The search for bark beetle infestation

Several conditions have to be fulfilled before the eight-toothed spruce bark beetle swarms and spruce are infested:

• For the beetles to swarm, the air temperature must be above 16.5 °C.
• The sum of the maximum daily temperatures above 8.3°C (development start of the eight-toothed spruce bark beetle) from 1. April must exceed 140 “day-degrees” (e.g.: maximum daily temperature 18.3 °C - 8.3°C = 10 “day-degrees”).
• A length of day such as is prevalent from mid-April onwards ensures that the eight-toothed spruce bark beetles all fly at the same time.

As soon as these conditions are met and the weather turns warm and dry, we can expect the first large-scale and sustained swarming of the year.

It is assumed that a simultaneous attack by a few hundred beetles is sufficient to overcome the defences (resin flow) of vital spruce trees. If the trees are already damaged or drought conditions prevail, the trees’ resistance is correspondingly lower. European eight-toothed spruce bark beetles - like their smaller relative, the smaller European six-toothed spruce bark beetle - have an enormous reproductive potential. Depending on the weather conditions, they can produce 2 - 3 generations per year under the current climatic conditions. In addition, there are several sibling broods per generation, as the parent animals leave the breeding gallery after successfully laying eggs, and continue breeding elsewhere. For a beetle year with favourable development conditions, the following scenario could be derived for the eight-toothed bark beetle:

• An infested old spruce releases at least 20,000 beetles, of which at least 50% are males.
• These 10,000 male beetles are able to infest at least 20 more neighbouring trees.
• More than 400,000 beetles (200,000 males) can in turn swarm from these, potentially resulting in the infestation of another 400 spruce trees.

The swarming spruce bark beetles first drill into fresh lying and standing wood. If the lying wood is occupied, the approaching beetles drill into standing spruce. In the following one to two weeks, the beetles continuously eject a lot of drill dust as they create the mother gallery. Targeted searches should be carried out to find this drill dust.

The search for drill dust is time-consuming, but it is the only reliable way of diagnosing an infestation at an early stage, and it ensures the maximum efficiency of control measures. The searches should be carried out in dry, preferably windless weather. Regular inspections are necessary from the start of swarming, and should be carried out weekly if possible during the waves of swarming.

Step 1 - Where should you look?

• Start by looking for drill dust at beetle nests from the previous year. The beetles tend to bore into trees on the sunlit edges of the forest first. Even now, infested spruces from the previous year are still only just becoming visible through crown discolouration. Be aware of this and make sure you search any spruce trees nearby for signs of infestation (Figure 3).
• Increased infestation is to be expected In the immediate vicinity of spruce wood that has been left lying around (wood piles, damaged timber, firewood, slash residues) (Figure 4).
• If the stand conditions have changed significantly, e.g. the edges of the stand have been exposed, check there as well.

At temperatures above 30 °C, the eight-toothed spruce bark beetle will also retreat into the interior of the stand to infest trees there. Then the search for infestation should also be extended into the interior of the stand.

Step 3 - What do you need?

• Spray paint: to mark infested trees (Figure 5).
• A bark peeler or pocket knife: to look under the bark to see how far the brood has developed.
• Glasses or magnifying glass and binoculars: the drill dust is very fine and there may not be much of it, so you have to look carefully. Spruce bark beetles prefer to drill their way in at the base of the crown, i.e. at a height of several metres. With binoculars you can then for example see drops of resin at the base of the crown.
• Time: take your time and look closely. Especially after rainfall, it is easy to miss an infested trunk. Allow about 15 minutes per critical point, plus travel time.

Step 4 - Be thorough

• It is usually sufficient to search the first two rows of trees in the places described above.
• Walk systematically from tree to tree.
• Look all the way round each tree.

Step 5 - Where exactly will you find signs?

• Drill dust is to be found especially behind scales of bark; for this it is helpful to look very close to the trunk and from the top downwards - this is the best way to see behind scales of bark (Figure 6).
• On dead branches, branch forks or other surfaces that catch the falling bore dust;
• On spiders’ webs (Figure 7);
• In the moss at the base of the trunk (Figure 8);
• On the ground vegetation; the drill dust remains lying especially on rough-leaved plants such as blackberries or hazelnut; hardly any drill dust is found on beech leaves after rainfall.