The horntail wasp poses a threat to pine forests that have been weakened by drought

Scots pine is the second most important economic tree species in the Czech Republic. Like other tree species, its health worsens due to prolonged periods of drought and rising air temperatures. Trees that experience drought are not only at physiological stress but are also more vulnerable to biotic threats, increasing their likelihood of dying.

Photo: Female horntail wasp (Sirex noctilio), VÚLHM archive

Economic pests are becoming more widespread, numerous, aggressive, and competitive due to climate change. As a result, they pose a greater threat to pine forests than ever before. In Central Europe, Scots pine is commonly attacked by more than thirty species of bark beetles and wood-destroying insects, a number that continues to rise as their range expands, including the spread of non-native species. The key economically undesirable insect species attacking pine include native species such as the larger bark beetle, the top bark beetle, the pine engraver beetle, the lesser pine shoot beetle, and the pine shoot beetle. In forest protection efforts, foresters are currently focusing mainly on bark beetles, but wood-destroying pests are equally important.

Photo: Weakened pine stands with dry trees in Central Bohemia, author Jan Řezáč

Wood-destroying insects are characterized by their habit of laying eggs in the trunks, branches, or roots of trees, or on their surfaces. Once the eggs hatch, the larvae burrow into the wood, feeding on its internal tissues. This feeding severely disrupts the structural integrity of the tree and complicates efforts to detect and control the infestation.

In addition to causing direct damage to the wood, these insects often carry fungal pathogens, which further negatively impact the health of the infected tree.

One example of a wood-destroying insect is the horntail wasp (Sirex noctilio). Until 2015, this pest caused little damage in Central Europe, but its population rapidly increased after that year, turning it into a significant economic threat to pine trees for several years.

Research conducted by scientists from VÚLHM, v. v. i., as well as the Forest Protection Services and the Forest Ecology departments, focused on a specific pest. They published their findings in an article titled Sucho jako klíčový faktor přemnožení pilořitky borové v Česku (Drought is the primary driver of Sirex noctilio outbreaks in Czechia),  which appeared in the journal Reports of Forestry Research, Volume 2, in 2025. This article was produced as part of the NAZV project QK22020062— „Identifikace přeživších jedinců lesních dřevin na kalamitních plochách, jejich záchrana a výzkum jejich rezistence“ (Identification of surviving individuals of forest tree species in disaster areas, their rescue, and research into their resistence).

Photo: Weakened pine stands with dry trees in Central Bohemia, author Jan Řezáč

High populations of Sirex noctilio, commonly known as the horntail wasp, have primarily been observed in the southern hemisphere, where it is considered a non-native species. As a result, this species is viewed as invasive and poses a significant threat, particularly to plantations of Monterey pine (Pinus radiata).

In Europe, Sirex noctilio is typically more abundant in regions characterized by low rainfall and higher temperatures. With climate change leading to an increase in dry and warm periods, which may also result in more frequent outbreaks of wood-destroying insects, concerns arise about whether the population of the horntail wasp and the associated economic damage will also rise in its native range.

To investigate this, scientists studied the factors affecting the number of trees attacked by the horntail wasp in the Czech Republic. They monitored the number of attacked pine trees in relation to decreasing precipitation and rising temperatures from 2015 to 2019, as well as considering climatic regions, soil types, altitudes, and the area of pine forests surrounding the studied stands.

The results indicated that the primary factors affecting the number of infested trees were the difference in precipitation between the reference season and the dry season, as well as the climatic region where the pine trees are located. The highest number of infested trees was found in areas experiencing a more substantial decrease in precipitation and in warmer climatic conditions.

Photo: Weakened pine stands with dry trees in Central Bohemia, author Jan Řezáč

This phenomenon can be explained by the primary weakening of pine trees due to abiotic stressors, particularly drought. Drought-induced stress is reflected in reduced resin pressure and changes in resin composition, which impair the trees’ ability to defend themselves against insect attacks. Consequently, the lack of precipitation indirectly affects sawfly populations by increasing the availability of resources for their reproduction.

Temperature plays a direct role in the growth and spread of forest insects. However, the changes in average temperatures between different periods did not show a significant impact. This indicates that the temperature during the current season is critical for sawfly populations, rather than variations from year to year.

Higher temperatures significantly influence the development of the horntail wasp. They enhance almost every aspect of its life cycle, including the speed of larval development, mating frequency, the number of eggs laid, and dispersal ability. For instance, when the temperature is 30 °C, the larval development period is shortened to just 10 days, whereas at 10 °C, it may extend to 60 days.

In addition to resource depletion, natural enemies play a significant role in the decline of forest insect populations. For instance, in the case of the horntail wasp, parasitoids such as the wasp Ibalia leucospoides and the ichneumon fly (Rhyssa persuasoria) can greatly contribute to a population collapse, particularly during times of overpopulation.

Natural enemies also include entomopathogenic nematodes. However, their impact on the population size of the horntail wasp can be diminished by unusual weather patterns that affect their development, reproduction, and spread.

For example, extremely warm weather can reduce the activity of Ibalia leucospoides and shorten the lifespan of adult wasps, limiting the time available for females to find hosts for reproduction. Similarly, parasitic nematodes tend to thrive in wetter wood conditions.

To protect pine stands, preventive measures primarily involve cultivating healthy forest stands and consistently searching for and removing damaged, weakened, or dying trees.

Additionally, the presence of natural enemies, particularly woodpeckers and parasitoids, can aid in preventing damage by reducing larval populations by up to 70%. Experience from other regions suggests the potential for inoculating trees with parasitic nematodes. However, the use of insecticides has proven to be less effective.

The paper „Drought is the primary driver of Sirex noctilio outbreaks in Czechia“ can be downloaded here.

Authors: Adam Véle, Kateřina Neudertová Hellebrandová, VÚLHM, v. v. i., e-mail: vele@vulhm.cz

Prepared by Jan Řezáč (rezac@vulhm.cz), based on the original.