Browsing by Author "Pelto-Arvo, Mikko"

The European spruce bark beetle Ips typographus (L.) is a severe pest of the Norway spruce Picea abies (Karst.). The species usually attacks weakened trees, but in a consequence of a strong abiotic disturbance event, population may increase sufficiently to threaten even healthy trees. In addition to available trees for feeding and reproduction, temperature is the most important factor limiting the damage of the pest. Limiting potential of the pest’s enemies is potentially important but poorly known. There are several predator and parasite species limiting the success of I. typographus. These species are commonly known as natural enemies or natural enemy complex. Occurrences of these species are investigated in this study. Regardless of intensive studies about natural enemies, there are still remarkable gaps in knowledge. While natural enemies could theoretically be used to control pest populations, practical applications in forestry are in a very limited use. Mechanical methods, mainly sanitation and salvation loggings, are instead used to prevent outbreaks from escalating. These management practices prevent outbreak from spreading, although damage caused by loggings may also harm trees and expose these to pathogens. Possibility of using natural enemies as a part of pest control in forestry is considered in this study. Field study was conducted in SE Finland. I. typographus and natural enemy complex were sampled in 2018 by means of trapping logs settled with fixed number of I. typographus individuals. This was conducted in three different stand classes representing different gradation phases in forests to find differences between regulating potential of enemies. Stand classes were chosen based on the visible symptoms of bark beetle infestation and disturbance history. Classes were healthy (no attack), early outbreak (first symptoms and increased pest population) and declined outbreak (earlier outbreak, pest already declined near endemic level). Trapping logs with settled I. typographus were allocated to these classes, and insects emerging from trapping logs were later collected for sampling in sealed funnel traps. I. typographus population level was simultaneously monitored using pheromone traps. Differences between insect occurrences in different stand classes were statistically analysed by Kruskal-Wallis test with 0.05-level of significance for each identified enemy species and families. Spearman correlation was also used to detect any possible relations between different enemy species. Predators emerging from the logs were identified to six coleopteran families. Species were Thanasimus formicarius (Cleridae), Plegaderus vulneratus (Histeridae), Epuraea spp. (Nitidulidae), Rhizophagus spp. (Monotomidae) and from Staphylinidae Quedius plagiatus, Nudobius lentus, Phloeonomus spp., Leptusa spp. and Placusa spp. Family Elateridae was inspected as a single tested group. Predatory flies and parasitic wasps also emerged from the logs, although these were excluded from the current study. Three species had significant preference to certain outbreak classes. P. vulneratus proved most promising regulator in this study. It reproduced rapidly already during the first year of infestation. T. formicarius had significant preference for stand class, but failed to occur in sufficient numbers during the first year of infestation to have impact on the I. typographus population. Staphylids Phloeonomus spp. were very common at all stand classes but preferred healthy class. For both P. vulneratus and Phloeonomus spp. preferences for stand classes were significant, but the regulation effectiveness of these species is relatively unknown. Results suggested that limited resources in the healthy class, similar to managed forests forces both bark beetles and predators into same space, potentially increasing mortality. This result was observed by increased total numbers of beetles in the trapping logs of healthy class in comparison to outbreak classes, while simultaneously prey-predator ratio was also lower. This means that in addition to removing resources, sanitation loggings could also have increasing effect on enemy induced mortality. Small amount of deadwood potentially sustains higher endemic population of enemies, potentially increasing stands resistance to further insect induced disturbances. While some enemies were more common at the declined outbreak class, this claim was not supported by this study.