Too soon for optimism despite bark beetle numbers falling in Estonia

The number of spruce bark beetles has decreased across nearly all of Estonia over the past year, according to the annual monitoring results of the Environment Agency. However, it is still too early to declare the end of the epidemic. In the longer term, forest owners are advised to continue making their forests more resilient to disturbances and to plant tree species other than spruce.
Kristjan Ait, a junior researcher at the Estonian University of Life Sciences studying the dynamics of bark beetle spread, confirmed that damage caused by the beetle decreased last year compared to recent years. This is also reflected in the Environment Agency's forest protection statistics, which show a slight reduction in the area designated for emergency logging.
However, Ait cautioned that there is no reason for major optimism just yet. "Looking at a longer time frame, 2024 was not particularly remarkable. In fact, more forest protection assessments for beetle-related logging were carried out only in 2022 and 2023. While the area of assessments conducted last year was about half of that in the previous two years, it was still significantly higher compared to years before that," the junior researcher noted.
When interpreting the Environment Agency's monitoring results, Ait noted that they provide a good overall picture of the situation in Estonia as a whole. Although monitoring points are located in every county on the Estonian mainland, they primarily reflect the conditions near each specific monitoring site.
For instance, forest owners in Lääne County need not worry that there are significantly more bark beetles in their county compared to the rest of Estonia. "We can take an example from Lithuania, where a denser monitoring network revealed that traps with the highest and lowest numbers of bark beetles were located relatively close to each other," Ait explained.
Natural deviation
The reasons behind the decline in the population of insects living under tree bark and feeding on phloem remain speculative. Kristjan Ait attributes this primarily to an increase in the number of bark beetles' natural predators. "Such changes in nature occur in waves. After several consecutive years with high bark beetle populations, the numbers of animals feeding on them have finally caught up," the junior researcher explained.
Another potential factor could be the availability of breeding material, namely weakened spruces. "In problem areas, it is possible that the number of spruces has locally been depleted, but over the past year, this factor cannot be considered the most significant," Ait suggested. On the contrary, climatic conditions in recent years have generally made life easier for bark beetles. Prolonged droughts cause water stress in trees, making them more susceptible to bark beetle damage.
Additionally, the number of degree days during the bark beetles' active period from April to September has increased. To calculate degree days, researchers sum up daily temperatures above the threshold at which bark beetles can develop. "I've analyzed degree days from 1945 to the present and last year's figure was the highest on record for that period," Ait noted. The exact impact of this will become evident next year.
A more extensive shift
It is already evident that there has been a significant shift in environmental conditions in Estonia and its neighboring countries over the past few decades. Before the 1990s, bark beetles typically produced one generation per year, but now they consistently manage two generations annually. This makes it increasingly difficult to combat them using traditional methods. "The rise in bark beetle populations in Estonia is very similar to that of our neighboring countries. While political decisions and sanitation logging can achieve some localized results, these measures alone will not completely solve the problem," Kristjan Ait observed.
"As a logical step, we need to look toward the future and consider how to shape our forests going forward. For example, we shouldn't put all our eggs in one basket and should regenerate forests with tree species other than just spruce," the junior researcher added. From the perspective of making forests more resilient to disturbances, spruces are relatively vulnerable not only to drought but also to storm damage.
In the context of bark beetle control, Ait and his colleagues at the Estonian University of Life Sciences are studying the effectiveness of trap trees and sanitation logging. Their research has shown, for instance, that trap trees prepared using chainsaws or harvesters attract spruce bark beetles to a similar degree. However, the same cannot be said for certain other beetle species.
It has also become clear that, at least in standing trees, the winter survival rate of beetles under the bark has been extremely low in recent years. "Most of the beetles have already left the trees by that time or have been consumed by natural predators," Ait explained.
Ait emphasized that even if bark beetle populations cannot be fully brought under control, Estonia's forests are not at risk of disappearing in the future. "Besides spruce, other tree species will continue to grow in these forests. From an economic perspective, the spruce bark beetle primarily targets trees with a diameter at breast height greater than 15 centimeters. In the future, we may simply focus on growing younger spruces," Ait concluded.
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Editor: Marcus Turovski