Why some beetles like alcohol

April 9, 2018    No. 3/2018 (187)

Alcohol used as a "weed killer" optimizes the harvest of ambrosia beetles

If on a warm summer's evening in the beer garden, small beetles dive into your beer, consider giving them a break. Referred to as "ambrosia beetles", these insects just want what’s best for themselves and their offspring. Drawn to the smell of alcohol in the cold liquid, the beetles are always on the lookout for a new environment to farm. And alcohol plays an important role in optimizing the agricultural yield of their crops, as an international team of researchers reports in the current issue of the journal PNAS.

The fruit-tree pinhole borer (Xyleborinus saxesenii) is a so-called ambrosia beetle. These bark beetle species cultivate fungal gardens in the tunnels they have excavated in the wood of trees. The fungal gardens are their sole source of nutrition and are called ambrosia (“food of the gods”). Photo: Gernot Kunz.

The black timber bark beetle and its fungal "crop"

Ambrosia beetles, which are a large group of several thousand species worldwide, belong to the bark beetles. All species are characterized by the ability to cultivate fungi. The researchers, including Peter Biedermann, from the Department of Animal Ecology and Tropical Biology at the University of Wuerzburg and the Max Planck Institute for Chemical Ecology in Jena, Christopher Ranger, Ohio State University, USA, as well as Philipp Benz from the Wood Research Institute of the Technical University of Munich (TUM), investigated the role played by alcohol in the farming of fungi as practiced by the black timber bark beetle (Xylosandrus germanus) and its fungal "crop".

"It has long been known that alcohol is produced by weakened trees and that these trees are recognized and colonized by ambrosia beetles," says Biedermann. Baiting traps with alcohol is a classic way to catch these bugs. "And often you will find the roughly two millimeter long beetles in glasses of beer, when a beer garden is surrounded by old trees," adds Biedermann.

Sustainable agriculture as a recipe for success

Thanks to the results of the new study, we now know why alcohol is so attractive to these insects. "An increase in the activity of alcohol-degrading enzymes allows the insects' fungi to grow optimally in alcohol-rich wood, while alcohol is toxic to other microorganisms," says Biedermann. More fungi mean more food for the beetles, and more food means more offspring. The beetles and their larvae feed on the fruiting bodies of the fungi, which grow best at an alcohol concentration of about two percent.

Electron microscope image of the nutritional fungal garden in the nests of ambrosia beetles. Copyright: Peter Biedermann, Elektronenmikroskopisches Zentrum Jena.

"At this level of alcohol, the omnipresent molds, which can also be considered the "weeds" of fungal agriculture, only grow weekly and cannot overgrow the fungal gardens”, says Prof. Benz. Given the beetle’s evolutionary success, the details of its sustainable farming strategy are worth noting. "For more than 60 million years, the animals have successfully and sustainably practiced agriculture, even though their crop – the ambrosia fungus – is a monoculture." Unlike human farmers, the insects seem to have had no problem with weed fungi becoming resistant to the alcohol.

Communal care of fungal gardens

It is not only the agricultural skill of the Ambrosia beetles that inspires Biedermann. "They show social behavior," says the ecologist. Beetles share the work of cultivating their fungal gardens: some clean the tunnel systems that are being eaten into the wood; others clear the dirt from the nest and clean their fellow workers – always with the aim of optimizing the symbiosis of beetle and fungus.

This system is so sophisticated that when they colonize new trees, the animals bring along the fungal spores in their own spore organs.  New fungal gardens grow from the “transplanted” spores. The fungi are even able to produce alcohol in order to optimize the environment.

"In this way, the fungi cultivated by the ambrosia beetles behave like beer or wine yeast, generating an alcoholic substrate in which only they can thrive and from which other competing microorganisms are excluded," explains Biedermann.

There is much more to learn from the beetles

Biedermann and Benz are plannung to collaboratively study these bark beetles and their fungal symbionts in the future. One of many open questions that remain about the lifestyle of these six-legged friends and their fungal crops is: What exactly enables them to survive in this boozy environment? “Of course, they have to be more resistant to alcohol than other creatures," says Biedermann. “These characteristics are also of high potential interest from a biotechnological point of view, since they might be transferrable to other systems when better understood”, adds Benz. Maybe humanity has something to learn from the bark beetle. [JMU/KG]

Black timber bark beetles (Xylosandrus germanusis) are ambrosia beetles. They cultivate fungal gardens in the tunnels they have excavated in the wood of trees.  Alcohol produced by weakened trees is important for the beetles as it increases the growth of their food fungi. Film: Reinhard Weidlich

Original Publication:
Ranger, C. M., Biedermann, P. H. W., Phuntumart, V., Beligala, G. U., Ghosh, S., Palmquist, D. E., Mueller, R., Barnett, J., Schultz, P. B.,, Reding, M. E., Benz, P. (2018). Symbiont selection via alcohol benefits fungus farming by ambrosia beetles. Proceedings of the National Academy of Science of the of the United States of America. DOI 10.1073/pnas.1716852115

Further Information:

Dr. Peter Biedermann, Biozentrum, Lehrstuhl für Tierökologie und Tropenbiologie (Zoologie III). T.: +49 (0)931 31-89589, E-Mail:  peter.biedermann@uni-wuerzburg.de, Arbeitsgruppe: http://www.insect-fungus.com/  

Prof. Dr. J. Philipp Benz, Professorship of Wood Bioprocesses, Holzforschung München, Technical University of Munich. T.: +49 (0)8161 71-4590, E-Mail: benz@hfm.tum.de, working group: http://www.hfm.tum.de/index.php?id=20&L=1 

Download of high resolution images and videos via http://www.ice.mpg.de/ext/downloads2018.html