Forest Pests: Insects, Diseases & Other Damage Agents

Controlling the European spruce bark beetle

Translation of French document (Jean-Pierre Renaud)

Controlling the European spruce bark beetle

To ensure a complete development, the European spruce bark beetle needs a sufficiently thick bark. This is the reason why one can only find this species on trees usually broader than 25 cm in diameter. However, the situation is not the same for Pityogenes chalcographus. This latter species can develop underneath thin barks and is able to colonize young stems and branches, as well as the upper part of bigger trees. As the European spruce bark beetle constitute a significant risk factor for spruce stands, this document mainly presents control methods for this species, unless particular cases. Management options proposed for the European spruce bark beetle can be used against Pityogenes chalcographus, whenever this latter species becomes at an epidemic level.

Control principle

Spruce colonization success by the European spruce bark beetle depends on:

1. Tree health. Weakened trees have a reduced resistance capacity.
2. Local beetle population level. With a massive population, beetles succeed in overcoming the defenses mechanisms (resin) of the trees.

After catastrophic events, such as droughts, or storms, there is no way to improve tree health. Only preventive stand management measures can be adopted: fitness of a species to the local conditions, modulating thinning frequencies… Once the outbreak has begun, the single management option left is to control beetle populations by limiting reproduction area and killing insects, in order to reduce population level below the epidemic threshold.

Limiting the reproduction sites

Reproduction sites consist of normally harvested or windthrown trees. During beetle outbreaks, the first control action is to postpone the normally planned harvesting activities, in order to reduce the amount of potential reproduction areas. This management option is particularly effective for limiting Pityogenes chalcographus populations, especially when suspension of early thinning or suspension of the pruning of living branches is concerned.

If the normal harvesting activities are not postponed, special attention needs to be paid to:

• haulage of harvested trees out of the forest. Harvested trees should be hauled out of the woods rapidly enough to prevent complete development of beetles, that is:
  • before mid-March to mid-June (depending on region) for trees harvested from October to May;
  • at a maximum of 6 weeks after harvest, when logging takes place between May and October.
  Harvested trees should be stocked at least 5 km away from the forest or should be processed quickly.
• wood neutralization in case haulage of logged trees out of the forest cannot be performed at appropriate time. Neutralization aims at preventing wood colonization by beetles or at preventing the complete insect development. When beetles are found under the bark of harvested trees (detection of entrance holes, reddish sawdust, insects living in galleries…), neutralization needs to be conducted rapidly (within 1 to 4 weeks, depending on the insect development stage). Debarking and insecticide application were proven to be effective methods. Permanent water aspersion is also effective for protecting non-infested wood.
• burning or chipping waste wood resulting from harvest. If standing infested trees are detected nearby a logging site, waste wood should be burned or chipped at latest 6 weeks after harvest, in order to prevent beetles from colonizing it. If such an option is not feasible, cutting of branches into small pieces could reduce the colonization risk by the beetles, since it is speeding wood drying. Such processing of big branches is effective in controlling European spruce bark beetle populations. It should also be implemented in case of high risk of Pityogenes chalcographus outbreak
.

Reducing the beetle populations

When damages are observed, efforts should be made to identify attacked trees that are sheltering beetles. This should be done before the adult initial flight. At this stage, external evidence of infestation is tiny (entrance holes, reddish sawdust). These signs are often located high in the tree, as colonization by beetles often begins at the top of the trunk. Needles discoloration happens at the end of beetle development cycle and is quite hard to detect. The bark peeling off and the complete needle reddish discoloration are occurring at the moment of adult initial flight or later. At this stage, it is too late for any effective control options. Surveys should be concentrated in the vicinity of trees turning reddish, and should be aimed at detecting infested trees that are not yet discolored. It is not recommended to harvest systematically all green trees growing around the reddish ones, as it leads to further disturbance of the stand stability, with no guarantees in terms of beetle control. In fact, beetles do not systematically attack neighboring trees.

Active management options

When infested trees are detected, the following control options should be implemented:

1. infested trees should be logged very rapidly (within a few days to a few weeks, depending on the beetles development stage);
2. logged trees should be neutralized:

• either by hauling them immediately out of the forest with their bark still on (logged trees should be stocked at least 5 km away from the stand)
• or by processing them as quickly as possible. These two options are the cheapest and most effective ones, although bark could eventually peel off partially during hauling which could lead to the release of mature beetles in the forest. If these options are note feasible, two other alternatives can be implemented:
• insecticide pulverization on logged trees: this should be done very quickly and only if logs are stocked at special location (outside the stands);
• or by debarking logged trees within the logging site and by killing the beetles that are present in the bark. This can be achieved simply through natural drying, as long as only larvae or white pupae are present (bark pieces should be left with the inner side up). At a later developmental stage, it is recommended, immediately after debarking, to burn the bark pieces, or to chip them very carefully.

Waste wood (branches, upper part of the trunk) coming out of infested trees should be burned preferably, or otherwise chipped. This is particularly recommended when the risk of a Pityogenes chalcographus outbreak exits.

What about pheromones?

Aggregative pheromones for the European spruce bark beetle can be artificially synthesized and purchased. The use of these pheromones enables the foresters to attract beetles at a given place, to estimate the population level, or to destroy them. Trapping has been widely used during previous outbreaks, with either artificial traps or trap trees. With an artificial trap, one can catch about 10 000 beetles, whereas with trap trees, several tens of thousands can be caught. These quantities have to be compared to the number of beetles released from a single cubic meter of wood (30 000 insects). Therefore, one trap tree, or 3 to 10 artificial traps are required to attract all the beetles released from a single infested tree. This method appears excessively expensive, if a sufficient number of traps are to be installed. So, that is why trapping, either with artificial traps or with trap trees, is not recommended any more in France.