Western Pine Beetle Populations in Arizona and California Differ in the Composition of Their Aggregation Pheromones

An enhanced lure for eastern populations of the North American spruce beetle, Dendroctonus rufipennis Kirby (Coleoptera: Curculionidae)

Regional variation in pheromone production and response has practical implications for the use of semiochemical lures to monitor and control bark beetle populations. We tested 4 lure formulations including 2 new formulations that reflect the pheromone production profiles of western and eastern populations of spruce beetles, Dendroctonus rufipennis Kirby (Coleoptera: Curculionidae), as well as 2 commercially available formulations (current Rocky Mountain lure and current Atlantic lure), in 2 locations in New Brunswick, Canada. In 2 separate years, the new eastern lure containing seudenol, MCOL, and spruce terpenes captured 4 times (2021) and 11 times (2022) more spruce beetles than the current Atlantic lure that consisted of frontalin, seudenol, and spruce terpenes. In 2021, we also captured more eastern larch beetles, Dendroctonus simplex LeConte (Coleoptera: Curculionidae), with the new eastern lure, whereas in 2022, we captured the most D. simplex with the current Atlantic lure, suggesting that more research is needed on D. simplex pheromone production and response across its range. The bark beetle predator, Thanasimus dubius (Fabr.; Coleoptera: Cleridae), did not respond well to the new eastern blend that lacks frontalin, suggesting that response to frontalin is important in finding prey and might be conserved in predator populations. The reduced trap catch of T. dubius to the enhanced lure is beneficial because it does not inhibit natural population control by removing predators from the community. Our study reveals an improved trap lure for eastern populations of spruce beetles and highlights gaps and research needs in bark beetle pheromone ecology.

Rearranging and completing the puzzle: Phylogenomic analysis of bark beetles Dendroctonus reveals new hypotheses about genus diversification

Studies carried out on bark beetles within Dendroctonus have been extensive and revealed diverse information in different areas of their natural history, taxonomy, evolution, and interactions, among others. Despite these efforts, phylogenetic hypotheses have remained obscured mainly due to limited information analyzed (taxonomic, gene sampling, or both) in studies focused on obtaining evolutionary hypotheses for this genus. With the aim of filling these gaps in the evolutionary history for Dendroctonus, we analyzed ∼1800 loci mapped to a reference genome obtained for 20 of the 21 species recognized to date, minimizing the impact of missing information and improving the assumption of orthology in a phylogenomic framework. We obtained congruent phylogenetic topologies from two phylogenomic inference strategies: loci concatenation (ML framework) and a multispecies coalescent model (MSC) through the analysis of site pattern frequencies (SNPs). Dendroctonus is composed of two major clades (A and B), each containing five and four subclades, respectively. According to our divergence dating analysis, the MRCA for Dendroctonus dates back to the early Eocene, while the MRCA for each major clade diverged in the mid-Eocene. Interestingly, most of the speciation events of extant species occurred during the Miocene, which could be correlated with the diversification of pine trees (Pinus). The MRCA for Dendroctonus inhabited large regions of North America, with all ancestors and descendants of clade A having diversified within this region. The Mexican Transition Zone is important in the diversification processes for the majority of clade A species. For clade B, we identified two important colonization events to the Old World from America: the first in the early Oligocene from the Arctic to Asia (via Beringia), and the second during the Miocene from the Arctic-Western-Alleghany region to Europe and Siberia (also via Beringia). Our genomic analyses also supported the existence of hidden structured lineages within the frontalis complex, and also that D. beckeri represent a lineage independent from D. valens, as previously suggested. The information presented here updates the knowledge concerning the diversification of a genus with remarkable ecological and economic importance.

Plasticity of aggregation pheromones in insects

Pheromone plasticity is widely observed in insects and enhances their survival, adaptation, and reproductive success. Aggregation pheromones, which cause notable individual aggregation and consequently impact agriculture and human life, are renowned for their special function. Here, we present a review of research progress regarding pheromone plasticity in three typical aggregative insects: locusts, bark beetles, and cockroaches. These insects are major pest species with considerable impacts on the social economy and public health. Numerous studies have demonstrated the plasticity of aggregation pheromones in different populations of these insect species. Although pheromone chemicals and compositions vary across the three groups, the plasticity of aggregation pheromones is significantly impacted by population density, location, food resources, and gut symbiotic microorganisms, indicating the complexity of pheromone plasticity regulated by multiple factors. Finally, we discuss the potential application of pheromone plasticity in basic research and pest management.

Improving Natural Enemy Selection in Biological Control through Greater Attention to Chemical Ecology and Host-Associated Differentiation of Target Arthropod Pests

Host-associated differentiation (HAD) refers to cases in which genetically distinct populations of a species (e.g., herbivores or natural enemies) preferentially reproduce or feed on different host species. In agroecosystems, HAD often results in unique strains or biotypes of pest species, each attacking different species of crops. However, HAD is not restricted to pest populations, and may cascade to the third trophic level, affecting host selection by natural enemies, and ultimately leading to HAD within natural enemy species. Natural enemy HAD may affect the outcomes of biological control efforts, whether classical, conservation, or augmentative. Here, we explore the potential effects of pest and natural enemy HAD on biological control in agroecosystems, with emphases on current knowledge gaps and implications of HAD for selection of biological control agents. Additionally, given the importance of semiochemicals in mediating interactions between trophic levels, we emphasize the role of chemical ecology in interactions between pests and natural enemies, and suggest areas of consideration for biological control. Overall, we aim to jump-start a conversation concerning the relevance of HAD in biological control by reviewing currently available information on natural enemy HAD, identifying challenges to incorporating HAD considerations into biological control efforts, and proposing future research directions on natural enemy selection and HAD.

Evidence for Semiochemical Divergence Between Sibling Bark Beetle Species: Dendroctonus brevicomis and Dendroctonus barberi

We investigated geographic variation in the semiochemistry of major disturbance agents of western North American pine forests, Dendroctonus brevicomis Le Conte and Dendroctonus barberi Hopkins (Coleoptera: Curculionidae: Scolytinae), species separated by the Great Basin in the USA that until recently were synonymous. At 15 sites in the western USA and northern Mexico, beetle populations were examined to determine (1) pheromone production by solitary, mining females, (2) male electroantennogram amplitudes in response to known semiochemicals for the genus, or (3) relative attractiveness of two female-produced pheromone components (endo- and exo-brevicomin) and two host odors (alpha-pinene and myrcene) to beetles in the field. Compared to female beetles collected east of the Great Basin (D. barberi), western females (D. brevicomis) produced a consistently higher proportion of, and male antenna were correspondingly more sensitive to, the exo- than the endo-isomer of brevicomin. With the exception of one sampling location (where no preference was observed), beetles west of the Great Basin were more attracted to exo- than endo- brevicomin trap lures, whereas eastern beetles displayed the reverse preference. In contrast, there was not a consistent difference between these populations regarding relative attraction or olfactory response to myrcene or alpha-pinene, although some geographic variability was evident. These data show that the semiochemical systems of D. brevicomis and D. barberi have diverged and corroborate genetic and morphological evidence that they are distinct, allopatric species.

Local, Geographical, and Contextual Variation in the Aggregation Pheromone Blend of the Spruce Beetle, Dendroctonus rufipennis (Coleoptera: Curculionidae)

Prior research from trapping experiments using synthetic pheromone components suggested the existence of local and geographical variation in the pheromone system of the spruce beetle, Dendroctonus rufipennis (Kirby). To test hypotheses concerning variation in the spruce beetle's aggregation pheromone blend, we extracted volatiles from the hindguts of individual spruce beetles sampled from sites in eastern Canada (Nova Scotia and Newfoundland) and western Canada (British Columbia and Alberta) and quantified the amounts of four known aggregation pheromone components within each sample. Chiral analyses were performed on a subset of samples. Frontalin (1,5-dimethyl-6,8-dioxabicyclo[3.2.1]octane) was more dominant in western Canada, whereas MCOL (1-methyl-2-cyclohexen-1-ol) and seudenol (3-methyl-2-cyclohexen-1-ol) were more dominant in the east. Verbenene (4-methylene-6,6-dimethylbicyclo[3.1.1]hept-2-ene) was prevalent only in one of our six sites (Rocky Mountain House, AB). Female beetles in Rocky Mountain House (AB) also produced a higher proportion of (+)-frontalin than females from the other sites, and a comparison to previously published data suggests additional geographic variation that was not captured among our sites. When paired with a male in a gallery, female spruce beetles produced pheromone blends with more frontalin and less MCOL compared to solitary female beetles. Our results show that variation in the spruce beetle's pheromone blend exists at both large and small spatial scales, possibly a consequence of local selective pressures and assortative mating.

Cryptic Species Discrimination in Western Pine Beetle, Dendroctonus brevicomis LeConte (Curculionidae: Scolytinae), Based on Morphological Characters and Geometric Morphometrics

The western pine beetle (WPB), Dendroctonus brevicomis LeConte, is a major mortality agent of pines in North America. A total of 706 adults of WPB from 81 geographical sites were analyzed with traditional and geometric morphometric methods to evaluate the variation of discrete and quantitative morphological characters with particular attention to the antenna, spermatheca, and seminal rod. Principal coordinates and canonical variate analyses supported three geographical groups in WPB: (1) West, from British Columbia to southern California along the Pacific coast, Idaho, and Montana; (2) East-SMOC, including Nevada, Utah, Colorado, Arizona, New Mexico, Texas, Chihuahua, and Durango; and (3) SMOR, including Coahuila, Nuevo Leon, and Tamaulipas. The pubescence length on the elytral declivity was a robust character for separating West specimens from the other groups. Additionally, the genitalia shape both female and male in dorsal view was a reliable character for discriminating among groups. Based on these results, which agree with genetic and chemical ecology evidence, we herein reinstate Dendroctonus barberi Hopkins (East-SMOC group) and remove it from synonymy with D. brevicomis (West group). Differences in the spermatheca and seminal rod shape of SMOR specimens suggest that these populations might be a different species from D. barberi and D. brevicomis.

Cascading speciation among mutualists and antagonists in a tree-beetle-fungi interaction

Cascading speciation is predicted to occur when multiple interacting species diverge in parallel as a result of divergence in one species promoting adaptive differentiation in other species. However, there are few examples where ecological interactions among taxa have been shown to result in speciation that cascades across multiple trophic levels. Here, we test for cascading speciation occurring among the western pine beetle (Dendroctonus brevicomis), its primary host tree (Pinus ponderosa), and the beetle's fungal mutualists (Ceratocystiopsis brevicomi and Entomocorticium sp. B). We assembled genomes for the beetle and a fungal symbiont and then generated reduced representation genomic data (RADseq) from range-wide samples of these three interacting species. Combined with published data for the host tree, we present clear evidence that the tree, the beetle, and the fungal symbionts are all genetically structured into at least two distinct groups that have strongly codiverged with geographical isolation. We then combine our genomic results with diverse population and laboratory-based data to show evidence for reproductive isolation at each level of the cascade and for coevolution of both antagonistic and mutualistic species interactions within this complex network.

Management of Western North American Bark Beetles with Semiochemicals

We summarize the status of semiochemical-based management of the major bark beetle species in western North America. The conifer forests of this region have a long history of profound impacts by phloem-feeding bark beetles, and species such as the mountain pine beetle (Dendroctonus ponderosae) and the spruce beetle (D. rufipennis) have recently undergone epic outbreaks linked to changing climate. At the same time, great strides are being made in the application of semiochemicals to the integrated pest management of bark beetles. In this review, we synthesize and interpret these recent advances in applied chemical ecology of bark beetles for scientists and land managers.

The Current Status of the Distribution Range of the Western Pine Beetle, Dendroctonus brevicomis (Curculionidae: Solytinae) in Northern Mexico

The distribution range of the western pine beetle Dendroctonus brevicomis LeConte (Coleoptera: Curculionidae) is supported only by scattered records in the northern parts of Mexico, suggesting that its populations may be marginal and rare in this region. In this study, we review the geographical distribution of D. brevicomis in northern Mexico and perform a geometric morphometric analysis of seminal rod shape to evaluate its reliability for identifying this species with respect to other members of the Dendroctonus frontalis Zimmermann (Coleoptera: Curculionidae) complex. Our results provide 30 new records, with 26 distributed in the Sierra Madre Occidental and 4 in the Sierra Madre Oriental. These records extend the known distribution range of D. brevicomis to Durango and Tamaulipas states in northern Mexico. Furthermore, we find high geographic variation in size and shape of the seminal rod, with conspicous differences among individuals from different geographical regions, namely west and east of the Great Basin and between mountain systems in Mexico.