Fig. 1: Typical gallery system of European eight-toothed spruce bark beetle (Picture: Florian Stahl, LWF).
Given that bark beetle infestation repeatedly causes major problems in spruce stands, it is important that all forest owners have as clear a picture as possible of the biology and infestation behaviour of the European and Smaller European spruce bark beetles. They can only make an accurate assessment of the risk of infestation and respond early enough and with the appropriate countermeasures to prevent the further spread of damage if they are well informed on the pest. To help provide this information, we have thus put together a list of the most frequently asked questions and uncertainties on the issue here, with a brief response to each.
Where do bark beetles spend the winter?
The European eight-toothed and Smaller European six-toothed bark beetles overwinter at all stages of development (larvae, pupa, adult beetle) in the bark of infested trees.
Adult beetles sometimes also retreat into the litter on the forest floor to overwinter. However, the proportion of beetles overwintering in the forest soil and the factors that promote this behaviour have not yet been clearly established.
Do bark beetle broods die off in winter?
Eggs and early stage larvae are sensitive to temperatures falling below -10 to -15°C and prevailing over several days. By contrast, older larvae stages, pupae and beetles can survive even long periods of cold without great losses.
When and under what conditions do the beetles begin to fly in the springtime?
European eight-toothed spruce bark beetles and Smaller European six-toothed spruce bark beetles swarm out of their winter quarters in the spring, from the middle/end of April. Both types of beetle fly from temperatures of 16.5° C and in dry weather conditions.
Do bark beetles swarm before April, given the right temperatures?
No. It is not sufficient for the temperature just to have reached 16.5° C - this can of course also occur in winter. The beetles are indeed active under the bark, but they do not leave their winter quarters before the middle/end of April. For the beetles to swarm out and infest other trees, a certain accumulated temperature must be reached.
Which standing trees are the beetles most likely to infest?
The European eight-toothed spruce bark beetle prefers vigorous, mature spruce trees. The first pioneer beetles select their host trees according to criteria that have not yet been fully clarified, or select them at random. Some beetles are killed off by exuding resin, caused to flow by their boring into the bark - but if the density of the attack is sufficient (a few hundred beetles/tree), the trees’ defence mechanisms are overcome. As soon as a few beetles have successfully bored their way into the bark, they produce pheromones to attract more beetles of the same species - and the trunk is subject to mass infestation.
The Smaller European six-toothed spruce bark beetle has another infestation strategy: It reacts to characteristic scent signals from damaged trees and focuses on infesting these trees. In situations of mass propagation, however, it is also able to attack healthy, vigorous mature spruce trees. In contrast to the European eight-toothed spruce bark beetle, it usually prefers small-diameter timber.
How do you recognise bark beetle infestation?
Signs of infestation with the European eight-toothed spruce bark beetle are recognisable almost immediately because of concentrations of brown bore dust that appear at the basal area of the trunk, in flakes of bark, cobwebs and on the ground vegetation. Later symptoms of infestation (too late for countermeasures) are pieces of bark broken off by woodpeckers, bark falling off, discolouration starting at the bottom of the crowns and moving upwards, or needles dropping from the crown when green.
In the case of infestation by the Smaller European six-toothed spruce bark beetle in mature stands, early diagnosis is not possible. Usually it is several months before signs appear in the afflicted trees - the crown turns reddish brown from the treetop downwards, and the needles drop off.
How fast is the propagation of bark beetles?
A European eight-toothed spruce bark beetle female will lay up to 150 eggs during the growing season. With losses taken into account, a beetle female can thus produce more than 100 000 offspring in years with three young beetle generations and two sibling broods.
Are there ways of combating the beetle apart from "clean" forestry practice?
Not at the moment. All sorts of attempts to combat bark beetles by using micro-organisms (e.g. fungi) have failed or are not yet viable for use in forestry practice. The use of insecticides is restricted to timber stored in the forest and can or should only be used on a limited scale. Because it is difficult to ensure complete wetting, there is a residual risk, which should not be underestimated in mass outbreaks of the bark beetle. One reasonable alternative to the use of pesticides is de-barking.
Does leftover timber (crown wood, branch material) pose a threat after harvesting and processing?
Yes - if there is a higher concentration of Smaller European six-toothed spruce bark beetles, any leftover timber poses an enormous risk. The Smaller European six-toothed spruce bark beetle can use this material over several months for brooding. If the material is already infested, the brood will usually develop successfully and the young beetles will fly.
As an early diagnosis of infestation is impossible and targeted countermeasures cannot thus be implemented as they can for the European eight-toothed spruce bark beetle, it is essential to remove all potential brooding material and to skim off the population by removing all already infested material.
Do the broods and beetles in peeled bark still present a risk?
No danger is posed by larvae, pupae and very light-coloured young beetles that have escaped mechanical destruction during the de-barking process. The stripped bark dries out and the insects cannot fully develop. Young beetles that have already been through their maturation feeding process (dark colour) can swarm out and cause infestation. This can be hindered by collecting the bark into piles (>50 cm high) and covering these with black plastic sheeting. The resulting high temperatures arising in the piles and the extensive growth of fungi kills off the beetles.
Is the "trap tree" method suitable as a control measure?
Trap trees are not an effective control measure, whether treated with insecticide or not.
- Trap trees can only absorb a relatively small number of beetles - the effort involved in felling and monitoring bears no relation to the benefits.
- Once infested with bark beetles, trap trees constitute a dangerous source of attractants in the middle of the risk area.
How can pheromone traps be used?
Pheromone traps are suitable for monitoring bark beetle populations:
- They provide information on the progress of swarming (start, peak times), and thus an insight into the number of subsequent generations.
- When observations are recorded over a number of years, they give an impression of the development of beetle population densities according to region.
What is important here is the informed selection of trap sites. Traps cannot be used successfully to control bark beetle infestation.
What influence do natural enemies have on beetle population density?
Bark beetles have many natural enemies, e.g. predators (checkered beetles such as the ant beetle (Thanasimus formicarius) or bark-chewing beetles (Trogossitidae), parasites (chalcid and parasitic wasps (Chalcidoidea and Ichneumonidae)), and pathogens (fungi, viruses, etc.). They can occur in large numbers - but they do not normally have enough influence to cause mass outbreaks to collapse.
What is the best time for tending and harvesting measures in years with a higher risk of bark beetle outbreaks?
In mass outbreaks of bark beetles, the suspension of such measures should be considered. Otherwise it is recommended that the work is carried out as far as possible in the late summer, once the beetles have finished swarming. Where there is a high population density of Smaller European six-toothed spruce bark beetles, it is essential that all leftover wood or all infested material is disposed of carefully (burning, chipping). Crown wood, branches and brushwood mats maintain their suitability for brooding for the Smaller European six-toothed spruce bark beetle for many months.