Developing semiochemical repellents for protecting Picea from Dendroctonus rufipennis (Coleoptera: Curculionidae) in Alaska and Utah, USA.

Spruce beetle, Dendroctonus rufipennis (Kirby) (Coleoptera: Curculionidae), is the most destructive pest of mature spruce (Picea) in western North America. Recent outbreaks in Alaska and other western US states highlight the need for tools to protect Picea from D. rufipennis. The primary antiaggregation pheromone of D. rufipennis (3-methylcyclohex-2-en-1-one, MCH) and various combinations of potential repellents (1-octen-3-ol, exo-brevicomin, endo-brevicomin, ipsdienol, ipsenol, limonene, and verbenone) were tested for their ability to disrupt the response of D. rufipennis to attractant-baited multiple-funnel traps. Two assays were conducted on the Kenai Peninsula, Alaska, in June and July 2021. All treatments significantly reduced the mean number of D. rufipennis caught compared to the baited control. No other significant differences were observed among treatments. Informed by these and other data, tree protection studies were established in Lutz spruce, Picea × lutzii, on the Kenai Peninsula in 2022 and in Engelmann spruce, Pi. engelmannii, in the Uinta Mountains, Utah, in 2021. All experimental trees were baited with frontalin. Repellent treatments included MCH (SPLAT MCH, ISCA Inc., Riverside, CA, USA) and at least 1 additional repellent combination. In Alaska, all treatments significantly reduced colonization (strip attacks + mass attacks) and mortality of individually treated Pi. × lutzii and all Picea within 11.3-m radius of each treated Pi. × lutzii compared to the control. In Utah, all treatments except for SPLAT MCH + octenol significantly reduced colonization compared to the control. Only SPLAT MCH + Acer kairomone blend (AKB) and SPLAT MCH + octenol reduced Pi. engelmannii mortality compared to the control. SPLAT MCH + AKB and SPLAT MCH + acetophenone and green leaf volatiles (PLUS) were the most effective across both studies. The implications of these and other results to the development of an effective semiochemical repellent for D. rufipennis are discussed.

MCH-Based Semiochemical Repellents for Protecting Engelmann Spruce Trees From Dendroctonus rufipennis (Coleoptera: Curculionidae).

Spruce beetle, Dendroctonus rufipennis Kirby (Coleoptera: Curculionidae), is a lethal pest of spruce trees in North America. Despite decades of research, a semiochemical repellent that consistently and effectively protects spruce trees remains elusive. We evaluated the efficacy of 3-methyl-2-cyclohexen-1-one (MCH) in a proprietary, volatile compound release technology (SPLAT) alone and with two adjuvants, Acer kairomone blend (AKB) and acetophenone + green leaf volatiles (PLUS) to protect individually treated Picea engelmannii Parry ex. Engelm. (Pinales: Pinaceae), and Pi. engelmannii within 11.3-m radius of the individually treated trees from colonization and mortality attributed to D. rufipennis in western Wyoming. Ninety-one Pi. engelmannii were baited with frontalin and randomly assigned to one of seven treatments (n = 13): 3.5 g of MCH applied as SPLAT MCH (SPLAT3.5), 3.5AKB, 3.5PLUS, 7 g of MCH applied as SPLAT MCH (SPLAT7), 7AKB, 7PLUS, and baited control (bait only). All repellents except SPLAT3.5 and SPLAT7 significantly reduced colonization of individually treated Pi. engelmannii compared to the baited control. 3.5PLUS, 7AKB, and 7PLUS reduced colonization most effectively, and all repellents significantly reduced mortality of individually treated Pi. engelmannii compared to the baited control. All repellents also significantly reduced colonization and mortality of neighboring Pi. engelmannii.

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.

Efficacy of Spring and Fall Treatments of Carbaryl for Protecting Ponderosa Pine From Mortality Attributed to Mountain Pine Beetle (Coleoptera: Curculionidae).

High-value trees, such as those growing in residential, recreational, or administrative sites, are often susceptible to colonization by bark beetles (Coleoptera: Curculionidae: Scolytinae) as a result of increased amounts of stress associated with off-site plantings, drought, soil compaction, and/or mechanical injury. The value of these trees, cost of removing dead trees, and loss of aesthetics often justify the use of insecticides to protect trees from mortality attributed to bark beetles. Carbaryl (1-naphthyl methylcarbamate) is among the most effective, economically-viable, and ecologically-compatible insecticides available for protecting conifers from several species of bark beetles in the western United States. Treatments are usually applied in spring prior to initiation of flight of the target species. We evaluated the efficacy of spring and fall applications of carbaryl for protecting individual ponderosa pine, Pinus ponderosa Dougl. ex Laws. (Pinales: Pinaceae), from mortality attributed to mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae), in Idaho. Both spring and fall treatments of 2.0% a.i. carbaryl (maximum label rate; Sevin SL, Bayer Environmental Science, Montvale, NJ 07645) provided one field season of protection, and thus should be applied annually if tree protection is desired for multiple years. Our research also provides some insight on the efficacy of carbaryl treatments after wildfire. We found no evidence that a mixed-severity wildfire negatively affected the efficacy of carbaryl treatments.

3-Methylcyclohex-2-en-1-one for area and individual tree protection against spruce beetle (Coleoptera: Curculionidae: Scolytinae) attack in the southern Rocky Mountains.

We tested 3-methylcyclohex-2-en-1-one (MCH) and an Acer kairomone blend (AKB) as repellent semiochemicals for area and single tree protection to prevent spruce beetle (Dendroctonus rufipennis Kirby) attacks at locations in Utah and New Mexico. In the area protection study, we compared host infestation rates of MCH applications at three densities (20, 40, and 80 g MCH ha-1) against a control treatment over 0.64 ha plots centered within ~1.25 ha treatment blocks. All treatments included two baited funnel traps within the plot to assure spruce beetle pressure. Following beetle attack, plots were surveyed for new spruce beetle attacks and to quantify stand characteristics. The probability of more severe spruce beetle attacks was significantly reduced, by ~50%, in each of the MCH area treatments compared with the control treatment but there was no significant treatment difference among the MCH deployment densities. For the single tree protection study, we compared attack rates of MCH, Acer kairomone blend (AKB), and MCH plus AKB on spruce beetle-baited trees against bait-only trees. Each treatment was applied over a range of host diameters to test for host size effects. Seventy-five percent of control trees were mass-attacked, about one-third of MCH- and AKB-alone spruce was mass-attacked, and no MCH plus AKB spruce were mass-attacked. These results suggest that MCH alone is a marginal area and single tree protectant against spruce beetle but that deployment with other repellents can significantly increase treatment efficacy.

Lethal Trap Trees and Semiochemical Repellents as Area Host Protection Strategies for Spruce Beetle (Coleoptera: Curculionidae, Scolytinae) in Utah.

We tested lethal trap trees and repellent semiochemicals as area treatments to protect host trees from spruce beetle (Dendroctonus rufipennis Kirby) attacks. Lethal trap tree treatments ("spray treatment") combined a spruce beetle bait with carbaryl treatment of the baited spruce. Repellent treatments ("spray-repellent") combined a baited lethal trap tree within a 16-m grid of MCH (3-methylcyclohex-2-en-1-one) and two novel spruce beetle repellents. After beetle flight, we surveyed all trees within 50 m of plot center, stratified by 10-m radius subplots, and compared attack rates to those from baited and unbaited control plots. Compared to the baited controls, spruce in the spray treatment had significantly reduced likelihood of a more severe attack classification (e.g., mass-attacked over strip-attacked or unsuccessful-attacked over unattacked). Because spruce in the spray treatment also had significantly heightened probability of more severe attack classification than those in the unbaited controls, however, we do not recommend lethal trap trees as a stand-alone beetle suppression strategy for epidemic beetle populations. Spruce in the spray-repellent treatment were slightly more likely to be classified as more severely attacked within 30 m of plot center compared to unbaited controls but, overall, had reduced probabilities of beetle attack over the entire 50-m radius plots. The semiochemical repellents deployed in this study were effective at reducing attacks on spruce within treated plots despite the presence of a centrally located spruce beetle bait. Further testing will be required to clarify operational protocols such as dose, elution rate, and release device spacing.