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Timber yards: possible threat to surrounding woodlands
A timber producing company stores large amounts of spruce and pine round timber with bark, partly infested by bark beetles, beside the processing plant. What consequences does this have for neighbouring forests? The dispersal of the European spruce bark beetle was studied in a release - recapture experiment. A proportion of the released bark beetles did not react to the strong pheromone sources directly at the release site, but rather were only attracted after a relatively long flight.

Although comprehensive data is available about the wide dispersibility of the European spruce bark beetle (Ips typographus), there is however no solid data to answer the question of how the ability of individual beetles to migrate can lead to a mass reproduction in forest stands in the area.

For this reason a concrete case-study was examined: a timber-processing company stored large amounts of spruce and pine round timber with bark, which was partly infested with bark beetles. The storage area was situated in the valley-floor beside the processing plant, the valley slopes were forested, including some secondary spruce monocultures. After infestations of standing trees by spruce bark beetles broke out in numerous stands in the direct and wide vicinity, a connection with the storage area was discussed.

Pheromone traps against dispersal

The plant undertook the following measures against the spread of beetles: for the mass trapping of bark beetles, a tight belt of slot traps was erected in a radius of 500m of the storage area, 16 baited with Pheroprax® and seven baited with Chalcoprax® (BASF, Germany). On the south west side, a pheromone baited StoraNet® (BASF) was erected as a barrier (4m high, 60m long, 1m above ground), additionally several TriNet® traps (BASF) were erected from July 2012. The latter method is based on the use of synthetic nets, which have been treated with the pyrethroid α-Cypermethrin and thereby have a contact insecticide effect.

None of these measures can offer comprehensive protection. In order to study how many beetles could migrate out into the surrounding forest areas, a release-recapture experiment with marked spruce bark beetles was carried out.

Marking, Release and Recapture

Four 4m long spruce bark beetle infected spruce blocks were transported from a neighbouring forest to the storage area on the 4th of July 2012. On two sides there were the round timber piles, on one side the street, and the fourth side was exposed to a fixed/stretched insecticide net (distance ca. 20m). Random samples showed that pupae and juvenile beetles were present in the larval galleries, few beetles had already emerged. In order to mark the beetles, the outer bark was densely treated with a fluorescent dying powder (Figure 1).


Figure 1: Trunks treated with fluorescent powder. The beetles get contaminated by the dye as they emerge, the marking is very well visible under UV light.

As they emerge from the treated breeding trunk, the beetles are contaminated with the dye and thereby marked. This is visible under UV-light, even at low dye levels (Figure 2). This method was introduced to us by Axel Schopf of the University of Life Sciences and Natural Resources, Vienna.



Figure 2: a marked spruce bark beetle, which was captured in a trap in the surrounding area (Trap W1, on 10.07.2012)

To recapture the beetles, five additional slot-traps treated with Pheroprax® were erected (Figure 3). To prevent beetles in the traps from removing the dye or mutually contaminating each other, the containers were treated with small amounts of StoraNet® poison. The marked beetles were counted, the total amount of beetles per trap were estimated by random sampling through volume measurements.

Around 2% of emerged, marked spruce bark beetles recaptured.

Of the 5718 (calculated by random sampling of exit holes) marked spruce bark beetles which emerged, 109 specimens were recaptured in the pheromone traps, of which 6 beetles were recaptured in the surrounding area and 103 beetles in the trap belt around the storage area. The vast majority of the dyed beetles (two in the surrounding areas and 80 at the timber yard) were discovered at the first check of the traps.


Figure 3: Number of recaptured, marked Ips typographus in the traps on the timber yard and in the surrounding area and experimental layout with site of the marked logs (Freilassungspunkt), pheromone traps and piles of round wood; sketch (not drawn to scale). Traps in surrounding are shown in light green (total number of marked beetles in traps in bold, numbers per control date in parentheses), traps on timber yard in orange (total number of marked beetles per trap).

There is no evidence for a preferred dispersal direction of the spruce bark beetle. All traps except for W2 caught one or two marked beetles, one beetle was also found in the farthest trap O3 (distance 995m) in the first collection. The ring of traps around the storage area, however, showed an irregular distribution of recaptured beetles: 72 beetles (70% of the recaptured beetles at the storage area) were caught in the 3 closest traps to the release point. Only few marked beetles were trapped on the opposite side of the round timber piles (two marked beetles in the four traps on the north-east side).

Spruce bark beetle mostly caught in vicinity

The majority (94.5%) of the marked spruce bark beetles were caught in the ring of traps around the storage area, 5.5% were found in the traps in the surrounding area from 420m to 995m distance. However, this only accounts for around 1.9% of the total of dyed spruce bark beetles released (1.8% were caught at the storage area, 0.1% in the surrounding area.) These results should be interpreted with adequate caution.
What happened to the 98.1% of beetles, which were not recaptured? There was an abundance of natural pheromone and kairomone sources in the roundwood storage area, as well as the surrounding area. At the storage area, beetle infected wood was found. Furthermore, fresh wood was stored every four weeks. In the forests in the surrounding areas there were active spruce bark beetle infestation nests as well as numerous attractive spruce trees. The not recaptured beetles may have bored into blocks still covered with bark in the store, or flew into the insecticide net and subsequently perished, some may have also died of natural causes (e.g. predators). Some also would have migrated out into the surrounding areas, without entering one of the five pheromone traps, not least because the distances between the traps in the area were quite large.
On the other hand, the ring of pheromone traps around the storage area was tightly closed, at a range of around 500m each trap covered an average length of 32m. If we follow the assumptions of Duelli et al. (1997), that the attractive radius of a pheromone trap lies around 20m, no spruce bark beetle left the storage area without coming into contact with pheromones. Beetles which migrated into the surrounding area ignored the attractant shortly after flying out of the breeding trunk.  This 5.5% of our recaptured beetles were first drawn to the attractant after a flight of many hundreds of meters.

Consistent forest hygiene

Even if the vast majority of emerged spruce bark beetles had remained at the timber storage area or were caught in the trap-ring, we still must assume that within many thousands of cubic meters of timber not an inconsiderable amount of beetles migrate. Whether they poses a threat for spruce depends upon how much breeding material is found at the site, and how high the local beetle population is. Given that under natural conditions 30% of new infestations occur between 100 and 500m distance from last year¿s infestations (Kautz et al. 2011), the spruce stands in this zone around the storage area should be observed more closely: the consistent removal of breeding material and combatting of local beetle infestations should offer an unsuitable environment for the initiation of infestations of standing trees.
This experiment was carried out ad hoc due to the current problem. This resulted in a few methodical inconsistencies which make the repetition of the experiment desirable. But even these preliminary findings are of interest to us.

Acknowledgements

Thanks to Univ.-Prof. Dr. Axel Schopf and Veronika Wimmer MSc. (University of Natural Resources and Life Sciences, Vienna) for the introduction to the marking method, also to the employees of the company and the relevant regional forest authorities.

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