Toxicity of two Engelmann spruce (Pinaceae) monoterpene chemotypes from the southern Rocky Mountains to North American spruce beetle (Coleoptera: Scolytidae)

Engelmann spruce, Picea engelmannii Parry ex Engelm. (Pinaceae), in the southern Rocky Mountains is composed of two distinct phloem monoterpene chemotypes that differ in relative abundances of multiple monoterpenes, particularly α-pinene and Δ3-carene (hereafter, the “α-pinene chemotype” and the “Δ3-carene chemotype”). Here, relative toxicity of these chemotypes is tested on spruce beetle (Dendroctonus rufipennis Kirby) (Coleoptera: Scolytinae), a phloeophagous herbivore that colonises trees of both types. Synthetic monoterpene blends representing each chemotype were tested across a range of concentrations (0, 10, 50, 100, 200, and 500 µg/L) in the lab, and probability of survival of adult beetles exposed to each blend was modelled using a logit function. Logit curves were solved to determine LC25, LC50, and LC75 of each monoterpene blend. On average, probability of beetle survival was lower when exposed to the Δ3-carene chemotype than when exposed to the α-pinene chemotype. However, both chemotypes were completely lethal to beetles at concentrations exceeding 100 µg/L. Adult body mass did not affect survival probability. It is concluded that spruce phloem chemotypes may differ in their toxicity to spruce beetles, with potential consequences for patterns of host-tree colonisation by spruce beetle.

Evaluations of Bole Injections for Protecting Engelmann Spruce from Mortality Attributed to Spruce Beetle (Coleoptera: Curculionidae) in the Intermountain West

Bark beetles are important disturbance agents in coniferous forests, and spruce beetle, Dendroctonus rufipennis (Kirby) (Coleoptera: Curculionidae), is one of the more notable species causing landscape-level tree mortality in western North America. We evaluated the efficacy of bole injections of emamectin benzoate (TREE-äge®; Arborjet Inc., Woburn, MA) alone and combined with propiconazole (Alamo®; Syngenta Crop Protection Inc., Wilmington, DE) for protecting Engelmann spruce, Picea engelmannii Parry ex Engelmann (Pinales: Pinaceae), from mortality attributed to colonization by D. rufipennis. Two injection periods in 2013 (the spring and fall of the year prior to trees first being challenged by D. rufipennis in 2014) and distributions of injection points (7.6- and 15.2-cm spacings) were evaluated. Tree mortality was monitored over a 3-yr period (2014–2017). Emamectin benzoate injected in spring at a narrow spacing (7.6 cm) was the only effective treatment. Two (but not three) field seasons of protection can be expected with a single injection of this treatment. We discuss the implications of these and other results regarding the use of emamectin benzoate and propiconazole for protecting western conifers from mortality attributed to bark beetles, and provide suggestions for future research. A table summarizing the appropriate timing of treatments in different bark beetle/host systems is provided.

Improved Identification and New Records of Dendroctonus Bark Beetles Attacking Pinus contorta in the Subalpine Forest of the Southern Rocky Mountains

Research Highlights: Atypical and poorly understood attacks by Dendroctonus rufipennis (Kirby) to Pinus contorta Doug. ex Loudon were detected in the southern Rocky Mountains (SRM). The phenomenon is confirmed across all examined area. Its reproduction is described for the first time as well as the first attacks of D. adjunctus Blandf. in that host. Improved detection and diagnostics of D. rufipennis will allow a simpler, and efficient identification of the species. It will improve the detection capacity by pest detection specialists and entomologists, which will increase our understanding of the phenomena within and beyond the known range. Background and Objectives: In addition to D. ponderosae Hopk. other Dendroctonus species, sometimes together, attacked P. contorta that grew intermixed with Picea engelmannii in the SRM’ subalpine forest. The identification of these beetles was difficult. The goal was to improve the detection and identification of the species from similar Dendroctonus spp. attacking that host and to uncover biological facts about the phenomena. Materials and Methods: Dendroctonus attacking P. contorta were collected along the entire SRM, their attack signs and behavior were recorded. These characteristics were revised from those in the literature and new characters were introduced and tested. Results: The identification of Dendroctonus bark beetles attacking P. contorta in the SRM was improved using revised and new characters including attack signs, attack behavior, and adult beetle characters. An improved identification key couplet is presented to effectively distinguish D. murrayanae from D. rufipennis. Conclusions: Simplified insect identifications that are both accessible to users with different levels of expertise and are based on insect characters, their attack pattern, and signs, like the present, improve detection of insects of interest. Efficient insect detections allow a better understanding of the capabilities they have and the impact they cause to the woodland ecosystems we study, protect, and manage around the globe.

Disturbance history modulates how litter and herbaceous cover influence conifer regeneration after fire

Climate-driven increases in disturbance frequency and extent augment the potential for compounded disturbances. Drawing on well-studied forests that experienced successive disturbances, we asked: (1) how does post-fire cover of litter, herbaceous cover and bare ground vary between stands affected by combinations of blow-down, insect outbreak, and fire? (2) How do post-fire relationships between ground cover and conifer regeneration vary with recent disturbance history? We measured ground cover and conifer regeneration from 2003 to 2014 following stand-replacing fires in 2002. Burned stands were either blown down in 1997, affected by a 1940s Dendroctonus rufipennis (spruce beetle; SB) outbreak, or neither. Implementing mixed-effects models, we measured the relationships between pre-fire stand attributes (structural stage, canopy dominance and combination of disturbances) and post-fire ground cover and between post-fire ground cover and conifer regeneration. Fire-only stands had more litter and herbaceous cover post fire than other stands (P<0.05). Fir regeneration increased with litter in stands that only burned, but decreased with litter in stands that were first blown down. Similarly, pine and fir regeneration increased with herbaceous cover after fire-only, but did the opposite in stands affected by the SB outbreak. Pre-fire legacies can modulate the effects of ground cover on plant regeneration.

Experimental Evidence Supporting an Obligate Adult Diapause for Spruce Beetle (Coleoptera: Curculionidae) from British Columbia

Insects respond to environmental conditions with a variety of developmental responses which include changing developmental rates, or suppressing development altogether in quiescence or diapause. Such responses have important implications for survival and voltinism. The spruce beetle, Dendroctonus rufipennis Kirby (Coleoptera: Curculionidae), has long been assumed to have an obligatory adult diapause based on field observations that adults always overwinter prior to dispersing and reproducing; however, it has never been experimentally demonstrated and recent evidence suggests that adults in some populations may exhibit facultative tendencies. We examined the emergence of adults at a constant 22°C after exposure to 2°C for 0 (no cold), 25, 50, and 75 d. Our observations of a delayed and extremely protracted emergence period in the absence of a substantive cold period, which could last several months, in combination with rapid, synchronous mass emergence after a substantive cold period, are most consistent with an obligate diapause. Approximately 50% of individuals could complete diapause in 55 d at 2°C, and 75 d of cold was adequate for almost all individuals to complete diapause. Beetles that emerged slowly over time may still have been in diapause; however, additional research would be required to test this hypothesis. Conditions throughout the beetle’s range in Canada should facilitate successful completion of diapause. An obligate adult diapause for spruce beetle populations in western Canada would limit the shortest possible life cycle to 1 yr, but would also serve to maintain an adaptive 1-yr life cycle, especially under a changing climate.

Advances in Semiochemical Repellents to Mitigate Host Mortality From the Spruce Beetle (Coleoptera: Curculionidae)

We tested 3-methyl-2-cyclohexen-1-one (MCH) and novel semiochemicals as potential spruce beetle (Dendroctonus rufipennis Kirby) (Coleoptera: Curculionidae, Scolytinae) repellents over multiple years in Utah and Colorado trapping bioassays. MCH is a known spruce beetle repellent and our testing revealed Acer kairomone blend (AKB) and isophorone plus sulcatone as repellents. We subsequently tested these semiochemicals for area and single tree protection to prevent spruce beetle attacks at locations in Utah, Colorado, Wyoming, New Mexico, and Alaska. Individual tree protection trials found MCH–AKB provided significant protection against spruce beetle attacks in the southern Rocky Mountains but not in Alaska. Adding sulcatone or doubling MCH–AKB pouches did not further enhance protection. A degree of protection was extended to spruce at least 10 m distant from the repellents, including in Alaska. Tree diameter was not a significant covariate among treated trees but was positively correlated with the probability of infestation for surrounding spruce. In area protection trials, spruce in control plots were 2.4 times more likely to be in a higher severity attack class compared with spruce in plots treated with MCH–AKB pouches deployed at 30 sets per hectare. Tree diameter had a significant, positive relationship to the probability of infestation. We found MCH–AKB to offer a high degree of protection against beetle attack in Engelmann spruce (Picea engelmannii Parry ex Engelm.) (Pinales: Pinaceae) (Picea engelmannii Parry ex Engelm.) (Pinales: Pinaceae), especially for single tree protection (66% of control trees were strip- or mass-attacked compared with 6% of repellent-treated trees). AKB requires registration and labeling, however, before this economical and environmentally benign semiochemical can be used operationally.

Effects of Site Thermal Variation and Physiography on Flight Synchrony and Phenology of the North American Spruce Beetle (Coleoptera: Curculionidae, Scolytinae) and Associated Species in Colorado

Spruce beetle, Dendroctonus rufipennis Kirby, is associated with forest mortality in Colorado and across western North America, yet it is not well understood how thermal variability affects basic population processes such as flight phenology. However, phenology–temperature relationships are important for understanding patterns of ecosystem disturbance, especially under projected climate warming. Here, we use a multiyear trapping study to test the hypothesis that spruce beetle flight synchrony, timing, and fitness traits (body size) are affected by variation in regional temperature and physiography. Large quantities of co-colonizing scolytines (Polygraphus convexifrons) (Coleoptera: Curculionidae, Scolytinae) and predatory beetles (Thanasimus undulatus) (Coleoptera: Cleridae) that may affect D. rufipennis populations also responded to spruce beetle synthetic pheromone lures. Relationships between flight patterns and environmental conditions were also analyzed for these species. The winter of 2018 was warmer and drier than winter 2017 and was associated with earlier flight for both scolytine species across most sites. The most important environmental factor driving D. rufipennis flight phenology was accumulated growing degree-days, with delayed flight cessation under warmer conditions and larger beetles following a warm winter. Flight was consistently more synchronous under colder growing season conditions for all species, but synchrony was not associated with winter temperatures. Warmer-than-average years promoted earlier flight of D. rufipennis and associated species, and less synchronous, prolonged flight across the region. Consequently, climate warming may be associated with earlier and potentially extended biotic pressure for spruce trees in the Rocky Mountain region, and flight phenology of multiple scolytines is plastic in response to thermal conditions.