Intensive forest afforestation with native pine species was developed in the 1960s on degraded and deforested lands in the region of the Eastern Rhodopes (south-eastern Bulgaria). Severe damage by wet snow was registered in the coniferous forests of the Rhodopes in March 2015. In the following years, bark beetle attacks were registered on the broken and felled fresh wood. As a result, bark beetle infestation spots appeared in the pine plantations. In the period 2019–2021, damage caused by bark beetles was assessed in the region of State Forestry Kirkovo (the Eastern Rhodopes, south-eastern Bulgaria). An integrated approach using the data of the information system of the Executive Forest Agency (ISEFA), remote sensing data obtained by an “eBee SQ” unmanned aerial vehicle (UAV) equipped with a “Parrot Sequoia” multispectral camera, and subsequent terrestrial observations, was applied. ISEFA data showed that there was no serious damage caused by abiotic and biotic factors in the pine forests of SF Kirkovo until 2014. Snow damage in 2015 affected 513 ha of pine plantations, and bark beetle infestations reached up to 1316 ha in 2016. In 2019, a total of 226.87 ha of pine plantations were captured in three localities—Fotinovo, Kirkovo, and Kremen. The relative share of damage caused by bark beetles was greater in P. sylvestris plantations (15.3–23.0%), compared to damage in P. nigra (2.3%). Four different categories of normalised difference vegetation index (NDVI) were separated in bark beetle infestation spots—living trees, dead trees, grass and shrub vegetation, stones and rocks. The NDVI values in locations with living trees varied between 0.500 (spaces between tree crowns) and 0.700 (central part of the crown projection) (an average of 0.617). In the locations with dead trees, the average values of NDVI of lying trees was 0.273, and in standing trees, NDVI varied between 0.275 (central part of crown projections) and 0.424 (spaces between tree crowns). In the locations with grass and shrub vegetation, stones and rocks, the average NDVI was 0.436 and 0.329, respectively. In the field study, average defoliation of 31.2–32.3% was registered in P. sylvestris plantations, and 47.4% in P. nigra plantations. Defoliations mainly were caused by pine processionary moth (Thaumetopoea pityocampa) and fungal pathogens (Dothistroma septosporum and Lecanosticta acicola). The damage was caused by Ips acuminatus (in P. sylvestris only), and I. sexdentatus, Tomicus piniperda and T. minor (in P. sylvestris and P. nigra). Infestations by other xylophages, such as Phaenops cyanea, Rhagium inquisitor, and Pissodes spp., were also found on pine stems.

The Chuprene Reserve was created in 1973 to preserve the natural coniferous forests in the Western Balkan Range in Bulgaria. The first infestations by European spruce bark beetle (Ips typographus) were registered in Norway spruce (Picea abies) stands in the mid-1980s. The aim of this study is to assess the damages caused by I. typographus in the Chuprene Reserve using remote sensing techniques – unmanned aerial vehicle (UAV) images, airborne images, and satellite images of European Space Imaging (EUSI), combined with terrestrial verification. High-resolution images in four bands of the electromagnetic spectrum and in a standard RGB channel were taken in 2017 via a multispectral camera ‘Parrot Sequoia’, integrated with a specialized professional UAV system eBee ‘Flying Wing’. The health status of Norway spruce stands in the reserve was assessed with the normalized difference vegetation index, based on the digital mixing of imagery captured in the red and near infrared range. The dynamic of bark beetle attacks was studied in GIS on the basis of maps generated from photographic surveys, airborne images taken in 2011 and 2015, and satellite images from 2020. In the UAV-captured area (314.0 ha), the size of Norway spruce stands attacked by I. typographus increased from 7.6 ha (2.4%) in 2011 to 44.9 ha (14.3%) in 2020. The satellite images showed that on the entire territory of the Chuprene Reserve (1451.9 ha), I. typographus killed spruce trees on 137.4 ha, which is 9.6% of the total area.

Strong winds uprooted more than 100 thousand m³ of coniferous trees in natural forest stands nearby the town of Smolyan (the Western Rhodopes) in January 2018. Although damaged trees were quickly removed from the stands, the European spruce bark beetle (Ips typographus) attacked the healthy Norway spruce trees near the windthrow areas in August 2020. Our hypothesis was that the trees were infested by the pest in previous years when no symptoms of attacks were observed. This study was conducted in three spruce stands, located near the windthrow areas and attacked by I. typographus, and in three control (healthy) stands located 5-10 km from the affected areas. We used satellite images captured by Sentinel-2 in September 2017-2020. It was established that in September 2017 (a year before the windthrow), the mean values of the Normalized difference vegetation index (NDVI) in the attacked stands were concentrated between 0.60 and 0.75 (with a maximum at 0.70), indicating that the trees were in good health. During the period 2018-2020 the distribution of mean values of NDVI was stretched between 0.35 and 0.75, which is an indication of evidence of pest attacks on the individual trees. The detail comparison of pixel values of the NDVI in the attacked and control sample plots was made on the base of images captured on 27.06.2020. The mean NDVI values in the three control plots (0.74-0.79) were much higher than the mean values in the sample plots attacked by the pest (0.57-0.65). These results showed that the values of NDVI based on satellite remote sensing data of Sentinel-2 can be used for early detection of I. typographus infestations in spruce stands around the windthrows. These data are important for rapid planning and implementing the sanitary feelings that reduce the pest population.

A system of field-protective forest belts was created in the 1950-60s in northeastern Bulgaria to maintain soil moisture, reduce wind speed and protect soil against wind erosion, improve the microclimate, and increase the fertility of agricultural lands. The deciduous tree species such as Fraxinus excelsior, F. angustifolia, F. americana, Gleditsia triacanthos, Quercus robur, Q. cerris, Q. rubra, Robinia pseudoacacia, Ulmus minor etc., were in common use for afforestation. Since 2020, the processes of deterioration, premature leaf fall and drying of ash trees have been observed. The deterioration affected Fraxinus excelsior and F. angustifolia on more than 200 ha, mainly on the territory of the State Hunting Enterprise Balchik, State Forest Enterprises Dobrich and General Toshevo. The results from the health status assessment carried-out in 2023 showed severely damaged ash forest belts with defoliation reaching up to 80-100% of tree crowns. Browning of leaves and petioles, and wilting of young shoots, were noticeable. Dam-age on shoots and stems of ash trees was caused by the insect pests Cicada orni, Cossus cossus, Zeuzera pyrina, Hylesinus fraxini, H. crenatus, and on leaves – by leaf-damaging and gall-forming fly Dasineura acrophila and D. fraxini. The complex of fungal species Armillaria mellea, Coriolopsis gallica, Cytospora pulchelia, Daldinia concentrica, Diplodia sp., Helicobasidium purpureum, Hysterographium fraxini, were identified on ash trees. Most of them are considered opportunistic pathogens that cause disease on trees under stress conditions. Deterioration of the health status of other major tree species was also observed. The most dangerous insect pest in oak forests – Lymantria dispar caused the defoliation of Quercus cerris belts. The endophytic fungal pathogen Biscogniauxia mediterranea was identified as a causal agent of charcoal disease on Q. rubra trees. Dutch elm disease caused by the fungus Ophiostoma novo-ulmi resulted in the wilting and death of 80-100% of Ulmus minor trees.

In 2022, the health status of the field protective forest belts on the territory of the State Hunting Enterprise Balchik and State Forest Enterprises General Toshevo and Dobrich was monitored. The assessment of tree crown condition covered 7069.3 ha (66% of all field protective belts in Bulgaria). The results showed that 70% of monitored belts were in good condition, 23% - in moderate condition, and 7% - in poor condition. Since 2014 the area of the protective forest belts in poor condition has increased 2.6 times, and 76% of them were situated in Dobrudzha region. Ash belts – Fraxinus excelsior (66% of all belts) and Fraxinus americana (10%) were in the worst conditon. In ash belts a process of crown dieback, premature falling of leaves and drying of whole trees were observed. In individual ash belts, drying reached up to 80% of tree crowns, regardless of their age and origin. The drying was mostly due to the biotic factors – insect pests and fungal pathogens. The protective forest belts of Quercus cerris, Q. petraea, Q., rubra and Gleditschia triacanthos stand out as sustainable tree species. It is necessary to significantly upgrade the methodology by which the monitoring of the health status of trees in the field protective forest belts is currently carried out. In the new methodology a detailed description of the type of data that need to be collected, the methods of assessments, and data reporting formats, have to be included.

In 2021, biological control programme against gipsy moth (Lymantria dispar) populations was carried out by introduction of the entomopathogenic fungus Entomophaga maimaiga on the territory of four State Forest Enterprises: Municipal Enterprise (ME) ‘Management of Municipal Forests, Agriculture and Forestry’, Nessebar; State Game Enterprises (SGE) Nessebar and Balchik; State Forestry (SF) Vidin. The pathogen was introduced during the period 15-26.03.2021 in 34 localities - five in ME Nessebar, eight in SGE Nessebar, ten in SGE Balchik and eleven in SF Vidin. The average number of gypsy moth population density in the locations of introduction was relatively high, ranging between 0.4-15.9 egg mass/tree in the area of ​​SGE Balchik and 11.9-65.0 egg mass/tree in the area of ​​ME Nessebar. The average mortality of young gypsy moth caterpillars (first-third instar) due to E. maimaiga varied between 2.6% (SGE Balchik) and 13.0% (SF Vidin), and of caterpillars in later fourth-sixth instar - between 20.7% (SF Vidin) and 52.4% (ME Nessebar). The overall mortality of the gipsy moth caterpillars due to E. maimaiga was lowest in the region of SGE Balchik (26.1%), followed by SF Vidin (33.7%), SGE Nessebar (48.5%) and ME Nessebar (55.9%). As a result of the introduction, gipsy moth severe outbreaks in the region of Nessebar was significantly suppressed. The high number of E. maimaiga resting spores persists in the surface layers of the soil in the other two areas (Vidin and Balchik) has the potential to suppress L. dispar attacks in next years.