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2 August 2026

15 Pages

Rapid Population Response of the Mediterranean Pine Engraver (Orthotomicus erosus Wollaston) Following a Severe Windthrow Event in Aleppo Pine Forest

,
and
Division of Forest Protection and Game Management, Croatian Forest Research Institute, Cvjetno Naselje 41, 10450 Jastrebarsko, Croatia
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Author to whom correspondence should be addressed.
Forests2026, 17(8), 910;https://doi.org/10.3390/f17080910
This article belongs to the Special Issue Impacts of Climate Change and Disturbances on Forest Ecosystems
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Abstract

Severe windthrow events can create highly favorable conditions for bark beetle outbreaks by generating large quantities of weakened breeding material. This study investigated the population response of the Mediterranean pine engraver, Orthotomicus erosus Wollaston, following a microburst windstorm that struck Marjan Forest Park, an urban Mediterranean Aleppo pine (Pinus halepensis Mill.) forest in Split, Croatia, in July 2025. Approximately 2000 m3 of damaged timber remained in the forest until February 2026, enabling assessment of colonization dynamics and outbreak development. Field inspections, laboratory analyses, and pheromone trap monitoring were conducted between July 2025 and April 2026. Insect sampling was conducted in the field through bark inspections and pheromone trap monitoring. For log inspections, bark sections were carefully removed to expose galleries and immature stages beneath the bark. Observations focused on the presence of entrance holes, boring dust, maternal galleries, larval galleries, pupal chambers, exit holes, adults, and immature developmental stages. In addition, ten pheromone traps were installed shortly after the windthrow event. Trap catches were collected regularly, and the captured insects were examined in the laboratory, where O. erosus individuals were identified and counted under a stereomicroscope. Colonization of windthrown material began within two weeks after the disturbance, and infestation rates reached 94% of examined logs by December 2025. At that point, 86% of infested logs already contained exit holes, indicating successful completion of beetle development. Pheromone traps captured a total of 131,588 adults, with more than 88% recorded during spring 2026 and a pronounced maximum recorded catch occurring on 8 April 2026. The appearance of newly attacked standing trees in spring (April) 2026 confirmed rapid population expansion from fallen material to living hosts. Results demonstrate the exceptional outbreak potential of O. erosus following extreme climatic disturbances and emphasize the importance of rapid sanitation measures and continuous monitoring in Mediterranean pine forests under climate change conditions. Delayed salvage logging after severe windthrow can rapidly trigger an outbreak of O. erosus.

1. Introduction

For the past few years, outbreaks of the Mediterranean pine engraver, Orthotomicus erosus Woll. (Coleoptera, Curculionidae, Scolytinae) on Aleppo pine (Pinus halepensis Mill.) have become a regular threat in Croatia [1]. Climate change and extreme drought along with the secondary attacks of bark beetles, eventually led to an increase in bark beetle populations, which subsequently began attacking healthy trees. [1]. Infestations by O. erosus have been reported in warmer mediterranean regions such as in Israel, Iran, Morocco, Turkey and Tunisia [2,3,4,5,6,7], while no outbreaks occurred in Croatia since 2018 [1]. Such increase in bark beetle population in conifers is well known and documented in European forestry, most commonly for the spruce bark beetle (Ips typographus L.) [8,9,10,11,12,13]. Previously, only local outbreaks of other bark beetle species, Tomicus piniperda L. or T. destruens Woll. were reported in the region [14,15].
O. erosus is naturally distributed in Central Asia, the Middle East, Europe, and China. Although it is widespread throughout Europe, it has so far only caused damage in very warm Mediterranean areas. In France, Morocco, and Turkey, two generations have been identified, three to four in South Africa and Tunisia, and three to five generations in Israel, where adults are active from March to October. Although previous research identified three [16] or four generations [2], more recent studies have shown that there could be even 5 generations in the outbreak site [1,17]. The reason could be climate changes, primarily aridification, and a significant extension of the vegetation period in this region. O. erosus is also associated with xylophagous fungi which probably play a role in the colonization of the host trees [18].
O. erosus is a reddish-brown beetle, 2.7 to 3.5 mm long. Its tunnels are located in the living part of the bark. The larvae are legless, white, and about 2.7 to 3.5 mm long, and their appearance does not change as they grow. The eggs are white, partly transparent, and about 1 mm long. The behavior of pest insects may change as a result of climate change, which can create conditions favorable for reduced mortality, increased reproduction, higher voltinism, and the expansion of their spatial distribution. [1,19]. Under our environmental conditions, O. erosus may produce 2–4 generations per year, including several sister broods [1]. Due to the extension of the growing season, several generations more than usual per year allowed an exponential growth of the population, the development of an extreme outbreak which in the end destroyed 50% of the trees in the Park-forest Marjan after 4 year of heavy attack (Pernek unpublished).
Pheromone traps in general are used in various designs for insect attraction, to predict the dates of swarming and tree infestation, and, for monitoring, while mass-trapping is rather an exception [20,21,22]. In order to find optimal pheromone traps and lures for O. erosus as a target species, have shown that Erosowit® exhibits significantly higher catches compared to Pheroprax® and is significantly more selective [23,24].
Marjan Forest Park, located in the city of Split on the eastern Adriatic coast, represents one of the most important urban Mediterranean forests in Croatia. Aleppo pine dominates the forest vegetation and has experienced repeated disturbances caused by drought, storms, pathogens, and bark beetles during recent decades. Previous outbreaks of O. erosus resulted in significant tree mortality and required extensive management interventions.
On 8 July 2025, Marjan Forest Park was struck by an exceptionally severe windstorm classified as a microburst event. Strong downburst winds caused widespread breakage and uprooting of 5000 trees and creating 2000 m3 of damaged wood material. Because the volume of affected timber exceeded immediate removal capacities, concerns arose regarding the potential development of bark beetle outbreaks.
The objectives of this study were therefore to: (1) establish a monitoring network for O. erosus populations following the windthrow event; (2) assess colonization dynamics within damaged timber; (3) quantify seasonal population trends using pheromone traps; and (4) evaluate the risk of outbreak development and subsequent attacks on standing Aleppo pine trees.
We hypothesized that the large volume of windthrown Aleppo pine material would enable rapid colonization by O. erosus, resulting in substantial population growth and increased attack pressure on standing trees during the subsequent growing season.

2. Materials and Methods

The woody material damaged by the windstorm (Figure 1) remained in the forest until February 2026. This provided an opportunity to investigate the condition of the forest following the windstorm and the subsequent colonization by O. erosus. In addition, Marjan is located on a peninsula next to the city of Split, so there is no influence of bark beetles coming from surrounding areas.
Figure 1. Broken Aleppo pine trees after windstorm on 8 July 2025.

2.1. Study Area

The study was conducted in Marjan Forest Park (43°31′ N, 16°25′ E), located on the Marjan Peninsula within the city of Split, Croatia (Figure 2). The area is characterized by a Mediterranean climate with hot dry summers and mild winters. Aleppo pine constitutes the dominant tree species and forms extensive even-aged stands across the peninsula. The visualization of the spatial extent of damage using Sentinel-2 satellite has shown high damage after the windthrow event (Figure 2).
Figure 2. Position of Marjan Forest Park, city of Split, Croatia (top panel); Forest condition in Marjan Forest Park prior to the windstorm from the Sentinel-2 images with high impact zone marked in red. The black squares indicate the positions of the pheromone traps.

2.2. Field Assessment

Field inspections were conducted repeatedly between July and December 2025, and between March and May 2026, to evaluate bark beetle colonization and developmental stages. Bark sections were carefully removed using chisels and knives to expose galleries and immature stages beneath the bark. Observations focused on the presence of: entrance holes, boring dust, maternal galleries, larval galleries, pupal chambers, exit holes, adults and immature developmental stages.
Particular attention was paid to determining the timing of colonization and the progression of infestation within windthrown material.
Wind-damaged trees were inspected immediately after the windstorm, confirming that all trees had green crowns and that none exhibited symptoms of bark beetle infestation. In December, a survey was conducted starting from the edge of the windthrow area along a random west–east transect. Fifty trees encountered along this transect were examined in detail to assess the extent of colonization by O. erosus, with the aim of determining how rapidly the beetle colonized the highly suitable breeding material created by the windstorm. Logs were classified into three categories: (i) Infested logs with exit holes; (ii) Infested logs containing galleries but few or no exit holes; (iii) Uninfested logs without evidence of bark beetle activity.

2.3. Preliminary Forest Damage Detection Using Sentinel-2 Imagery

The detection and visualization of the spatial extent of damage were carried out using Sentinel-2 satellite imagery. The analysis was based on a change detection methodology, focusing on variations in canopy cover resulting from wind disturbance [25]. To compare differences, the NDVI (Normalized Difference Vegetation Index) was used, which is one of the most widely applied indicators in remote sensing for assessing vegetation health and greenness. NDVI is calculated as the ratio of the differences between the near-infrared (B8) and red (B4) spectral bands. NDVI values were computed from cloud-free average Sentinel-2 pixel data for two periods: 15 June–7 July 2025 (pre-disturbance), and 8 July–8 August 2025 (post-windthrow) (Figure 2). The same approach was also applied to assess the O. erosus outbreak in the following year, by comparing NDVI differences between the two dates (7 March–26 May 2026). Areas classified as forest damage were defined using a threshold NDVI difference of −0.2. The applied method represents a rapid, ad hoc approach suitable for the preliminary detection and visualization of forest disturbances, particularly larger-scale events such as windthrows and pest outbreaks.

2.4. Installing Pheromone Traps

Ten pheromone traps were installed on 9 September 2025 for monitoring O. erosus. Traps were mounted on specially designed metal frames with collection containers positioned approximately 1 m above ground level [24]. Trap locations were selected according to operational forest protection guidelines: minimum distance of 20 m from healthy standing pines; minimum spacing of 50 m between neighboring traps. The traps were positioned along the windthrow corridor, following the spatial extent of the windstorm damage shown in Figure 2.
Modified trap designs previously developed for conservation of predatory entomofauna were employed. Traps were baited with commercially available Erosowit® pheromone dispensers (Witasek, Feldkirchen in Kärnten, Austria), previously demonstrated to provide effective attraction of O. erosus populations. Trap contents were collected weekly. Captured insects were preserved in 70% ethanol and transported to the Croatian Forest Research Institute for laboratory processing.

2.5. Laboratory Analyses of Insect Samples

Samples were dried, sorted, identified, and counted under stereomicroscopes in the laboratory of the Croatian Forest Research Institute. Since other insect and bark beetle species were also present in the catches, although in small numbers, the identification keys by [22] were used to avoid misidentification of the specimens.

2.6. Pheromone Trap Catches and Meteorological Data

Bark beetle catches were summarized for each sampling date using the median number of O. erosus adults per trap, while variability among traps was expressed as the interquartile range (IQR; 25th–75th percentile). The median was selected because beetle catches showed a highly skewed distribution with pronounced differences among traps and occasional extreme values, making it a more robust measure of central tendency than the arithmetic mean.
Daily trap catches were graphically summarized using the median number of O. erosus adults per trap, with variability expressed as the interquartile range (IQR, 25th–75th percentile). Median and IQR were selected for graphical presentation because trap catches were highly right-skewed and contained several extreme observations among traps. Descriptive statistics include both arithmetic means and medians to facilitate comparison of central tendency and variability. Differences in trap catches between autumn 2025 and spring 2026 were evaluated using the Mann–Whitney U test because trap catches were strongly right-skewed and did not meet the assumptions of parametric tests. Statistical significance was assessed at α = 0.05.

3. Results

3.1. Colonization Dynamics

The first inspection conducted on 16 July 2025 revealed that colonization had already begun within two weeks after the windthrow event. Although most examined logs remained unattacked, several exhibited fresh boring dust and newly established maternal galleries containing eggs.
Subsequent inspections demonstrated rapid population development. By early September, larval galleries were fully developed, while pupae, newly emerged adults, and exit holes were frequently observed. These findings confirmed successful completion of a new generation within windthrown material.
By late October, nearly all suitable breeding material had been colonized. Exit holes were abundant, indicating extensive adult dispersal. Simultaneously, the first symptoms of attacks on standing trees appeared, including crown discoloration and progressive needle reddening.
During the final inspection in December, virtually all suitable material exhibited exit holes. Pupae, immature adults, and mature adults ready for overwintering were present beneath the bark, suggesting that infested material represented a substantial source of spring-emerging beetles.
Finally, in December inspection of fifty logs on a random east–west transect revealed extremely high infestation rates. The examined trees represent the average characteristics of windthrown trees after the windstorm and include different diameter classes of pine. The mean DBH was 34.92 ± 10.32 cm (mean ± SD), corresponding to an interval of 24.60–45.24 cm, with a minimum of 20.00 cm and a maximum of 59.00 cm. Forty-seven logs (94%) contained evidence of bark beetle colonization. Among these, forty-three logs (86%) already exhibited exit holes, indicating successful completion of development and dispersal of adult beetles. Only three logs (6%) remained uninfested (Table 1). Beneath the bark of the infested logs, no fresh galleries or larvae were found. Instead, only pupae and light-brown, immature adults were present, which had most likely overwintered there before completing their maturation feeding.
Table 1. Infestation status of inspected trees by Orthotomicus erosus by 12 December 2025 (DBH = 34.92 ± 10.32 cm (range: 20.00–59.00 cm)).
In the spring of 2026, by the end of April, groups of infested trees with sawdust on the bark and moderately to severely damaged crowns appeared in the field (Figure 3), indicating exceptionally high bark beetle activity following a period of latency since 2022 [26]. These trees were completely green and intact after the windstorm, confirming that they represented new infestations.
Figure 3. Occurrence of dead trees within a group infested by Orthotomicus erosus in spring 2026.

3.2. Spatial Distribution of Forest Damage

The ΔNDVI map, derived from pre- and post-windthrow satellite imagery, illustrates the spatial distribution of vegetation changes within Marjan Forest Park following both disturbance events, windthrow in 2025 (Figure 4). Red areas indicate a decrease in NDVI relative to pre-disturbance conditions, reflecting canopy damage, vegetation loss, and reduced biomass. The largest declines were concentrated in the northwestern part of the forest park and along sections of the northern coast.
Figure 4. Spatial distribution of the forest damages caused by windstorm in 2025 detected by Sentinel-2 NDVI change detection in Marjan Forest Park (the yellow rectangle represents the broader windthrow area).

3.3. Pheromone Trap Captures

The total number of captured individuals of O. erosus between 16 September 2025 and 8 April 2026 amounted to 131,588, of which 15,713 (11.9%) were captured in 2025 and 115,875 (88.1%) in 2026 (Table 2). The highest catch was recorded on 8 April 2026 (Figure 5). In spring, the catches in the traps increase sharply, and only two catches exceeded the catch recorded in 2025 (Table 2, Figure 6).
Table 2. Catch of Orthotomicus erosus individuals in pheromone traps by sampling date.
Figure 5. Heat map of the number of Orthotomicus erosus individuals captured per trap on each sampling date during the study period (October 2025–April 2026). Colour intensity represents the number of captured individuals, with lighter colours indicating higher catches.
Figure 6. Seasonal variation in Orthotomicus erosus trap catches and air temperature. Black circles indicate the median number of adults per trap (IQR shown as error bars). The shaded area represents the daily temperature range (Tmin–Tmax), and the orange line represents the 7-day moving average of mean daily air temperature. The left y-axis is shown on a logarithmic scale.
Descriptive statistics revealed pronounced spatial heterogeneity of O. erosus populations within the study area (Table 3). Total catches ranged from only 344 individuals in trap IV to 27,920 individuals in trap II. The five most productive traps (II, V, VI, VIII, and IX) accounted for approximately 86% of all captured beetles, indicating strong spatial aggregation of the population.
Table 3. Trap performance and spatial distribution of Orthotomicus erosus during the study period (October 2025–April 2026), expressed as the total number of individuals per trap.
Mean catches were substantially higher than median values in most traps, particularly in traps II, V, VI, VIII, and IX. For example, trap II recorded a mean catch of 1994.3 individuals per sampling date, whereas the median was only 69 individuals. This discrepancy indicates that seasonal totals were largely driven by a few exceptionally large flight events.
Temporal variability was extremely high. Coefficients of variation exceeded 300% in traps II (354.5%), V (333.7%), VI (315.9%), and IX (314.3%), confirming highly aggregated and eruptive population dynamics. Such variability is characteristic of bark beetle populations developing in recently disturbed forest stands.
Trap catches revealed a marked population increase from autumn 2025 to spring 2026, indicating rapid population build-up during the study period rather than differences in seasonal flight activity. The Mann–Whitney U test detected significant differences between the two monitoring periods (U = 0.0, p = 0.036). This result confirms that the major population expansion occurred during spring emergence following successful development within the windthrown material.
The statistical results therefore support field observations indicating that the windthrow event created highly favorable breeding conditions, enabling rapid population growth and resulting in a synchronized mass flight during spring 2026 (Figure 6).

4. Discussion

Orthotomicus erosus is known to attack Pinus species following drought and fire [27,28], and cases of attacks resulting in the mortality of apparently healthy trees have also been reported [29]. Climate change is altering many of the key factors regulating bark beetle population dynamics, including thermal conditions, host water stress, and tree resistance. Together, these factors create favorable conditions for the gradual development of bark beetle outbreaks [30].
In the present study, we demonstrate the extraordinary capacity of O. erosus populations to exploit large-scale windthrow disturbances in Aleppo pine forests. Colonization of the wind-felled material commenced within only two weeks after the disturbance, highlighting the remarkable efficiency with which beetles locate suitable breeding substrates. The large volume of freshly windthrown pine material provided abundant breeding habitat, enabling the population to increase explosively within a single season. This disturbance should also be viewed in the context of climate change, as severe windstorms of this type have become increasingly frequent and intense in recent years, thereby substantially increasing the risk of bark beetle outbreaks.
To monitor the population response to the large volume of breeding material created by the windthrow, a pheromone-trap monitoring system was established, providing valuable insight into population status and dynamics. Pheromone traps have been widely used in numerous previous studies [31,32], which, similarly to our findings, reported considerable heterogeneity in trap catches associated with the microsite conditions of individual trap locations. In our study, the monitoring system enabled the rapid population response of O. erosus following the severe windstorm in July 2025 to be documented. Trap catches increased rapidly after the disturbance, reaching a peak on 16 October and remaining high during the second half of October before gradually declining throughout November and December. A temporary reduction in trap catches was recorded in early October, coinciding with a period of strong bora winds, lower temperatures, and rainfall. Following the return of favorable weather conditions, trap catches increased sharply again, indicating that the population continued to develop despite the short-term reduction in flight activity caused by adverse weather conditions.
Because the wind-felled timber remained in the forest and was not removed, the beetles successfully overwintered within the breeding material. Following emergence from hibernation, the population increased dramatically, creating a substantial threat to the remaining forest through increased attack pressure on standing, healthy trees.
As demonstrated by our results, the seasonal pattern reflects a strong functional response of the population to the large quantity of freshly available breeding substrate generated by the windthrow event.
Temporal dynamics also revealed a strong seasonal concentration of catches. The overwhelming majority of individuals were recorded during the spring sampling period, with the dominant sampling date of maximum catch occurring on 8 April 2026. This pattern indicates that the main dispersal and colonization activity of O. erosus was concentrated within a very narrow time window, emphasizing the importance of timely sanitation measures and monitoring activities before the onset of the spring flight peak. Overall, the results confirm that the windthrow event created highly favorable conditions for the development of O. erosus, while the delayed removal of damaged woody material likely enabled successful reproduction and a strong population increase during 2026.
At the same time, the pronounced spatial aggregation of catches suggests that infestation intensity was not uniform throughout the study area, but instead concentrated around specific micrlocalities with suitable breeding conditions. Windthrow events eliminate host defensive mechanisms, particularly resin-based defenses, thereby creating highly favorable conditions for bark beetle reproduction. The extensive volume of damaged timber generated by the July 2025 storm provided abundant breeding resources and substantially reduced density-dependent limitations on population growth. This is well known in the spruce bark beetle [10,33]. The observed infestation rate of 94% indicates near-complete utilization of available breeding material. Such levels of colonization are characteristic of rapidly expanding populations and have frequently preceded outbreak development in other bark beetle systems.
The seasonal dynamics observed in pheromone trap captures further support this interpretation. Contrary to normal seasonal declines typically observed toward late autumn, captures remained exceptionally high throughout October and early November. Such trends suggest that population growth continued despite progressively cooler conditions. Climate change likely represents an important underlying factor influencing outbreak risk. Rising temperatures have previously been associated with increased voltinism in O. erosus, allowing more generations to develop annually. Simultaneously, increased frequencies of droughts and extreme storms create favorable conditions both for host weakening and for production of large quantities of breeding material. Particularly concerning is the transition from colonization of windthrown material to attacks on standing trees. Evidence of crown discoloration observed during October indicates that populations had already begun exploiting weakened living hosts. If infested material remains within the forest until spring emergence, a substantial increase in attack pressure on standing Aleppo pine trees can be expected. From a management perspective, sanitation measures remain the most effective strategy for reducing population growth. Removal or destruction of infested material before spring emergence would substantially decrease the number of dispersing adults and reduce the probability of mass attacks on healthy trees. Bark beetles are integral components of forest ecosystems and contribute to nutrient cycling and decomposition processes under endemic population conditions. However, when favorable breeding conditions coincide with suitable climatic conditions, populations may rapidly increase and reach outbreak levels capable of killing physiologically weakened or even healthy trees. Such transitions from endemic to epidemic population phases have been documented worldwide and are increasingly associated with climate change and extreme weather disturbances. As well as the other pine bark beetles, O. erosus is also known to vector a diverse range of associated microorganisms, including bacteria, yeasts, and filamentous fungi, among which ophiostomatoid fungi represent some of the most ecologically significant symbionts due to their frequent occurrence and roles in host colonization [34,35]. In Kovač et al. [18], six ophiostomatoid fungi were identified in association with O. erosus in Croatia: Ophiostoma ips, O. piceae, O. floccosum, Graphilbum cf. rectangulosporium, Sporothrix pseudoabietina, and Ceratocystiopsis cf. minuta. Same as in above mentioned studies, O. ips was reported as the most frequently isolated species, indicating it may be the dominant fungal associate of this beetle. The study also notes that, although these fungi are clearly linked to blue-staining of sapwood and are likely involved in weakening host defenses, their exact ecological roles and degree of pathogenicity in the O. erosus–pine interaction still remain insufficiently understood. Given that rapid host colonization may depend not only on beetle activity but also on the fungi’s ability to quickly invade host tissues and impair tree defenses, further research is needed to clarify the relative contribution of individual fungal species to this process.
The biological potential of O. erosus is strongly influenced by temperature [6]. Depending on climatic conditions, the species may develop between two and seven generations annually [36]. Previous studies in Croatia documented three generations per year during the mid-twentieth century, whereas more recent investigations indicate the occurrence of five or even six generations under contemporary climatic conditions [1,22]. This increase in voltinism is widely attributed to rising temperatures and prolonged vegetation periods. The findings indicate that immediate sanitation measures are required to prevent outbreak development during the 2026 growing season. Priority actions should include removal of all infested windthrown material before spring emergence and early detection and removal of newly attacked standing trees. Given projected increases in storm frequency and drought stress under climate change, bark beetle monitoring should become a permanent component of management strategies in Mediterranean pine forests.
One limitation of this study is the shorter monitoring period during the spring compared with the autumn sampling period. However, the objective of this study was not to describe the complete seasonal trend in trap catches, but rather to demonstrate the population response following delayed sanitation after the windthrow event. Even the initial spring catches clearly indicate the magnitude of the population increase when damaged material is not removed in a timely manner. Trap catches at the same locality during the previous latency phase were substantially lower than those recorded following the windthrow event [22], further supporting the conclusion that delayed sanitation resulted in a rapid population increase.
In this study, satellite information, primarily Sentinel-2 multispectral imagery, was efficiently used for geospatially quantifying and visualizing windstorm damage. The spatial layer produced using the simple change detection principle based on Sentinel-2 imagery, before and after the events, not only provided an overview of the damage extent but also confirmed the spatial coincidence between the two events. Specifically, the onset and progression of the O. erosus outburst initially occurred in areas where trees had been damaged by the windstorm, and subsequently expanded into the surrounding forest area. In the case of windthrows, by analysing imagery acquired immediately before and after the events, the focus is on mapping structural changes in the canopy, i.e., the loss of biomass that leads to a decline in the NDVI signal. In this case vegetation indices that highlight damaged-areas differences in the visible and the SWIR (Short Wave Infrared Regions) are strongly recommended for the detection of windthrows [37]. The combination of various indices also provides sufficient sensitivity for the precise quantification of the percentage of structural canopy damage at the Sentinel-2 sub-pixel level [37].

5. Conclusions

This study demonstrates the rapid population response of Orthotomicus erosus following a severe windthrow event in a Mediterranean Aleppo pine forest. The main conclusions are as follows:
  • The severe windthrow that occurred in Marjan Forest Park in July 2025 created highly favorable conditions for rapid population growth of O. erosus.
  • Colonization of wind-felled material began within two weeks after the disturbance, and infestation reached 94% of the examined logs by December. Pheromone trap monitoring confirmed a strong population response to the large amount of newly available breeding material.
  • The occurrence of newly attacked standing trees, together with the extensive volume of infested windthrown timber, indicates a high risk of outbreak development during the following spring if sanitation measures are delayed.
  • Delayed salvage logging after severe windthrow can rapidly trigger population outbreaks of O. erosus. Continuous pheromone-based monitoring and prompt removal of infested material are therefore essential for reducing attack pressure, preventing further forest decline, and maintaining the ecological stability of Mediterranean urban pine forests under increasingly frequent climate-related disturbances.

Author Contributions

Conceptualization, M.P.; methodology, M.P., I.P. and M.K.; field investigation, M.P.; data analysis, M.P. and I.P.; writing—original draft preparation, M.P., M.K. and I.P.; writing—review and editing, M.P. and M.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financed by Park Forest Marjan under the project: “Establishment of a Monitoring System for the Mediterranean Pine Engraver Beetle (Orthotomicus erosus)”.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The author thanks the Forest Park Marjan for logistical support and access to study sites. Special thanks to Blaženka Ercegovac for the field and laboratory work. We gratefully acknowledge Andrija Jukić, Tomislav Krcivoj and Filip Cvjetković for their contribution to the deployment of monitoring traps in the field. Many thanks to Ante Kodžoman and Frane Pletikošić from Forest park Marjan.

Conflicts of Interest

The author declares no conflicts of interest.

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