Do pheromone trapping always reflect Ips typographus (L.) population level? A study from the Tatra National Park in Poland

Pheromone traps are used for monitoring I. typographus populations in Norway spruce stands of the Tatra National Park (TPN) in Poland. The presented study is based on the set of pheromone traps of precisely known location (23) located in the whole area of the TPN and operated continuously in 2010–2019. The data on the captures of beetles were compared with two kinds of data concerning the mortality: the area covered by standing dead trees (airborne photographs) in the no-intervention zone, and the volume of trees infested by bark beetles processed in the active protection zone. No relationship was found between the mean numbers of beetles captured yearly in all pheromone traps in the whole TPN area and the volume of infested trees removed from the stands in the active protection zone. The captures in the two selected study areas were correlated with the area of spots with dead trees in the 500 m circle around the traps, however, this correlation is not statistically significant. There is no relation of captures to the volume of processed infested trees. The captures decreased in the growing seasons after the wind damage, and increased markedly after the drought started in 2015. The results of pheromone trapping are affected by several factors, as wind damage and defence potential of trees resulting from their physiological status. Pheromone traps represent valuable source of information about the bark beetle I. typographus population dynamics, although the collected data do not enable direct definition of its population level, especially in the protected areas with different and unstable (changed in 2017) approach to the protection of stands. As most of the information on beetles is captured in the first half of the growing season, the data collected till the end of July are sufficient for monitoring purposes; thus, the trapping should be reduced to the period May–July.