Resource Sharing between the Invasive Sirex noctilio and Native Woodborers and Beetles in Pinus Plantations

Simple Summary The invasive woodwasp Sirex noctilio occasionally shares hosts with the native S. nitobei and other colonizers. The coexistence of these species may significantly impact individual species. We compared coexistence patterns among several colonizers at both stand and tree scales. Spatial scales revealed negative associations (e.g., Asemum striatum, Phaenops sp.) and neutral ones (Ips acuminatus) between woodwasps and other co-colonizing insects. A positive correlation was found between the frequency of woodwasp attacks and the density of trees damaged by Sirex in the current and previous years. At the tree scale, S. noctilio is more abundant in sections where it occurs alone than in sections where it co-occurs with S. nitobei. The distribution and body size of S. noctilio within the tree are unaffected by the presence of S. nitobei. However, S. nitobei was more prevalent in bottom sections when S. noctilio was present and larger in bottom sections where it occurred alone. This result can be attributed to early S. noctilio attacks enhancing the micro-environment of S. nitobei, which provides nutrients for larval development. Abstract Sirex noctilio, a European woodwasp, occasionally shares resources with the native S. nitobei and other colonizers in northeast China. The impact of its coexistence on individual species remains unclear. Random sampling was conducted to assess the patterns and extent of insect co-colonization across various spatial scales. Additionally, we analyzed wood sections to determine the density, adult size, and distribution of the two Sirex species. Spatial scales revealed negative associations (Asemum striatum and Phaenops sp.) and neutral ones (Ips acuminatus) between woodwasps and other co-colonizers. Clustering of woodwasps and Phaenops sp. occurred at a small scale (0–7.3 m). Regression analysis showed a positive correlation between the chance of woodwasp attacks and past attacks on the same host, with little impact from other colonization factors. The distribution and body size of S. noctilio within the tree appeared unaffected by S. nitobei’s presence. In the presence of S. noctilio, S. nitobei tended to lay eggs in damaged sections. At the stand level, the overall impact of S. noctilio on S. nitobei population density is likely positive because S. nitobei prefer weaker trees, a preference potentially influenced by initial attacks from S. noctilio on healthier hosts.


Introduction
Biological invasions are occurring at an unprecedented rate due to increased global trade and transportation, posing a threat to indigenous communities in invaded areas [1,2].
Insects 2024, 15, 478 2 of 12 Species diversity amplifies the likelihood of interaction between sympatric invasive and native species, potentially influencing their ecology and evolution [3].Within forest ecosystems, interactions (facilitative or antagonistic) among insects, whether direct or indirect, are common [4][5][6].Primary bark beetles consume resin through collective attacks, which makes the tree more accessible to secondary pests and bark beetles [7].For instance, pioneer species Dendroctonus frontalis Zimmermann renders hosts vulnerable, thus facilitating colonization by numerous secondary species [8].The damage caused by Agrilus planipennis Fairmaire can temporarily provide a resource supply for local saproxylic insects and predators such as woodpeckers [9].Furthermore, it can facilitate the expansion of the range of local buprestid beetle populations in Europe [10].
Sirex noctilio Fabricius, a woodwasp native to Europe, was first investigated in northeast China in 2013 [11].Female S. noctilio drill eggs, symbiotic fungi, and toxins into the host tree [11][12][13].The synergistic action of the fungus Amylostereum areolatum (Fr.)Boiden and phytotoxic mucus overwhelms the tree's defense mechanisms and provides a suitable environment for larvae [14,15].Sirex noctilio completes its generation development in 1-2 years, with most completing development within a year in China [16].A 2015 field survey found sympatric distribution of S. noctilio and S. nitobei in northeast China [17,18].Research shows that the Asian native S. nitobei (found in China, Japan, and Korea) has one generation per year and is associated with A. areolatum or A. chailletii (Pers.: Fr.) Boidin [12,[19][20][21].Direct competition is expected between two woodwasps as they develop in the sapwood, phloem, and xylem.These species may interact indirectly by inducing tree defenses, altering tree physiological state, or affecting tree decline, thus influencing nutrition or other resources.Based on the literature, S. noctilio prefers stressed trees, and it may even infest healthy trees [22,23].In contrast, S. nitobei primarily prefers severely weakened or recently dead trees [24].It can be concluded that the host niche width of S. noctilio is larger than that of S. nitobei.Considering that S. noctilio attacks healthy trees, which are later co-colonized by S. nitobei as the trees decline, S. noctilio likely contributes to S. nitobei at this level [24].The extent to which the damage caused by S. noctilio may provide a habitat for S. nitobei is unclear.The sympatric distribution of two woodwasps in Northeast China provides sample sites to study their coexistence extent and impacts of their interaction.
Similar to the damage observed in North America [25,26], S. noctilio has encountered some native co-colonizers in Northeast China.Ips acuminatus (Gyllenhal, 1827) (Coleoptera: Curculionidae) is prominent among the diversity of scolytines that infest the interface between bark and xylem, while various woodborers (Cerambycidae and Buprestidae) burrow deep into the xylem tissue.Interactions among Sirex woodwasps, bark beetles, and woodborers are probably indirect [27][28][29], which can influence their distribution and abundance [30][31][32].In North America, competition between native colonizers and S. noctilio might have restrained significant outbreaks of this invader [33].Therefore, primary treeattacking insects may experience negative effects from co-colonization with heterospecifics due to resource competition or intraguild predation [26,31,[34][35][36].Interactions between S. noctilio and other insects remain unexplored in its native European range due to its limited economic significance.Consequently, our understanding of these complex species interactions remains incomplete.
The temporal and spatial niches of S. noctilio and/or S. nitobei have been documented in certain regions (e.g., Dumeng (DM), Hegang (HG)) [17,37,38], but analyses of dispersal, colonization, as well as the abundance and quality of host materials are lacking for populations in Northeast China.Additionally, woodwasps are influenced not only by current-year colonizers but also by those from previous years.Here, we have specifically investigated the patterns of native S. nitobei and invasive S. noctilio, in mixed coniferous forests in Jinbaotun (JBT) and Yushu (YS).These forests represent the primary habitat for S. nitobei and S. noctilio since their introduction [12,17].We examined colonizers contributing to pine mortality: Sirex species, bark beetles, and woodborers.Specifically, we aimed to determine the extent of co-occurrence of Sirex spp.and other competitors at the stand scale; evaluate the impacts of co-occurrence on body size and density at the scale of individual trees; and investigate resource partitioning in co-colonized trees through differential density based on trunk height.

Study Sites
The study was conducted in JBT and YS, which primarily consisted of 30-year-old plantations of Pinus.sylvestris var.mongolica Litv.and P. tabuliformis Carrière.Four locations were chosen for this study due to their documented history of significant infestations by S. noctilio and S. nitobei between the years 2017 and 2019.

Degree of Co-Occurrence among Colonizers at the Stand Level
We observed exit holes and frass (Table 1) of co-colonizers within the lower 3 m of the trees.Binary data (yes/no) were collected for each dead tree regarding the presence of the four prevalent co-colonizers (Sirex woodwasps, Asemum striatum (Linnaeus, 1758), Phaenops sp. and I. acuminatus).Siricid circular exit holes can be distinguished from those of other woodborers (Table 1).Colonization by Siricid woodwasps can be determined for the current and preceding years, using two key diagnostic markers: resin beads for the current year and circular holes for the previous year (Figure 1).Dead/dying trees were marked using a handheld GPS (Global Positioning System, G310, Guangzhou Taike Electron Technology Co., Ltd., Guangzhou, China, accuracy < 3 m), and their diameter at breast height (DBH) and species (P.sylvestris var.mongolica or P. tabuliformis) were recorded.scale; evaluate the impacts of co-occurrence on body size and density at the scale of individual trees; and investigate resource partitioning in co-colonized trees through differential density based on trunk height.

Study Sites
The study was conducted in JBT and YS, which primarily consisted of 30-year-old plantations of Pinus.sylvestris var.mongolica Litv.and P. tabuliformis Carrière.Four locations were chosen for this study due to their documented history of significant infestations by S. noctilio and S. nitobei between the years 2017 and 2019.

Degree of Co-Occurrence among Colonizers at the Stand Level
We observed exit holes and frass (Table 1) of co-colonizers within the lower 3 m of the trees.Binary data (yes/no) were collected for each dead tree regarding the presence of the four prevalent co-colonizers (Sirex woodwasps, Asemum striatum (Linnaeus, 1758), Phaenops sp. and I. acuminatus).Siricid circular exit holes can be distinguished from those of other woodborers (Table 1).Colonization by Siricid woodwasps can be determined for the current and preceding years, using two key diagnostic markers: resin beads for the current year and circular holes for the previous year (Figure 1).Dead/dying trees were marked using a handheld GPS (Global Positioning System, G310, Guangzhou Taike Electron Technology Co.,Ltd., Guangzhou, China, accuracy <3 m), and their diameter at breast height (DBH) and species (P.sylvestris var.mongolica or P. tabuliformis) were recorded.

Effect of Co-Occurrence on Woodwasps in Wood Segments
A total of 52 trees with resin beads on their bark were selected for felling in this experiment, comprising 40 from JBT and 12 from YS.Those included several infestation types: [S.nitobei only (X), S. noctilio only (S), co-occurrence (S + X)].Every tree was sectioned into two parts (bottom and middle), each 200 cm in length, since Sirex species are rarely found in the upper sections [17].Prior to felling, some bark beetles and woodborers

Effect of Co-Occurrence on Woodwasps in Wood Segments
A total of 52 trees with resin beads on their bark were selected for felling in this experiment, comprising 40 from JBT and 12 from YS.Those included several infestation types: [S.nitobei only (X), S. noctilio only (S), co-occurrence (S + X)].Every tree was sectioned into two parts (bottom and middle), each 200 cm in length, since Sirex species are rarely found in the upper sections [17].Prior to felling, some bark beetles and woodborers emerged from the trees, leaving only a few hosts infested.Thus, the analysis the wood segments focused solely on the co-occurrence of the two woodwasps.

Density of Woodwasps
The DBH of infested trees was measured to standardize estimates of insect density by wood volume.Quantifying density in the field is challenging due to the possibility of two woodwasps infesting the same tree, the fact that both types have identical exit holes and resin beads (Figure 1a,b), and the difficulty in distinguishing larval stage morphology (Figure 1c).Therefore, woodwasp species were confirmed based on morphological characteristics of the adults that emerged after rearing.We identified the species of emergence holes using two indices: the emergence periods of the two wasp species are relatively distinct, allowing for the differentiation of most adults based on the timing of emergence [17].For the few adults with overlapping emergence periods, we marked the species and locations of emerging adults at different times each day to facilitate distinction.Emergence holes of Sirex species were then counted to compare densities and the spatial distribution of them.

Body Size of Woodwasps
Significant variations in body size characterize the biology of dimorphic woodwasps [39].We analyzed the correlation between pronotum width, ovipositor length, wing length, and body length (n = 100).Indices that were easy to measure, relatively stable, and highly correlated with body length were selected as criteria for evaluating body size.

Statistical Analysis
The percentages of emergence holes of Sirex woodwasps were evaluated using the nonparametric Kruskal-Wallis H test followed by Dunn's test for post-hoc multiple comparisons among four sites.The presence (denoted as 1) or absence (denoted as 0) of colonized pests on each tree was recorded, with trees without pest damage being excluded.The phi coefficient, a measure of correlation between two dichotomous variables, was employed in SPSS 26.0.0 to examine the correlation between woodwasps and other pests (A.striatum, I. acuminatus, and Phaenops sp.).Statistical significance was determined using the chi-square test.Ripley's K-function with border correction for spatial inhomogeneity [40] was employed to assess the distribution of co-colonizing populations within trees.Based on the two-dimensional point coordinate distribution of different populations, the spatial distribution patterns of individual colonizer populations were quantitatively analyzed.This method tests complete spatial randomness (CSR) by employing an extended search radius centered on each target tree.The maximum search radius was set to 15 m.Point pattern analysis was conducted using R Studio (version 2024.04.2-764.for Windows) and the 'spatstat' package (version 3.0-8).A 95% confidence envelope was calculated through Monte Carlo random simulation of 299 bootstrapping replications.Aggregation is indicated by space above the envelope, while regularity is indicated by space below it.Dead tree samples from field surveys, encompassing data from the current year (y) and the previous year (y − 1), were analyzed as response variables, and a spatial lag model was constructed.All data were processed using ArcGIS version 10.8 and analyzed with GeoDa version 1.22.0.4.
Due to the non-significant interaction between host sections and infestation types, separate analyses were performed for the bottom and middle sections of the wood segment.Considering the normality of the data and homogeneity of variances, unpaired t-tests, Welch's t-test, or Mann-Whitney tests were conducted for assessing the body size and density of wood wasps (GraphPad Prism version 10.0.0 for Windows).To evaluate changes in the distribution patterns of two woodwasp species, their colonization percentages within the mid-section of trees were compared.

Colonizer Damage at the Stand Scale
The emergence-hole diameters (S. noctilio: 3.89-6.91mm; S. nitobei: 3.04-5.63mm) and near emergence-hole orientation (Figure S1) (n = 40 S. noctilio: 15% upward, 85% downward; n = 40 S. nitobei: 70% up, 30% down) partially overlapped between species.Therefore, Insects 2024, 15, 478 5 of 12 our statistics did not allow for accurate differentiation of the damage characteristics between the two woodwasp species.Consequently, spatial distribution analysis at the stand level was confined to the genus level.The majority of damage to P. sylvestris var.mongolica was attributed to several colonizing agents, including Sirex woodwasps, A. striatum, I. acuminatus, and Phaenops species.Overall, the complex damage caused by colonizers across various genera amounted to 1.45%.Sirex woodwasps alone accounted for damage in up to 12.32% of host trees.At the four study sites, 82.75% of the trees affected by woodwasps exhibited 1 to 30 emergence holes at standard visual height (Figure 2).

Colonizer Damage at the Stand Scale
The emergence-hole diameters (S. noctilio: 3.89-6.91mm; S. nitobei: 3.04-5.63mm) and near emergence-hole orientation (Figure S1) (n = 40 S. noctilio: 15% upward, 85% downward; n = 40 S. nitobei: 70% up, 30% down) partially overlapped between species.Therefore, our statistics did not allow for accurate differentiation of the damage characteristics between the two woodwasp species.Consequently, spatial distribution analysis at the stand level was confined to the genus level.The majority of damage to P. sylvestris var.mongolica was attributed to several colonizing agents, including Sirex woodwasps, A. striatum, I. acuminatus, and Phaenops species.Overall, the complex damage caused by colonizers across various genera amounted to 1.45%.Sirex woodwasps alone accounted for damage in up to 12.32% of host trees.At the four study sites, 82.75% of the trees affected by woodwasps exhibited 1 to 30 emergence holes at standard visual height (Figure 2).

Within-Tree Associations at the Tree Scale
This study analyzed a total of 276 dead trees with substantial colonizer damage.Sirex woodwasps were negatively associated with most colonizing agents (p < 0.001), except for I. acuminatus, with which they maintained a neutral association (p > 0.05).Phaenops sp.exhibited a positive relationship with both A. striatum and I. acuminatus (p < 0.001), and it demonstrated neutrality between A. striatum and I. acuminatus (Table 2).The autologistic regression model revealed a significant increase in the frequency of Sirex woodwasp attacks with attack density of Sirex over two years, density of Phaenops sp.attacks from the previous year (y − 1), and A. striatum attack density from the current year (Table 3).The presence of Phaenops sp. in the current year and A. striatum in the previous year were negatively associated with Sirex woodwasp attacks.The influence of I. acuminatus on woodwasp attacks was not statistically significant (p > 0.05).

Within-Tree Associations at the Tree Scale
This study analyzed a total of 276 dead trees with substantial colonizer damage.Sirex woodwasps were negatively associated with most colonizing agents (p < 0.001), except for I. acuminatus, with which they maintained a neutral association (p > 0.05).Phaenops sp.exhibited a positive relationship with both A. striatum and I. acuminatus (p < 0.001), and it demonstrated neutrality between A. striatum and I. acuminatus (Table 2).The autologistic regression model revealed a significant increase in the frequency of Sirex woodwasp attacks with attack density of Sirex over two years, density of Phaenops sp.attacks from the previous year (y − 1), and A. striatum attack density from the current year (Table 3).The presence of Phaenops sp. in the current year and A. striatum in the previous year were negatively associated with Sirex woodwasp attacks.The influence of I. acuminatus on woodwasp attacks was not statistically significant (p > 0.05).

Body Size of Woodwasps
Given the ease of measurement and the strong correlation with body length (ranging from 0.87 to 0.97), wing length was ultimately employed as an indicator of body size.There was no significant difference in the body size of S. noctilio in S + X trees compared to S trees, both in the bottom (14.79 ± 1.84 mm vs. 12.29 ± 0.40 mm, F = 2.314, p = 0.0554) and middle (16.26 ± 0.64 mm vs. 13.86 ± 0.69 mm, F = 1.559, p = 0.3847) tree sections (Figure 5A).In contrast, S. nitobei was larger in X trees (11.62 ± 0.66 mm vs. 10.36 ± 0.26 mm, F = 1.567, p < 0.05) compared to S + X trees in the bottom tree section.No significant difference was observed in the body size of S. nitobei between S + X and X trees (10.18 ± 0.32 mm vs. 9.78 + 0.29 mm, F = 2.314, p = 0.4190) in the middle section (Figure 5B).

Body Size of Woodwasps
Given the ease of measurement and the strong correlation with body length (ranging from 0.87 to 0.97), wing length was ultimately employed as an indicator of body size.There was no significant difference in the body size of S. noctilio in S + X trees compared to S trees, both in the bottom (14.79 ± 1.84 mm vs. 12.29 ± 0.40 mm, F = 2.314, p = 0.0554) and middle (16.26 ± 0.64 mm vs. 13.86 ± 0.69 mm, F = 1.559, p = 0.3847) tree sections (Figure 5A).In contrast, S. nitobei was larger in X trees (11.62 ± 0.66 mm vs. 10.36 ± 0.26 mm, F = 1.567, p < 0.05) compared to S + X trees in the bottom tree section.No significant difference was observed in the body size of S. nitobei between S + X and X trees (10.18 ± 0.32 mm vs. 9.78 + 0.29 mm, F = 2.314, p = 0.4190) in the middle section (Figure 5B).

Within-Tree Distribution of Woodwasps in Different Infestation Types
The spatial distribution of both Sirex species within trees varied among S, S + X, and X trees (Figure 6).Sirex noctilio exhibited a more concentrated distribution in the bottom section in both S (95% CI = 0.38-0.46)and S + X (95% CI = 0.28-0.40)trees.In X trees, S. nitobei was more concentrated in the middle section (95% CI = 0.48-0.80),whereas it was more abundant in the bottom section of S + X trees (95% CI = 0.19-0.28).

Within-Tree Distribution of Woodwasps in Different Infestation Types
The spatial distribution of both Sirex species within trees varied among S, S + X, and X trees (Figure 6).Sirex noctilio exhibited a more concentrated distribution in the bottom section in both S (95% CI = 0.38-0.46)and S + X (95% CI = 0.28-0.40)trees.In X trees, S. nitobei was more concentrated in the middle section (95% CI = 0.48-0.80),whereas it was more abundant in the bottom section of S + X trees (95% CI = 0.19-0.28).

Within-Tree Distribution of Woodwasps in Different Infestation Types
The spatial distribution of both Sirex species within trees varied among S, S + X, and X trees (Figure 6).Sirex noctilio exhibited a more concentrated distribution in the bottom section in both S (95% CI = 0.38-0.46)and S + X (95% CI = 0.28-0.40)trees.In X trees, S. nitobei was more concentrated in the middle section (95% CI = 0.48-0.80),whereas it was more abundant in the bottom section of S + X trees (95% CI = 0.19-0.28).

Discussion
Research on the co-occurrence patterns of invasive and native species can help elucidate and clarify their potential interactions.The field survey revealed co-occurrence of S. nitobei, A. striatum, Phaenops sp., and I. acuminatus with S. noctilio.Across spatial scales, neutral (I.acuminatus) and negative (A. striatum and Phaenops sp.) associations were observed between Sirex woodwasps and other co-occurring insects.The emergence period of I. acuminatus in the Inner Mongolia region spans from May to July, relatively preceding the eclosion of two woodwasp species.Studies indicate that ophiostomatoid fungi exhibit potent repellent properties against unmated female woodwasps [41].As a result, later-emerging woodwasps typically avoid Ophiostoma clavatum (Mathiesen-Käärik) Hunt, a fungus associated with I. acuminatus, during oviposition site selection.This behavior mitigates the antagonistic impact of ophiostomatoid fungi on woodwasp symbionts, thus facilitating a conducive environment for the growth of these symbiotic fungi and the development of their offspring [42].This likely explains the neutral association between the beetle and the woodwasp.Asemum striatum has an extended flight period, and the early-emerging adults can utilize the weakened wood, following woodwasp attacks, to supplement their nutrition in the current year.Once winter sets in, the growth environment for the late-emerging individuals deteriorates, hindering their development.Trees severely damaged by A. striatum typically weaken and die within a year, which is not conducive to woodwasp egg-laying and development.Consequently, a positive correlation was observed between woodwasps and A. striatum in the current year, whereas a negative correlation was detected in the previous year.In North America, mixed associations were also observed between S. noctilio and woodborers [33].Sirex noctilio has a net positive effect on Pissodes sp.(arrived after S. noctilio) population densities at a landscape scale in North America [43] and a weakly positive association with Tomicus piniperda (Linnaeus) and Cerambycidae in Spain [44].Co-habiting beetles could have a negative impact on the population dynamics of S. noctilio over time [26,29,45].Mixed associations observed between them may be attributed to the differences in host physiological state preferences among different species.Studies have shown that S. nitobei and S. noctilio exhibit overlap in their preferences and behavior towards declining or recently dead trees [13,17,19,24,38,43].Consequently, these species notably aggregate to share resources.Interspecific competition between them is relatively weak [17], as the population density of woodwasps is low compared to outbreaks in the Southern Hemisphere which can reach 50-100 individuals per meter of log [46].This study suggests a minimal effect of S. nitobei on S. noctilio.The ability of S. noctilio to kill healthier trees at higher densities [22,47,48] likely creates additional habitat for S. nitobei in trees that would otherwise be too healthy and unavailable.We hypothesize that S. noctilio may have a net positive effect on S. nitobei, as its presence appears to enhance S. nitobei numbers at the stand level, necessitating additional investigation.Research indicates a positive correlation between the body size of Sirex nigricornis Fabricius and DBH of their host trees [49], a relationship not observed in S. noctilio and S. nitobei in this study.Consequently, the impact of DBH on woodwasp body size was not considered in the analysis.Ryan et al. (2012) suggested a negative correlation between body size and density [45].Smaller (but higher densities of) S. nitobei emerged from the bottom part of S + X trees compared to X trees.The body size of S. noctilio, which generally arrive first [17], seems unaffected by S. nitobei.The density of woodwasps within a forest stand may lead to differences in inter-landscape-scale interactions.Therefore, S. noctilio may have both positive (density) and negative (body size) impacts on S. nitobei in current study.Similarly, Pissodes sp.(arrived after S. noctilio) had no effects on the body size of S. noctilio [43].Conversely, a larger body size was observed in S. noctilio, especially in males, within beetle-infested trees in Canada [45].Ryan et al. (2012) suggested that the accelerated drying of sapwood, induced by ophiostomatoid fungi [50], promotes the growth of symbiotic fungi [51], enhancing larval nutrition in beetle-infested trees, which may explain the phenomenon [45].The emergence period appears to be a significant factor influencing the body size of woodwasps.
This study identified the degree of co-occurrence of S. noctilio and S. nitobei in northeastern China.Ignoring the influence of other colonizers (trees with only resin beads), the changes in the spatial distribution of S. nitobei within trees potentially indicate interactions between the two species of woodwasps.This suggests that the presence of S. noctilio influences the distribution of S. nitobei within the tree, indicating resource partitioning in the current year [52][53][54].In trees infested with both S. noctilio and S. nitobei (S + X), a greater proportion of S. nitobei was found in the lower sections compared to trees infested only with S. nitobei (X).Consequently, S. nitobei aggregation and dispersal patterns, as well as egg-laying preferences, seem to be influenced by S. noctilio, particularly in the lower sections of infested trees with higher insect densities and more emergence holes [44,55,56].Sirex noctilio predominantly colonizes in the bottom section (S + X or S) of the tree, irrespective of the presence of S. nitobei.This is consistent with the oviposition habits of S. noctilio observed in multiple countries, including in the Southern Hemisphere, such as South Africa, New Zealand, Australia, Brazil, and in the Northern Hemisphere, such as Canada [44,[55][56][57].In the autumn of the current year, native woodwasps do not appear to avoid trees already colonized by S. noctilio within the stands, according to our findings.Xeris species do not form a symbiotic relationship with fungi; instead, they deposit eggs in trees that have been previously attacked by other siricid species and infected with their fungi [58].In North America, Xeris species may feed on A. areolatum or A. chailletii.Xeris spectrum Linné, which lacks a symbiotic fungus, can utilize the fungal symbionts of S. nitobei [58,59].During summer, X. spectrum adults emerge with woodwasps carrying fungi, laying their eggs on trees already colonized by fungi.In the subsequent spring, they deposit eggs on trees previously infected with fungi [59][60][61].These findings suggest that horizontal transmission of interspecies symbiotic fungi between siricid woodwasps may be enhanced, regardless of whether the woodwasp carries the fungus [12].This can occur when fungi are acquired from the environment, potentially complicating pest management strategies.

Figure 2 .
Figure 2. Density for emergence holes of Sirex woodwasps in damaged Pinus sylvestris var.mongolica.a, b: p < 0.05.Ripley's K function analysis revealed that colonizing agents exhibited spatial aggregation within a moderate spatial scale, specifically within a radius of 0-8 m.Sirex woodwasps demonstrated clustering within a distance range of 0-7.3 m, with their distribution becoming random beyond 7.3 m.The highest aggregation degree of Sirex occurred at a radius of 4.1 m.Conversely, the distribution of A. striatum and I. acuminatus appeared random within the 0-8 m radius.Phaenops sp.exhibited the highest concentration within a 3.1 m radius (Figure 3).

Figure 2 .
Figure 2. Density for emergence holes of Sirex woodwasps in damaged Pinus sylvestris var.mongolica.a, b: p < 0.05.Ripley's K function analysis revealed that colonizing agents exhibited spatial aggregation within a moderate spatial scale, specifically within a radius of 0-8 m.Sirex woodwasps demonstrated clustering within a distance range of 0-7.3 m, with their distribution becoming random beyond 7.3 m.The highest aggregation degree of Sirex occurred at a radius of 4.1 m.Conversely, the distribution of A. striatum and I. acuminatus appeared random within the 0-8 m radius.Phaenops sp.exhibited the highest concentration within a 3.1 m radius (Figure 3).

Figure 3 .
Figure 3. Spatial distribution of woodwasps and other woodborers based on Ripley's K function.Red line: completely random spatial distribution.Gray area indicates a 95% confidence.Black line: degree of aggregation; Upper gray areas: clustered, Lower part: uniform distribution.

Figure 3 .
Figure 3. Spatial distribution of woodwasps and other woodborers based on Ripley's K function.Red line: completely random spatial distribution.Gray area indicates a 95% confidence.Black line: degree of aggregation; Upper gray areas: clustered, Lower part: uniform distribution.

Figure 4 .
Figure 4. Mean density of emergence holes for (A) Sirex noctilio and (B) S. nitobei in the bottom and middle sections of trees co-inhabited by both species, as well as in trees inhabited by only one species.Error bars represent the SE, *: p ≤ 0.05; ns: p > 0.05.

Figure 4 .
Figure 4. Mean density of emergence holes for (A) Sirex noctilio and (B) S. nitobei in the bottom and middle sections of trees co-inhabited by both species, as well as in trees inhabited by only one species.Error bars represent the SE, *: p ≤ 0.05; ns: p > 0.05.

Insects 2024 , 13 Figure 5 .
Figure 5. Mean body size of (A) Sirex noctilio and (B) S. nitobei in the bottom and middle sections of trees co-inhabited by both species and those inhabited by only one species.Error bars represent the SE, *: p ≤ 0.05; ns: p > 0.05.

Figure 5 .
Figure 5. Mean body size of (A) Sirex noctilio and (B) S. nitobei in the bottom and middle sections of trees co-inhabited by both species and those inhabited by only one species.Error bars represent the SE, *: p ≤ 0.05; ns: p > 0.05.

Figure 6 .
Figure 6.Distribution of Sirex noctilio and S. nitobei within Pinus sylvestris var.mongolica trees in Northeast China.The distribution patterns are illustrated as follows: S. noctilio in trees with both S and X (light rose red line), S. noctilio in trees with S only (red line), S. nitobei in trees with both S and X (grey line), S. nitobei in trees with X only (green line), slope = 0.5 (blue line), and slope = 1.0 (black line).S: Sirex noctilio; X: Sirex nitobei; S + X: Sirex noctilio and S. nitobei.
Research on the co-occurrence patterns of invasive and native species can help elucidate and clarify their potential interactions.The field survey revealed co-occurrence of S. nitobei, A. striatum, Phaenops sp., and I. acuminatus with S. noctilio.Across spatial scales, neutral (I.acuminatus) and negative (A. striatum and Phaenops sp.) associations were observed between Sirex woodwasps and other co-occurring insects.The emergence period of I. acuminatus in the Inner Mongolia region spans from May to July, relatively preceding the eclosion of two woodwasp species.Studies indicate that ophiostomatoid fungi exhibit potent repellent properties against unmated female woodwasps [41].As a result, later-

Figure 6 .
Figure 6.Distribution of Sirex noctilio and S. nitobei within Pinus sylvestris var.mongolica trees in Northeast China.The distribution patterns are illustrated as follows: S. noctilio in trees with both S and X (light rose red line), S. noctilio in trees with S only (red line), S. nitobei in trees with both S and X (grey line), S. nitobei in trees with X only (green line), slope = 0.5 (blue line), and slope = 1.0 (black line).S: Sirex noctilio; X: Sirex nitobei; S + X: Sirex noctilio and S. nitobei.

Table 1 .
Damage characteristics of Sirex woodwasps and other bark-and woodboring insects.

Table 1 .
Damage characteristics of Sirex woodwasps and other bark-and woodboring insects.

Table 3 .
Spatial lag model testing effects of colonizers from the current year (y) and the previous year (y − 1).

Table 3 .
Spatial lag model testing effects of colonizers from the current year (y) and the previous year (y − 1).*** p < 0.001.