Some fungi are not so fun if you're a bark beetle.

Some fungi are not so fun if you're a bark beetle. 

Take a moment with me to imagine you are a mountain pine beetle. You're about a quarter of an inch long, your body is covered in hard plate armor. You live in a gorgeous old ponderosa pine tree in the cool wet forests of northern California. You have endless food. Your offspring are thriving. You’ve stopped dying off in the winter. Lately, the forests haven't been as wet or cool as they used to be, and dry patches of forest become more plentiful every year. You and your mate are set to have 12 million offspring this season. Everything is perfect.

But then the lightning strikes. 

The wood you call home is dry because you and your ancestors have been eating the living tissue from inside for years. The forest is a tinderbox. You and every beetle you know are burned in a fire that destroys 1,032,648 acres in 2020 (Cal Fire.gov).

Known research on tree-killing bark beetles goes back more than 200 years (Krieger, 1998). Most bark beetles don’t kill trees; the ones that do have been identified and humans have been fighting them to save trees from them for centuries. Despite all that time and research, we still do not know enough about what drives their population size to combat or control them when their numbers get out of control. We do know, however, when they do get out of control, it can result in longer and more severe wildfires due to plentiful fuel in the form of stands of dead trees throughout forests. Bark beetles are one of the most economically impactful insects in the world today (Biedermann et al., 2019).  

Bark beetles used to fill a delicate and important niche in the ecosystem. When a stand of trees would become infected with a pathogen like a virus or parasitic fungus and start to decline, bark beetles would move into the area and kill dying trees by tunneling in and eating the living tissues between the bark and wood (Biedermann et al., 2019). When this happens, the tree loses the ability to move water from its roots to its leaves, so photosynthesis stops working, and it dies. This sounds like a pitiful tale from the tree’s point of view, but it is good for the forest because it shortens the lifespan of whatever was killing it, helping to prevent pathogenic pandemics. However, this forest-saving superpower was not made for climates as hot and dry as the ones we have seen grow increasingly more common in the last 25 years. Dry conditions and uncommonly low amounts of rainfall have led to an explosion in the number of trees that are weak enough to be afflicted by bark beetles, and the beetles have not missed out on this opportunity. 

According to a 2020 paper in Forest Ecology and Management using United States Forest Service aerial data, the ten most common species of bark beetles in the western United States destroyed 4.3% of forested areas from the Rocky Mountains to the West Coast from 1997-2008 (Hicke, 2020). By far the most damaging species in the survey was the mountain pine beetle. These tiny beetles alone, practically indistinguishable from mouse droppings, did almost half the total damage observed at almost 1.9%. What’s worse, mountain pine beetles do not arrive gradually in an area, but in great population explosions: in 2006 alone, nearly half of Colorado’s lodge pole pines were infested by mountain pine beetles — nearly 330 million acres. Mountain communities are terrified of them because of the increased fire risk they present, and many governments are working to stop them from spreading. In 2018, I was personally stopped at the California state line and had the firewood from the top of my van confiscated for fear of spreading bark beetles, that’s when I first got a glimpse into the severity of this issue. It has even gotten to the point where humans are destroying trees before bark beetles can just to discourage them from taking up residence in an area (USFS, 2011). How can these little beetles cause so much destruction? Well, They don’t work alone…

Fungus is integral to a bark beetle’s life cycle. The mountain pine beetle, the most destructive of the bark beetles, spends the first 3 weeks of its adult life eating the fungal spores that line its pupal chamber before emerging to find its own host tree (Six, Klepsig, 2004). When the beetle finds a new host, it arrives covered in millions of fungal spores from its pupal chamber. Some beetles even have organs called mycangia dedicated to carrying fungal spores around so they can release them when needed. (Hofstetter et al., 2005) Once the beetle gets into the tree via a hole in the bark or anything that exposes the phloem, this fungal symbiont, Ophiostoma clavigerum, sets to work digesting the phloem, attacking the tree and, keeping the beetles healthy by acting as an enrichment to the food source as the beetles eat tunnels through the fungi-lined phloem (Hofstetter et al., 2005). This is especially important as the tree's health declines. The living tissue becomes less nutritious for the beetles as the cells die, but the fungus can digest the dead tissue and turn it into a good source of nutrients. It seems that symbiotic fungal species may be the key to the very aggressive nature of many bark beetles such as the mountain pine beetle (Kandasamy et al., 2016). If their food source was not supplemented by this fungus, they would have to leave the host tree sooner, giving it a chance to survive the attack, as is often the case with many other species of bark beetles (Six Klepsig, 2004). We do, however, have a couple of tools to slow the bark beetles down. 

How do you slow growing populations of a beetle that is working with a fungus to kill trees? They are hardly big enough to see if you are not looking closely. Conventional pesticides have not worked in a spray format because the bark protects the beetles within. In the past, foresters have tried trapping beetles, cutting a notch into the roots, and injecting pesticides into the tree directly, which would kill the beetles as they infest the tree, but this also kills the tree, and comes with the added cost of having to come back to cut down and process the now poisonous trees (Krieger 1998). However, this injection method is not recommended for pine bark beetles. According to Texas A&M University, conifer resin in the tree clogs injecting devices like syringes, and even if a pesticide is successfully injected, it is transported to the top of the tree via the xylem rather than to the inner bark where bark beetles are devouring phloem (Manage Forests and Land, 2022). The only really effective tool that has been used against mountain pine beetles is the clearing of affected trees and potential habitats from the forest before bark beetles complete their one-year life cycle and reproduce (Manage Forests and Land, 2022). This comes with its own set of challenges. First, it is very hard to tell if a tree is infested until it is too late, and falling trees being cut down can damage healthy nearby trees, thus creating a breach in the tree’s protective layer and allowing bark beetles to easily access the inner bark (Manage Forests and Land, 2022). Moreover, dead and decaying matter is key to building healthy soils in forests, so the more organic matter we remove, the more our soils, and consequentially our forests, will suffer. To put it bluntly, we don't have a good way to fight these beetles. We have seen drought conditions for 25 years in some parts of the US, and a team of scientists in 2019 proved that dry conditions in an area made for a reliable predictor of bark beetle infestation (Netherer et al., 2019). Luckily, we have imaginative scientists that know when nature presents a problem, it also presents the solutions if we look closely enough.

Enter Dr Richard Hofstetter, PhD of ecology and evolution, and professor of forestry and entomology at Northern Arizona University. He has been working on this problem for more than twenty years. He has found that while fungi are the key to the bark beetles’ success, it may also be their downfall, and subsequently, the answer to slowing down their destruction (Vega & Hofstetter, 2016). In 2010, Hofstetter’s research led him to the conclusion that he could control bark beetle behavior using a speaker and the recorded sounds that bark beetles use to socialize. The team did this by inserting a speaker system directly into the inner bark of the wood by cutting a small hole just deep enough and sealing it with a battery-powered speaker, the likes of which one might find in a greeting card. They found that they could manipulate aggression and mating behavior in the bark beetles depending on what sound they played, including one particularly poignant account where the research team was playing mating sounds and recorded 3 matings between a male and female in an observation box, then turned on an aggression call and watched in horror while the male bark beetle tore the female apart (Stevens, 2010). This kind of behavior is particularly striking because it happened immediately after mating. In many cases in the animal world, a male will protect a female he has mated with because it increases his fitness (Hofstetter et al., 2013). This is a bizarre and very promising discovery that has led to a lot of current research. Hofstetter has spent more than 2 decades of his life studying eruptive and destructive insects like bark beetles, and using what he learned in the experiment above he has been getting closer and closer to figuring out how to slow or even stop their growth.

A few years later, in 2013 Hofstetter and his team had an idea about how to stop bark beetles from entering logs in the first place. They connected a soundproof chamber to a log with a tube for bark beetles to move through. The log was treated by inserting a speaker into the phloem layer, where the beetles live and eat, and playing bark beetle stress calls (Aflitto & Hofstetter, 2013). The team found a decrease in entry into the log by 72% in the southern pine beetles, meaning the majority of the beetles decided to stay away from the food source in the soundproof chamber (Aflitto & Hofstetter, 2013). This serves as a viable solution on a small scale for select trees, but could never be used to protect a whole forest. Luckily, this experiment was just another step in Hofstetter’s path of knowledge. His awareness as a forester and entomologist, that the answer to nature's problems is built into nature itself, is present throughout his early experiments, and his interest was soon to spread into the realm of fungi.

Bavaria bassiana is a fungus native to the forests of the Western U.S. that specializes in destroying arthropods. This particular fungus is quite common in soils throughout the world (Kandasamy et al., 2016). Dr Hofstetter is using this fungus to combat bark beetle invasions and seeing results of up to 90% mortality in test piles of dead pine (the Nau Review, 2015). The issue with this experiment back in 2015 was that B. bassiana was considered a generalist and dangerous to all arthropods, including beneficial ones like pollinators, so they could not be widely used (Mann & Davis, 2021). Now, there are promising studies revealing certain strains of B. bassiana that are virulent against bark beetles but not bees or bark beetle predators (Mann & Davis, 2021). Fungal prevention techniques are ideal for a few reasons: first, beetles are naturally exploratory as it is their main mechanism for obtaining food, so they are more likely to interact with some fungal spores in the meantime. Second, fungi can produce billions and billions of spores in an hour. To make a sprayable slurry that creates 10 million spores per square inch of coverage is not at all difficult. Finally, it proliferates on its own and spreads through your target tree because it is alive, which provides longer-lasting effects compared to chemical pesticides (Mann & Davis, 2021). These experiments have led to a body of research that is growing all the time in a very exciting direction.

The knowledge about bark beetles has many gaps, but thanks to Hofstetter and a driven team of scientists around the world, it expands every day. Recently, teams have used semiochemical traps, which attract bark beetles with scent compounds, cover them in fungal spores once they enter, and contaminate their living space when they get back to the tree. However, research in this area has been small and limited (Manage Forests and Land, 2022). Some teams are even getting close to working out how to inoculate pine trees with B. bassiana through the root systems, so the fungus lives inside the tree and is available to defend it when needed (Mann & Davis, 2021). Last year, a USDA paper was published in Microorganisms detailing findings that mites are symbionts to multiple bark beetle-killing species of fungi, like blue rot fungus, Grosmannia clavigera and describing how to encourage mite growth to dampen bark beetle population growth (Mercado et al., 2021). The more we learn about every aspect of bark beetle life, the more power we have over them. 

Dr. Hofstetter’s research was just the tip of the iceberg, and it has opened a door of discovery that may allow us to put up a good fight for our forests and protect them as best we can in these very uncertain times on planet earth. There is so much potential for us to beat these beetles. The more we understand them, their life cycles, and the organisms they interact with, the more effectively we can stop them. Hofstetter and his team have given us the tools to protect individual trees and we are using their findings more every day on larger and larger scales. We are only a few steps away from protecting entire forests, and if we are lucky, humankind may continue to enjoy them for generations to come.

References

Aflitto, N. C., & Hofstetter, R. W. (2013). Use of acoustics to deter bark beetles from entering tree material. Pest Management Science, 70(12), 1808–1814. Retrieved September 19, 2022, from https://doi.org/10.1002/ps.3720

Batta, Y. (2007). Biocontrol of almond bark beetle (scolytus amygdali geurin-meneville, coleoptera: Scolytidae) using beauveria bassiana (bals.) vuill. (deuteromycotina: Hyphomycetes). Journal of Applied Microbiology, 103(5), 1406–1414. Retrieved September 6, 2022, from https://doi.org/10.1111/j.1365-2672.2007.03369.x

Biedermann, P. H., Müller, J., Grégoire, J.-C., Gruppe, A., Hagge, J., Hammerbacher, A., Hofstetter, R. W., Kandasamy, D., Kolarik, M., Kostovcik, M., Krokene, P., Sallé, A., Six, D. L., Turrini, T., Vanderpool, D., Wingfield, M. J., & Bässler, C. (2019). Bark beetle population dynamics in the anthropocene: Challenges and solutions. Trends in Ecology & Evolution, 34(10), 914–924. Retrieved September 13, 2022, from https://doi.org/10.1016/j.tree.2019.06.002

Hicke, J. A., Bingbing, X., & Arjan, J. H. (2020, June 3). Characterizing recent bark beetle-caused tree mortality in the western United States from aerial surveys [PDF]. Forest Ecology and Management. Retrieved September 15, 2022, from https://webpages.uidaho.edu/~jhicke/outgoing/barkbeetle_mortality_dataset/hicke_AS_recent_outbreaks_200703_accepted_FEM.pdf

Hofstetter, R. W., Cronin, J. T., Klepzig, K. D., Moser, J. C., & Ayres, M. P. (2005). Antagonisms, mutualisms and commensalisms affect outbreak dynamics of the southern pine beetle. Oecologia, 147(4), 679–691. Retrieved September 6, 2022, from https://doi.org/10.1007/s00442-005-0312-0

Hofstetter, R. W., Dunn, D. D., McGuire, R., & Potter, K. A. (2013). Using acoustic technology to reduce bark beetle reproduction. Pest Management Science, 70(1), 24–27. Retrieved September 19, 2022, from https://doi.org/10.1002/ps.3656

Kandasamy, D., Gershenzon, J., & Hammerbacher, A. (2016). Volatile organic compounds emitted by fungal associates of conifer bark beetles and their potential in bark beetle control. Journal of Chemical Ecology, 42(9), 952–969. Retrieved September 13, 2022, from https://doi.org/10.1007/s10886-016-0768-x

Krieger, C. (1998). An overview of bark beetle control methodologies. Forestry, 17. Retrieved September 12, 2022, from http://www.gov.pe.ca/photos/original/af_bark_beetle.pdf

Manage forests and land. (2022, September 19). Texas A&M Forest Service. Retrieved September 19, 2022, from https://tfsweb.tamu.edu/PreventionAndControlOfPineEngraverBeetle/

Mann, A. J., & Davis, T. S. (2021). Entomopathogenic fungi to control bark beetles: A review of ecological recommendations. Pest Management Science, 77(9), 3841–3846. Retrieved September 19, 2022, from https://doi.org/10.1002/ps.6364

Mercado, J. E., Ortiz-Santana, B., & Kay, S. L. (2021). Endemic jeffrey pine beetle associates: Beetle/mite fungal dissemination strategies and interactions that may influence beetle population levels. Microorganisms, 9(8), 1641. Retrieved October 18, 2022, from https://doi.org/10.3390/microorganisms9081641

Nau scientist uses fungus to destroy bark beetles – the nau review. (2015, February 15). The Northern Arizona University review. Retrieved September 19, 2022, from https://news.nau.edu/nau-scientist-uses-fungus-destroy-bark-beetles/

Netherer, S., Panassiti, B., Pennerstorfer, J., & Matthews, B. (2019). Acute drought is an important driver of bark beetle infestation in austrian norway spruce stands. Frontiers in Forests and Global Change, 2. Retrieved October 19, 2022, from https://doi.org/10.3389/ffgc.2019.00039

Six, D. L., & Klepzig, K. D. (2004). Dendroctonus Bark Beetles as Model Systems for Studies on Symbiosis. Symbiosis, 37, 207–232. Retrieved September 6, 2022, from

Stevens, B. (2010, February 2). Beetle mania rocks northern arizona lab [PDF]. Northern Arizona University Ecological Restoration Institute. Retrieved September 19, 2022, from http://openknowledge.nau.edu/id/eprint/2485/1/Stevens_B_2010_BeetleMania.pdf

Stevens, B. (2015, January 22). Brain food: Nau bark beetle researcher fights back with fungus. KNAU Arizona Public Radio. Retrieved September 6, 2022, from https://www.knau.org/knau-and-arizona-news/2015-01-22/brain-food-nau-bark-beetle-researcher-fights-back-with-fungus

U.S. Forest Service. (2011). final bark beetle strategy [PDF]. U. S. forest service. Retrieved October 1, 2022, from https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5337222.pdf