How to Protect Spruce Forests from the Pine Bark Beetle

Extensive infestations of the pine bark beetle often occur after abiotic disturbances, such as windstorms, snowstorms, and droughts. These events create optimal conditions for the reproduction of the pine bark beetle by providing enough suitable trees. As a result, the beetle population can rapidly increase and subsequently begin to colonise healthier trees that are not yet weakened.

Photo: Younger and older spruces destroyed by pine bark beetles, Vysočina, author Jan Řezáč

The spruce bark beetle is a harmful pest that threatens spruce forests and is often found alongside the pine bark beetle, a significant pest of coniferous forests in Europe.

This species has been identified as a serious threat in six Central and Eastern European countries: Austria, the Czech Republic, Germany, Hungary, Romania, and Slovakia. In the Czech Republic, the pine bark beetle is even classified as one of the most significant pests in current legislation (Decree 76/2018 Coll.).

The spruce bark beetle typically prefers older spruce stands but can also attack other tree species and younger trees during overpopulation events. In contrast, the pine bark beetle primarily targets younger trees and has a broader range of host tree species. Its overpopulation occurs unpredictably at irregular intervals.

In recent years, foresters and forest owners have been actively seeking effective methods to reduce further damage caused by the pine bark beetle. However, integrated protection strategies for this pest are not nearly as advanced as those developed for the spruce bark beetle or the larch bark beetle.

Photo: Bark beetles in the initial phase of the attack show themselves inconspicuously as a few withered trees, author Jan Řezáč

Scientists from the Czech University of Life Sciences in Prague and Mendel University in Brno have developed a certified methodology for integrated protection against the pine bark beetle. This work was part of their research project, NAZV QL24010235, titled “Optimization of the Representation of Deciduous Tree Species in Mixed Forest Stands Leading to a Reduction in the Attractiveness of Norway Spruce for Bark Beetles.” („Optimalizace zastoupení listnatých dřevin ve smíšených lesních porostech vedoucí ke snížení atraktivity smrku ztepilého pro lýkožrouty“). The methodology has been published by the Forestry and Game Management Research Institute in its Forestry Guide (Lesnický průvodce) edition.

The publication summarizes current knowledge about the biology and potential harmfulness of the pine bark beetle, assesses its significance, and outlines the principles of integrated protection. It includes strategies for defence and protection against the beetle. A key section of the methodology focuses on the reproductive potential of this species, as understanding its dependence on various conditions is crucial for choosing effective defence and protection measures.

The development of the pine bark beetle is significantly influenced by temperature, which affects key life stages such as flight activity, tree invasion, and the development of the next generation. In Central Europe, the pine bark beetle typically produces two generations per year, both in lowland and mountainous regions. Flight activity occurs continuously from April to August, beginning at temperatures around 16.8 to 17 °C. The development of one generation requires approximately 652.8 (± 22.7) degree days above the threshold temperature of 7.4 °C.

Photo: Rapid, catastrophic spread of bark beetles significantly contributes to drought, author Jan Řezáč

The length of daylight likely influences the development of the pine bark beetle. For instance, during its adult stage, the beetle enters a diapause that is regulated by photoperiod, beginning in late August when the length of daylight drops below 13.6 hours. Scientists have found a meaningful relationship between day length and the beetle’s development at lower temperatures, supporting the idea of photoperiod-induced diapause. However, this process can be affected by temperature conditions, as higher temperatures may delay or suppress the onset of diapause.

The pine bark beetle is an oligophagous species that primarily feeds on conifers. This feeding strategy provides ecological and evolutionary advantages, allowing the beetle to survive even when its preferred host is unavailable—such as when it spreads to new areas. Its most favoured host is the Norway spruce, which is why the beetle is found in spruce stands ranging from Scandinavia in the north to the Balkans in the south.

Additionally, the pine bark beetle benefits from cohabiting with the spruce bark beetle. When the spruce bark beetle occupies the lower sections of the trees and the pine bark beetle inhabits the upper parts of the tree canopy, the reproductive success is significantly enhanced. In such cases, the average reproductive success is 1.20 offspring per female for the parental generation. In contrast, when pine bark beetles colonize trees without the presence of spruce bark beetles, the reproductive success drops to only 0.75 offspring per female.

Photo: Pine bark beetle gallery system, VULHM archive

Scientists have discovered an interesting reaction between the spruce bark beetle and the pine bark beetle concerning their pheromones, which are different for each species. In simple terms, the pheromones produced by the pine bark beetle repel the spruce bark beetle, while the pheromones of the spruce bark beetle attract the pine bark beetle. As a result, the spruce bark beetle tends to avoid trees that have previously been colonized by the pine bark beetle, whereas the pine bark beetle shows no response to the presence of the spruce bark beetle.

This interaction provides an evolutionary advantage for the pine bark beetle. It tends to inhabit the upper sections of trees already infested by the spruce bark beetle—specifically, those trees that have already had their defences compromised. Additionally, the pine bark beetle can fill vacant spaces on the thicker portions of trees where spruce bark beetles have fed. In some cases, the pine bark beetle can even occupy areas already taken by the spruce bark beetle with a slight delay.

When population densities are high, the pine bark beetle can damage healthy, younger trees, as well as those that are already weakened or damaged. However, its effectiveness in overcoming the defences of older, healthy trees appears to be limited, making it unlikely that it would cause their death under normal circumstances.

The mortality rate of trees infested by both spruce bark beetle and pine bark beetle is significantly higher than that of trees infested solely by pine bark beetle. When only pine bark beetle attacks the upper part of mature trees, the top may die, but these trees can still survive the attack.

Photo: Bark beetle outbreaks in the landscape, Vysočina, author Jan Řezáč

The pine bark beetle poses a significant threat as it leads to high tree mortality, particularly in young spruce stands, at irregular intervals. This overpopulation is typically caused by low tree vitality, which results from water stress combined with a high population density.

The methodology „Integrated protection against pine bark beetle“ (Integrovaná ochrana proti lýkožroutu lesklému)  can be downloaded here.

Authors: Prof. Ing. Bc. Jaroslav Holuša, Ph.D., Česká zemědělská univerzita v Praze; Ing. Tomáš Fiala, Ph.D., Mendelova univerzita v Brně

Prepared by Ing. Jan Řezáč, VÚLHM, v. v. i., e-mail: rezac@vulhm.cz