Chemical ecology of bark beetle associations with microbes

We aim to understand how bark beetles interact with fungi and bacteria and how this influences their success in colonizing trees.

Bark beetles (Coleoptera: Curculionidae) are a highly diverse group of insects found in forests throughout much of the world. Some species are major pests of wild trees and trees on plantations, with mass outbreaks that kill millions of hectares of trees per year. Bark and ambrosia beetles have a remarkable ability to colonize the bark and wood of trees, substrates that have a very low nutrient content and are often well protected by chemical defenses. This success is often attributed to their associated microbes.

We are investigating how fungal and bacterial associates might help beetles to survive on trees by providing nutrients, protecting beetles from antagonistic microbes, and detoxifying tree defense compounds. Our research includes a variety of different microbes (bacteria, yeasts and filamentous fungi) found within the beetle or as free-living associates.

We apply various methods, including microbiological culturing, sequencing and transcriptomics, beetle behavioral and performance assays, chemical isolation and characterization, and metabolic pathway elucidation. One goal is to isolate microbial and insect genes encoding enzymes involved in nutritional acquisition, detoxification of tree defenses and the biosynthesis of microbial defenses active against antagonistic microbes. Such genes will be manipulated to test hypotheses about the ecological functions of biosynthetic or catabolic pathways, and their expression monitored to determine how insect-microbe and microbe-microbe interactions are regulated.

Projects focused on Ips typographus bark beetle and its associated microbes

Millions of hectares of Norway spruce (Picea abies) in Germany and surrounding countries are killed each year by the Eurasian spruce bark beetle, Ips typographus. Outbreaks are increasing due to warming temperatures that speed up the beetle’s life cycle and increase tree stress. The colonization success of I. typographus relies on the presence of symbionts, in particular free-living blue stain fungi from the order Ophiostomatales. Although the exact benefits and extent of these associations are not yet well understood, these ectosymbiotic fungi may exhaust host tree defenses, metabolize tree defense compounds, provide nutritional benefits to larvae and adults, or protect them from harmful microbes. The many questions surrounding this multipartite ecosystem are the focus of various projects in our group, all aiming to gain knowledge about the ecological interactions and strategies responsible for the tree-killing success of I. typographus.

Effect of ophiostomatoid fungi on Ips typographus nutrition and fitness

Participating researcher: Emily Puckett

Collaborators: Dr. Dineshkumar Kandasamy (Lund University, Sweden), Dr. Axel Schmidt (Conifer Defense Group)

Tree bark represents a challenging substrate for insects due not only to its defense compounds, but also to its low concentration of accessible nutrients. Like many insects, bark beetles face a nutritional mismatch between what they require and what their hosts can provide. This mismatch is often overcome with the help of a microbial symbiont that either lives within the insect’s organs or can be found in its environment. In addition to provisioning nutrients that cannot be found in high enough quantities within the host tissue, microbes can also provide insects with access to otherwise nutritionally inert substrates. Using a combination of chemical and behavioral assays, we are attempting to establish whether one or more of the microbial associates of Ips typographus function as its nutritional symbionts.

The role of bacterial symbionts for bark beetle ecology

Participating researchers: Ana Patricia Baños Quintana

Co-supervisor: Prof. Dr. Martin Kaltenpoth, Department of Insect Symbiosis

The Ips typographus microbiome has already been characterized, but its role in the bark beetle colonization success is not well understood. Bacterial symbionts in the gut could also play an important role in promoting beetle attack by supplying critical nutrients or detoxifying host tree defenses. We are investigating the role of the gut microbial community in promoting bark beetle development on spruce by a combination of culture-dependent and -independent microbiota profiling, fluorescence in-situ hybridization, metagenomics and metatranscriptomics of beetle guts, manipulative bioassays, and chemical analytics. Knowledge about the role of microbial gut symbionts in the bark beetle life cycle may yield new insights into how insects are able to survive on trees with low nutrient content and abundant chemical defenses, as well as suggesting new strategies to combat this pest.

Metabolism of spruce defensive phenolic compounds by a bark beetle and a symbiotic fungus

Participating researchers: Baoyu Hu, Dr. Ruo Sun

Natural interactions involving plants, herbivore insects and their symbiotic fungi are ubiquitous in terrestrial ecosystems. Norway spruce is susceptible to attack by the Eurasian spruce bark beetle Ips typographus and its symbiotic fungus. The symbiont is vectored into the host tree during bark beetle attack, and this infestation is hypothesized to enhance bark beetle development by reducing the levels of spruce defensive chemicals. Besides oleoresins, defensive phenolic compounds are thought to protect Norway spruce from pest attack and fungal infection. However, the mechanism by which bark beetles and their fungal associates have counter-adapted to spruce defensive phenolic compounds remains unclear. In this project, we aim to reveal the detoxification strategies employed by the bark beetle and its symbiont to overcome spruce defense. First, bioassays will be conducted to detect the influence of spruce phenolic chemicals on bark beetles with or without their symbiotic fungi present. Secondly, the pathways of phenolic catabolism will be determined in both the bark beetle and its fungal associate. Subsequently, metabolic mechanisms will be explored at the protein and gene levels. Last, we hope to understand how the interaction of the bark beetle and its associated fungi achieve successful colonization of the host tree.

The role of phenolic substances in the interaction of bark beetle and an entomopathogenic fungus

Participating researchers: Baoyu Hu, Dr. Ruo Sun

Norway spruce (Picea abies) produces chemical defenses such as terpenes and phenolics to protect itself from herbivores and pathogens. However, in natural systems, Norway spruce is susceptible to be attacked by the Eurasian spruce bark beetle Ips typographus. Entomopathogenic fungi are natural insect pathogens that can cause disease and thus control a large number of arthropod pest species, including bark beetles. It is known that certain herbivores can recruit plant chemical defenses and use them to fend off pathogens. Thus, in our project, we will investigate the metabolic ability of the entomopathogenic fungus to degrade phenolic defense compounds found in bark beetles attacking Norway spruce trees. Moreover, we will search for the genes and proteins of the entomopathogen involved in these degradative processes.

Role of endosymbiotic gut bacteria of Hylobius abietis in detoxification of terpenes

Participating researchers: Kristina Kshatriya, Dr. Axel Schmidt (Conifer Defense Group)

The pine weevil (Hylobius abietis) is a major pest in European conifer forests where adults feed on the bark and cambium of Norway spruce and pine seedlings. Conifers are protected against bark-feeding herbivores by a complex mixture of secondary metabolites, mainly terpenoid-based oleoresins, and phenolic compounds. Therefore, adult pine weevils must cope with a complex mixture of noxious secondary metabolites in their diet, in addition to the problem of utilizing a food source that mainly consists of lignocellulose.

Many insects are known to harbor symbiotic microorganisms in their digestive system that allow the host to subsist on suboptimal diets by enhancing digestion efficiency, supplementing the diet with limiting vitamins or amino acids, or detoxifying plant secondary metabolites. However, little is known about degradation of lignin or mono-, and sesquiterpenes, as well as diterpene resin acids, the components of coniferous resin by symbiotic microorganisms.

We are exploring how the pine weevil copes with high concentrations of terpenes present in the host bark and cambium, by elucidating the metabolic fate of these compounds and their effect on the beetle. In order to understand the possible role of gut microorganisms in terpene detoxification as well as in wood digestion, we are using culture-dependent and -independent methods as well as metabolic and genomic analyses to functionally characterize the pine weevil’s gut microbiota. Elucidating the fate of terpenes in this system will shed some light on how insects cope with plant induced defenses and how some symbionts allow their hosts to exploit otherwise inaccessible food sources.