Brian H. Aukema

Cross-scale Drivers of Natural Disturbances Prone to Anthropogenic Amplification: The Dynamics of Bark Beetle Eruptions

ABSTRACT Biome-scale disturbances by eruptive herbivores provide valuable insights into species interactions, ecosystem function, and impacts of global change. We present a conceptual framework using one system as a model, emphasizing interactions across levels of biological hierarchy and spatiotemporal scales. Bark beetles are major natural disturbance agents in western North American forests. However, recent bark beetle population eruptions have exceeded the frequencies, impacts, and ranges documented during the previous 125 years. Extensive host abundance and susceptibility, concentrated beetle density, favorable weather, optimal symbiotic associations, and escape from natural enemies must occur jointly for beetles to surpass a series of thresholds and exert widespread disturbance. Opposing feedbacks determine qualitatively distinct outcomes at junctures at the biochemical through landscape levels. Eruptions occur when key thresholds are surpassed, prior constraints cease to exert influence, and positive feedbacks amplify across scales. These dynamics are bidirectional, as landscape features influence how lower-scale processes are amplified or buffered. Climate change and reduced habitat heterogeneity increase the likelihood that key thresholds will be exceeded, and may cause fundamental regime shifts. Systems in which endogenous feedbacks can dominate after external forces foster the initial breach of thresholds appear particularly sensitive to anthropogenic perturbations.

Interactions among conifer terpenoids and bark beetles across multiple levels of scale: An attempt to understand links between population patterns and physiological processes

Introduction 80 Bark Beetles and Associated Microorganisms in Host Conifers 80 Localized Reactions: Constitutive and Induced Defenses 83 Whole Trees: Individual Tree Defenses and Group Colonization 85 Populationand LandscapeLevel Dynamics: Bimodal Equilibria, Allee Effects, and Extended Phenotypes 89 Constraints on Population Eruptions 99 How to Link the Scales? 107

Mountain Pine Beetles Colonizing Historical and Naïve Host Trees Are Associated with a Bacterial Community Highly Enriched in Genes Contributing to Terpene Metabolism

ABSTRACT The mountain pine beetle, Dendroctonus ponderosae, is a subcortical herbivore native to western North America that can kill healthy conifers by overcoming host tree defenses, which consist largely of high terpene concentrations. The mechanisms by which these beetles contend with toxic compounds are not well understood. Here, we explore a component of the hypothesis that beetle-associated bacterial symbionts contribute to the ability of D. ponderosae to overcome tree defenses by assisting with terpene detoxification. Such symbionts may facilitate host tree transitions during range expansions currently being driven by climate change. For example, this insect has recently breached the historical geophysical barrier of the Canadian Rocky Mountains, providing access to näive tree hosts and unprecedented connectivity to eastern forests. We use culture-independent techniques to describe the bacterial community associated with D. ponderosae beetles and their galleries from their historical host, Pinus contorta, and their more recent host, hybrid P. contorta-Pinus banksiana. We show that these communities are enriched with genes involved in terpene degradation compared with other plant biomass-processing microbial communities. These pine beetle microbial communities are dominated by members of the genera Pseudomonas, Rahnella, Serratia, and Burkholderia, and the majority of genes involved in terpene degradation belong to these genera. Our work provides the first metagenome of bacterial communities associated with a bark beetle and is consistent with a potential microbial contribution to detoxification of tree defenses needed to survive the subcortical environment.

Does aggregation benefit bark beetles by diluting predation? Links between a group-colonisation strategy and the absence of emergent multiple predator effects

Abstract.  1. Aggregation in bark beetles (Coleoptera: Scolytidae) aids in mate attraction and resource procurement when colonising well‐defended plants; however, some species colonise primarily poorly defended plants, and intraspecific competition increases mortality. The hypothesis that decreased risk of predation was a potential benefit to aggregation in such circumstances was tested, using the pine engraver, Ips pini (Say) and its two major predators Thanasimus dubius (F.) (Coleoptera: Cleridae) and Platysoma cylindrica (Paykull) (Coleoptera: Histeridae). Both single‐ and multiple‐predator effects, across a range of prey densities, were tested.

Improved Population Monitoring of Bark Beetles and Predators by Incorporating Disparate Behavioral Responses to Semiochemicals

Abstract Estimating populations of both pest and natural enemy species is important in the planning and implementation of biological control. For example, synthetic pheromone lures are used to sample bark beetles, and sometimes their predators, in forest ecosystems. However, insect attraction to natural pheromone sources may differ from attraction to synthetic pheromone lures. Moreover, these differences may vary systematically between the target pest and some important natural enemies. Thus, the accuracy of both absolute and relative abundances of bark beetles and predators could vary with lure selection. We evaluated a series of synthetic lures to determine which lure gave the closest approximation to actual numbers of Ips pini (Say) and predators arriving at hosts infested with I. pini in Wisconsin. We deployed synthetic lures containing various ratios of the (+) and (−) enantiomers of the principal I. pini pheromone component, ipsdienol, with or without an additional component, lanierone. I. pini showed strong preferences for specific enantiomeric ratios of ipsdienol, and these responses were synergized by lanierone. Predators showed equally strong attraction to ipsdienol, but preferred different ratios of the stereoisomers. The addition of lanierone had no effect on predators. The most abundant predator, Thanasimus dubius (F.), showed greater preference for host material infested with I. pini than any synthetic lure. These disparities in responses, combined with strong disparities in seasonal flight patterns, provided estimates of pest to predator ratios that varied by as little as 12% to as much as 12 times, from pest:predator ratios arriving at host material infested with I. pini. These results suggest that variation between herbivores and their natural enemies in their phenology, preferred pheromone blends, and infested host material should be considered when developing estimates of relative pest and predator densities for subsequent management options.

Exploiting Behavioral Disparities Among Predators and Prey to Selectively Remove Pests: Maximizing the Ratio of Bark Beetles to Predators Removed During Semiochemically Based Trap-Out

Abstract Mass-trapping using semiochemical lures is a potentially useful control measure against bark beetle pests. A serious problem, however, is the inadvertent removal of predators that respond to these baits as kairomones. Ips pini (Say) infests hard pines in the western, Great Lakes, and eastern forests of North America. In Wisconsin, I. pini responds primarily to 50(+)/50(−) and 75(+)/25(−) blends of ipsdienol, its principal pheromone component. Its response is increased by a synergist, lanierone. Its most abundant predators in Wisconsin include Thanasimus dubius (F.), which responds to similar blends of ipsdienol, and Platysoma cylindrica (Paykull) and P. parallelum (Say), which respond to primarily (−) enantiomers of ipsdienol. These predators do not show increased response when lanierone is released in addition to ipsdienol. We conducted a no-choice assay using rotating blends of ipsdienol and lanierone to simulate a trap-out treatment. Lures that contain enantiomers of ipsdienol most preferred by I. pini, in combination with lanierone, can selectively remove up to three to six times more pests than predators during the spring. Moreover, delaying deployment of the same lures until summer can result in removal of up to 39 times more pests than predators. In contrast, lures that contain enantiomers of ipsdienol most preferred by predators can inadvertently remove two or more predators per each bark beetle trapped. Exploiting these behavioral differences between pests and predators can improve biological control by conserving predators during trap-out programs.

Population Dynamics of Ips pini and Ips grandicollis in Red Pine Plantations in Wisconsin: Within- and Between-Year Associations with Predators, Competitors, and Habitat Quality

Abstract We sampled bark beetle (Coleoptera: Scolytidae) populations in 17 declining and healthy red pine plantations in Wisconsin over 3 yr. We tested for potential relationships among numbers of bark beetles, conspecifics and competitors, and predators within and among flight seasons to help identify factors affecting population densities. The two most common bark beetle species obtained were Ips pini (Say) and Ips grandicollis (Eichhoff). The predominant predators obtained were Thanasimus dubius (F.) (Cleridae), Platysoma cylindrica (Paykull) (Histeridae), and Platysoma parallelum Say. Declining stands contained significantly more Ips than did healthy stands during the early portion of the season. Healthy stands had more predators than declining stands. There were strong delayed inverse relationships between I. pini and predators at the site level, both within and between flight seasons. The number of I. pini caught during the late portion of the season was lower when each of the above predators was more abundant earlier in the season, during both 1998 and 1999. Likewise, numbers of I. pini and I. grandicollis caught during the early portion of the year were inversely related to numbers of predators caught during the previous year. Although Ips trap counts showed significant correlations with each predator species, simple predator-prey models did not necessarily improve fits based on habitat quality (i.e., Ips numbers regressed on prior Ips numbers). We did not observe evidence for interspecific competition among Ips spp. This pattern is consistent with the view that host plant quality and predation jointly affect I. pini and I. grandicollis population dynamics. These results emphasize the importance of interactions among host tree physiology, predation, and dispersal in the population dynamics of phloeophagous herbivores, and have implications to forest management.

Spatial-Temporal Modeling of Forest Gaps Generated by Colonization From Below- and Above-Ground Bark Beetle Species

Studies of forest declines are important, because they both reduce timber production and affect successional trajectories of landscapes and ecosystems. Of particular interest is the decline of red pines, which is characterized by expanding areas of dead and chlorotic trees in plantations throughout the Great Lakes region. Here we examine the impact of two bark beetle groups, red turpentine beetles and pine engraver bark beetles, on tree mortality and the subsequent gap formation over time in a plantation in Wisconsin. We construct spatial-temporal statistical models that quantify the relations among red turpentine beetle colonization, pine engraver bark beetle colonization, and mortality of red pine trees while accounting for correlation across space and over time. We extend traditional Markov random-field models to include temporal terms and multiple-response variables aimed at developing a suitable set of statistical models for addressing the scientific questions about the forest ecosystem under study. For statistical inference, we adopt a Bayesian hierarchical modeling approach and devise Markov chain Monte Carlo algorithms for obtaining the posterior distributions of model parameters as well as posterior predictive distributions. In particular, we implement path sampling combined with perfect simulation for autologistic models while formally addressing the posterior propriety under an improper uniform prior. Our data analysis results suggest that red turpentine beetle colonization is associated with a higher likelihood of pine engraver bark beetle colonization and that pine engraver bark beetle colonization is associated with higher likelihood of red pine tree mortality, whereas there is no direct association between red turpentine beetle colonization and red pine tree mortality. There is strong evidence that red turpentine beetle colonization does not kill a red pine tree directly, but rather predisposes the tree to subsequent colonization by pine engraver bark beetles. The evidence is also strong that pine engraver bark beetles are the ultimate mortality agents of red pine trees.

Autologistic Regression Analysis of Spatial-Temporal Binary Data via Monte Carlo Maximum Likelihood

This article considers logistic regression analysis of binary data that are measured on a spatial lattice and repeatedly over discrete time points. We propose a spatial-temporal autologistic regression model and draw statistical inference via maximum likelihood. Due to an unknown normalizing constant in the likelihood function, we use Monte Carlo to obtain maximum likelihood estimates of the model parameters and predictive distributions at future time points. We also use path sampling to estimate the unknown normalizing constant and approximate an information criterion for model assessment. The methodology is illustrated by the analysis of a dataset of mountain pine beetle outbreaks in western Canada.

Species Assemblage Arriving at and Emerging from Trees Colonized by Ips pini in the Great Lakes Region: Partitioning by Time Since Colonization, Season, and Host Species

Abstract The pine engraver, Ips pini (Say), colonizes jack, red, and white pines in the Great Lakes region. Males select suitable hosts, bore through the bark into phloem tissue, and emit aggregation pheromones. Pheromones attract conspecifics, which aid in overcoming tree defenses, and predators, which exploit these cues as kairomones. Sampling was conducted over 2 yr to characterize the assemblage of insects that arrive at and reproduce in trees colonized by I. pini, and how this assemblage is partitioned by host species, time after colonization, and seasonal phenology. Over 70 species from three orders were obtained. I. pini was most abundant, especially during late summer. The first natural enemy to arrive was Medetera bistriata Parent, which came simultaneously with I. pini. Other Diptera such as Lonchaea corticis Taylor and Zabrachia polita Coquillett were also abundant. Roptrocerus xylophagorum (Ratzeburg), a late instar parasitoid, arrived last. Its emergence most closely coincided with I. pini emergence, whereas the other species emerged substantially after I. pini. Host species did not affect total I. pini emergence but strongly affected natural enemies. Most R. xylophagorum and Monochamus spp. emerged from white pine, and most Z. polita emerged from red pine. I. pini had the highest ratio of emergence to arrival per log. Only the predator T. dubius and the parasitoid R. xylophagorum showed numerical responses to the number of emerging I. pini. Exclusion of insects during the first 2 wk of colonization decreased reproduction of I. pini and other wood borers in the spring, but not summer.