M.L., 1943. Sampling methods in population studies of the European spruce sawfly, Gilpinia hercyniae (Hartig), in Eastern Canada. Trans. R. Soc. Canada (Ser. 3, Sect. V), 37: 93-126. Simandl, J., 1989. Seasonal changes in the synusia of pine sawflies (Hym., Diprionidae) during their latency. J. Appl. Entomol., 108: 217-226. Stark, R.W. and Dahlsten, D.L., 1961. Distribution of cocoons of a Neodiprion sawfly under open-growth conditions. Can. Entomot., 93: 443-450.
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Styles, J.H., 1959. Observations on the spinning of cocoons by larvae of the sawfly Neodiprion sertifer (Geoffr.) (Hym., Diprionidae). Entomol. Mon. Mag., 95:178-179. Wilson, L.F. and Gerrard, D.J., 1971. A new procedure for rapidly estimating European pine sawfly (Hym., Diprionidae) population levels in young pine plantation. Can. Entomol., 103: 1315-1322.
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Elsevier Science Publishers B.V., Amsterdam
The distribution of pine sawfly cocoons (Diprionidae) in Scots pine stands in relation to stand edge and tree base Jii-i Simandl
Institute of Entomology, CzechoslovakAcademy of Sciences, Branigovskd 31, 370 05 ~esk~ Bud~jovice, Czechoslovakia (Accepted 7 November 1991 )
ABSTRACT Simandl, J., 1992. The distribution of pine sawfly cocoons (Diprionidae) in Scots pine stands in relation to stand edge and tree base. For. Ecol. Manage., 54:193-203. The distribution of cocoons of the sawfly community (Diprionidae) within Scots pine (Pinus silvestris L. ) stands was quantified for endemic populations in relation to stand edge and distance from tree trunks on the basis of old and living cocoons that had accumulated for several years in the litter. Pine stands were 40 and 80 years old and located in southern Bohemia (Czechoslovakia). The density of living sawfly cocoons overwintering in the litter was low, 0.25-0.48 m- 2, (average 0.37 ) and comprised 2.7% of 2346 cocoons in the samples. Numbers of cocoons decreased with increasing distance from stand edge with 40.5% of the cocoons near the stand edge, in contrast to 25.1% in the stand. Significant differences in cocoon densities also occurred in stands of both ages examined. The cocoon densities decreased nearly linearly with increasing distance from the host tree trunk. About 37% of the cocoons occurred less than 0.3 m from the tree base. The method of circular samples may be more effective at lower population densities.
INTRODUCTION D e t a i l e d d a t a o f e n d e m i c sawfly c o c o o n p o p u l a t i o n s are scarce in the litera t u r e a n d are n e e d e d for b e t t e r u n d e r s t a n d i n g o f t h e i r regulating factors ( H a n s k i , 1987 ). T h e d i s t r i b u t i o n o f c o c o o n s a n d s a m p l i n g m e t h o d s h a v e been i n v e s t i g a t e d m a i n l y d u r i n g o u t b r e a k s or high p o p u l a t i o n levels o f e c o n o m i cally i m p o r t a n t species, a n d t h o s e w h o s e l a r v a e feed in colonies ( M c L e o d , 1961, 1966; Stark a n d D a h l s t e n , 1961; L y o n s , 1964). T h e d i s t r i b u t i o n patt e r n o f a single species c a n be i n f l u e n c e d b y high larval densities. N i k l a s a n d F r a n z ( 1957 ) f o u n d c o c o o n s o f N e o d i p r i o n sertifer o n l y in the g r o u n d , in contrast M o r r i s a n d C a m e r o n ( 1 9 3 5 ) r e c o r d e d m o s t c o c o o n s o f this species o n
Correspondence to: J. Simandl, Institute of Entomology, Czechoslovak Academy of Sciences, Branigovsk~i 31,370 05 (~esk6 Budrjovice, Czechoslovakia.
© 1992 Elsevier Science Publishers B.V. All rights reserved 0378-1127/92/$05.00
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J. SIMANDL
the trunks or in the crowns of host trees. During the periods of high density of diprionid larvae, and at outbreaks, most of the summer cocoons of the outbreak species (N. sertifer) as well as autumn generations (Diprion pini) occurred on tree trunks or in the crowns (Escherich, 1942; Styles, 1959) - up to 90% was reported by Kurir (1977a,b) in Diprion pallidum and by Luterek et al. (1982) in the complex of pine sawflies. In general, incidence of cocoons is usually higher under infested trees in most sawfly species as reported, for example, by Ives and Turnock ( 1959 ) for Pristiphora erichsonii (Htg.) or for Neodiprion swainei (McLeod, 1966). However, some larvae crawl away from the trees before spinning (Lyons, 1964). Sawfly cocoons are exposed to natural enemies during prolonged diapause when the mortality rate partly depends on the duration of exposure. The aim of the present study was to quantify long-term trends in the sawfly cocoon (populations) distribution under circumstances of latency in managed pine woods. MATERIALS A N D M E T H O D S
Area of investigations Two plots were established in managed, pure Scots pine stands (Pinus silvestris L. ) in southern Bohemia ( Czechoslovakia ): (A) near ~ev~tin and (B) Blatmi. In each area plots were established in a mature (over 80 years old) and medium-aged (about 40 years) stands (see Table 1 ). Cocoons were sampled during winter periods (November-March) in 1984 and 1985. Ground vegetation in the mature stands was mainly mosses (Leucobryum glaucum, Dicranum spp. ) and lichens ( Cetraria islandica, Cladonia spp. ) with areas of Vaccinium myrtillus and Vaccinium vitis-idaeae, the vegetation not being continuous. There were litter layers 35-120 mm thick in the investigated stands. TABLE 1 Characteristics of the investigated Scots pine stands of Piceeto-Pinetum forest association in 1986 in southern Bohemia (CS) Plot t
Mean age (years)
Mean DBH ( mm )
Meantree height ( m )
Tree density h a - 1
A
48 81
110 190
12.5 17.5
2200 1300
B
44 90
120 250
12.0 22.0
2400 1100
JA, ~ev~tin plot, 500 m above sea level; B, Btatmi plot, 450 m above sea level.
PINE SAWFLY COCOONS IN SCOTS PINE STANDS
195
Methods Three zones were established in each stand from the south-southeastern edge, bordering a meadow, or a wide road in the stand: Zone I, less than 5 m; Zone II, 5.1-15 m; Zone III, greater than 15.1 m. The abundance (density) of cocoons was examined in relation to the stand edge and to trees selected at random in each zone. The distribution of cocoons was determined within each zone with two methods: ( 1 ) a circle of 0.3 m diameter was marked around the base of each of three trees in each zone; (2) squares of 0.25 m per side were selected in four directions and spaced at 0.25 m intervals from the base of each tree. In mature stands the squares (n = 80) extended to 1.75 m from the base of each of five trees in each zone, and to 1.25 m in medium-aged stands (n = 60) in each zone (five trees). Cocoons were extracted from the sample units with sieves in the field or by thorough examination of the samples cut out from the ground and transported to the laboratory. All the cocoons were counted: living, empty and those parasitized and preyed upon. Cocoons comprised eight species of pine sawflies in the region (Simandl, 1989) and, no outbreak of sawflies had been recorded during and prior to the year of sampling. The total area of pine litter sampled was 188 m 2.
Statistical analysis Statistical analysis of differences among sample sizes was performed using analysis of variance (three-way ANOVA) followed by the Student-NewmanKeul test. Multiple regression was used for evaluation of dependence of density of cocoons on zone and distance from tree trunk. RESULTS A total of 2346 cocoons occurred in the pine litter sampled. Of these 1893 cocoons were collected in square and 453 in circular units. Only 2.7% (density 0.25-0.48 cocoons m - 2, X= 0.37 ) of the sampled cocoons were living. I n the sample of old cocoons, sawflies had emerged from 23.1%, parasitoids from 39.3%, 34.8% had been destroyed by predators, and 2.9% could not be classed. The most frequent number of cocoons per square unit was one (n-- 118 ), but the highest proportion of cocoons was recorded in 41 sample units containing six cocoons each. The cumulation of cocoons was found in some sites such as around tree bases (in bark crevices as well) but the frequency of cocoons per square sample unit varied greatly. For example, no cocoons occurred in many samples and several contained more than 20. However the number of square units without cocoons (the average density of cocoons) decreased with increasing distance from the tree; the percent of sample units with and without cocoons related to the distance from the trees are shown in Fig. 1. Only 17.5% (n--441) of all square sampling units contained at least one cocoon.
196
J. SIMANDL I00 86.5
u) t-
82.2
8].1
78.4
..I
z nl ,,,"
I 50
Z
,~ ,7
10
6.2 . . . . .. .. . . . .
•" . " - " - -
5.1 .
.
.
.~. ; :.' ; ; ; ..~ . . ... ' : .
j
.
.
].1
. . . .2.4 ....
. . . ... . . . .
Fig. 1. Proportions of square sample units without sawfly cocoons (n = 2520, empty columns) and units containing cocoons ( n = 720, hatched columns) within each sampled distance in Scots pine stands (~ev~tin and Blatn~i, 1984-1985 ). a, only mature stands (over 80 years old, n = 360).
A. 30-
25-
v 15
;
z
j
r-
1
I
I
i
--.,:
i
:¢;i!l :..:::
"" i
I. •
c II.m
c III.m
~,~
c i.m
c ii.m
i
c iii.m
zone
age
Fig. 2. Distribution of cocoon densities (n m -2, .~___SE) related to the stand edge and the comparison of samples from circular and square sample units (94 m 2 in each plot). Probes: circular, []; square, rt. Locality (P<0.001): (A) ~ev6tin; (B) Blatn~i. Age (ns): c, 40 years; m, greater than 80 years. - - - , trend in stands 40 years old; - - , trend in stands greater than 80 years old.
PINE SAWFLYCOCOONS IN SCOTS PINE STANDS
197
TABLE 2 Total cocoon densities (n m-2, ~ + SE) of pine sawflies sampled per plot, age and zone (square plus circular sample units) in 1984-1985 in southern Bohemia (three-way ANOVA) Plot
Age
Zone
~
SE
A
40
I II III I II III
16.4 13.0 8.9 10.8 11.1 8.4
2.10 1.32 2.10 1.40 2.16 1.90
I II III I II III
15.5 11.9 10.3 22.9 19.7 11.6
1.78 1.45 1.71 2.80 1.69 2.10
I*** II III
16.4b 13.9b 9.8a
1.13 0.90 0.98
80
B
40
80
Total All***
All ns
***P < 0. 001. "SNon-significant.
Density related to stand edge For endemic population levels, the edge effect was the main factor influencing the dispersion of sawflies overwintering in the litter. The densities of cocoons decreased in each zone towards the stand interior (Fig. 2, Table 2 ). ~EV~TtN
20
]LATNA
I0
0-0.25
0.5-0.75
1.0-1.25
1.5-1.75
DISTANCE FRON TREES ( N )
Fig. 3. Distribution of pine sawfiy cocoons (n m -2) in relation to distances from tree trunks (square sample units, 1984-1985 ).
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Total differences among individual zones were significant ( P < 0.001 ), Zone III (25.1% of the sampled cocoons) being distinctly separate from Zones I (40.5%) and II (34.4%). The densities of cocoons in each zone of the individual plots ranged from 8.4 to 22.9 cocoons m -2 and were sub-equal in the mature stand at ~ev~tin where the difference of the cocoon densities between Zones I and III was lowest (Table 2 ). Therefore cocoons were concentrated along the edges and in gaps under the most infested trees. It was distinct particularly in mature stands where the most distant sampled areas (1.5-1.75 m) showed a higher cocoon density in comparison with the previous two distances sampled where the decrease in densities was distinct (Fig. 3 ). Density in relation to the tree
The distribution pattern also was influenced by the aggregation of cocoons at the base of trees. Changes of the cocoon density in relation to distance from tree were distinct and diminished nearly linearly with the increasing distance from the tree trunk (Fig. 3 ). The highest cocoon density was recorded around the tree base as found by both sampling methods (15.0 cocoons m -2 in circular and 15.9 in square units (Table 3). The differences between samples near the tree bases at 0-0.25 m and those lying farthest at 1.0-1.25 m was about 16.3%. Sawfly larvae most frequently spin cocoons near the base of the TABLE 3 Mean density (n m-2) of sawfly cocoons in Scots pine stands in relation to distances from tree trunks (square sampling units, 1984-1985 ). Standard error within parentheses Distance from trees (m)
Zone
Localities gev~tin
Blatn~t
0 -0.25
I II III
20.7 (2.15) 13.9 (1.33) 9.2 (1.39)
20.3 (2.12) 20.9 (1.11) 10.5 (1.26)
0.5-0.75
I II III
11.2 (0.84) 6.8 (0.78) 6.4 (0.40)
18.5 (0.58) 14.3 (0.88) 10.1 (0.68)
1.0-1.25
I II III
8.5 (1.10) 4.9 (0.55) 4.3 (0.24)
11.7 (0.68) 13.7 (0.64) 11.0 (0.75)
1.5-1.751
I II III
15.7 (2.00) 15.5 (0.42) 8.0 (0.64)
9.3 (1.08) 12.5 (1.10) 9.6 (1.33)
lOnly mature stands.
PINE SAWFLY COCOONS IN SCOTS PINE STANDS
199
trees if the litter layer is thick enough. More cocoons per square metre were collected by the method of circular samples than by square samples in general (Fig. 2) as well as in comparison with samples taken within 25 cm of tree base. The differences in cocoon density between mature and medium-aged stands were not statistically significant ( P > 0.05 ) (Table 2). The numbers of old cocoons, accumulated in the litter, were higher ( P < 0.001 ) at Blatn~i than at ~ev~tin (see Table 2 ) differing by 15.7% (368 cocoons). The largest number of cocoons occurred in the mature stand at Blatn~i (density 18.1 cocoons m-2; SE= 1.41 ) with the southern-edge exposure. There the number of Gilpin& frutetorum (F.) was relatively high (J. Simandl, personal observations, 19851988). The regression equation of the number of cocoons per sample unit dependent on zone and distance from tree is as follows:
Cd=a-bxDe-cXDs where a = 21.386; b = 2.904; c = 1.441; Do is distance from the stand edge (m) expressed as values of 1, 2, or 3; Ds is distance from tree trunk expressed as values of 1, 2, 3 or 4. All regression coefficients (a, b, c) are significantly different from zero (for a, b: P<0.001; for c: P<0.05). The cocoon populations of the sawfly community at relatively low densities for a long time had a distinct tendency to aggregate towards sunny edges (see Fig. 2) of pine stands as well as within managed stands within 0.3 m of host tree trunks (see Fig. 3). The dependence discovered will facilitate a more effective sampling of sawfly species overwintering in the litter during the period of low population densities. DISCUSSION
Most of the sawflies defoliating coniferous trees exhibit the same way of overwintering, i.e. in the cocoon stage in forest litter (one exception are Neodiprion sawflies). At the same time a small part of the cocoon population of sawflies overwintering in the litter can be placed outside the litter (Luterek et al., 1982), especially at higher population density of the insects (see Kurir, 1977a). During the period of endemic population levels of sawflies most of larvae were traveling to the stand litter every autumn (J. Simandl, personal observations, 1985 ). A tendency for cocoon numbers to decrease with increasing distance from tree trunk was reported in Gilpinia hercyniae (Htg.) (Prebble, 1943 ) and P. erichsonii (Htg.) (Ives and Turnock, 1959). Ohnesorge and Thalenhorst ( 1956 ) and Pointing ( 1957 ) reported that cocoons of some spruce sawflies were concentrated under tree crowns. The distribution of cocoons found in
200
J. SIMANDL
the vicinity of trees indicates that the distribution of endemic sawfly populations in managed pine stands over 40 years of age corresponds to the dispersion of Neodiprion spp. cocoons found at higher population densities on insolated young pines (Stark and Dahlsten, 1961 ). However, Lyons (1964) concluded that "cocoon density within stands is not strongly related to the distance from the nearest tree" in two Neodiprion species. It seems then that at low population densities the dependence on distance from a tree can be distinct in a species complex overwintering in the litter (Gilpinia spp., Diprion spp., Macrodiprion ). The prespinning mobility of sawfly larvae is good and they may crawl on the ground up to a distance of 70 cm before spinning (Lyons, 1964). However, when the litter is sufficiently thick (about 1 cm and more) near the host tree base and neither extremely dry nor moist (Ives, 1955 ), most larvae travel (not fall) to the litter down the trunk on sunny days (J. Simandl, personal observations, 1985 ) and spin their cocoons in the vicinity of the tree. The cocoon distribution pattern depends not only on the habits of larvae, but also on environmental factors, including the character of the ground cover, the thickness of litter, the density of tree canopy, etc. Adult females are partly responsible for the distribution pattern as well, because they oviposit more frequently on insolated parts of tree crowns where their offspring will find optimal life conditions (Lyons, 1962, 1964). The pattern of spinning is very irregular in open stands (Lyons, 1964) in contrast to dense stands where the pattern can be clearly discernible. The incidence of cocoons was very low under some trees although it was relatively high under the adjoining ones; one explanation can be the dispersion of populations according to negative binomial series in an area (Wilson and Gerrard, 1971 ). The suitability of the negative binomial in describing frequency distribution of cocoons does not mean that aggregation is in any sense explained, since the negative binomial has been deduced from a number of widely contrasting hypotheses regarding the mechanism of dispersal. The high abundance of old cocoons, in particular, in the mature stand at Blatn~i (22.9 m - 2) indicated that the population level of sawflies had been relatively higher for some time. In managed, medium-aged stands where the tree canopy is maintained dense, with relatively small crown projections of interior trees, it was impossible in some cases to decide which tree the cocoons (larvae) originated from when many were found farther than 1 m from the trees. A predisposition to increased mortality caused by parasitoids or by predators existed in sites with cocoon aggregations (Hanski and Parviainen, 1985 ). Thus distribution affects the role of beneficials in the regulation of latent populations. Aggregation of cocoons at tree bases or in stand edges did not involve one or two species whose larvae live in colonies (e.g.D. pini), because the majority of species with solitary larvae formed the sawfly community of adjoining stands of similar character (Simandl, 1989 ). Many forest insects prefer forest edges, an ecotone that is very suitable for
PINE SAWFLY COCOONS IN SCOTS PINE STANDS
201
development of defoliators and characterized by large tree crowns with a large biomass of needles and optimal abiotic conditions. In general, similar aggregation patterns of endemic populations along forest edges was reported in other forest insect defoliators (see Campbell et al., 1976; Bellinger et al., 1989). Diprionids number among the most abundant insects among Scots pine defoliators (Larsson and Tenow, 1980) and with a relatively high representation among pine-dwelling arthropods (J. Simandl, unpublished data, 1988 ) therefore relatively vacant sites (niches) can be successfully occupied by the diprionid complex. The results concerning the stand edge effect were confirmed by the measurement of falling frass, most of which was usually recorded along the edges (J. Simandl, personal observations, 1985-1988 ). The number and configuration of sample units performed seemed to have been sufficient, as the same unit size was often used (see Ohnesorge and Thalenhorst, 1956) for prognosis of sawflies. For cocoon densities from 2 to 3 m -a, Borodin (1975) recommended the unit size (0.0625 m 2) used in the investigation. The circular samples around trunks seem to be a more effective method of sampling at low population densities of sawflies. Thus Scots pine stand management should be focused on maintaining stands below 80 years of age dense, which is better both for the tree quality (as generally recommended for P. silvestris by forest managers) and a certain limiting of development of sawfly populations. If the sampling of lower sawfly populations in large managed pine woods endangered by sawflies is to be effective the prognoses should be based on more frequent sampling near tree bases (if stand gaps are present) and along the edge areas than within the stands to avoid underestimating the populations. ACKNOWLEDGMENTS
I thank Prof. H.H. Eidmann, Dr. E. Olofsson (University of Uppsala, Sweden), Dr. V. Zumr (Institute of Entomology, Czechoslovakia) and two anonymous referees for their valuable comments and editorial assistance on the first version of the manuscript and Dr. J. Lep~ (Centre of Biomathematical Methods, Czechoslovak Academy of Sciences) for statistical assistance.
REFERENCES Bellinger, R.G., Ravlin, F.W. and McManus, M.L., 1989. Forest edge effects and their influence on Gypsy Moth (Lepidoptera Lymantriidae) egg mass distribution. Environ. Entomol., 18: 835-840. Borodin, A.L., 1975. Optimalization of a method for counting the cocoons of the rusty pine sawfly Neodiprion sertifer (Hymenoptera, Diprionidae). Ekologiya, 3:60-67 (in Russian). Campbell, R.W., Miller, M.G., Duda, E.J., Biazak, C.E. and Sloan, R.J., 1976. Man's activities
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and subsequent gypsy moth egg-mass density along the forest edge. Environ. Entomol., 5: 273-276. Escherich, K., 1942. Die Forstinsekten Mitteleuropas. Bd. 5, Berlin, pp. 51-243. Hanski, I., 1987. Pine sawfly population dynamics - - patterns, processes, problems. Oikos, 50: 327-336. Hanski, I. and Parviainen, P., 1985. Cocoon predation by small mammals, and pine sawfly population dynamics. Oikos, 45: 1 2 5 - 1 3 6 . Ives, W.G.H., 1955. Effect of moisture on the selection of cocooning sites by the Larch Sawfly, Pristiphora erichsonii (Hartig). Can. Entomol., 87:301-311. Ives, W.G.H. and Turnock, W.J., 1959. Estimation of cocoon populations of the larch sawfly, Pristiphora erichsonii (Htg.). Can. Entomol., 91:650-661. Kurir, A., 1977a. Einspinnen and Clberwinterung der 1. Generation der Gew6hnlichen Kiefernbuschhornblattwespe, Diprion pini Linnaeus (Hym., Diprionidae) in der Baumkrone der Weisskiefer (Pinus sylvestris Linnaeus). Z. Angew. Entomol., 84: 47-52. Kurir, A., 1977b. Beobachtungen zur Bionomie der Blassen Kiefernbuschhornblattwespe, Diprion pallidum Klug. (Hym., Diprionidae) wiihrend der Gradation in Kiirnten 1971/1972. Z. Angew. Entomol., 84:155-163. Larsson, S. and Tenow, O., 1980. Needle-eating insects and grazing dynamics in a mature Scots pine forest in Central Sweden. Ecol. Bull. (Stockholm), 32: 269-306. Luterek, R., Gralicki, L. and Krystek, J., 1982. Places of cocoons formation by pine sawflies (Hymenoptera, Tenthredinidae). Pr. Kom. Nauk Roln. Kom. Nauk Lesn., 54:61-66 (in Polish, English summary). Lyons, L.A., 1962. The effect of aggregation on egg and larval survival in Neodiprion swainei Midd. (Hymenoptera, Diprionidae). Can. Entomol., 94: 49-58. Lyons, L.A., 1964. The spatial distribution of two pine sawflies and method of sampling for the study of population dynamics. Can. Entomol., 96:1373-1407. McLeod, J.M., 1961. A technique for the extraction of cocoons from soil samples during population studies on the Swaine sawfly, Neodiprion swainei Midd. (Hymenoptera, Diprionidae). Can. Entomol., 93: 888-890. McLeod, J.M., 1966. The spatial distribution of cocoons of Neodiprion swainei Midd. in Jack pine stand. I. A cartographic analysis of cocoon distribution, with special references to predation by small mammals. Can. Entomol., 98: 430-447. Morris, R.S. and Cameron, E., 1935. The biology ofMicroplectronfuscipennis Zett. (Chalcididae), a parasite of the pine sawfly (Diprion sertifer Geoff. ). Bull. Entomol. Res., 26: 407419. Niklas, O.F. and Franz, J., 1957. Begrenzungsfaktoren einer Gradation der Roten Kiefernbuschhornblattwespe (Neodiprion sertifer (Geoff.)) in Stidwestdeutschland 1953 bis 1956. Mitt. Biol. Bundesanst. Berlin, 89:39 pp. Ohnesorge, B. and Thalenhorst, W., 1956. Zur Kenntnis der Fichten-Blattwespen. IV. Die Dispersion. Z. Pflanzenkr. Pflanzenschutz., 63:197-211. Pointing, P.J., 1957. Studies on the comparative ecology of two sawflies Piconema alaskensis Roh. and Pikonema dimmockii Cress. (Tenthredinidae, Hymenoptera). Ph.D. Thesis, Univ. Toronto. Prebble, M.L., 1943. Sampling methods in population studies of the European spruce sawfly, Gilpinia hercyniae (Hartig), in Eastern Canada. Trans. R. Soc. Canada (Ser. 3, Sect. V), 37: 93-126. Simandl, J., 1989. Seasonal changes in the synusia of pine sawflies (Hym., Diprionidae) during their latency. J. Appl. Entomol., 108: 217-226. Stark, R.W. and Dahlsten, D.L., 1961. Distribution of cocoons of a Neodiprion sawfly under open-growth conditions. Can. Entomot., 93: 443-450.
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Styles, J.H., 1959. Observations on the spinning of cocoons by larvae of the sawfly Neodiprion sertifer (Geoffr.) (Hym., Diprionidae). Entomol. Mon. Mag., 95:178-179. Wilson, L.F. and Gerrard, D.J., 1971. A new procedure for rapidly estimating European pine sawfly (Hym., Diprionidae) population levels in young pine plantation. Can. Entomol., 103: 1315-1322.