A METHOD OF ESTIMATION OF THE TOTAL DENSITY OF INFESTATION OF SCOTS PINE STEMS BY THE LESSER PINE SHOOT BEETLE (TOMICUS MINOR)

Andrzej Borkowski
Department of Ecology and Environment Protection, Institute of Biology, Jan Kochanowski University of Kielce, Poland

ABSTRACT

Results of studies on colonization of Pinus sylvestris trap trees by Tomicus minor are presented. The scheme of spatial distribution of density of this bark beetle nests agrees with spatial distribution of colonization frequency in individual sections of the bole. Beetles most frequently colonized the middle parts of the bole, occurring most frequently (59.5%) and reaching the greatest density of nests (14.8%) in the eighth one-meter section of the trap tree bole. The linear relationships between nest numbers in distinguished one-meter sections of the bole and the total density of the bole colonization are determined. In the case of the eighth section, counting from the butt, the correlation was strongest (R² = 0.5877, P < 0.0001), and the mean relative error of estimation was smallest (49.46%).

Key words: Pinus sylvestris, Tomicus minor, attack pattern, stem infestation.

INTRODUCTION

The lesser pine shoot beetle Tomicus minor (Hart.) (Col., Curculionidae, Scolytinae) is a species widely distributed in the Palearctic region developing mainly on Scots pine (Pinus sylvestris L.) within its natural range [9,17,22]. This bark beetle breeds in the bole sections with thin bark of felled or strongly weakened trees, and according to Långström and Hellqvist [16] colonized earlier by Tomicus piniperda (L.). In addition, the harmfulness of this species is heightened by introduction to wood by parent beetles of stain fungi (Ophiostoma canum) consumed by larvae [24]. In Europe, feeding of adults of Tomicus minor and T. piniperda in pine shoots causes significant losses in tree increments [1,2,3,4,14,15,18,19,21].

In spite of many papers devoted to pine shoot beetles, only few discuss directly the biology and ecology of T. minor [9,10,11,13,23]. So far no method of population density estimation was worked out for this insect species. The estimation of the population size with methods based on collection of pruned and fallen shoots is of a relative value, and does not reflect the actual numerical relations of both species of pine shoot beetles. The problems with synthesis of synthetic pheromones, most likely resulting from a small attraction force of aggregation pheromones produced by beetles [12] cause, that the only method of estimation of the population size of this bark beetle is the estimation of its density on trap trees or felled sample trees.

The estimation of population density of the lesser pine shoot beetle and investigations on its population dynamics may be useful for the theory as well as practice of forestry. The presence of nests of pine shoot beetles in every Scots pine stand makes the studies on their numbers easier. Such studies are very important from the theoretical (e.g. permit to recognize in forest ecosystems the mechanisms connected with functioning of the interactions parasite-host and predator-prey) as well as practical (providing additional data helping to estimate more exactly the degree of threat to Scots pine stands) point of view.

The exact determination of the number of pine shoot beetle nests is very labor-consuming because the whole bole needs to be debarked precisely.

The purpose of this study was to work out the method of estimation of the total density of colonization of pine boles based on the relationship between the number of beetle nests in selected bole sections and the total density of bole colonization.

STUDY AREA AND METHODS

The studies were carried out during 1991–2005 in Scots pine stands of age classes IV–V (61–100 years) with a moderate crown closure, growing on sites of the following types: fresh coniferous forest, fresh mixed coniferous forest, and upland mixed broadleaved forest (Table 1).

In total 175 trees were felled. The following bole elements of each tree were measured:

1. diameter outside bark at the thicker end;

2. diameter outside bark of the successive one-meter sections beginning from the butt;

3. length.

After debarking, the colonized boles were divided into one-meter sections and nests of the lesser pine shoot beetle were counted on their upper and lower parts (after turning them over). Then, the bole surface area and the total density of the bole colonization were computed.

In order to determine the relationship between the number of nests in selected one-meter sections and the total density of colonization of boles the analyses of regression and correlation were used. Four (n = 4) one-meter bole sections were distinguished: (1) situated between 6^th and 7^th meter, (2) between 7^th and 8^th meter, (3) between 8^th and 9^th meter, and (4) between 10^th and 11^th meter. The following linear functions of regression were used:

y = a_0n + a_1nx_n            (1)

where y is the total density of infestation [m^-2], x_n is the number of nests in distinguished one-meter stem sections (n = 1, 2, 3, 4) and a_0n, a_1n parameters of linear functions.

The parameters of linear functions of regression were estimated using the least squares method. The coefficients of rectilinear correlation (r_n), determination (R²), mean relative errors of estimation (sw_n) were computed, and zero hypotheses H₀ were checked, according to which [5]:

1. slopes a_0n, a_1n are equal to zero;

2. differences between coefficients of correlation (r_n) are equal to zero.

The analyses were carried out using packet Statistica 6.1 [25]. The comparison of differences between coefficients of correlation computed for more than one pair required taking into account the error of first kind of the higher order (α_n1n2), arising during multiple comparisons [8]. To calculate the error of first kind the formula derived from Bonferroni inequality was used [20]:

(2)

where α_n1n2 is the adjusted significance level for the compared pair of coefficients of correlation (r_n), α is the total significance level for all comparisons and n number of coefficients of correlation analyzed (n = 4).

For four coefficients of correlation tested in this study and for the total significance level α = 0.2 the adjusted significance level α_n1n2 was equal to 0.05.

RESULTS AND DISCUSSION

In stands under investigations the lesser pine shoot beetle colonized most of felled trap trees (Table 1). The mean density of colonization was from 0.007 nests/dm² in 1999 in the Jędrzejów Forest District to 0.062 nests/dm² in the Łążek Ordynacki Forest District (Table 1). This bark beetle occurred in the entire bole length of trap trees. The scheme of the spatial distribution of density of nests agreed with the spatial distribution of colonization frequency in individual bole sections. Beetles most frequently colonized middle parts of the bole, occurring most frequently (59.5%) and reaching the greatest density of nests (14.81%) in the eighth (between 7^th and 8^th meter) one-meter bole section (Fig. 1).

The analyses of regression and correlation showed that there were statistically significant linear relationships between the number of nests in distinguished one-meter bole sections and the total density of bole colonization (Table 2). Coefficients of determination were significant and their value ranged from 0.3085 to 0.5877. The mean error of estimation for the eighth bole section did not exceed 50% (Table 2). Slopes of a straight line were different from zero assuming positive values. At the assumed levels of significance α = 0.2 and α = 0.05 the coefficient of correlation r₂, computed for the relationship between the number of nests occurring between 7^th and 8^th m of the bole and the total density of colonization was significantly greater than coefficients r₁ and r₃. The remaining coefficients of correlation did not differ significantly between one another (Table 3). The results presented in this paper showed the highest usefulness of regression function computed for the eighth one-meter bole section (n = 8).

The method of estimation of the total density of colonization of Scots pine boles by the lesser pine shoot beetle, presented above, is little labor-consuming, and therefore may be used in studies on insect ecology as well as in the forestry practice. The proposed method is little invasive because only a one-meter-long bole section needs to be debarked, and therefore it may also be used in ecological studies conducted in zones of partial protection, and even in zones of strict protection. In national parks no trap trees should be felled, but there is a possibility to utilize for example Scots pine windfalls.

A complex estimation of the colonization of Scots pine boles in area under investigations should be carried out in the following three basic stages:

1. sample selection, i.e. the number and distribution of sample boles;

2. estimation of the total density of bole colonization of individual felled and colonized trees;

3. estimation of the mean total density of colonization for the area under investigations (computation of the mean and confidence intervals).

To carry out the complex estimation of the density of colonization of Scots pine boles according to the scheme presented above the representative method (stages one and three) should be combined with the method proposed in this paper (stage two). The representative method makes the objective sample selection and estimation of colonization density possible, and for this reason, among other reasons, it is more and more commonly used in forestry sciences [6,7].

SUMMARY OF RESULTS AND CONCLUSIONS

1. When analyzing linear relationships between the numbers of nests of the lesser pine shoot beetle in distinguished one-meter bole sections and the total density of colonization of the Scots pine bole the strongest correlation (R² = 0.5877, P < 0.0001) and the smallest mean relative error of estimation (49.46%) were found for the eighths one-meter section of the bole, counting from the butt.

2. The method presented in this paper permits to make a very quick estimation of the density of colonization of Scots pine boles by the lesser pine shoot beetle. In this estimation, the simple regressions computed for the eighth bole section should be used.

3. Results of this study suggest the undertaking of further investigations on elaboration of relationships of this type for Scots pine stands in various regions of Poland.

4. The estimation of the total colonization of Scots pine boles by the pine shoot beetles, carried out using the method presented in this paper, is little invasive because it requires debarking of only a one-meter-long bole section. For this reason this method could also be used in zones of partial protection and even in zones of strict protection in national parks, where there is a possibility to utilize for example the windfalls for estimation of population density of the pine shoot beetles.

ACKNOWLEDGMENTS

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Accepted for print: 12.11.2010

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