Life cycle

Norway spruce (Picea abies) is the preferred host of the European spruce bark beetle, but it can also be found on fir, pine, larch, and other spruces species.

The initial adult flight occurs in spring with the first warm days (a few consecutive days with temperatures higher than 18-20°C are needed). Male starts the breeding process and one to two females join him (it is a polygamous species). Once fecundated, females bore through the bark and construct egg galleries that are parallel to the wood grain. Eggs are laid by groups alternatively along opposite sides of the galleries. Newly hatched larvae mine outward and perpendicularly to the main gallery. These secondary galleries are sinuous and are widening out, ending by a pupal chamber.

Young adults become sexually mature only after they have spent some time under the bark.

Overwintering in galleries occurs either at the larvae, pupae, or young immature stages. Mature adults overwinter in the litter.

The annual life cycle largely depends on local climate: there are usually two generations per year in the plain locations of France, whereas in the mountains only one generation is realized. Flights take place almost any time from the initial spring flight until October, as long as weather conditions remain favorable (no rain and temperature warmer than 20°C). Beetles of different generations can be flying at a given time.

Damages

Once a tree has been attacked, and larvae have bored galleries underneath its bark, fungi settle and occasionally destroy the sap conducting tissues. The settlement of the bark beetles is always a massive process since they are producing aggregative pheromones. For an infested tree, it takes from a few weeks to a few months to die after that the beetles have completed their development under the bark.

The European spruce bark beetle is a secondary parasite that mainly attacks weakened, injured or freshly harvested trees. Under certain circumstances (e.g. storms…), when beetle populations build up rapidly, it becomes a primary epidemic parasite, being even able to attack successfully healthy trees.

The fungi, carried by beetles, are responsible for a blue wood staining process (Ophiostoma spp.)which causes wood depreciation, even for trees that are rapidly harvested.

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 in 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 areas 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 number of potential reproduction areas. This management option is particularly effective for limiting Pityogenes chalcographus populations, especially when the 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 the 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 an appropriate time. Neutralization aims at preventing wood colonization by beetles or at preventing 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 not 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 a 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.

Resources

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• Screening Aids for Exotic Bark Beetles in the Northeastern United States - USDA Forest Service
• The Atlas of Forest Insect Pests - The Polish Forest Research Institute
• Xilofág rovarok - Xylophagous insects - Hungarian Forest Research Institute
• Data Sheets on Quarantine Pests - European and Mediterranean Plant Protection Organization
• Exotic Forest Pest Information System For North America - North American Forest Commission

Selected Images

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Invasive Listing Sources

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• National Cooperative Agricultural Pest Survey Target Species
• National Cooperative Agricultural Pest Survey Target Species - 2015
• New Hampshire Prohibited Invasive Species
• North American Forest Commission Exotic Forest Pest Information System

References

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Common Name Reference: Web Search - http://www.fs.fed.us/foresthealth/technology/invasives_ipstypographus_riskmaps.shtml
Scientific Name Reference: Die Kafer Mitteleruopas Katalog - http://www.faunaeur.org/full_results.php?id=249683