Bark Beetles and Symbiotic Fungi

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Introduction

Bark Beetle galleries in wood. Photo taken by Deborah Bell, Smithsonian Institution.


A symbiotic relationship is held between bark beetles and fungi. This interaction has been known to be devastating to forest stands all over the world. Fungi in this interaction lead to the devastating effects in certain arboreal species. There have been many important studies and research performed in order to understand and perhaps to prevent or contain the spread of certain diseases to valued tree stands.

Biological Interaction

Mutualism

Mutualism occurs between two organisms when they both mutually benefit from interacting with one another [1]. Bark beetles transport the fungi to new trees and the symbiotic fungi protect the beetles by preventing the tree from decimating the bark beetles.

Commensalism

Commensalism is an ecological term in which a specific interaction between two organisms allows one organism to benefit while the other organism remains unaffected [1]. This interaction between bark beetles and symbiotic fungi is a specific interaction where the bark beetles are unaffected when they carry and spread the fungi through the host tree.

Microbial populations

The interaction between bark beetles and symbiotic fungi not only affect the host plant they are occupying but also microbial populations. When the bark beetles burrow and reside in the host tree they are increasing surface area where other microbes, pathogens, and insects can have a deleterious effect. The interaction between bark beetles and symbiotic fungi is a positive influence. Furthermore, allowing other organisms access to the inside of the tree is a positive influence.

Ecological effects

This interaction between the bark beetles and the symbiotic fungi eventually leads to the death of the tree, then the beetles and fungi move on to their next host. There are ecological consequences to an unregulated population of bark beetles and their symbiotic fungi. Ecologically the continued destruction of types of species of trees can affect the surrounding habitats in that there is less water uptake by a diseased tree, when a tree perishes then it provides an available food source for bacteria and fungi in the environment. This cycle aids in the development of younger trees that are then allowed to grow and take the place of the affected trees.

Niche

Bark beetles have been creating mazes in trees for a long while. These bark beetles live in the dead phloem tissues of trees. Most bark beetles live in dead or decaying trees, however some are known to actively penetrate healthy trees, such as the mountain pine beetle (Dendroctonus ponderosae). Female bark beetles burrow into mature trees, signal males, mate, and then deposit their eggs deep within the tree’s tissue. When bark beetles attack trees that are healthy, these trees may produce resin or latex as a defense.

Elm Trees

In Elm trees, bark beetles spread the fungi through sexual contact. With this, the fungus spreads and due to a tylotic response in the xylem, the tree prevents the fungus from spreading. This response, however, also blocks water from moving up and photosynthates from moving down the trunk of the tree. This Dutch Elm Disease has been spreading across North America killing unresistant elm species.

Pine Trees

In pine trees, bark beetles infest by laying eggs under the bark. Once present in the tree, these beetles inoculate the tree with a blue stain fungus. This specific fungus is injected into the sapwood. This action prevents the tree from controlling or exterminating the beetle larvae with sap. The introduction of this particular fungus blocks water and nutrient transportation within the xylem and phloem of the tree.


Microbial processes

The fungi in these interactions are relied upon by some bark beetles. The blue stain fungus and also the fungi related to Dutch Elm Disease all can prevent the tree's xylem and phloem from functioning properly. The response from the tree eventually will kill the it.

Ecosystem-level Effects

Millions of trees perish each year due to this symbiotic relationship between these two organisms.

Environmental Effects

This microbial process can cause an environmental effect because of the reduction of susceptible species of trees. Having decreasing amounts of trees reduces the amount of carbon sequestering that trees do each year.


Key Microorganisms

Close up of Ophiostoma ulmi the pathogen responsible for Dutch Elm Disease. Photo taken by William Jacobi, Colorado State University.


Fungi are the major microorganisms that are involved with this symbiotic interaction.

Ophiostomatales Fungi

This fungi genus of pathogens is responsible for the Dutch Elm Disease.

Ophiostoma novo-ulmi

This fungi species is extremely destructive and it was first described in both Europe and North America in the 1940s and has devastated elm stands in both areas since the late 1960s.

Ophiostoma himal-ulmi

This fungal species is very devastating to elms located in the western Himalaya

Ophiostoma ulmi

This fungal species affected elm stands in Europe around 1910 and was transported to North America in 1928

Grosmannia Fungi

This pathogenic fungi genus is responsible for the destruction by mountain pine beetles.

Grosmannia clavigera

This fungal species affects Lodgepole pine, Ponderosa pine, Douglas-fir, and Whitebark pine trees.



Current Research

Resistant Varieties

Fungicides are only useful as a protective measure, are not very cost effective. Research has therefore focused on selection of elm varieties that are both resistant to Dutch elm disease and well suited to European environments. Research conducted by INRA and CEMAGREF has shown that European elms are susceptible to Dutch elm disease. Resistant varieties are found in Asian species, but they do not look similar. A project began in 1975 that led to the creation of new varieties through cross-breeding. The Lutèce® variety is a result of this research. It combines resistance to Dutch elm disease, ornamental qualities, and also is adapted to the European climate. Furthermore, other varieties are currently being selected to restore the genetic diversity necessary for the future of the elm.

Solar Treatments

Experiments were conducted to evaluate the use of solar radiation for reducing survival of mountain pine beetle populations in infested logs. Plastic sheeting, routine turning of the logs, and stacking of logs were utilized in these experiments. All treatments in all experiments caused drastic reductions in brood survival. Also in all experiments brood survival was regularly decreased when the logs were exposed to the sun. High temperatures were consistently greater in the treatments with plastic sheeting, the exposed surfaces of the logs to the sun, and the upper layer of logs in the two-layer treatments. This information suggests that heat is directly responsible for the observed reductions in survival. Solar treatments are an effective alternative for reducing mountain pine beetle survival in infested trees.

Utilizing Cloning Techniques

An efficient procedure for the conservation of mature American elm trees that have survived the epidemics of Dutch elm disease and are potential sources of disease resistance is reported. This experiment utilizes in vitro propagation of buds from mature trees to clone 100 year old American elm trees. An important factor which is used for the optimization of culture process is auxin metabolism in the source tissue. In this experiment use of blocking antiauxins was utilized so that auxins would not be metabolized. This was important because a high shoot rate was necessary. Plantlets that had roots were easily adapted to the greenhouse with 90 percent surviving. This will aid in making Dutch elm disease resistant clones, and this will also provide an approach to advance preservation of other endangered tree species.


References

[1]Myer, J. (1998, Jan 03). College of agriculture and life sciences.

Kirisits, T.2004. "Fungal Associates of European Bark Beetles With Special Emphasis on the Ophiostomatoid Fungi" pg. 198-236.

Klepzig, K. D. and Wilkens, R. T. 1997. "Competitive Interactions among Symbiotic Fungi of the Southern Pine Beetle." Appl. Environ. Microbiol. vol. 63 no. 2: 621-627.

Mukund R. Shukla, A. Maxwell P. Jones, J. Alan Sullivan, Chunzhao Liu,* Susan Gosling,† Praveen K. Saxena. 2012. "In vitro conservation of American elm (Ulmus americana): potential role of auxin metabolism in sustained plant proliferation". Department of Plant Agriculture, University of Guelph.

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Edited by Kord Nolte, a student of Angela Kent at the University of Illinois at Urbana-Champaign.