In Finland and Central Europe, damage caused by the European spruce bark beetle could be prevented by using its natural enemies, explains Mikko Pelto-Arvo, a researcher at the University of Helsinki (Mikko Pelto-Arvo).
Utilising the bark beetle's natural enemies could help to reduce the population of this destructive forest pest (whose name in Finnish means “printer beetle” (printer beetle)) before it manages to cause significant damage. The best results can be achieved by combining biological pest control with good forest management.
According to M. Pelto-Arvo, the bark beetle's natural enemies include predatory beetles and parasitoid wasps, although the three-toed woodpecker and black woodpecker can also limit the spread of the bark beetle in its initial stages. The researcher hypothesises that in a favourable environment, the number of insects dangerous to the bark beetle is much higher, which means they can attack more effectively during the period when the spruce bark beetle population begins to increase (in the early stage).
According to data from the Finnish Forest Centre, more than 6,000 hectares of forest were felled in Finland last year due to insect damage, and the need for such action is increasing. In Germany and the Czech Republic, the spruce bark beetle has destroyed vast areas of forest in recent years, also causing problems for the timber market. The determining factors for beetle infestations are drought and prolonged heatwaves.
The European spruce bark beetle is a dark brown beetle about five millimetres long. As the name suggests, it lives on spruce trees and reproduces mainly in trees broken or blown down by storms (windthrow), piles of spruce logs, and spruce stands in poor condition. If the population is large enough, the beetle can also infest living trees.
The predator population must be sufficiently large
M. Pelto-Arvo hopes that owners will not keep their forests too “tidy”; rather, when planning good forest management measures, they should also include natural interaction processes. “If every branch and twig is removed from the forest during thinning, the continuity needed by predators will also be broken,” the researcher explains.
During thinning, it is beneficial to leave a mixture of tree species in the forest. When felling, all dead trees that are not attractive to spruce bark beetles should also be left in the forest. This will help maintain the population of predatory beetles and parasitoid wasps at a sufficient level. “Predators, such as ant beetles, are capable of travelling quite long distances to look for spruce bark beetles, but if there are no enemies in the forest area, the beetle can ‘run riot’ before enough enemies arrive, or manage to multiply,” says M. Pelto-Arvo.
Newly dead trees must be removed
M. Pelto-Arvo believes that an ideal situation would be a compromise solution, where trees that attract spruce bark beetles, i.e., those that have just died or are weakened, are removed if there are many of them due to abiotic disturbances. Abiotic disturbances are caused by “non-living” environmental factors such as storms and drought. In the long term, it must be taken into account that both dead wood and a mixture of species are needed in the forest.
“Many of the spruce bark beetle's natural enemies are generalists in terms of food, meaning that if there are no bark beetles, they can feed on other insects. The necessary continuity of the food chain is ensured if other prey can be found in dead wood or other tree species. For example, ant beetles can feed on Tomicus beetles that live on pine,” explains M. Pelto-Arvo. (Ant beetles (Thanasimus sp.) are predators whose adults hunt bark beetles on the surface of the tree, while the larvae hunt bark beetle larvae in the tree bark.)
M. Pelto-Arvo believes that it is very important to maintain forest conditions that are favourable to the enemies of pests, which allows the bark beetle population to be kept low. Furthermore, forest stands must be healthy and resistant to infestation. “First of all, it is important to have suitable tree species in every location. If, for example, spruce is grown in areas that are too dry or nutrient-poor, the trees will be subjected to stress, which will create a breeding ground for an increase in spruce bark beetles; subsequent generations of pests will multiply and attack even healthy trees in adjacent areas,” says M. Pelto-Arvo.
Dead wood does not create a risk
M. Pelto-Arvo's main message to forest owners is that dead wood does not always pose a risk for the spread of the spruce bark beetle. Beetles usually feed on the bark of a specific tree for only one summer; in rare cases, for a few years, until the “edible layer” is depleted or becomes too dry for the larvae to use.
“By the time a tree has no more bark, it will be completely useless to the spruce bark beetle and much more useful to other forest organisms to feed on. You might want to make the most of your forest by removing all dead trees for firewood, but that can be bad news for the bark beetle's natural enemies,” points out M. Pelto-Arvo.
If a decision is made to fight spruce bark beetles by felling trees, all trees with green crowns and fresh brown dust at the base of the trunk left by the beetles must be felled. Infestation is also indicated by abundant, thin resin streaks on the trunk.
“As long as the resin is flowing, the tree is still to some extent capable of defending itself. By the time the bark dust is found, it shows that the defence has failed and nothing more can be done,” is how M. Pelto-Arvo describes the situation.
He argues that it is not useful to fight spruce bark beetles by introducing beneficial predators into the forest, similar to how it is done in biological pest control in greenhouses. “It is not useful to rear bark beetle enemies to release them into the forest. Such active measures are too expensive, given that the forest management cycle lasts for decades,” explains M. Pelto-Arvo.
The range of species in Central Europe is wider
It is not possible to compare the forests of Finland and Central Europe directly. Central European forests have many more species and the region has a higher total annual temperature. Mikko Pelto-Arvo says that in Central European spruce forests, the natural enemies of the bark beetle behave the same way as they do in the north. The fact that spruce bark beetle damage has increased in Central Europe is linked to climate change.
“If these changes become permanent and trees continue to be affected by, for example, drought, it may be necessary to consider growing other tree species in Europe. In southern Finland, too, we will have to think about whether it would be useful to increase the proportion of birch and mixed forests,” M. Pelto-Arvo concludes.
https://forest.fi/article/natural-enemies-may-prevent-damage-by-spruce-bark-beetle-researcher-explains-how/#669791a2