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mm ± 0.12 (0.22 – 3.26 mm) and 1.04 mm ± 0.10 (0.25 – 2.61 mm) in uninfested trees, respectively (Figure 1a, b, c). Values were significantly different between infested and uninfested trees (t test, t = 2.013, df = 58, p < 0.05 for MAI-5 and t = 2.484, df = 58, p < 0.05 for MAI-10). In the infested and uninfested trees PGR was 1.01 mm ± 0.06 (0.29 – 2.27 mm) and 1.08 mm ± 0.07 (0.46 – 2.53 mm), respectively. The difference was not significant (t test, t = 0.752, df = 58, p > 0.05). Mean annual increment in the last year until the sampling date was 1.50 mm ± 0.13 (0.24 – 3.40 mm) and 1.01 mm ± 0.10 (0.16 – 2.42 mm) in infested and uninfested trees, respectively. The difference in mean annual increment in the last year until the sampling date for infested and uninfested trees was significant (t test, t = 2.934, df = 58, p < 0.05). DBH was 33.20 cm ± 0.57 (26 – 39 cm) and 33.77 cm ± 0.62 (28 – 39 cm) in infested and uninfested trees, respectively. The difference was not significant (t test, t = 0.672, df = 58, p > 0.05). In the infested and uninfested trees number of xylem cells was 46.27 ± 4.39 (13 – 112) and 36.67 ± 3.89 (9 – 88), respectively. The difference between the infested and uninfested trees was not significant (t test, t = 1.637, df = 58, p > 0.05). DISCUSSION RASPRAVA As a whole, oriental spruce trees that had higher phloem thickness, MAI-5, MAI-10, mean annual increment in the last year until the sampling date and number of xylem cells were infested successfully by D. micans. Within the scope of our study, we can remark that spruce trees with higher recent tree growth rates in the same stand are more likely to be infested by D. micans than others. Phloem thickness of the infested trees was 4.19 mm on average, and it was 1.18 mm thicker than uninfested trees. There are other studies that have indicated the positive effect of phloem thickness for bark beetles (Amman, 1972; Berryman, 1976; Haack et al., 1987a, b; Alkan Akıncı and Erşen Bak, 2016; Alkan Akıncı et al., 2018). Storer and Speight (1996) expresses that D. micans host selection may be determined in part by the suitability of the substrate for larval development. So, by attacking trees with thicker phloem, D. micans parental females may assure their larvae to have enough nourishment throughout development stages. There was a contrasting result that phloem thickness had no unique effect on a bark beetle species -pine engraver- reproduction reported by Reid and Robb (1999). In their case, the mean thickness of phloem in the focal stand was 1.07 mm that is less than 1.45 mm the body width of pine engravers, and required beetles also to consume outer bark and xylem of the trees (Reid and Robb, 1999). MAI-5 and MAI-10 in the infested trees was higher than the uninfested ones. D. micans preferred vigorously growing trees during attacks. In the former experimental studies that D. micans females were inserted on healthy oriental spruce trees, parental females were tended to be successful on vigorously growing trees (Alkan Akıncı and Erşen Bak, 2016) or codominant trees that have decreasing growth in the last 10 years (Alkan Akıncı et al., 2018). These trees provide thicker phloem and larger phloem surface area to D. micans larvae for feeding. And being stressed in some way also benefits beetles. Reid and Robb (1999) have reported comparable results for Ips pini. Their results showed that tree growth rate parameters are positively related to beetle reproductive performance on freshly dead Pinus banksiana (Reid and Robb, 1999). There was also a significant difference between mean annual increment in the last year until the sampling date in infested and uninfested trees. Mean annual increment in the last year until the sampling date in infested trees was 0.49 mm higher than the uninfested ones. Infested trees have grown vigorously until the sampling date in the last year as well. The difference between the DBH of the trees and number of xylem cells in infested and uninfested trees was not significant. The number of xylem cells was higher in infested trees (Figure 1a, b, c). Infested trees have higher cambium activity so they have higher wood and phloem formation. Oriental spruce is one of the dominant conifer species in Artvin and Eastern Black Sea region, and natural spruce forests are of great economic and ecological importance in its distribution area. Tree mortalities associated with outbreaks cause reductions in tree density that cause changes in microclimate that favor beetles (Fettig and Hilszczański, 2015). Such a microclimate alteration in a stand involves changes in reproduction of beetles such as phenology, and voltinism (Fettig and Hilszczański, 2015) and trees’ exposure to direct sunlight. Death of larger trees in a stand creates neighboring trees to get more sunlight. Trees at the stand edges and in sun-exposed patches are preferably infested by bark beetles (Schopf and Köhler, 1995; Mezei et al., 2012). Eventually, loss of vigorously growing larger trees reduces resistance of remaining living trees in the stand. CONCLUSION ZAKLJUČAK In our study scale, we can conclude that parental females that start attacks on new hosts have preferred vigorously growing oriental spruce trees. D. micans, during its natural |