by Chrissy Dodge, Ph.D. Candidate
Twitter: @christinedodge_
Agriculture is arguably one of humanity’s best ideas. By actively cultivating our food supply, we were able to settle down in one place instead of following migrating herds. Through this practice, we also ensured that we would have a steady food supply for years to come. This enabled us to develop into the complex societies that characterize our species today.
However, humans aren’t the only animals that have come up with this successful strategy – in fact, we weren’t even the first. There are several insect groups that have been practicing agriculture for millions of years, and they all farm the same crop: fungi. You’ve probably heard of leaf-cutter ants, but did you know they usually aren’t eating the leaves they collect? Those leaves are fed to a tended garden of fungus, and it is the fungus that makes up the diet of the entire ant brood. You may also have heard of fungus-farming termites, which exhibit a similar strategy of cultivating and feeding on a fungus “comb.” The difference is the termites nourish this fungus using their own frass, which is a fancy word for insect poop. (Yes, they poop on their crop to help it grow. Hey, it’s fertilizer!) But there is a third group of insects that also practices agriculture, and they are both the most diverse and the least recognized. This group is known collectively as the ambrosia beetles, and they are the first known insects to farm fungi.
Ambrosia beetles are tiny wood-boring beetles that are part of the weevil family, although they lack the distinctive weevil “nose” called the rostrum. Over evolutionary time the rostrum was lost in this group, which makes sense – it would probably get in the way when trying to bore into a hard substrate like wood. Ambrosia beetles got their name because of their close association with fungi. Such a tight-knit relationship between different species is known as a symbiosis. (For another blog post on symbiosis, see Amelia’s “A Whole New World (of Wasps)” from 2016.) The first observations of ambrosia beetles feeding on fungi were reported almost 200 years ago, before we knew that it was fungi they were feeding on – to the observer who saw it, it was just an unknown white substance. He called the substance “ambrosia,” a term from Greek mythology that refers to the “food of the gods” that bestowed immortality to those who ate it. We now know that this substance is fungus, but our current knowledge of most ambrosia fungi still just scratches the surface. There are about 3,200 known species of ambrosia beetle, but we’ve identified the fungal symbionts of only about 5% of them.
Twitter: @christinedodge_
Agriculture is arguably one of humanity’s best ideas. By actively cultivating our food supply, we were able to settle down in one place instead of following migrating herds. Through this practice, we also ensured that we would have a steady food supply for years to come. This enabled us to develop into the complex societies that characterize our species today.
However, humans aren’t the only animals that have come up with this successful strategy – in fact, we weren’t even the first. There are several insect groups that have been practicing agriculture for millions of years, and they all farm the same crop: fungi. You’ve probably heard of leaf-cutter ants, but did you know they usually aren’t eating the leaves they collect? Those leaves are fed to a tended garden of fungus, and it is the fungus that makes up the diet of the entire ant brood. You may also have heard of fungus-farming termites, which exhibit a similar strategy of cultivating and feeding on a fungus “comb.” The difference is the termites nourish this fungus using their own frass, which is a fancy word for insect poop. (Yes, they poop on their crop to help it grow. Hey, it’s fertilizer!) But there is a third group of insects that also practices agriculture, and they are both the most diverse and the least recognized. This group is known collectively as the ambrosia beetles, and they are the first known insects to farm fungi.
Ambrosia beetles are tiny wood-boring beetles that are part of the weevil family, although they lack the distinctive weevil “nose” called the rostrum. Over evolutionary time the rostrum was lost in this group, which makes sense – it would probably get in the way when trying to bore into a hard substrate like wood. Ambrosia beetles got their name because of their close association with fungi. Such a tight-knit relationship between different species is known as a symbiosis. (For another blog post on symbiosis, see Amelia’s “A Whole New World (of Wasps)” from 2016.) The first observations of ambrosia beetles feeding on fungi were reported almost 200 years ago, before we knew that it was fungi they were feeding on – to the observer who saw it, it was just an unknown white substance. He called the substance “ambrosia,” a term from Greek mythology that refers to the “food of the gods” that bestowed immortality to those who ate it. We now know that this substance is fungus, but our current knowledge of most ambrosia fungi still just scratches the surface. There are about 3,200 known species of ambrosia beetle, but we’ve identified the fungal symbionts of only about 5% of them.
Ambrosia beetles bore into trees and create galleries, or tunnels, in the wood, where they produce offspring and rear their young. They carry their fungi with them from tree to tree inside of specialized organs called mycangia, which protect and nourish the fungi during transport. They grow their fungi inside the galleries, and feed on it exclusively throughout their development. But how did this relationship evolve? The answer likely has to do with where they live. Most ambrosia beetles live in dead or dying trees, and they are typically not the only inhabitants. Dead trees provide a readily available source of food and shelter. Bacteria, fungi, and other decomposers are abundant, as well as other wood-boring insects looking for a place to live and reproduce.
Most wood-boring insects, including ambrosia beetles’ close relatives, the bark beetles, feed directly on tree tissues. Since much of a dead tree sooner or later becomes infested with microbes, it’s not hard to imagine a scenario where beetles begin to feed on fungus-infested tissue. Fungi are decomposers after all, and are capable of extracting and concentrating nutrients from wood that the insects wouldn’t be able to get to easily on their own. The beetles probably preferred fungus-infested wood to non-infested wood, since it took less time and energy to consume the same amount of nutrients – the fungi were doing all the work! This is likely how the ambrosia symbiosis evolved: from a facultative relationship, one that is not essential but is useful when the opportunity presents itself, to an obligate one – one upon which both partners depend. Ambrosia beetles cannot sustain themselves without their fungi, and ambrosia fungi are not able to move from tree to tree without their beetle partners. Through this close association, everybody wins! We call a symbiosis in which both partners benefit a mutualism.
There are two main groups of ambrosia beetles, the scolytines (like the one in the first picture) and the platypodines. The platypodines are thought to have started cultivating fungus about 96 million years ago, making them the oldest known fungus-farming insects! Whereas fungus farming was taken up only once in the platypodines, this strategy arose independently at least 14 times in the scolytines, which indicates that it’s a pretty successful lifestyle.
The vast majority of ambrosia beetles live in dead or dying trees, where they play an important role in forest succession and decomposition. However, there are a few species that bore into healthy trees, which can result in tree death. Because of this, some ambrosia beetles are serious pests in natural, urban, and agricultural landscapes. These tree-killing beetles usually only cause problems when they are introduced to a new place where they are free of whatever competitors they had to deal with in their native range – in other words, when they are invasive. However, climate change plays a big role as well. Warming temperatures can allow beetle species to range into new territory and produce more generations annually than usual, which can lead to more tree-killing. Additionally, some of the fungal symbionts of ambrosia beetles are plant pathogens that can seriously injure or kill the tree. One example of this is the redbay ambrosia beetle Xyleborus glabratus and its fungal symbiont Raffaelea lauricola. Invasive to Florida, this beetle-fungus complex is responsible for the devastating plant disease called laurel wilt, which affects plants in the laurel family, including avocado.
Another example is the group that I work on, the Euwallacea fornicatus species complex, which also attacks avocado. These beetles associate with Fusarium and Graphium fungi, which are mild to robust plant pathogens. The dual effect of beetle boring activity and fungal growth causes the emerging plant disease known as Fusarium dieback. These beetles are invasive in California, Hawaii, Florida, and dozens of other areas around the globe. This is one reason why knowing about fungal symbionts of these beetles is important – it can help us better understand how to deal with pest species. Another good reason is that it’s just plain cool! These tiny beetles were practicing agriculture millions of years before humans even walked the earth. So the next time you think about the great advancements of human society, just know that, at some point in evolutionary history, an insect probably did it first.
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Chrissy is a 5th year Ph.D. candidate working in Richard Stouthamer’s lab. She is studying the biology of the polyphagous and Kuroshio shot hole borers, two invasive ambrosia beetles in Southern California, and their fungal symbionts.
If you’d like to learn more about any of the subjects covered in this blog, I am happy to provide papers for readers behind a pay wall. Most of the information in this post is referenced in the following papers:
Hulcr, J., and L. L. Stelinski. 2017. The Ambrosia Symbiosis: From Evolutionary Ecology to Practical Management. Annual Review of Entomology 62: 285-303.
Li, Y., D. R. Simmons, C. C. Bateman, D. P. Short, M. T. Kasson, R. J. Rabaglia, and J. Hulcr. 2015. New Fungus-Insect Symbiosis: Culturing, Molecular, and Histological Methods Determine Saprophytic Polyporales Mutualists of Ambrosiodmus Ambrosia Beetles. PloS one 10: e0137689.
Mueller, U. G., N. M. Gerardo, D. K. Aanen, D. L. Six, and T. R. Schultz. 2005. The Evolution of Agriculture in Insects. Annual Review of Ecology, Evolution, and Systematics 36: 563-595.
Vanderpool, D., R. R. Bracewell, and J. P. McCutcheon. 2017. Know your farmer: Ancient origins and multiple independent domestications of ambrosia beetle fungal cultivars. Mol Ecol.
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Chrissy is a 5th year Ph.D. candidate working in Richard Stouthamer’s lab. She is studying the biology of the polyphagous and Kuroshio shot hole borers, two invasive ambrosia beetles in Southern California, and their fungal symbionts.
If you’d like to learn more about any of the subjects covered in this blog, I am happy to provide papers for readers behind a pay wall. Most of the information in this post is referenced in the following papers:
Hulcr, J., and L. L. Stelinski. 2017. The Ambrosia Symbiosis: From Evolutionary Ecology to Practical Management. Annual Review of Entomology 62: 285-303.
Li, Y., D. R. Simmons, C. C. Bateman, D. P. Short, M. T. Kasson, R. J. Rabaglia, and J. Hulcr. 2015. New Fungus-Insect Symbiosis: Culturing, Molecular, and Histological Methods Determine Saprophytic Polyporales Mutualists of Ambrosiodmus Ambrosia Beetles. PloS one 10: e0137689.
Mueller, U. G., N. M. Gerardo, D. K. Aanen, D. L. Six, and T. R. Schultz. 2005. The Evolution of Agriculture in Insects. Annual Review of Ecology, Evolution, and Systematics 36: 563-595.
Vanderpool, D., R. R. Bracewell, and J. P. McCutcheon. 2017. Know your farmer: Ancient origins and multiple independent domestications of ambrosia beetle fungal cultivars. Mol Ecol.