Bark beetle attacks reduce survival of wood ant nests
Central European coniferous forests are facing the most significant bark beetle outbreak, from 2015. The consequence is the creation of large clearings. In 2021, the total area of clearings in Czech Republic had increased over three-fold compared to the areas recorded in the years before the bark beetle outbreak. Large clearings, which are associated with changes in environmental parameters – increasing of the amount of light on ground Vegetation, soil temperature, together with changes of soil humidity, etc.
Photo: Wood ants‘ nest on clear cutting area, FGMRI (VÚLHM) archive
Plants and animals have different reactions to these changes. The species diversity of some taxa increases (e.g., bees and wasps, non-saproxylic beetles, lichens, birds); however, for other species, diversity does not change (e.g., moths, Mollusks, Carabids), or it decreases (e.g., wood-inhabiting fungi, some soil taxa). Grass coverage and the appearance of pioneer species increase.
The diversity and abundance of ant taxa sensitively and ambiguously respond to the clear-cutting of spruce plantations. The researchers from both the Forestry and Game Management Research Institute and Charles University conducted a study on the relationship between clearings and the presence of ants. They published their findings in an article titled “Bark Beetle Attacks Reduce Survival of Wood Ant Nests” (Kůrovcové kalamity omezují životnost mravenišť lesních mravenců) in the journal Forests.
Red wood ants are important for forest ecosystems as they improve soil pH and nutrient content, create microsites, and provide habitat for various organisms. This helps reduce the infestation of trees by bark beetles, which is especially valuable in coniferous forests with acidic soil and susceptible forest stands.
Wood ants are experiencing local population declines due to factors such as insufficient forest management and a lack of understanding about their habitat needs. Current regulations do not adequately protect them, and their conservation should be prioritized in light of potential climate change impacts.
Photo: Wood ants‘ nest on the edge of forest stand, author Jan Řezáč
Out of the total 1070 historical red wood ant nests revisited, 24.9% had disappeared between the years of the historical and recent inventories. In forests that were not subjected to bark beetle attacks, only 16.8% of the nests disappeared; in contrast, in bark beetle-affected forests, about 38.5% of the nests disappeared.
However, survival differed between the two dominant ant species. Out of the 1070 nests, 736 belonged to Formica polyctena, 186 belonged to F. aquilonia, and 148 belonged to F. rufa. However, all F. aquilonia nests were located in forests unaffected by bark beetles; therefore, the effect of bark beetles on ant species could only be evaluated for F. polyctena and F. rufa.
In case of F. polyctena, 22.9% of nests disappeared, and in F. rufa, 78.3% of nests vanished by 2021. Nest loss was greater in bark beetle-affected forests for both species. F. polyctena lost 16.7% (intact forests) and 31.5% (affected forests) of nests observed at the turn of the century. F. rufa lost 67.7% (intact forests) and 86.0% (affected forests) of the nests observed in 2000.
Obtained results clearly indicated that bark beetle attacks significantly reduced Formica sp. nest survival.
Photo: Wood ants‘ nest inside the forest stand, author Jan Řezáč
This may be contra intuitive as wood ants are sun-loving species commonly occurring on sun-exposed edges of forest stands. Based on that, one can expect that clear-cuts may form more edges suitable for wood ants. This may hold for small clear-cuts.
The bark beetle infestation and logging created large clearings in the forest, which seems to be harming the survival of ant nests. This supports the idea that the decline in nest numbers is due to habitat loss, as clearings have different environmental conditions such as radiation, moisture, and temperature changes.
Obtained data shows that clearings resulting from the removal of bark beetle-affected trees increased vegetation coverage and height on ant mounds, leading to reduced nest survival. The thick vegetation made it difficult for worker ants to hunt and disrupted the nest microclimate, affecting the successful development of the ant population and the production of winged ants.
Moreover, when more radiation falls on the clearings, it supports a rise in metabolic rates and, consequently, a loss of winter energy stores. Food resources are mandatory for maintaining stable, optimal nest temperatures and for the production of offspring.
Ants obtain most of their food from trees (e.g., honeydew, arthropods); therefore, clear-cutting causes a reduction in the food supply. The abundance of aphids can be considered essential because aphids′ honeydew constitute the main food source for wood ants.
Additionally, ants‘ „workers“ harvest less honeydew mass from clearings compared to the amounts they harvest in middle-aged and mature stands. When nests are located in the central parts of clearings, the ants cannot find sufficient food on trees.
The surrounding older stands can serve as food resources for wood ants, but the average distance of foraging routes is relatively short (16 m). In the event of a lack of resources, the ants will extend routes up to distances of 50–100 m and, exceptionally, up to 164 m. However, bringing honeydew from greater distances is not ideal because the amount of honeydew harvested declines with the distance to the trees.
Food availability may determine the body size and fat resources of ant workers. Just large „worker ants“ may be nest defenders.
The use of insecticides is widely applied to defend forests against bark beetle infestations. All approved preparations contain parts derived from pyrethroids, which act non-selectively and have a significant negative impact on many groups of invertebrates, including ants.
The typical method of protecting a forest after bark beetle infestations is the timely clearing of infested trees. This clearing can prevent the formation of extensive clearings and, in addition, it contributes to maintaining the mosaic environment. It is necessary to maintain a balance between sunlit forest soils and forage trees and to retain the connectivity between habitat patches without applying insecticides near ant nests to conserve ant populations
Large clearings created after a bark beetle outbreak are unsuitable for maintaining ant populations. Moreover, replacing coniferous trees with deciduous seedlings, which are now largely preferred in planting, is unsuitable for the conservation of wood ants.
The paper „Bark Beetle Attacks Reduce Survival of Wood Ant Nests“(Kůrovcové kalamity omezují životnost mravenišť lesních mravenců) can be downloaded here.
Authors of paper: Adam Véle, Forestry and Game Management Research Institute, Strnady 136, 252 02 Jíloviště; Jan Frouz, Institute for Environmental Studies and Environmental Centre, Charles University, Benátská 2, 128 01 Prague; Institute of Soil Biology and Biochemistry, Biology Centre, Czech Academy of Sciences, Na Sádkách 7, 37005 České Budějovice
According to the original, prepared by Jan Řezáč, FGMRI (VÚLHM, v. v. i.), e-mail: rezac@vulhm.cz