Do harvesters reduce the risk of bark beetles?

The current bark beetle calamity shows clearly just how important bark beetle management is, particularly in the case of the European spruce bark beetle (Ips typographus) in spruce forests. The share of mechanical processing has increased considerably in recent years within the context of “clean” woodland management and the sanitation of infested trees. It was thus reported from the ForstBW forest district “Südschwarzwald” (southern Black Forest) that about 90 % of the storm-damaged and bark beetle-infested timber that arose during the calamity in 2020 was processed by harvester (by word of mouth,. Emmerich 2021).

In this article, we take account of the great importance of this technology by examining the machine processing methods and relating them to established methods of bark beetle management. The background to this is a recently completed research project on the risk of infestation with the European eight-toothed spruce bark beetle spreading from spruce trees mechanically processed by harvester. A summarised presentation of this study is followed by recommendations on how mechanical processing can be integrated in bark beetle management.

The proportions of deformed or completely sheared off bark differed significantly between the methods investigated and the motor-manual processing; but there were also striking differences between the respective partially mechanised processing variants. This resulted in different rates of efficiency in killing the insects at the brown stages (young beetles, adult beetles) and white stages (eggs, larvae and pupae).

Each of the three partially mechanised processing variants tested resulted in reductions of the insect presence with an efficiency (WG)* of ≥ 80 % when white stages of the insect were present. For beetles in the white stages, these processing variants thus achieved an adequate degree of efficiency for a therapeutic measure, comparable with the generally accepted level of efficiency expected of plant protection products (PPP). For beetles at the brown stage (developed young beetles), only the variant Tm_Mod achieved a rate of efficiency of ≥ 80 %. Compared with this, the rate of efficiency of the Tm and Tm_Dbh variants was somewhat lower at 65-75 % (Fig. 4 Decision tree). It should also be borne in mind that large pieces of the bark are peeled off during the processing, especially with the Tm_Mod and Tm_Dbh variants. If young beetles (brown stage) are already present at the time of processing, they remain with the peeled off bark in the stand and continue to present a risk to the forest!

*The rates of efficiency of the treatment variants in comparison with the motor-manual processing as a reference procedure are determined on the basis of the number of hatched spruce bark beetles relative to one square metre of the mantle (area) of the logs (formula according to Abbott (1925)):  = 100∗-/ (w = Efficiency, u = number of animals in control group, b = number of animals in treatment group).

Preventive processing by harvester, i.e. the processing of uninfested wood, also resulted in an insufficient rate of efficiency in comparison with the efficiency expected of plant protection products. For all the variants examined, the efficiency of preventive processing was between 50 and 70 %. Although the supply of suitable breeding space was reduced, the timber could thus still be extensively colonised (Fig. 4 Decision tree).

The study shows that processing with a harvester is generally preferable to processing by the motor-manual method, not only because of the higher level of work safety, the lower harvesting costs and the higher processing output, but also from the point of view of forest protection. If motor-manual felling is unavoidable, e.g. due to wide distances between extraction tracks or to protect the stand, an attempt should be made to integrate a harvester in the processing (partial mechanisation). The results show that infestation is significantly less likely to spread from logs that have been processed by harvester, and that there is thus less of a threat to the forest from logs processed in this way. However, it should be borne in mind that it is only at the time of the development of the first generation in spring that exclusively white stages of the beetle are present. After the appearance of the first brown beetles, there are usually both white and brown stages of the insect present at the same time during the rest of the year, due also to the establishment of sibling broods!

At the brown stage, only the processing variant Tm_Mod achieves a sufficient rate of efficiency. For this reason, further measures are necessary to manage the beetles after the Tm and Tm_Dbh treatment variants once there are beetles at the brown stage (Fig. 4 Decision tree). In this case, timely removal to the sawmill, storage at a sufficient distance from coniferous stands or storage in deciduous stands, storage in foil, debarking, or chipping should be considered. As a last resort, an approved plant protection product can be sprayed to sanitise the infested timber before the beetles can fly on to other trees. In the case of both the Tm_Mod and Tm_Dbh variants, it is necessary to collect and destroy the peeled off bark when beetles at the brown stage are present, as they are mature enough to survive and may thus remain in the stand with the peeled bark and cause secondary infestations.

The positive effects of the Tm_Mod and Tm_Dbh variants in terms of forest protection are offset by a reduced processing capacity compared with the conventional Tm variant. The Tm_Mod procedure also requires an increased level of concentration and skill on the part of the harvester operator. With the Tm_Dbh variant, the required contact pressure and the diagonally positioned shanks on the feed rollers can result in increased damage to the wood body, which may have a negative influence on the timber revenues.

The decision for or against mechanised processing must ultimately be made by the forest owner himself after weighing up the advantages and disadvantages of the individual methods described here. Financial support under the “debarking of damaged wood” measure in accordance with Part F of the “Administrative Regulation on Sustainable Forest Management” (for the elimination of the consequences of extreme weather events in the forest) is not available for any of the three harvester-based processing methods described here.