If you’re a fan of HBO’s popular series, “The Last of Us,” (or you’ve played the video game the show is based on), then you’re already familiar with the concept behind the paper I’m covering in this episode. But, in case you aren’t up to speed on the show, here’s a very brief cliff notes version. Basically, a brain fungus called Cordyceps that is known to infect arthropods makes the terrifying leap to infect humans and turn them into zombies. While there isn’t any truth to the idea that Cordyceps can infect people in the way portrayed on the show, there are in fact hundreds of fungi that are known for turning arthropods into real-life zombies! And, just like the show, these fungi manipulate insect host behavior turning infected individuals into mindless food bags that are being eaten alive from the inside out by the predatory fungus.

Some zombifungi make adult insects wander to a high point on a wall or a tree branch, where they die frozen in place so the fungal spores can rain down and disperse more widely. Another fungus that infects houseflies causes the female carcass to give off an odor that attracts males to mate with the carcass and become covered with spores. And the list of crazy zombie side effects goes on!

Because these fungi self-replicate and the end result of infection is death, researchers have explored the use of certain entomopathogenic fungi as biological control agents against invasive insects for years. One widely used and well-studied species of fungi is Metarhizium anisopliae (MA). MA is known for being target specific and easy to mass-produce making it an ideal candidate for the development of mycoinsecticides that can be used to control invasive pests.

The red imported fire ant, Solenopsis invicta Buren, is one of the most notorious and ecologically damaging invasive ants on the plant. Efforts to stop the spread of this species around the world have fallen short thanks to human mediated transport and this ant’s ability to quickly dominate non-native habitats. While there are bait and liquid insecticide options for the RIFA, any chance to add another control tool to the arsenal is always a positive. Previous research showed that the fungus, MA, exhibited some toxicity to RIFA, demonstrating that it could be a viable option to explore further. Which brings us to the research by Hassan et al. (2024).

 

 

Remember, the way these fungi work is they change the host’s behavior. The hope is the change is impactful enough to reduce the target pest’s survival. To test the viability of MA as a possible mycoinsecticides against the RIFA, Hassan and colleagues directly applied MA spores at different concentrations to RIFA workers and measured their foraging behavior and movement patterns for up to36 hours after exposure. They even measured biochemical changes and antifungal activity to determine what internal changes could be occurring and how effective the ants were at fighting off the infection

What they found was that fungal infection impacted everything they measured. For foraging behavior, infected ants spent significantly less time in or near the feeding area compared to uninfected ants. For locomotion, infected ants traveled shorter distances and made fewer turns when traveling. Infected ants also measured significantly lower enzymatic activity of enzymes related to detoxification after fungal stress. Lastly, researchers found that antifungal activity did increase post exposure, but that survival of ants exposed decreased.

Overall, the results from this study were consistent with previous studies evaluating MA and should encourage further research into the use of MA to control RIFA. However, there are some important caveats to point out with this study that could impact the application of MA in the field. 

First, this study was conducted in a very controlled laboratory setting, and MA was physically applied to workers at varying concentrations. These exposure settings and rates are likely not ecologically relevant and would differ dramatically from how ants may be exposed in the field. So, more work needs to be done to determine if MA is a viable candidate. Secondly, and most importantly, is the fact that ants are NOTORIOUSLY difficult to control with entomopathogenic fungi for a number of reasons. Ants are packed with some highly sophisticated and impressive antifungal weapons at the individual and colony-wide levels that have evolved over millions of years to combat these natural pathogens. So, finding something that’s going to punch through these defenses is tough. Having said that, if MA does show through further research to be a viable option, it could prove to be an important support tool in helping to control one of the most damaging invasive ants on the planet.  

Article by: Mike Bentley, BCE, PhD

References

Ali Hassan, Lidong Kang, Kaixiong Zhang, Lei Wang, Xianjiao Qin, Guobin Fang, Yongyue Lu, Qiuying Huang, Effect of entomopathogenic fungi on behavior and physiology of Solenopsis invicta (Hymenoptera, Formicidae), Journal of Economic Entomology, 2024;, toae068, https://doi.org/10.1093/jee/toae068

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