The decline of forests has occurred in the last decade in many regions of Europe (Colombari et al. 2013; Andreieva and Goychuk 2018; Andreieva et al. 2019). Drought and increasing anthropogenic impact led to an increase in the susceptibility of trees to colonisation by bark beetles, among which the multivoltine species
So far, the main way to reduce the consequences of outbreaks of bark beetles remains selective or clear sanitary felling (Anonimous 2016). However, it can be effective only if it is carried out after the colonisation of trees by bark beetles, but before the emergence of a new generation (Meshkova 2019). Removing dead trees is not useful since bark beetles that are dangerous to living trees do not colonise dead trees (Selikhovkin 2017). This action is even harmful since entomophages and saproxylic insects concentrate under the bark of dead trees (Sanginés de Cárcer et al. 2021).
Numerous studies show that entomophages (parasitoids and predators) can significantly reduce the populations of bark beetles (Herard and Mercadier 1996; Sarikaya and Avci 2009; Fora et al. 2012; Yaman et al. 2016; Cebeci and Baydemir 2018; Akkuzu et al. 2021). However, usually, during the outbreaks of bark beetles, the populations of predators increase later than those of prey (Kenis et al. 2004). In order to accelerate the collapse of bark beetles’ population, certain species of predatory insects are reared in the laboratory and released into the forest (Kharitonova 1972; Kenis et al. 2004). When choosing a predator species for rearing, it is important to study its habitats, seasonal development, hibernation sites and other biological features (Kenis et al. 2004). Preference is given to polyphagous species that attack various stages of prey, have high fecundity, are able to develop without diapause, reproduce over a long period and have several generations per year. These conditions are satisfied by several species of coleopterous predators, which are reared and successfully released into the foci of bark beetles, particularly,
In 2018–2019, the State Specialized Forest Protection Enterprise ‘Kharkivlisozahyst’ (Kharkiv region, Ukraine) began rearing
The purpose of this research was to recognise the species composition of coleopterous predators of bark beetles and their occurrence in different parts of the stem depending on the health condition of Scots pine trees in the collapsing foci of bark beetles.
The research was carried out in 2019–2021 in the foci of Scots pine (
Location of inspected forest stands
State Forestry Enterprise | Forestry | Latitude, N | Longitude, E | Altitude, m a.s.l. | Number of plots | Number of gallery systems |
---|---|---|---|---|---|---|
Svessky | Prudishchenske | 51°57″17¢ | 33°43″38¢ | 159 | 3 | 54 |
Svessky | Olynske | 51°56″51¢ | 33°47″15¢ | 165 | 6 | 110 |
Seredyno-Budsky | Kamyanske | 52°05″49¢ | 33°53″06¢ | 196 | 2 | 36 |
Seredyno-Budsky | Golubivske | 50°48″01¢ | 34°25″10¢ | 170 | 6 | 110 |
Seredyno-Budsky | Ochkinske | 52°13″36¢ | 33°22″41¢ | 130 | 2 | 36 |
Krolevetsky | Dubovytske | 51°38″05¢ | 33°34″21¢ | 162 | 7 | 126 |
Krolevetsky | Gruzchanske | 51°12″32¢ | 33°31″45¢ | 158 | 4 | 72 |
Krolevetsky | Krolevetske | 51°33″10¢ | 33°22″57¢ | 166 | 1 | 18 |
Krolevetsky | Khreshchatynske | 51°38″23¢ | 33°22″07¢ | 154 | 1 | 18 |
Lebedinsky | Mezhyrichske | 50°41″58¢ | 34°29″39¢ | 145 | 1 | 18 |
Lebedinsky | Borovenkivske | 50°27″52¢ | 34°24″32¢ | 142 | 1 | 18 |
Lebedinsky | Ukrainske | 50°29″28¢ | 34°30″23¢ | 140 | 1 | 18 |
Trostyanetsky | Makivske | 50°53″10¢ | 34°97″15¢ | 165 | 1 | 18 |
Trostyanetsky | Lytovske | 50°36″15¢ | 34°86″12¢ | 143 | 1 | 18 |
Trostyanetsky | Trostyanetske | 50°28″30¢ | 34°58″18¢ | 118 | 1 | 18 |
Total | 38 | 688 |
All sample plots are located in pure Scots pine stands in relatively poor forest site conditions – B2 (Ostapenko and Vorobyov 2014). The relative density of stocking is 0.6–0.7, and the age of stands is between 60 and 110 years.
Considering that the population level of bark beetles and their predators depends on the number of trees susceptible to colonisation, the health condition class of each sample tree was assessed.
The health condition for each tree was evaluated on a range of visual characteristics according to the ‘Sanitary Forest Regulations in Ukraine’ (Anonimous 1995) and was divided into the following classes: first – healthy; second – weakened; third – severely weakened; fourth – drying up; fifth – recently died and sixth – died over a year ago. The health condition index (HCI) was calculated as a weighted average of the trees in each class of health condition. Sample plots were taken for analysis, where the health condition of trees did not differ significantly (Kruskal–Wallis [K–W] test: H = 2.74,
Bark beetles’ nuptial chambers and predators were counted on 25 × 25 cm pallets, which were located at the parts of the stem with thin, thick and transitional bark. Bark thickness at the lower part of Scots pine stems was over 2 cm, in the upper part was below 0.3 cm and the width of transitional bark in the middle part of the stem was 0.5–0.8 cm. For this research, the number of nuptial chambers of bark beetles was recalculated on 1 dm2.
The prevalence of predator species in different complexes was assessed on a scale: single – up to 0.1% of the total, rare – 0.1–1%, common – 1–5% and abundant – more than 5% (Bieliavtsev and Skrylnik 2020).
To analyse the performance of different predator species depending on the tree part, health condition and year, normality tests were performed. The significance of differences in studied traits was analysed using the nonparametric K–W test because the conditions of normality were not met. When the differences were significant, the Dunn procedure for multiple pairwise comparations was applied.
The species composition of predators in different sample plots was compared using the Sorensen–Chekanovsky index (1) (Leontyev 2008):
Microsoft Excel software and statistical software package Paleontological Statistics (PAST) Software Package for Education and Data Analysis (Hammer et al. 2001) were used.
In the inspected foci of bark beetles, most pine trees were characterised by the third to sixth classes of health condition. However, the bark beetles and their predators were found in the trees in the fourth to sixth classes of health condition.
The health condition of the examined trees tended to worsen (increase in the HCI) during 2019–2021 (K–W test: H = 61.65–149.1 for different pairs of groups,
HCI for Scots pine trees sampled in different stem parts
Sampled part of stem | Mean HCI ± st. err. for years* | ||
---|---|---|---|
2019 | 2020 | 2021 | |
Upper (thin bark) | 4.4 ± 0.13a (27) | 5.3 ± 0.07b (72) | 6.0 ± 0.0c (30) |
Middle (transitional bark) | 4.4 ± 0.13a (27) | 5.3 ± 0,07b (72) | 6.0 ± 0,0c (30) |
Lower (thick bark) | 4.5 ± 0.08a (72) | 4.7 ± 0.04d (265) | 5.3 ± 0.05e (90) |
All samples | 4.5 ± 0.06a (126) | 4.9 ± 0.04f (409) | 5.6 ± 0.04g (150) |
Note:
Number of trees in parentheses. Means followed by different letters are significantly different at the 95% confidence level. HCI – health condition index.
Two bark beetle species,
In all foci, the population of bark beetles was collapsing. The average (±standard error) infestation density (number of nuptial chambers per 1 dm2) calculated from all samples collected was 0.66 ± 0.27, 0.47 ± 0.18 and 0.42 ± 0.23 in 2019, 2020 and 2021, respectively.
Infestation density (number of nuptial chambers per 1 dm2) calculated from all 1443 samples collected was 0–2.72 for
At the same time, the coefficient of variation of this parameter was quite high (81.7% and 80% for
No significant differences were found in the colonisation of the upper and lower parts of the stem by the two bark beetle species (K–W test: H = 2.74,
Nine species of coleopterous predators were found in the analysed trees. In the galleries of bark beetles, seven species coleopterous predators were collected:
Two more species –
The distribution of predators on different parts of the stem differed significantly (K–W test: H = 9.45,
The Sorensen–Chekanovsky index (Csc) showed different similarities of predator species composition between stem parts, class of tree health condition and years.
For all years and classes of the tree health condition, the similarity between upper and lower parts of the stem was the lowest (Csc = 0.83) and the similarity between the predator species composition in the trees of the fifth and sixth classes of health condition as well as between 2019 and 2021 was the highest (Csc = 1.0) (Tab. 3).
Calculation of Sorenson–Chekanovsky index (Csc) for predator complexes in the different parts of stem, class of tree health condition and years of assessment
Group of samples A | Species number in A | Group of samples B | Species number in B | Number of common species for A and B | Csc | ||||
---|---|---|---|---|---|---|---|---|---|
HCC | Stem part | Year | HCC | Stem part | Year | ||||
All | U | All | 5 | All | L | All | 7 | 5 | 0.83 |
All | U | All | 5 | All | M | All | 6 | 5 | 0.91 |
All | M | All | 6 | All | L | All | 7 | 6 | 0.92 |
4 | All | All | 7 | 5 | All | All | 6 | 6 | 0.92 |
4 | All | All | 7 | 6 | All | All | 6 | 6 | 0.92 |
5 | All | All | 6 | 6 | All | All | 6 | 6 | 1.00 |
All | All | 2019 | 6 | All | All | 2020 | 7 | 6 | 0.92 |
All | All | 2019 | 6 | All | All | 2021 | 6 | 6 | 1.00 |
All | All | 2020 | 7 | All | All | 2021 | 6 | 6 | 0.92 |
5 | U | 2020 | 3 | 5 | M | 2020 | 4 | 2 | 0.57 |
5 | U | 2020 | 3 | 5 | L | 2020 | 6 | 3 | 0.67 |
5 | M | 2020 | 4 | 5 | L | 2020 | 6 | 4 | 0.80 |
6 | U | 2020 | 2 | 6 | M | 2020 | 5 | 2 | 0.57 |
6 | U | 2020 | 2 | 6 | L | 2020 | 5 | 2 | 0.57 |
6 | M | 2020 | 5 | 6 | L | 2020 | 5 | 4 | 0.80 |
6 | U | 2021 | 4 | 6 | M | 2021 | 6 | 4 | 0.80 |
6 | U | 2021 | 4 | 6 | L | 2021 | 5 | 4 | 0.89 |
6 | M | 2021 | 6 | 6 | L | 2021 | 5 | 5 | 0.91 |
Note: The groups of sample plots without predators are not considered. HCC – health condition class of the trees. Stem part – U – upper; M – middle; L – lower
If we specify the location of the bark samples and the class of tree health condition, then the values of the Sorenson–Chekanovsky index (Csc = 0.57) are minimal when comparing the samples from the upper part of trees of the sixth class during the surveys of 2020 and the middle and lower parts of the same trees. Predator complexes in the middle and lower parts of trees of the fifth and sixth classes of the health condition in 2020 were of high similarity (Csc = 0.80). A similar Sorenson–Chekanovsky index was calculated at the 2021 assessment in trees of the sixth class of the health condition for the samples from the upper and middle parts of the stem. The highest similarity of predator complexes was found between the samples from the lower and middle parts of trees of the sixth class of health condition in 2021.
The proportion of the number of individual predator species and bark beetles depended on the year of assessment, tree health condition (Tab. 4) and stem part (Tab. 5).
Species composition of coleopterous predators in sample trees of different health condition
Predator species | Health condition classes | ||
---|---|---|---|
4 – drying up | 5 – recently died | 6 – died over a year ago | |
1 | 2 | 3 | 4 |
Upper stem part (thin bark) | |||
0 | 58.4 ± 14.23a | 15.6 ± 6.41b | |
0 | 0 | 34.4 ± 8.40c | |
0 | 33.3 ± 13.60c | 21.9 ± 7.31c | |
0 | 0 | 28.1 ± 7.95d | |
0 | 8.3 ± 7.96e | 0 | |
Middle stem part (transitional bark) | |||
0 | 45.5 ± 10.62a | 7.3 ± 3.50b | |
0 | 22.7 ± 8.93c | 18.2 ± 5.20c | |
0 | 0.0 | 20.0 ± 5.39c | |
0 | 13.6 ± 7.32d | 25.5 ± 5.87d | |
0 | 18.2 ± 8.22d | 25.5 ± 5.87d | |
0 | 0.0 | 3.6 ± 2.52e | |
Lower stem part (thick bark) | |||
15.3 ± 3.64a | 20.0 ± 2.67a | 4.7 ± 3.21e | |
13.3 ± 3.43a | 18.7 ± 2.60a | 11.6 ± 4.89a | |
9.2 ± 2.92a | 15.6 ± 2.42a | 9.3 ± 4.43a | |
40.8 ± 4.96b | 28.0 ± 2.99c | 58.1 ± 7.52b | |
2.0 ± 1.43d | 0.0d | 0.0d | |
14.3 ± 3.53a | 13.3 ± 2.27a | 4.7 ± 3.21e | |
5.1 ± 2.22d | 4.4 ± 1.37d | 11.6 ± 4.89d |
Note: Means followed by different letters in every row are significantly different at the 95% confidence level
Species composition of coleopterous predators in different years of assessment
Predator species | Years | ||
---|---|---|---|
2019 | 2020 | 2021 | |
Upper stem part (thin bark) | |||
0 | 29.2 ± 9.28a | 0 | |
0 | 0 | 25.0 ± 9.68a | |
0 | 41.6 ± 10.06a | 25.0 ± 9.68a | |
0 | 0 | 35.0 ± 10.67a | |
0 | 29.2 ± 9.28a | 15.0 ± 7.98a | |
Middle stem part (transitional bark) | |||
0 | 20.0 ± 5.16a | 9.1 ± 6.13a | |
0 | 20.0 ± 5.16a | 13.6 ± 7.32a | |
0 | 8.3 ± 3.57a | 27.3 ± 9.50b | |
0 | 20.0 ± 5.16a | 22.7 ± 8.93a | |
0 | 23.3 ± 5.46a | 18.2 ± 8.22a | |
0 | 8.3 ± 3.57a | 9.1 ± 6.13a | |
Lower stem part (thick bark) | |||
3.6 ± 2.52a | 20.2 ± 2.63b | 20.7 ± 4.48b | |
1.8 ± 1.80a | 19.3 ± 2.59b | 17.1 ± 4.16b | |
41.8 ± 6.65c | 6.4 ± 1.61d | 12.2 ± 3.61e | |
18.2 ± 5.20b | 36.5 ± 3.15f | 40.2 ± 5.42f | |
0 | 0.9 ± 0.60g | 0 | |
25.5 ± 5.87h | 10.7 ± 2.03i | 8.5 ± 3.09i | |
9.1 ± 3.88j | 6.0 ± 1.56j | 1.2 ± 1.21k |
Note: Means followed by different letters in every row are significantly different at the 95% confidence level
Predators were not found in the samples with thin and transitional bark from the trees of the fourth class of health condition (Tab. 4).
In the lower part of the stem, seven predator species were found with the dominance of
In the transition bark of the trees of the fifth and sixth health condition classes, four and six predator species were found, respectively.
An analysis of the dynamics of the proportion of individual predator species showed that in 2019, they were found only in the trees of the lower part of the stem, with
In the samples with thin bark in 2020 and 2021, three and four species of predators were found, respectively. At the same time,
In the samples with transitional bark in 2020 and 2021, six species of predators were found and their ratio did not differ much in these years. One can only note a decrease in the proportion of
In the samples with thick bark, the proportions of
In 2019, predators were found only under thick bark. In 2020, the predator–prey ratio increased from thin to thick bark and was the highest in 3 years (Tab. 6). In 2021, the predator–prey ratio was the lowest on thin bark and did not differ significantly from the transitional and thick bark. For 3 years, in all stem parts, the maximum predator–prey ratio was in 2020, and by stem sections, it was on the thick bark.
Coleopterous predator–prey ratio in different years and parts of stem
Sampled part of stem | Predator–prey ratio by years, % | ||
---|---|---|---|
2019 | 2020 | 2021 | |
Upper (thin bark) | 0.0a | 2.75 ± 0.55b | 5.78 ± 1.25c |
Middle (transitional bark) | 0.0a | 7.46 ± 0.93d | 7.94 ± 1.62cd |
Lower (thick bark) | 7.27 ± 0.94e | 12.56 ± 0.77g | 7.95 ± 0.84d |
All samples | 7.27 ± 0.71e | 8.97 ± 0.48d | 7.50 ± 0.65d |
Note: Means followed by different letters are significantly different at the 95% confidence level
On the trees of the fourth class of health condition, the predators were found only under thick bark (Tab. 7).
Coleopterous predator–prey ratio in different parts of stem depending on health condition of the trees*
Sampled part of stem | Predator–prey ratio by the health condition tree classes | ||
---|---|---|---|
4 – drying up | 5 – recently died | 6 – died over a year ago | |
Upper, thin bark | 0.0a | 1.9 ± 0.54c | 4.1 ± 0.70e |
Transitional bark | 0.0a | 3.6 ± 0.76c | 8.3 ± 1.08f |
Lower, thick bark | 16.1 ± 1.49b | 9.3 ± 0.59d | 7.0 ± 1.02f |
All samples | 12.4 ± 1.17b | 7.1 ± 0.42g | 6.3 ± 0.53f |
Note:
Trees of the first and second classes of health condition were absent, and no gallery systems of bark beetles and their predators were found in the trees of the third class of health condition. Means followed by different letters are significantly different at the 95% confidence level.
On the trees of the fifth class of health condition, the predator–prey ratio increased from thin to thick bark, and on the trees of the sixth class of health condition, the highest predator–prey ratio was on the transitional bark. In the samples with thin and transitional bark, the predator–prey ratio was the highest in the trees of the sixth class of health condition. It may be explained by favourable microclimate or to be as a result of consuming other prey. In the lower stem part and in general, in the trees, the predator–prey ratio was the highest in the trees of the fourth class of health condition and decreased as the health condition of the trees deteriorated.
Cambiophagous insects are characterised by physiological and technical harmfulness. Physiological harmfulness depends on the ability of these insects to colonise the trees of a certain class of health condition, damage trees during maturation feeding and vector the pathogens into the trees (Skrylnik et al. 2019). In this regard,
In our research, trees of the third to sixth classes of health condition prevailed in the collapsing foci and the health condition of the trees continued to deteriorate in 2019–2021 (Tab. 2). This was facilitated by vectoring the ophiostomatoid fungi by bark beetles (Davydenko et al. 2021).
Bark beetles are attracted to trees by volatiles, and predators are attracted to bark beetle pheromones (Schroeder 1999; Kenis et al. 2004). In the collapsing focus, bark beetles concentrate on drying up and dead trees and their predators also concentrate there.
The population density of bark beetles
In the galleries of bark beetles, seven species of predatory beetles were found. These species have a fairly wide range and are also known from other regions.
The analysis showed the features in the distribution of predators on different parts of the stem and on the trees of different classes of health condition.
The ratio of predator–prey also varied by years and stem parts. This parameter averaged 7.27–8.97% for all stem parts (Tab. 6). In 2020, it significantly increased from the upper to the lower part of the stem, which could be due to the greater attractiveness of the thick bark to predators and the presence of a higher density of
In studies conducted using flight barrier traps in central Sweden, the ratio of
In the collapsing foci of bark beetles (Coleoptera: Curculionidae: Scolytinae), the health condition of Scots pine in 2019–2021 tended to worsen. The infestation density of
In the galleries of bark beetles, seven species of coleopterous predators were collected:
The ratio of predator–prey significantly increased from the upper to the lower part of the stem with thick bark; however, it decreased in this stem part from the fourth to the sixth class of the health condition of the host tree.