Late Glacial and Holocene records of tree-killing conifer bark beetles in Europe and North America: Implications for forest disturbance dynamics

Outbreaks of conifer bark beetles in Europe and North America have increased in scale and severity in recent decades. In this study, we identify existing fossil records containing bark beetle remains from the end of the Last Glacial Maximum (~14,000 cal. yr BP) to present day using the online databases Neotoma and BugsCEP and literature searches, and compare these data with modern distribution data of selected tree-killing species. Modern-day observational data from the Global Biodiversity Information Facility (GBIF) database was used to map recorded distributions from AD 1750 to present day. A total of 53 fossil sites containing bark beetle remains, from both geological and archeological sites, were found during our searches. Fossil sites were fewer in Europe ( n = 21) than North America ( n = 32). In Europe, 29% of the samples in which remains were found were younger than 1000 cal. yr BP, while in North America, remains were mainly identified from late Glacial (~14,000–11,500 cal. yr BP) sites. In total, the fossil records contained only 8 of 20 species we consider important tree-killing bark beetles in Europe and North America based on their impacts during the last 100 years. In Europe, Ips sexdentatus was absent from the fossil record. In North America, Dendroctonus adjunctus, Dendroctonus frontalis, Dendroctonus jeffreyi, Dendroctonus pseudotsugae, Dryocoetes confusus, Ips calligraphus, Ips confusus, Ips grandicollis, Ips lecontei, Ips paraconfusus, and Scolytus ventralis were absent. Overall, preserved remains of tree-killing bark beetles are rare in the fossil record. However, by retrieving bulk material from new and existing sites and combining data from identified bark beetle remains with pollen, charcoal, tree rings, and geochemistry, the occurrence and dominance of bark beetles, their outbreaks, and other disturbance events can be reconstructed.

Evaluation of Two Pheromone Baits for Containment and Concentration of Attack by the Western Balsam Bark Beetle, Dryocoetes confusus (Coleoptera: Curculionidae)

In past studies, the greatest response by the western balsam bark beetle Dryocoetes confusus Swaine (Coleoptera: Scolytidae) to traps and baited trees was obtained with blend of (+)-exo- and (±)-endo-brevicomin, which mimic the natural male-produced aggregation pheromone. We conducted a tree-baiting experiment to determine whether low-release enantiospecific blends [9:1 (+) exo-brevicomin:(+)-endo-brevicomin, or 9:2 (+)-exo-brevicomin: (±)-endo-brevicomin released at 0.3, 0.1, or 0.03 mg/day] could compete with or improve the efficacy of the standard (±)-exo-brevicomin bait released at 1.2 mg/day. In this experiment, the standard and experimental baits were almost 90% effective in inducing attack by D. confusus. Two subsequent 9-ha block experiments compared the ability of the standard and 9:2 (+)-exo-brevicomin:(±)-endo-brevicomin baits to contain and concentrate infestations before harvesting. Again, both baits were equally effective as potential stand management tools based on almost 100% attack on baited trees, green-to-red tree ratios between 3:1 and 5:1, and redistribution of the majority of attack around baited rather than red trees from which beetles could emerge. We recommend that the standard bait continue to be offered commercially for treatment of stands infested by D. confusus.

Response of Dryocoetes confusus and D. autographus (Coleoptera: Scolytidae) to enantiospecific pheromone baits

In past field studies, the greatest response of western balsam bark beetles, Dryocoetes confusus Swaine, to traps was obtained with blends of (+)-exo-brevicomin and (+)- or (±)-endo-brevicomin, which imitate the natural male-produced aggregation pheromone. We conducted a trapping experiment comparing low-release enanti ospecific blends (9:1 (+)-exo-brevicomin:(+)-endo-brevicomin or 9:2 (+)-exo-brevicomin:(±)-endo-brevicomin released at 0.3, 0.1, or 0.03 mg per day) with the standard commercial (±)-exo-brevicomin bait released at 1.2 mg per day. Multiple-funnel traps baited with the experimental blends caught more D. confusus than the unbaited traps, but only traps with the 9:2 (+):(±) blend released at 0.3 and 0.03 mg per day caught significantly more male and female beetles than those baited with the standard bait. Thus, trap sensitivity can be improved with the addition of (±)-endo-brevicomin. The sympatric bark beetle D. autographus Ratzeburg was captured in significant numbers in traps baited with (±)-exo-brevicomin. A subsequent trapping experiment showed that D. autographus responded to (+)- or (±)-exo-brevicomin, but not to (−)-exo-brevicomin, suggesting that (+)-exo-brevicomin is the principal aggregation pheromone component in this species.

Fast- and slow-growing subalpine fir produce lesions of different sizes in response to inoculation with a blue-stain fungus associated with Dryocoetes confusus (Coleoptera: Scolytidae)

Trees with low vigor and reduced radial growth may be more susceptible to attack by bark beetles because of reduced host defenses. Fungi associated with bark beetles may be used to elicit an induced defense response from the host. A blue-stain fungus isolated from Dryocoetes confusus Swaine was used to examine the morphology of the hypersensitive response of fast- and slow-growing subalpine fir trees in British Columbia. Twenty fast-growing and 20 slow-growing trees were inoculated with the blue-stain isolate, and the dimensions of the resultant lesions were compared between fast- and slow-growing trees and between fungus and control treatments at 3, 7, 10, 17, and 41 d after inoculation. The length and width of the lesions was greater in response to fungus versus control treatments at 7, 10, 17, and 41 d after inoculation. The length of the lesions was significantly greater in fast- than in slow-growing trees at 7, 10, and 17 d after inoculation. There was no significant difference in the size of the lesions between fast- and slow-growing trees 41 d after inoculation. The temporary difference in the size of the lesions between fast- and slow-growing trees suggests that host vigor affects the induced defense response within a certain time frame.Key words: Abies lasiocarpa, Dryocoetes confusus, host vigor, defense, blue-stain fungi, Ophiostoma.

Effects of Western Balsam Bark Beetle on Spruce-Fir Forests of North-Central Wyoming

Western balsam bark beetle, Dryocoetes confusus (Coleoptera: Scolytidae), has caused widespread mortality of subalpine fir (Abies lasiocarpa) in western North America throughout the past decade. The objectives of this study were to document the effects of this mortality, relate mortality to pre-existing stand conditions, and investigate the role of storm-damaged fir in beetle population dynamics in north-central Wyoming. Transect cruise lines and pairs of infested and uninfested plots were installed to detect changes in the forest overstory and understory and to determine associations between stand conditions and beetle-caused fir mortality. On average, beetles killed more than 70 trees/ac over the last several years. This mortality resulted in significant decreases in: subalpine fir basal area, trees per acre, stand density index, and the percentage of subalpine fir stems in the overstory. Small, but significant increases were detected in the understory; herbaceous plant abundance increased in the infested plots compared with the noninfested plots. Moreover, significant positive linear relationships were found between the amount of fir mortality and the percentage of subalpine fir trees in a stand, subalpine fir basal area, and subalpine fir stand density index. In addition, a significant positive linear relationship was found between the percentage of wind-caused downed fir logs in an area and the percentage of logs utilized by western balsam bark beetle. The blowdown events that occurred in the mid-1990s in combination with a high percentage of fir component has provided ideal conditions for continued beetle expansion. West. J. Appl. For. 18(4):259–266.

Characteristics of subalpine fir susceptible to attack by western balsam bark beetle (Coleoptera: Scolytidae)

A diameter distribution survey at three sites in the interior of British Columbia revealed that the western balsam bark beetle (Dryocoetes confusus Swaine) predominately attacked trees from the three to four largest diameter classes at each site. However, the mean diameter of attacked trees was significantly different among sites, indicating that factors other than diameter contribute to the susceptibility of subalpine fir (Abies lasiocarpa (Hook.) Nutt.) to the western balsam bark beetle. A number of tree characteristics, including measures of growth, age, crown size, and phloem thickness, were compared among a total of 22 successfully attacked, 26 unsuccessfully attacked, and 28 unattacked trees at three sites. Of the 12 tree characteristics measured, five showed significant differences between successfully attacked and unattacked trees. Successfully attacked trees had a lower percentage of the bole covered with constant crown, lower crown volume, lower radial growth in the last 5 years, and were older than unattacked trees. Successfully attacked trees also produced less induced resinosis than unsuccessfully attacked trees. The results of this study suggest that western balsam bark beetle mortality may be limited by the presence and distribution of susceptible hosts. The study also identifies a number of variables that could be used in a susceptibility and risk rating model for western balsam bark beetle.