Celia K. Boone

Parasitoids and Dipteran Predators Exploit Volatiles from Microbial Symbionts to Locate Bark Beetles

Abstract Host location by parasitoids and dipteran predators of bark beetles is poorly understood. Unlike coleopteran predators that locate prey by orienting to prey pheromones, wasps and flies often attack life stages not present until after pheromone production ceases. Bark beetles have important microbial symbionts, which could provide sources of cues. We tested host trees, trees colonized by beetles and symbionts, and trees colonized by symbionts alone for attractiveness to hymenopteran parasitoids and dipteran predators. Field studies were conducted with Ips pini in Montana. Three pteromalid wasps were predominant. All were associated with the second and third instars of I. pini. Heydenia unica was more attracted to logs colonized by either I. pini or the fungus Ophiostoma ips than logs alone or blank controls (screen with no log). Rhopalicus pulchripennis was more attracted to logs colonized by I. pini than logs alone or blank controls. Dibrachys cavus was attracted to logs but did not distinguish whether or not they were colonized. Two dolichopodid predators were predominant. A Medetera species was more attracted to colonized than uncolonized logs and more attracted to logs than blank controls. It was also more attracted to logs colonized with the yeast Pichia scolyti than uncolonized logs, but attraction was less consistent. An unidentified dolichopodid was more attracted to logs colonized with I. pini, O. ips, and the bacteria Burkholderia sp., than to uncolonized logs. It was also attracted to uncolonized logs. Its responses were less consistent and pronounced than H. unica. These results suggest some parasitoids and dipteran predators exploit microbial symbionts of bark beetles to locate hosts. Overall, specialists showed strong attraction to fungal cues, whereas generalists were more attracted by plant volatiles. These results also show how microbial symbionts can have conflicting effects on host fitness.

Responses of Bark Beetle-Associated Bacteria to Host Monoterpenes and Their Relationship to Insect Life Histories

Bark beetles that colonize living conifers and their microbial associates encounter constitutive and induced chemical defenses of their host. Monoterpene hydrocarbons comprise a major component of these allelochemicals, and many are antibiotic to insects, fungi, and bacteria. Some bark beetle species exhaust these defenses by killing their host through mass attacks mediated by aggregation pheromones. Others lack adult aggregation pheromones and do not engage in pheromone-mediated mass attacks, but rather have the ability to complete development within live hosts. In the former species, the larvae develop in tissue largely depleted of host terpenes, whereas in the latter exposure to these compounds persists throughout development. A substantial literature exists on how monoterpenes affect bark beetles and their associated fungi, but little is known of how they affect bacteria, which in turn can influence beetle performance in various manners. We tested several bacteria from two bark beetle species for their ability to grow in the presence of a diversity of host monoterpenes. Bacteria were isolated from the mountain pine beetle, Dendroctonus ponderosae Hopkins, which typically kills trees during colonization, and the red turpentine beetle, Dendroctonus valens LeConte, which often lives in their host without causing mortality. Bacteria from D. ponderosae were gram-positive Actinobacteria and Bacilli; one yeast also was tested. Bacteria from D. valens were Actinobacteria, Bacilli, and γ-Proteobacteria. Bacteria from D. valens were more tolerant of monoterpenes than were those from D. ponderosae. Bacteria from D. ponderosae did not grow in the presence of α-pinene and 3-carene, and grew in, but were inhibited by, β-pinene and β-phellandrene. Limonene and myrcene had little inhibitory effect on bacteria from either beetle species. Tolerance to these antibiotic compounds appears to have resulted from adaptation to living in a terpene-rich environment.

The enemy of my enemy is still my enemy: competitors add to predator load of a tree-killing bark beetle

1 The mountain pine beetle Dendroctonus ponderosae is a major tree‐killing bark beetle in North America. We evaluated how the subsequent arrival of a competing bark beetle Ips pini influences the arrival of predators and their impact on both species.

How the mountain pine beetle (Dendroctonus ponderosae) breached the Canadian Rocky Mountains

The mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins), a major pine forest pest native to western North America, has extended its range north and eastward during an ongoing outbreak. Determining how the MPB has expanded its range to breach putative barriers, whether physical (nonforested prairie and high elevation of the Rocky Mountains) or climatic (extreme continental climate where temperatures can be below −40 °C), may contribute to our general understanding of range changes as well as management of the current epidemic. Here, we use a panel of 1,536 single nucleotide polymorphisms (SNPs) to assess population genetic structure, connectivity, and signals of selection within this MPB range expansion. Biallelic SNPs in MPB from southwestern Canada revealed higher genetic differentiation and lower genetic connectivity than in the northern part of its range. A total of 208 unique SNPs were identified using different outlier detection tests, of which 32 returned annotations for products with putative functions in cholesterol synthesis, actin filament contraction, and membrane transport. We suggest that MPB has been able to spread beyond its previous range by adjusting its cellular and metabolic functions, with genome scale differentiation enabling populations to better withstand cooler climates and facilitate longer dispersal distances. Our study is the first to assess landscape-wide selective adaptation in an insect. We have shown that interrogation of genomic resources can identify shifts in genetic diversity and putative adaptive signals in this forest pest species.

Dispersal and edge behaviour of bark beetles and predators inhabiting red pine plantations

1 Quantifying dispersal in predator–prey systems can improve our understanding of how these species interact in space and time, as well as their relative distributions across complex landscapes. 2 We measured the dispersal abilities of three forest insects associated with red pine decline: the eastern five spined pine engraver Ips grandicollis (Coleoptera: Curculionidae), its main predator Thanasimus dubius (Coleoptera: Cleridae) and the basal stem and root colonizer Dendroctonus valens (Coleoptera: Curculionidae). We also examined the edge behaviours of these species and the predator Platysoma spp (Coleoptera: Histeridae) between red pine stands (habitat) and clearings (nonhabitat). 3 Thanasimus dubius dispersed 12 times farther than its prey I. grandicollis, with 50% of predators dispersing farther than 1.54 km. This profound difference in dispersal behaviour between prey and predator may contribute to the clumped distribution of I. grandicollis. 4 Most T. dubius and D. valens were confined in the pine forest, thus showing strong edge behaviour. This differed from I. grandicollis and Platysoma spp., which were commonly found in open areas adjacent to red pine plantations. 5 The bark beetle I. grandicollis and one of its main predators, T. dubius, exhibited different patterns of movement within a fragmented landscape. Despite a greater dispersal ability of T. dubius within forests, the spatial distribution of this predator may be restricted by fragmentation of its habitat, and provide an opportunity for partial escape of its prey. 6 The present study contributes to our knowledge of top‐down forces within red pine stands undergoing decline. Differences of dispersal patterns and edge behaviour could contribute to the initiation of new pockets of decline, as well as the connectedness among existing ones.

Tree response and mountain pine beetle attack preference, reproduction and emergence timing in mixed whitebark and lodgepole pine stands

Mountain pine beetle (Dendroctonus ponderosae) is an important disturbance agent in Pinus ecosystems of western North America, historically causing significant tree mortality. Most recorded outbreaks have occurred in mid elevation lodgepole pine (Pinus contorta). In warm years, tree mortality also occurs at higher elevations in mixed species stands. Mountain pine beetles relative preference for and performance in Pinus species that either commonly or less frequently encounter this insect has received little direct testing. Further, knowledge of the relative proportions of secondary compounds, which can differ among Pinus species and play important roles in attack rates and outcomes, is important to understanding host suitability. We monitored mountain pine beetle attacks, adult emergence timing and reproductive capacity in lodgepole and whitebark (Pinus albicaulis) pines growing in mixed stands at relatively high elevation. Phloem monoterpene chemistry of trees prior to and during attack was compared within and between species. Although beetles attacked lodgepole pine more frequently, lodgepole pines also resisted attacks more frequently. Overall, there were equal numbers of lethal attacks between species. Brood production and adult emergence timing did not differ between tree species. The relative composition of secondary compounds differed by tree species, although both species contained compounds that affect mountain pine beetle attack and reproductive success.

Genetic and genomic evidence of niche partitioning and adaptive radiation in mountain pine beetle fungal symbionts

Bark beetles form multipartite symbiotic associations with blue stain fungi (Ophiostomatales, Ascomycota). These fungal symbionts play an important role during the beetles life cycle by providing nutritional supplementation, overcoming tree defences and modifying host tissues to favour brood development. The maintenance of stable multipartite symbioses with seemingly less competitive symbionts in similar habitats is of fundamental interest to ecology and evolution. We tested the hypothesis that the coexistence of three fungal species associated with the mountain pine beetle is the result of niche partitioning and adaptive radiation using SNP genotyping coupled with genotype–environment association analysis and phenotypic characterization of growth rate under different temperatures. We found that genetic variation and population structure within each species is best explained by distinct spatial and environmental variables. We observed both common (temperature seasonality and the host species) and distinct (drought, cold stress, precipitation) environmental and spatial factors that shaped the genomes of these fungi resulting in contrasting outcomes. Phenotypic intraspecific variations in Grosmannia clavigera and Leptographium longiclavatum, together with high heritability, suggest potential for adaptive selection in these species. By contrast, Ophiostoma montium displayed narrower intraspecific variation but greater tolerance to extreme high temperatures. Our study highlights unique phenotypic and genotypic characteristics in these symbionts that are consistent with our hypothesis. By maintaining this multipartite relationship, the bark beetles have a greater likelihood of obtaining the benefits afforded by the fungi and reduce the risk of being left aposymbiotic. Complementarity among species could facilitate colonization of new habitats and survival under adverse conditions.

Adaptive and neutral markers both show continent‐wide population structure of mountain pine beetle (Dendroctonus ponderosae)

Abstract Assessments of population genetic structure and demographic history have traditionally been based on neutral markers while explicitly excluding adaptive markers. In this study, we compared the utility of putatively adaptive and neutral single‐nucleotide polymorphisms (SNPs) for inferring mountain pine beetle population structure across its geographic range. Both adaptive and neutral SNPs, and their combination, allowed range‐wide structure to be distinguished and delimited a population that has recently undergone range expansion across northern British Columbia and Alberta. Using an equal number of both adaptive and neutral SNPs revealed that adaptive SNPs resulted in a stronger correlation between sampled populations and inferred clustering. Our results suggest that adaptive SNPs should not be excluded prior to analysis from neutral SNPs as a combination of both marker sets resulted in better resolution of genetic differentiation between populations than either marker set alone. These results demonstrate the utility of adaptive loci for resolving population genetic structure in a nonmodel organism.

Assemblage of Hymenoptera arriving at logs colonized by Ips pini (Coleoptera: Curculionidae: Scolytinae) and its microbial symbionts in western Montana.

Abstract Colonization of a tree by bark beetles and their symbionts creates a new habitat for a diverse assemblage of arthropods, including competing herbivores, xylophages, fungivores, saprophages, predators, and parasitoids. Understanding these assemblages is important for evaluating nontarget effects of various management tactics and for subsequently evaluating how changes in climate, the presence of invasive species, and altered forestry practices and land-use tenure may affect biodiversity. We characterized the assemblage of hymenopterans attracted to logs of ponderosa pine (Pinus ponderosa C. Lawson (Pinaceae)) colonized by the bark beetle Ips pini (Say) and its microbial symbionts. In one experiment, the composition and relative abundances of species arriving at hosts colonized by I. pini, and possible sources of attraction, were determined. Treatments consisted of a log containing I. pini with its natural complement of microorganisms, a log alone, and a blank control. A second experiment was carried out to determine whether or not Hymenoptera were attracted to microbial symbionts of I. pini. Treatments consisted of a blank control, a log alone, a log containing I. pini with its natural complement of microorganisms, either Ophiostoma ips, Burkholderia sp., or Pichia scolyti, and a log inoculated with a combination of these three microorganisms. Over 2 years, 5163 Hymenoptera were captured, of which over 98% were parasitoids. Braconidae, Platygastridae, Encyrtidae, Pteromalidae, and Ichneumonidae were the most abundant. Seven known species of bark beetle parasitoids (all Pteromalidae) were captured. However, parasitoids of Diptera, Lepidoptera, Hymenoptera, and non-wood-boring Coleoptera were also common. Nineteen species showed preferential attraction to host plants infested with I. pini and its complement of microorganisms, host plants inoculated with I. pini microbial symbionts, or host plants alone. Interestingly, many of these species were parasitoids of phytophagous, fungivorous, and saprophytic insects rather than of bark beetles themselves. These results suggest that a diverse assemblage of natural enemies that attack various feeding guilds within a common habitat exploit common olfactory cues.

Monochamus species from different continents can be effectively detected with the same trapping protocol

Pine wilt disease is one of the most serious introduced threats to coniferous forests worldwide. Its causal agent, the pinewood nematode (PWN), Bursaphelenchus xylophilus, is vectored primarily by cerambycids of the genus Monochamus Dejean throughout its native (North America) and introduced (Japan, China, Korea, Taiwan, Portugal) ranges. Despite strict import regulations and phytosanitary measures, interception records indicate that PWN and Monochamus species continue to be moved worldwide. Following its introduction in Portugal in the late 1990s, extensive monitoring programs for PWN and its vectors have been conducted throughout the European Union, using locally developed and tested lures and traps. The trapping system developed in Europe and used in this study is composed of a Crosstrap® and Galloprotect Pack® lures. These trapping systems were deployed in two locations in the USA, two locations in Canada, and one location in China in order to test their capacity to detect Monochamus species exotic to Europe. Large numbers of M. carolinensis, M. mutator, M. notatus, M. s. scutellatus, M. clamator, and M. titillator were trapped in North America, while large numbers of M. alternatus were trapped in China. The trapping systems developed in Europe for monitoring the European Monochamus species are also effective for the detection of many exotic Monochamus species and could thus be used as an early detection tool in ports and other high-risk sites.