Ambrosia beetles can recognise their food fungi by their scents
IMAGE: NEST OF A BLACK STEM BORER (XYLOSANDRUS GERMANUS) IN A HAZELNUT BRANCH WITH ADULT FEMALES (LARGE), A MALE (SMALL) AND INDIVIDUAL LARVAE. THE GREYISH FUNGAL COATING OF THE FOOD FUNGUS IS VISIBLE ON THE WALLS OF THE TUNNEL SYSTEM. view more
CREDIT: ANTONIO GUGLIUZZO
Certain ambrosia beetles species engage in active agriculture. As social communities, they breed and care for food fungi in the wood of trees and ensure that so-called weed fungi spread less. Researchers led by Prof. Dr. Peter Biedermann, professor of Forest Entomology and Forest Protection at the University of Freiburg, now demonstrate for the first time that ambrosia beetles can distinguish between different species of fungi by their scents. "The results can contribute to a better understanding of why beetles selectively colonise trees with conspecifics and how exactly their fungiculture works," says Biedermann. "In addition, the scents of the fungi could be used to develop attractants to control non-native ambrosia beetles."
Beetles orientate themselves by fungal scents
A research team led by Biedermann and the environmental scientist Dr. Antonio Gugliuzzo from the University of Catania/Italy was able to show for the first time that the black stem borer (Xylosandrus germanus) perceives scents of its food fungi and that these act as so-called aggregation pheromones. This means that the beetle uses the scent of the food fungus to find trees that are already colonised by conspecifics. The beetle is an invasive species that is now widespread in Germany and is mainly found in fruit trees. The results have just been published in the journal Frontiers in Microbiology.
"Until now, we could not explain how these beetles attack trees in groups," says Biedermann - because no corresponding scent of the insects' own had been found so far. The experiment now showed that the beetles react to the scents of the specific food fungi that their conspecifics have already cultivated in the branches of a tree. "This enables the beetles to colonise weakened trees in greater numbers and to overcome the tree's defences more easily, thus causing the tree to die," says Biedermann. Further chemical analyses can now be done to determine a component of the fungus scent, which could then be used as an attractant for traps in fruit growing.
Even larvae can distinguish fungi
In another study, environmental scientist Denicia Kassie and biologist Janina Diehl were able to experimentally demonstrate for the first time that another ambrosia beetle species, the fruit-tree pinhole borer (Xyleborinus saxesenii), can recognise and distinguish between its food fungi and so-called weed or harmful fungi based on their scents. Diehl is a doctoral student with Biedermann at the University of Freiburg. "Depending on the condition of the fungi, the beetles in the experiments either specifically sought out the fungal cultures or avoided them," says Diehl. The ability to recognise a potential threat to food fungi or their own health gives the beetles the opportunity to react - and either avoid or specifically combat the harmful fungi. The results of the study have been published in the journal Symbiosis.
The scientists were able to demonstrate the ability to distinguish between different food and harmful fungi in both larvae and adult individuals of the fruit-tree pinhole borer - which each take on their own tasks in the social network in the social maintenance of the food fungus cultures. "These findings are another building block to better understand how the control of fungal breeding by ambrosia beetles works functionally," says Biedermann. "This could also result in ideas for our agriculture to control harmful organisms in a sustainable and environmentally friendly way."
CREDIT
Davide Vallotto
Cultures of the food fungus Ambrosiella grosmanniae of the black stem borer in the laboratory of the University of Freiburg
CREDIT
Antonio Gugliuzzo
Factual overview:
- Original publications: Gugliuzzo, A., Kreuzwieser, J., Ranger, Ch. M., Tropea Garzia, G., Biondi, A., Biedermann, P. H. W.: Volatiles of fungal cultivars act as cues for host-selection in the fungus-farming ambrosia beetle Xylosandrus germanus. In: Front. Microbiol. 14:1151078 (2023).https://www.frontiersin.org/articles/10.3389/fmicb.2023.1151078/full
Diehl, J.M.C., Kassie, D., Biedermann, P.H.W.: Friend or foe: Ambrosia beetle response to volatiles of common threats in their fungus gardensSymbiosis (2023). https://link.springer.com/article/10.1007/s13199-023-00914-y - Peter Biedermann is Professor of Forest Entomology and Forest Conservation at the University of Freiburg. His research interests include bark beetles, symbioses between insects and microorganisms, especially fungi, and the social behaviour of insects. Janina Diehl is a doctoral student at the University of Freiburg, Denicia Kassie wrote her bachelor's thesis on the topic at the University of Freiburg. Antonio Gugliuzzo is a research assistant at the University of Catania/Italy.
- The research was supported by a DAAD scholarship for Antonio Gigliuzzo, by an Emmy Noether Grant from the DFG for Peter Biedermann and by funding from the University of Freiburg for open access publications.
DOI
Farmer’ beetle finds suitable host trees by tracing scent of its fungus crop
First known chemical cue for aggregation of ambrosia beetles that isn’t synthesized by the species itself
Peer-Reviewed PublicationIMAGE: ALNUS AMBROSIA BEETLES (XYLOSANDRUS GERMANUS) IN THEIR GALLERIES, TENDING THE BROOD AND FUNGUS view more
CREDIT: ANTONIO GUGLIUZZO
The alnus ambrosia beetle Xylosandrus germanus, also known as the black stem borer, was accidentally introduced by humans from its native east Asia to North America and Europe around the beginning of the 20th century. X. germanus is a so-called ambrosia beetle, which means that it farms its own food: a specialized fungal symbiont which it ‘sows’ and tends inside the galleries that it digs inside wood. It is a destructive invasive pest, known to attack more than 200 species from 51 families of broadleaf and conifer trees. While it prefers to colonize dead wood, it can also infest and ultimately kill weakened or stressed trees.
A proven way to monitor or even control insect pests is by hijacking their communication system and manipulating it to trap them to their doom. Females of the alnus ambrosia beetle are known to aggregate on individual trees, suggesting that they use chemical signals to find each other and identify suitable trees. And now, scientists have cracked this chemical code. The results are published in Frontiers in Microbiology.
“Here we show that the alnus ambrosia beetle doesn’t produce its own pheromones during tree-host colonization, but rather uses the volatile compounds of their own fungal symbionts to aggregate en masse and potentially kill trees,” said the study’s senior author Dr Peter Biedermann, a professor at the Forestry Institute of the University of Freiburg in Germany.
First evidence of aggregation pheromones in ambrosia beetles
First author Antonio Gugliuzzo, a PhD student at the Department of Agriculture, Food and Environment of the University of Catania in Italy, added: “This is the first evidence for the existence of aggregation pheromones in ambrosia beetles of the tribe Xyleborini.”
Like all ambrosia beetles, X. germanus females carry their fungal symbionts in special pockets inside their head. The authors here used molecular methods to culture and identify the two most common symbionts as Ambrosiella grosmanniae, most likely the beetles’ primary food source, and an unknown species of Acremonium that the beetles may or may not eat. In 2020, the researchers caught mated dispersing X. germanus females near Freiburg, and allowed these dig galleries in an artificial medium based on beech sawdust, grow these two fungi, and rear their offspring.
They then used these fungi as bait in two-choice experiments in an apparatus called a still-air olfactometer. Here, X. germanus females were placed in an arena with two dead ends to enter and hide in: for example, one containing an isolate of the two symbionts A. grosmanniae and Acremonium sp., and another with an isolate of an unrelated fungus expected to be unattractive to the beetles – here, an uncharacterized Cladosporium species. The beetles’ preference will depend on their relative attraction to microbial volatile organic compounds (MVOCs) released by the fungi.
In a variant of this experiment, previously healthy beech branches infected with different fungi were presented to the beetles as options for choice.
Beetles prefer odor of symbionts and colonized branches
The results showed that the beetles were most attracted to A. grosmanniae MVOCs and least to Cladosporium MVOCs, and more to branches already colonized by A. grosmanniae than to uncolonized branches. The authors conclude that X. germanus uses MVOCs released by its two symbiont strains as a ‘synomone’ – a chemical signal that benefits both the emitter like and the receiver.
“Occupied tree branches may signal the beetles suitable substrate for the food fungi they depend on. This suitability is probably a delicate balance: trees need specific requirements to allow the beetles to farm the fungus successfully,” said Biedermann.
“But this isn’t an absolute requirement: if a female can’t find any conspecifics, or detect any MVOCs, she could still independently colonize a new tree branch.”
Promising avenue for biological control
These results immediately suggest a new method for biological control.
“Now that we know that the fungi produce attractive volatiles, we may be able to develop new trapping lures for the beetles on the basis of these compounds,” said Biedermann.
“Moreover, the use of specific volatile compounds for trapping purposes could contribute to the development of innovative and selective trapping methods targeting specific ambrosia beetle species, thereby minimizing catches of non-target arthropods living in the same environment.”
Alnus ambrosia beetles (Xylosandrus germanus) in their galleries, tending the brood and fungus
CREDIT
Antonio Gugliuzzo
JOURNAL
Frontiers in Microbiology
METHOD OF RESEARCH
Experimental study
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Volatiles of fungal cultivars act as cues for host-selection in the fungus-farming ambrosia beetle Xylosandrus germanus
ARTICLE PUBLICATION DATE
14-Apr-2023
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