When it comes to treating subterranean termite infestations of existing structures, the two most common control approaches include applying a liquid termiticide to the soil and installing termiticide bait stations around the foundation of the structure. In post-construction treatments, the liquid termiticide is applied to the soil directly against the foundation of the structure while the bait stations are installed a few feet to a few meters away from the foundation. Both control options work similarly by incorporating slow-acting active ingredients that allow termites to survive long enough after exposure to pass the active ingredient onto colony mates through trophallaxis (sharing of food through regurgitation) or through social contact.  

Decades ago, liquid termiticides were formulated with repellent insecticides under the premise that the repellency of the products would help to deter termites. Unfortunately, the repellent nature of the products also meant that termites could more easily find breaks in the barrier treatment that may unintentionally occur when soil is disturbed after application, such as when a homeowner decides to build a new deck or re-landscape one side of the home. A major breakthrough in the liquid termiticide game came with the development of non-repellant active ingredients such as fipronil. The absence of repellency meant that subterranean termites who encountered treated soil would not know when they were in a treated area and would be less likely to find any possible gaps in treatment coverage.  

As is the case with other non-repellent liquid termiticide chemistries, fipronil is intended to go undetected by hungry termites as they forage through treated soil. Additionally, fipronil should be slow acting enough to allow termites enough time to leave the treated area and share the active ingredient with other colony mates before succumbing to the effects of the toxicant. In a recently published study, Dr. Thomas Chouvenc is challenging the full extent of fipronil’s non-repellent nature in an interesting way…through what he’s calling the death zone!  

In a series of laboratory experiments, Thomas used specially constructed, 3x15 meter foraging arenas to measure how far subterranean termites would travel after encountering fipronil-treated soil before they started to die off. This die-off effect is significant because termites are “necrophobic” meaning they will actively avoid dead nestmates. So, areas where the dead start to pile up…the death zone...could have repellent effects on foraging termites.   

Using the foraging arenas, Dr. Chouvenc introduced termite colonies to arenas where soil was treated with fipronil (0.06% concentration) at either 1.5m, 7.5m, and 12.5 meters away from the colony central nest (See Figure 1). Then he measured if they started dying, and how far that death zone would be away from the treated area. What he found was the “death zone” for colonies set at the shortest 1.5m distance from the treated area averaged just over 1 meter away from the from the treatment. For colonies set at 7.5m and 12.5m away from the treated area, the “death zone” averaged about 2.56 meters away from the treatment. He also noted that mortality was low across all three treatment distances over a 200+ day span, with most of the mortality being recorded in the first few days of the experiments. The low mortality was largely assumed to be a result of the death zone cutting other termites off from accessing the treated soil once the dead started to pile up. 

Dr. Chouvenc’s conclusions from the study are that, despite fipronil’s non-repellent chemistry, the faster rate of kill of the active ingredient could lead to the establishment of a death zone around structures that would act to repel termites from continuing to access the treated soil. And, that does seem to be the case in his study.   

 

But, as is the case with all experiments, there are always some considerations to take into account. First, Dr. Chouvenc is the protégé of, and works out of the same lab as, Dr. Nan Yao Su who is known for his development of Sentricon, a well-known commercial termite baiting system. 

And second, this study was done in a laboratory setting using experimental foraging arenas which inherently impart limitations on the study and influence termite behavior. While Dr. Chouvenc does an excellent job at trying to minimize other variables, it’s nearly impossible for foraging arenas to replicate real world environments. So, more work needs to be done in the field to fully evaluate this death zone effect on subterranean termites in the field. 

Having said all that, I find this research very compelling and have had several conversations with Thomas about this work over the past year. What’s most interesting to me is his hypothesis of how the death zone effect could impact overall treatment efficacy of structures that utilize both a liquid termiticide and a bait station treatment. Essentially, Thomas’s laboratory experiments suggest that the distance of the death zone from the treated foundation of a structure could fall in line with the distance where bait stations are typically installed away from a structure. That would mean stations could be installed in this repellent area, potentially having negative impacts on the efficacy of bait stations. He plans to follow up this study with field work to continue testing this hypothesis, so stay tuned for future updates! 

Article by Mike Bentley, PhD, BCE

References

Thomas Chouvenc, Death zone minimizes the impact of fipronil-treated soils on subterranean termite colonies by negating transfer effects, Journal of Economic Entomology, 2024;, toae150, https://doi.org/10.1093/jee/toae150

 

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