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.
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.